Guido Petrucci

Emilie Rioust

José-Frédéric Deroubaix

Bruno Tassin

Do stormwater source control policies deliver the right hydrologic outcomes?

Introduction

In the last five decades, stormwater Source Control (SC) has gained relevance over traditional sewer approaches, mainly for its potential to cope with fast urbanization and non-point source pollution (Bergue and Ruperd, 1994;[START_REF] Delleur | The evolution of urban hydrology: past, present, and future[END_REF][START_REF] Urbonas | Summary of emergent urban stormwater themes[END_REF]. The principle of SC is to develop, simultaneously to urban growth, facilities to manage stormwater at a small-scale (about 10 2 -10 3 m 2 ) to solve or prevent catchment-scale 10 6 -10 7 m 2 stormwater problems. These parcel-scale facilities are usually called Best Management Practices (BMPs) in the USA, Sustainable Urban Drainage Systems (SUDSs) in the UK, techniques alternatives in France. Terms like Low Impacts Development (LID) in the USA or Water Sensitive Urban Design (WSUD) in Australia are used today to identify the application of this principle to the whole design of new urban developments, which is typical of an intermediate scale 10 4 -10 5 m 2 (Morison and Brown, 2011;[START_REF] Revitt | The best management practice catalogue developed within the DayWater project[END_REF][START_REF] Roy | Brown Impediments and solutions to sustainable, watershed-scale urban stormwater management: lessons from Australia and the United States[END_REF]Williams and Wise, 2006). In this paper, we will use "BMP" to address individual stormwater facilities, and "SC" when speaking of the catchment scale strategy.

In a first development phase, SC was realized, in most cases, by large reservoirs built to prevent sewer overflows.

Then, the principle of diffused small facilities started to be applied, and a pioneering phase of SC policies began [START_REF] Chouli | La gestion des eaux pluviales urbaines en Europe: analyse des conditions de développement des techniques alternatives[END_REF]. The first regulations enforceable for all new developments date back to the 1980s: the city of Bordeaux, in France (Bourgogne, 2010) started implementing SC policies in 1982, while the State of Maryland, in the USA, in 1984 (Comstock and Wallis, 2003).

Even if no official inventories of local SC policies are available, in several countries (e.g. France, USA, UK, Brazil) their diffusion strongly accelerated in the last ten years [START_REF] Ellis | Identifying success factors in urban surface BMP implementation: mission impossible?[END_REF]. In the USA, EPA regulation urges local authorities to adopt policies [START_REF] Epa | B. Faulkner The control of surface water runoff from new development -UK national 'policy' in need of review? Urban Water[END_REF], and a similar effect is expected, in France, for the SDAGE Seine-Normandie (a large-scale catchment management plan, Section 3.4 of this paper). This change of pace is due to a convergence of technical and political rationalities: SC offers an efficient opportunity for urban stormwater drainage and, in the last years, it acquired an halo of sustainable development that increased SC policies appeal for many local authorities [START_REF] Novotny | Cities of the future: towards integrated sustainable water and landscape management[END_REF].

Because of the accelerated diffusion of regulations demanding widespread construction of BMPs, we can expect in the near future that the hydrologic behavior of many urban and periurban catchments will be influenced, if not determined, by SC. In most cases, this effect will be driven by the policies that are discussed and implemented today. In France, the Seine-Saint-Denis county (Section 3.2), up to 2009, has prescribed 470,000 m 3 of BMPs on a territory of 236 km 2 . Considering an approximate impervious cover of 20%, this represents about 10 mm of storage on the whole impervious area of the county: a value comparable, for example, to weekly mean evaporation.

In view of this relevance, since the 1960s researchers investigate BMPs performances and efficiency. This effort had practical effects, diffusing good design and construction practices. Several BMPs' selection and design manuals have been published (e.g. [START_REF] Azzout | Techniques alternatives en assainissement pluvial: choix, conception, réalisation et entretien. Technique & Documentation[END_REF]Bergue and Ruperd, 1994;[START_REF] Clar | Stormwater best management practice design guide[END_REF]Debo and Reese, 2002;[START_REF] Woods-Ballard | The SUDS Manual[END_REF], that contributed to the actual diffusion of BMPs. However, studies on the global effect of BMPs on a catchment hydrology are still scarce, and so are directions on how to define a suitable policy [START_REF] Roesner | Are best-management-practice criteria really environmentally friendly?[END_REF][START_REF] Roy | Brown Impediments and solutions to sustainable, watershed-scale urban stormwater management: lessons from Australia and the United States[END_REF]. The transition from a pioneering phase to a rapid and wide diffusion of SC policies makes these questions a priority. If, in the past, efficient design of individual BMPs was the main concern, today it is important to investigate what an effective SC policy at the catchment's scale is.

The hydrological analysis of global effects of SC faces two main difficulties. The first one is that policies effects emerge slowly, at the rhythm of urban growth and renovation. Hence, catchment-wide measurements are still scarce and incomplete (see, for an example, [START_REF] Meierdiercks | Heterogeneity of hydrologic response in urban watersheds[END_REF]. Even when data are available, it is difficult to distinguish gradual SC effects from catchment uncontrolled evolutions [START_REF] Petrucci | Tassin Rainwater harvesting to control stormwater runoff in suburban areas. An experimental case-study[END_REF] or other stormwater management actions, like sewer system developments. The second difficulty concerns spatial scales: passing from parcel-scale BMPs to catchment-scale effects demands a good knowledge of scale-transition processes in urban and peri-urban settings [START_REF] Cantone | Potential dangers of simplifying combined sewer hydrologic/hydraulic models[END_REF]Chocat and Cabane, 1999). A similar issue has been pointed out for rural small scale surface runoff control measures [START_REF] O'connell | Quinn Is there a link between agricultural land-use management and flooding?[END_REF].

In the absence of comprehensive measurements on catchment-scale effects of SC, researches about this topic have been mainly based on hydrological modeling. Because the purpose of this modeling effort was to extrapolate smallscale known processes (i.e. the behavior of individual BMPs) to predict large-scale unknown effects, most researchers adopted a bottom-up scaling approach [START_REF] Blöschl | Scale issues in hydrological modelling: a review Hydrol[END_REF]. In practice, researches on this topic relies mostly on physically-based distributed models (e.g. SWMM, HEC-HMS, MOUSE), allowing for a detailed description of both the BMPs and the large-scale processes. These researches, exploring both water quality (e.g. [START_REF] Freni | GaspareViviani Urban storm-water quality management: centralized versus source control[END_REF][START_REF] Wu | Zou A water quality-based approach for watershed wide bmp strategies[END_REF] and water quantity effects (e.g. Carter and Jackson, 2007;[START_REF] Emerson | Watershed-scale evaluation of a system of storm water detention basins[END_REF]Faulkner, 1999;[START_REF] Goff | Gentry The influence of watershed and development characteristics on the cumulative impacts of stormwater detention ponds[END_REF]Gentry, 2006,: McCuen, 1979;[START_REF] Mouy | Faisabilité hydraulique du zonage pluvial à Paris[END_REF][START_REF] Urbonas | Potential effectiveness of detention policies[END_REF][START_REF] Zimmer | Low-impact-development practices for stormwater: implications for urban hydrology Can[END_REF], allowed to identify some important discrepancy between hydrological studies and actual SC policies (e.g. Booth and Jackson, 1997;Goff and Gentry, 2006;[START_REF] Mccuen | Downstream effects of stormwater management basins[END_REF].

This paper focuses on these discrepancies: the question is how the policy-making process comprehendor nothydrological considerations. In order to answer, this paper integrates two complementary approaches to study SC regulations: at first, it presents a regulations' analysis that aims to make explicit the logics behind SC policymaking; then, it discusses these logics through an hydrological analysis of the catchment-scale effects of SC regulations. Section 2 presents a review of researches on water-quantity catchment-scale effects of SC, focusing on the discrepancies between SC policies and hydrological outcomes. The regulations' analysis is presented in Section 3: three case-studies of SC policy-making are discussed to find how, and on which logics, regulations are developed today. Section 4, through the physically-based distributed model of a peri-urban catchment, assesses consequences, at the catchment-scale, of widely applied regulations. Sections 3 and 4 are based on French cases, but both the methodology and several results could be extended to other countries where SC is developing.

Background and research approach

Despite the difficulties highlighted in the introduction, hydrological analysis of global effects of SC provided some general guidance on how to design policies. For example, as BMPs must be adapted to their specific site, SC policies should be conceived according to the specific catchment's characteristics [START_REF] Ellis | A paradigm switch in urban stormwater management: achieving integrated strategic approaches; the Belo Horizonte example[END_REF][START_REF] Epa | B. Faulkner The control of surface water runoff from new development -UK national 'policy' in need of review? Urban Water[END_REF].

Another example is that many researchers, starting with the early analysis of [START_REF] Mccuen | Downstream effects of stormwater management basins[END_REF], agree on the fact that hydrographs timing must be taken into account when planning storage facilities: a local reduction of hydrographs' peak flow can produce a catchment-scale increase due to peaks' superposition (Fig. 1). Fig. 1. Peaks' superposition due to storage facilities (adapted from [START_REF] Azzout | Techniques alternatives en assainissement pluvial: choix, conception, réalisation et entretien. Technique & Documentation[END_REF].

It is surprising that, even if these general ideas gather relevant consensus among scientists, many implemented policies are in contrast with them. Many French policies impose a unique value-often very low-of admitted flow-rate from parcels (i.e. in the form of a x l s -1 ha -1 constraint) over entire regions, without considering catchments' specificities or hydrographs' superposition. Some UK regulation shows the same shortcomings (Faulkner, 1999). In the USA, regulations often demand to preserve pre-development peak flow-rates downstream of parcels (Balascio and Lucas, 2009;[START_REF] Fennessey | Changes in runoff due to stormwater management pond regulations[END_REF], ignoring peaks' superposition effects [START_REF] Emerson | Watershed-scale evaluation of a system of storm water detention basins[END_REF].

A complementary remark about current policies is that, even if they often involve different instruments to develop SC (e.g. recommendations to infiltrate or reuse stormwater, financial or technical support), most of them fix a regulation on maximum flow-rate downstream of parcels as the only quantified constraint. Today, many scientific works converge in criticizing this kind of flow-rate regulations.

The first critic is that SC policies explicitly or implicitly aim, in general, to preserve pre-development water balance: it has been shown that flow-rate constraints are, in most cases, unable to achieve this goal. In particular, these constraints do not cope with reduced infiltration volumes due to imperviousness, and distort downstream low-flow regimes (Booth and Jackson, 1997;[START_REF] Fennessey | Changes in runoff due to stormwater management pond regulations[END_REF]. [START_REF] Meierdiercks | Heterogeneity of hydrologic response in urban watersheds[END_REF] analyzed ten years of runoff data to compare three catchments: one undeveloped and two developed, respectively, before and after the adoption of a SC regulation (flow-rate based). In terms of hydrologic behavior, the catchment developed with SC is closer to the one without SC than to the one undeveloped.

The second critic is about peak flow-rate: as stated above, provisions demanding to preserve pre-development peak flow-rate locally, can actually worsen the situation at the catchment-scale [START_REF] Emerson | Watershed-scale evaluation of a system of storm water detention basins[END_REF]Goff and Gentry, 2006;[START_REF] Mccuen | Downstream effects of stormwater management basins[END_REF], depending on catchment's timing characteristics. Regulations demanding a specific value of flow-rate can have adverse effects on peak flow-rate until the catchment is not completely urbanized, because superpositions between regulated and non-regulated flows can occur. This last type of regulation can also affect intermediary flows, responsible of combined sewer overflows (CSOs). A study of the city of Paris [START_REF] Mouy | Faisabilité hydraulique du zonage pluvial à Paris[END_REF] showed that limiting flow-rates could be effective in reducing peak flow-rate and sewer overflows, but could increase frequency and duration of CSOs. Booth and Jackson (1997) proposed, in order to avoid these problems, to base regulations on runoff characteristics different from flow-rates; [START_REF] Emerson | Watershed-scale evaluation of a system of storm water detention basins[END_REF], more specifically, suggested limiting runoff volumes.

In contrast with the traditional regulations quantifying flow-rates, some authority is adopting policies that prescribe a volume that must be treated at the parcel-scale, without sending it to the sewer network. In the city of Paris, redevelopments have to integrate an infiltration area capable to collect, store and infiltrate a volume of rainwater corresponding to 4-12 mm [START_REF] Mouy | Faisabilité hydraulique du zonage pluvial à Paris[END_REF]. In the absence of infiltration possibilities green roofs can be used.

In the USA, [START_REF] Bollig | Diffusion of innovations: how local governments shape public policy to promote progressive stormwater management[END_REF] reports of a urban runoff mitigation ordinance in Sun City, Arizona, requiring a reduction in runoff corresponding to 0.75″(19mm). On a larger scale, the 2000 Maryland Stormwater Design Manual prescribes that stormwater ponds must retain volumes for water quality and groundwater recharge (Comstock and Wallis, 2003).

In general, the effect of SC on a catchment will depend on both the characteristics of the catchment (topography, geology, climate, urban development) and of the regulation (type, level of constraint). Most of the studies focus on catchments' variability, while regulations' variability is seldom considered in depth, despite its relevance for policy-making (among the cited studies, only [START_REF] Fennessey | Changes in runoff due to stormwater management pond regulations[END_REF] and [START_REF] Mouy | Faisabilité hydraulique du zonage pluvial à Paris[END_REF] compare several regulations). Studies on catchments' variability support the idea that flow-rate regulations can be ineffective and even harmful for some catchments, but they do not help defining which values of the limit can be effective, ineffective or harmful. Similarly, there are evidences that runoff volume regulations can be effective for peak flowrate reduction at the catchment-scale, but we do not know the link between constraint level and effectiveness.

To analyze the effect of regulations' variability on a periurban catchment, one option is to consider the "transient state", where the catchment is only partially equipped with BMPs, or the "steady state", where all the urban areas are equipped. In the first case, the focus can be on peaks' superpositions between controlled and uncontrolled areas. In the second case, regulation's effect does not depend on spatial distribution of BMPs in the urban area, but on the regulation type and on the level of constraint. In Section 4 of this paper, the "steady state" approach is applied to the study of a periurban catchment in the Paris region, representative of the drainage conditions of the regional 1950-2000 urbanization. The model of this catchment is used to perform a set of simulations of flow-rate and volume regulations, with varying constraint values.

Because of the dependence on catchment's characteristics, generalization of results obtained on a single catchment should be done carefully. Nevertheless, the analysis of a specific case-study can provide information about the possible behavior (or type of behavior) that a catchment can have when submitted to SC, and thus orient subsequent research and policy-making efforts. In particular, it can help to identify potentially harmful shortcomings in policymaking.

Regulations' analysis

Methodology

To analyze logics behind SC policies, we present three French case-studies representative of several spatial scales and degrees of urbanization (Fig. 2). For each case, we conduct a survey based on official documents, technical presentations and semi-structured interviews [START_REF] Quivy | Manuel de recherche en sciences sociales[END_REF] of technical staff. The purpose of the surveys was to reconstruct the historical process of SC policy-making. We focused on the policies' objectives, the relations to territorial characteristics and the technical framing. The various written sources were collected in order to construct, for each case, an initial "factual framework" of SC policy evolution. The interviews, cross-checked and compared with the available data, were treated like witnesses' accounts: they allowed to fill the factual frameworks with the arguments and motivations that driven SC evolution. Following this historical reconstruction, we analyzed the arguments retrieved from an hydrological point of view. In Sections 3.2, 3.3 and 3.4, we resume the local histories of SC policies for the three cases selected:

1.

the SC strategy of the Seine-Saint-Denis county. This county covers a densely urbanized area (236 km 2 , 1500,000 inhabitants). It is considered one of the most advanced local authorities in terms of stormwater management in France and an example of a "pioneer" local authority in SC development. For this case, four county's technicians have been interviewed between 2008 and 2010.

2.

The SC strategy of the Syndicat Mixte de la Vallée de l'Orge Aval (SIVOA), an authority in charge of stormwater management in the catchment of the Orge river in the Paris region. This catchment is still mostly rural, but facing a growing urbanization (207 km 2 , 370,000 inhabitants). The SIVOA applies a SC regulation since the 1990s. Two SIVOA technicians have been interviewed in 2009 and 2010.

3.

The Schéma Directeur d'Aménagement et de Gestion des Eaux Seine-Normandie 2010-2015 (SDAGE), is the River Basin Management Plan for the Seine river and some other small coastal catchments (globally 100,000 km 2 , 18,000,000 inhabitants). This plan (Comité de Bassin Seine-Normandie, 2009) includes SC regulations at a large catchment scale. About this plan, two technicians have been interviewed (2010): one of the catchment authority, and one of the navigation authority for the Seine river, having police power for water discharges in the river.

These cases are representative of many French current regulations. Moreover, in the discussion we will find that several observations on these cases can be extended, or at least adapted, to SC regulations in other countries.

The SC strategy of the Seine-Saint-Denis county

The territory of the Seine-Saint-Denis county incurred severe stormwater flooding starting from the 1960s. Today, 25 municipalities out of 40 are classed as "strongly" or "quite strongly" exposed to stormwater flooding risk (Préfecture de Seine-Saint-Denis, 2008).

The reasons of this risk exposure are the county's territory configuration (the county is, topographically, extremely flat, with off-centered stormwater natural outfalls) and fast urbanization. Since the 1960s, the county developed a set of strategies to prevent stormwater flooding.

The first strategy adopted was to improve and optimize the sewer network, both building large reservoirs to regulate inflows to the sewer, and developing real-time control of the system. Despite this strategy displayed consistent successes, it also showed its limits: fast urbanization continued, further aggravating flooding risk.

County's resources were not sufficient to solve existing flooding problems and, in the same time, to compensate this aggravation.

This situation drove to adopt SC: the county began to demand developers to contribute to stormwater management, building ponds. Dimensioning was based on a 10 l s -1 ha -1 flow-rate value. In 2009, the estimated (i.e. prescribed, not verified on the ground) volume of BMPs on the county was about 470,000 m 3 .

Since 2000, the county undertook two main actions to coordinate municipalities developing local SC strategies: the drawing of a sewer capacity map and of an infiltration map of the county.

The sewer capacity zoning was justified by enduring flood risk in some areas: the sewer was not able to receive 10 l s -1 ha -1 everywhere. Thus, the county's technicians identified the critical sections of the sewer and, through a hydraulic model of the sewer system, found the maximal flow capacity in these sections. Upstream area was not considered in the identification of the critical sections, but used for tracing back maximal capacities to an areal flow-rate (i.e. x l s -1 ha -1 ): the ratio between the capacity in a section and the total catchment area upstream of the section was calculated.

The infiltration map was motivated by pollution concerns: even if limiting flow-rate reduced urban flooding, it was not able to reduce CSO to the Seine river. Thus, it was important to reduce runoff volumes infiltrating stormwater wherever this was possible. An analysis of the pedology of the county has been performed, defining soil permeability and other constraints (e.g. presence of clay or gypsum).

We can remark, in this short history of SC in Seine-Saint-Denis, a trend that is common to most French "pioneer" authorities: SC starts as a reaction to urban flooding, simultaneously to a strategy based on large reservoirs to optimize sewer management. Only later (since the end of the 1980s) SC begins to include water quality as a new objective, that still remains secondary. This evolution has consequences on the actual policies: the x l s -1 ha -1 form is considered the norm because defining an output flow-rate (and a design rainfall) is the simplest way to define a reservoir that should limit the peak flow-rate in the downstream sewer.

A difference between reservoirs and BMPs is about infiltration. Reservoirs drain large urban areas and thus the water entering them is, in general, polluted or exposed to high pollution's risks. For this reason, in order to protect the underlying groundwater, infiltration is often proscribed (Bergue and Ruperd, 1994). For reservoirs where infiltration is admitted, it is considered less reliable than storage, because of its natural variability. On the contrary, BMPs drain small areas, with smaller water volumes to be infiltrated and smaller risks of pollution. Thus, infiltration is a main strength of BMPs.

The persistence of reservoir dimensioning practices in SC strategies is evident in the preeminence of the flow-rate approach over infiltration prescriptions. This fact emerges in the duality of the mapping effort of the Seine-Saint-Denis county: volume control through infiltration (or water harvesting and reuse) is not seen as an opportunity to limit peak flow-rates in the sewer, but just to protect water quality. Further, while flow-rate is quantified, the main concern about infiltration is only to define if it is possible or impossible.

The SC strategy of the Syndicat Mixte de la Vallée de l'Orge Aval

The SIVOA territory is crossed by the Orge river, a tributary of the Seine. Consequently, storm sewers are mainly structured as parallel linear collectors draining water from municipal networks to the Orge river. In some cases, small tributaries of the Orge are used as collectors and some have been buried under urban areas.

The urban growth of the last fifty years proceeded from downstream areas, close to the river or its tributaries, toward upstream areas. This dynamic is common of many periurban areas in France and other Countries, and it is responsible for a progressive lengthening and saturation of stormwater collectors (Bergue and Ruperd, 1994).

Straightened and buried creeks are a common feature of many urban areas, and are often subject to sewer overflows. In the SIVOA only a few areas are prone to this problem, but many collectors (like in the catchment described in Section 4.1.1) are close to saturation while urbanization of their catchments is still going on.

Then, differently from the Seine-Saint-Denis, where earlier flooding problems were frequent, SC policies of the SIVOA are more linked to severe and episodic floods of the Orge river, in particular those of 1978 and 1999 [START_REF] Fromon | Évaluation de la qualité des cours d'eau. Une approche par la méthode des prix hédonistes appliquée à quatre cas d'étude[END_REF]. In 1992, a study compared available capacity of the Orge river sections with expected urban growth. The result was that the ratio between available capacity at the most critic section and the projected impervious area was 2 l s -1 ha -1 . The conclusion of the study was to set a precautionary limit of 1 l s -1 ha -1 that would be applied in the form of stormwater ponds. Note that, differently from Seine-Saint-Denis, that used for calculations the total upstream area of a critic section, SIVOA used impervious upstream area. We will discuss this point in the hydrological analysis (Section 4.3.1).

The 1999 flooding produced a reinforcement of SC policies. The current regulation of the SIVOA [START_REF] Sivoa | Règlement d'assainissement[END_REF] prescribes that:

"Stormwater collected at the scale of private parcels is not directly accepted in the sewer system. This water has to be infiltrated, regulated or treated. […] the general rule is to search solutions allowing for the absence of stormwater runoff. […] In cases where infiltration […] demands disproportionate works, stormwater is stored and sent to the sewer system at a limited flow-rate." (translated by the authors).

The limited flow-rate is set to 1 l s -1 ha -1 for a rain event of 55 mm in 4 h, corresponding to a 20-year return period [START_REF] Sivoa | Règlement d'assainissement[END_REF]. The 2003 regulation results, thus, from the superposition of a "no-runoff" prescription over the pre-existing 1 l s -1 ha -1 limit.

The superposition of an infiltration prescription before a constraint on flow-rate is a diffusing practice in France.

It is based on the idea that reducing runoff volume is even better than reducing flow-rate. However, in this case like in many others, infiltration is left vague, while flow-rate constraint is clearly defined and quantified. For SIVOA, no-runoff is "the general rule", and the developer should find by himself the way to realize it; if these ways appear "disproportionate", the developer can apply the ready-to-use storage volumes. According to the authors' opinion, the risk connected with this formulation is that in the absence of a clear (and rigid) praxis of what "disproportionate" means, most developers will chose the easiest way to cope with regulation and build ponds.

The infiltration prescription will be ignored in most developments.

The Schéma Directeur d'Aménagement et de Gestion des Eaux Seine-Normandie, 2010-2015

From the point of view of watersheds' management, the entire France is divided into six parts, and for each a Schéma Directeur d'Aménagement et de Gestion des Eaux (SDAGE) is elaborated (Fig. 2).

The current version of the SDAGE (2010-2015) Seine-Normandie (Comité de Bassin Seine-Normandie, 2009) prescribes, for urban areas, the development of stormwater zoning that "can define criteria relative to […] maximal flow-rate". While this tool is already provided-without specific quantification-by French law, the SDAGE fixes a "default value":

"[…]
In the absence of studies or local doctrines determining [an acceptable downstream flow-rate], it will be limited to 1 l s -1 ha -1 for a storm event having a return period of 10 years. […] In order to prevent floods and to take into account the natural water cycle, zoning regulations should foster infiltration of stormwater and restoration of pervious soils." (translated by the authors).

As discussed in Section 2, there is growing consensus about the necessity of local and specific regulations of stormwater management. The SDAGE recognizes this necessity when it calls for local studies and doctrines but, at the same time, it sets a general standard of 1 l s -1 ha -1 for a 10-year storm. An interview with a member of the technical staff of AESN confirmed that the rationale of this default value comes more from communication than from hydrology: "I think that, behind the display of this value [of 1 l s -1 ha -1 ], was the will to make things evolve. This means that 0, 1, 2-well, any low value-was not important by itself. The important thing behind the current formulation […] is that stormwater management must be studied locally. […] The unique value has no meaning. Well, it has a

meaning only if you watch it as a really low value that pushes, by the constraints it creates, people to study what else can be done of intelligent." (translated by the authors).

This approach is perfectly coherent with the scope of a large-scale planning tool like the SDAGE: urban stormwater floods are a local problem that must be solved locally; the large-scale role is just to encourage and support good practices. However, from the point of view of our analysis, the fact of setting a low flow-rate value hides a major shortcut: what happens if people just apply the rule as it is?

The answer to the question which is implicit in the SDAGE formulation is that people (or authorities) applying the rule as it is will provide an effort bigger than necessary, improving the situation even more than required. The underlying assumption is that oversized BMPs are always beneficial, and thus a low flow-rate value is always better than a higher one.

As a final remark, the question about the application of the rule "as it is" is not just hypothetical: past experiences of stormwater management rules showed that general numerical standards are often followed strictly, without adaptation (Chocat and et al., 1997, p. 693).

Discussion

Some observation about SC policies can be drawn from the three cases discussed:

• BMPs are "small-scale large reservoirs". SC policies, at least in France, are a direct descendant of large stormwater reservoirs. Even if the objective of SC policies is no more flood prevention alone, but also pollution control, receiving water protection and water cycle mimicking, many authorities still focus on flow-rates constraints while elaborating SC policies. Flood prevention and the other objectives are still considered as different topics. This duality is not limited to France: in the USA, stormwater ponds and LID are often described as different technical objects, having different purposes (Williams and Wise, 2006). In a catchment-scale approach to catchment's problems, all these techniques should be seen as parts of the same set of tools.

The situation is however evolving: in France, the diffusion of infiltration prescription is a positive signal, as well as the implementation of the first stormwater zoning including both flow-rate and volume quantified prescriptions [START_REF] Mouy | Faisabilité hydraulique du zonage pluvial à Paris[END_REF]. In the USA, the 2000 Maryland Stormwater Design Manual integrates the two aspects in a unified dimensioning approach (Comstock and Wallis, 2003).

• SC is the sum of individual BMPs. Scale-transition between parcels and catchment is assumed to be linear. The approach followed to define parcel-scale constraints is, for Seine-Saint-Denis and SIVOA, to divide an admitted flow-rate by the upstream area. Beside the choice in the area measurement (total or impervious), this approach supposes that catchment-scale flow-rate is the sum of the flow-rates from parcels, ignoring all about scale effects like peaks' attenuation and delay.

Regulations demanding to preserve pre-development flow-rates, typical in the USA, are based on the same assumption [START_REF] Petrucci | Hydrographs' attenuation in sewers. effects on stormwater source control regulation[END_REF]. The rationale behind them can be summarized as: "if every parcel of the catchment does not produce a runoff peak higher than the pre-development one, the whole catchment too will not produce a runoff peak higher than the pre-development one". Peaks' increased durations and consequent superpositions are ignored (Booth and Jackson, 1997;Goff and Gentry, 2006).

• Two BMPs are better than one. All the approaches analyzed make the implicit assumption that more BMPs have a better global effect than less, and thus that a more strict regulation is always more suitable than a less strict one.

Both for Seine-Saint-Denis and SIVOA the precautionary choice is always the more strict. As discussed in Section 2, perhaps this assumption was acceptable in a pioneering phase of SC development, but in wide diffusion scenarios it has to be verified. Together with higher implementation costs, an excessive SC policy could involve negative side effects.

Hydrological analysis

The object of the modeling part of this paper is the analysis of the catchment-scale effects of SC regulations, under the hypothesis that they are widely applied to a catchment. The purpose is to assess the long term consequences of the present SC regulations, as they emerged from regulations' analysis.

Material and methods

To compare scenarios of SC regulations, we use the hydrologic model of a catchment in the Paris region (Section 4.1.1). In order to have reliable simulations of the catchment behavior, a comprehensive setup, calibration and validation process (Section 4.1.3 and 4.1.4) is performed using the available data. In the analysis, we consider two types of scenarios: flow-rate limitations and volume limitations (Section 4.1.5). For each type, we consider variable constraint values. Scenarios are compared on the basis of three indicators (Section 4.1.6).

The experimental catchment

The ZOH catchment (Zone opérationnelle d'habitat, 451 ha, Fig. 3) is 20 km south of Paris, and it is mainly under the authority of SIVOA (Section 3.3 and Fig. 2). While municipalities own proximity stormwater collectors on their territories (not modeled in the current study), SIVOA is in charge of the major collector. Topographically, the catchment is on a plateau (see collector's profile in Fig. 3): its upstream two-thirds have small slopes (<0.5%), while the downstream parton the hillsideis much steeper (5-6%). The catchment was rural until 1960 and drained by small creeks. Since the construction of the main collector from the SIVOA (1968/1969), many urban developments took place and were connected to it. Impervious cover in the catchment is approximately 30%, and urbanization of the area is still in progress. Most of the urban development occured before the enforcement of the actual SC regulation and, thus, it is equipped with conventional drainage. This type of plateau development-and in particluar its drainage conditions-is typical of the urbanization of the second half of the 20th century in the Paris region and in other French urban areas (Bergue and Ruperd, 1994;carre, 2002). The behavior of the considered catchment, therefore, can be considered representative of several periurban catchments in France.

The stormwater outlet is connected to a group of small sand-pit lakes linked to the Seine river. The outlet is equipped, since 2003, with a settling unit to protect the lakes from suspended solids and eutrophication [START_REF] Catherine | Troussellier Collapse of a planktothrix agardhii perennial bloom and microcystin dynamics in response to reduced phosphate concentrations in a temperate lake[END_REF].

Model choice and characteristics

The model used to perform the analysis is SWMM 5 [START_REF] Rossman | Storm Water Management Model (SWMM version 5.0) user's manual[END_REF]. It is a widely diffused urban hydrology model, flexible and well suited to represent BMPs (Carter and Jackson, 2007;Elliott and Trowsdale, 2007;[START_REF] Gironás | Storm Water Management Model Applications Manual[END_REF]. Elliott and Trowsdale, 2007 review ten different models suitable for SC modeling. They discuss a set of features of these models that should drive the choice of one specific model depending on the purpose of the study. Among others, they cite temporal resolution and scale, spatial resolution and scale, representation of runoff generation and routing.

The choice about temporal parameters and processes' representations depends on the variables of interest: if the model should well describe hydrographs' peaks in sewers, the modeling time-step should be short and hydrographs' routing detailed. In urban areas, peaks are mainly driven by direct runoff on impervious areas and sewer routing (Chocat and Cabane, 1999;[START_REF] Rodriguez | Creutin Surface runoff in urban catchments: morphological identification of unit hydrographs from urban databanks[END_REF]. Overland flow on a 10 2 -10 3 m scale, according to [START_REF] Blöschl | Scale issues in hydrological modelling: a review Hydrol[END_REF], corresponds to temporal scales smaller than one hour. Flow in the sewer system is even faster. We adopted a reporting time step of 5 min, while routing calculation (integration of de Saint Venant one-dimensional equations) is performed on shorter and varying time steps (<60 s). On the opposite, to model the global hydrological balance, a so high resolution is not necessary (1 h or even 1 day is a typical time step, [START_REF] Mitchell | McMahon Modelling the urban water cycle[END_REF], but time scale is relevant: to account for a catchment water balance, the model has to simulate long periods (one or more years). Some recent researches on SC (e.g. [START_REF] Zimmer | Low-impact-development practices for stormwater: implications for urban hydrology Can[END_REF] use a combined approach, simulating both single rain events and long time-series. To have a broader view of SC policies, we adopted the same approach. SWMM is adapted to this use, as it allows continuous simulations over long periods with short time steps.

From a spatial scale and resolution point of view, SWMM describes the catchment as a set of subcatchments and a drainage network. Subcatchments, whose size and number (i.e. spatial resolution) depend on the modeler's choice, are in charge of runoff generation, while the network accounts for routing processes.

Each subcatchment is described as a non-linear reservoir, with an outflow equation given by the kinematic wave model (Singh, 1988). The model takes into account initial losses, and evaporation is computed at each time step on the basis of a monthly average. Infiltration from pervious areas is described by the Green-Ampt model.

Subcatchments can be split in two parts: impervious and pervious. Runoff from one part can be routed over the other. Subcatchments are linked to a node of the sewer network: at each time-step the outflow of the subcatchment is injected in the corresponding node and routed in the network. Special devices can be added to the sewer network, like reservoirs, outlets and weirs.

Simulations' outputs are the values, at the reporting time-step, of relevant variables for all subcatchments and elements of the sewer network.

Model setup

Model setup involves the division of the catchment in subcatchments and the corresponding data-analysis to define each subcatchment's characteristics.

The division in subcatchments (32, Fig. 3) has been made according to catchment topography and sewer network.

Subcatchments' characteristics (area, slope, pervious ratios, roof versus road area, etc.) have been determined by maps and aerial photos of the catchment.

Differently from most cases in the literature-see for example the manual of SWMM applications by EPA [START_REF] Gironás | Storm Water Management Model Applications Manual[END_REF]-we did not considered each "real" subcatchment as a "modeled" subcatchment, but we parted it in four areas according to land-cover. For each subcatchment on the ground, we defined four subcatchments in the model corresponding to green-, roof-, road-and "other"-areas. An example of the model structure is shown in Fig. 4. This strategy has two main reasons:

1. to take into account differences among impervious areas. Average slope and flowpath length for roofs are not the same than for roads, and describing roofs and roads as different subcatchments allows to parametrize them differently.

2. To take into account uncertainties in subcatchments' characteristics. Pervious area on a subcatchment is determined by pixel classification on aerial photos. This process involves some error, and lets some areas unclassified. The "other" areas correspond to unclassified ones, and are used to compensate this uncertainty: in the model they are partially pervious and partially impervious, and the ratio is determined through calibration to better fit runoff data. This approach has already been used with good performances for a previous case-study, described in Petrucci et al., 2012. 4.1.4

. Model calibration and validation

Calibration and validation phases were performed on data issued from two flow-rate measurement campaigns operated by SIVOA in 2009 and 2010. Two three-month campaigns were performed at a 5-min frequency, the first from October to December 2009, the second from May to July 2010. The 2009 campaign involved one measurement point (point B in Fig. 3), while the 2010 campaign used two points (A and C). Rainfall data were issued from a rain gauge installed over the settling unit, at the catchment's outfall.

Data analysis showed that data collected on point C were of poor quality: they are affected by strong measurement noise (Fig. 10) and some missing value. Because of the importance of point C, downstream of all subcatchments, we decided to use these data anyway, after filtering and correction, still considering them less reliable.

The model parameters to calibrate are of two types: global, which are valid for the whole catchment, and local, valid for individual subcatchments. The global parameters are eleven: sewer roughness, soil characteristics (suction head, conductivity, initial deficit), initial losses and roughness for green, roofs and roads areas, impervious ratio of "other" areas. Only one local parameter, defined at the subcatchment scale, is used for calibration: the subcatchment width, a shape parameter (Singh, 1988;[START_REF] Rossman | Storm Water Management Model (SWMM version 5.0) user's manual[END_REF]. We defined, for each parameter, a variability interval as narrow as possible according to hydrologic literature and catchment specific informations (e.g. soil type is mainly loam).

While global parameters, assumed to be unique over the catchment, can be calibrated on any rainfall-runoff time series, the local parameter for a given subcatchment can be calibrated and validated only, obviously, on runoff data measured downstream of it. As available data are collected at three different points, to use all of them we performed a three-step calibration and validation: All calibrations and validations used, as a goodness-of-fit measure, the Nash criterion (Nash and Sutcliffe, 1970):

(1)

where Qt and Q^t are respectively the observed and simulated flow-rates at time t, and Q¯ is the observations' mean value. This indicator ranges from -∞ to 1, with higher values for more accurate models. A value of 1 denotes a model perfectly matching the observed data, while a value of 0 denotes a model as accurate as the observations' mean value.

Calibrations were performed using a genetic algorithm. This type of evolutionary algorithm has been used for hydrologic models' calibration or optimization during the last 20 years and it is now well diffused in hydroinformatics' applications [START_REF] Savic | Evolutionary computing in hydrological sciences[END_REF]. The main interest of this algorithm is its efficiency in finding close-to-optimal solutions of high-dimensional optimization problems, without strong hypotheses on the problem setting (e.g. linearity or continuity of the objective function).

Scenarios of SC policies

Flow-rate limitations. Scenarios of regulations limiting flow-rate are in the form q * = x l s -1 ha -1 typical of French cases. We simulate 15 scenarios, ranging from q * = 0.5 l s -1 ha -1 to 50 l s -1 ha -1 . The lower part of the range (0.5-10 l s -1 ha -1 ) corresponds to current practices in France (and also in the UK, according to Faulkner, 1999), while the upper limit (50 l s -1 ha -1 ) is chosen in order to include the reference value of the specific peak flow-rate (46 l s -1 ha -1 ).

Flow-rate limitations are modeled by installing a reservoir downstream of the roof and road areas of each subcatchment (Fig. 5). Each reservoir has an outfall and a spillway, both connected to the sewer network. Spillways are over-sized, in order to allow reservoirs' overflow to reach the sewer. Reservoirs' volume and outfalls' stagedischarge curve vary according to q * . Reservoirs' volume is calculated using the rainfall design method (Chocat and et al., 1997, p. 627) for the SIVOA's design rain-event (triangular, 55 mm in 4 h, see Section 3.3). Specific volumes for each scenario are reported in Table 1. Table 1. Flow-rate regulations and corresponding storage volumes.

q * (l s -1 ha -1 ) 0. where Q and h are, respectively, the reservoir's outflow and water level. Parameter a defines flow-rate magnitude, while b the steepness of the curve. Fig. 6 shows an adimensional plot of this curve for two values of b. In principle, a small value of b could assure a step-shaped curve: even for small water levels, the outflow is close to the regulation value. However, some tests showed that this choice produces numerical instabilities in the model. If the curve is too steep, in fact, for small stored volumes emptying is too fast and a discontinuous behavior takes place.

The consequence is oscillating values of stored volume and outflow. This saw-tooth profile has a low magnitude (10-3m3s-1), but produces substantially wrong results for the computation of low flows. Thus, our choice for b was a compromise between steepness and numerical stability. The selected value is b = 0.3, that gives consistent values, for q * < 15 l s -1 ha -1 .

Fig. 6. Adimensional stage-discharge curve.

Parameter a is defined, for each reservoir, as:

a=q * •Adrained
where Adrained is the impervious area contributing to the reservoir.

Volume limitations. SC scenarios with limitations on runoff volume are defined by the specific volume to be retained i * = x mm. It is the regulation form applied in Paris (4-12 mm) and in Sun City, Arizona (19mm). We consider 18 scenarios, ranging from i * = 0.01 mm to i * = 50 mm. The first half of the range corresponds to the cited existing policies, while the second half is to explore the potential of stronger regulations.

Volume regulations are modeled as filter strips downstream of each impervious subcatchment (roads and roofs), that store and infiltrate the prescribed volume of runoff from the impervious area. A scheme of the modified subcatchments' model is presented in Fig. 7. If the subcatchment has an area As, the strip stores (as initial losses) a runoff volume of v=Asi * . This water is then infiltrated and can be replaced by further runoff.

Remark 1

The two different forms of regulation are defined according to the current practices. It is important to note that, using q * and i * as defined, they vary in opposition: stronger regulations (i.e. regulations demanding more resources to comply with) lead to lower values of q * and to higher values of i * . Beside the graphical approach and in order to compare a large number of scenarios, it is useful to define numerical indicators. With this purpose, we considered two typical objectives for SC policies: to avoid sewer overflows and to preserve receiving waters.

For the first objective, we used the peak flow-rate in response to the SIVOA's design rain event, the same used for reservoir's dimensioning in flow-rate limitation scenarios. Thus, to evaluate the first objective, the indicator chosen is the peak flow-rate for this rain event, normalized on the impervious area of the catchment (141 ha). This indicator is noted qpeak (l s -1 ha -1 ); smaller values denote better performances.

As the stability and ecological status of the receiving waters depend, in general, both on low-flow regimes and on bypasses of treatment units, for the second objective we adopted two indicators: the fraction of time during which a flow is detected at the outlet (i.e. the frequency of flow), and the runoff volume bypassing treatment. The first is the x-axis intersection in flow-frequency curves, and it is noted fflow (-). The second is the total volume of flow exceeding 0.8m3s-1, which is the settling unit capacity. It is noted V0.8 (m 3 ). Considering the actual imperviousness of the catchment, 0.8 m 3 s -1 correspond to 5.7 l s -1 ha -1 . We consider that, for both indicators, smaller values denotes better performances.

V0.8 is the more pertinent indicator for the ZOH catchment, as low-flow regimes are not considered a problem for the downstream lakes. However, as fflow can be relevant for similar catchments, according to the outfall characteristics, both are considered in the analysis with a purpose of generality.

For the numerical problems on low flows discussed in Section 4.1.5, fflow is not computed for q * >15 l s -1 ha -1 . fflow and V0.8 are computed for a 23-month rainfall series (1/1/2009 to 1/12/2010). The series is not long enough to give statistically relevant results for return periods longer than 1 year, but it is sufficient for current rain events (T < 3 months). The reference case used for comparison is the calibrated model, as we have no information on the pre-development catchment.

Outcomes of calibration and validation

The calibration process involved three steps (Section 4. The results obtained by the last calibration and validation (point C) show a worse correspondence between measurements and simulations. It is reasonable to suppose that this poor performance is more linked to the poor reliability of measurements (Fig. 10), than to modeling errors. The modeling procedure is, in fact, the same that performs well for the rest of the catchment. Still, without other datasets on point C, it is impossible to know the actual accurateness of the model for the subcatchments between point B and point C (about 19% of the catchment area). As our purpose is analyzing scenarios, we can accept the calibrated model as a good representation of the entire catchment. If the purpose was to actually decide a policy for the catchment, a further measurement campaign at point A should be necessary.

Results and discussion

In Fig. 11 and 12 several flow-frequency curves are plotted and compared to the reference. The two figures reveal the different behaviors of the two regulations:

• Volume regulations (Fig. 11) have flow-frequency curves always lower than the reference. This means that, for any given frequency, the corresponding flow-rate will be smaller in presence of this kind of regulations. Obviously, low constraint values (e.g. 5 mm) affect strongly the frequent events (f > 0.01), while infrequent ones (f <0.001) are less changed. Increasing the constraint value (25 mm and 50 mm in Fig. 11) further reduces flow-rates. Also infrequent events are affected by high constraint values.

• On the contrary, flow-rate regulations (Fig. 12) have different effects for low and high frequencies. In fact, their flow-frequency curves have intersections with the reference curve. A reduction in low frequency flow-rates corresponds to an increase of flow-rates for higher frequencies, resulting in a "flattening" of the flow-frequency curve. Decreasing the limitation value (i.e. strengthening the constraint, as explained in paragraph 1) increases this "flattening" effect: the 1 l s -1 ha -1 flow-frequency curve is almost flat for f < 0.1, with flow-rates lesser than the reference for f < 0.03, and higher elsewhere.

Reduction of sewer overflows

This first objective (avoiding sewer overflows) is described by the indicator qpeak, as stated in Section 4.1.6. For both types of regulations, Fig. 13 reveals that qpeak monotonously decreases with strengthening regulations. Thus, for both regulation forms, no worsening of catchment-scale peak flow-rate occurs with specific regulation values.

Comparing volume and flow-rate regulations shows that the corresponding ranges of qpeak are about the same.

Thus, for a given target value of the objective in this range (e.g. qpeak = 20 l s -1 ha -1 ), it is always possible to find two corresponding regulations, one based on limitation of flow-rate (in the example q * = 5 l s -1 ha -1 ) and one on limitation of volume (i * = 37 mm).

A further remark is that, for flow-rate regulations, qpeak is greater than q * . For intense rain events, in facts, pervious areas (not controlled by reservoirs) contribute significantly to the catchment-scale peak flow-rate. The procedure applied by the SIVOA to define q * (Section 3.3) was to divide a maximum acceptable flow-rate by the upstream impervious area: this corresponds to taking a value of q * on the q * = qpeak curve (plotted in Fig. 16).

Because of the contribution of not controlled areas, this procedure leads to values of q * too high to assure the expected value of q peak . Fig. 16. qpeak values for flow-rate regulations, compared to q * = qpeak and q * = qpeak,tot lines.

As a comparison with the Seine-Saint-Denis procedure, based on the total upstream area (Section 3.2), we plotted in Fig. 16 the curve q * = qpeak,tot. qpeak,tot is an indicator analogous to qpeak, but normalized on the total area of the catchment. This second curve shows the limits of a linear approach in SC regulation (Section 3.5): using the total area for the calculation is, intuitively, a highly precautionary procedure. Fig. 16 shows that this is true for high values of q * (>8 l s -1 ha -1 ), for which the constraint is largely overestimated. But for low values of q * , the total area procedure underestimates the values of the constraint like the impervious area one. More in general, this is an evidence of the unfitness of a linear approach to the transition from the parcel-to the catchment-scale for SC regulation.

Preservation of receiving waters

This second objective (preserving receiving waters) is described by the two indicators fflow and V0.8 (Section 4.1.6).

Fig. 14 shows flow frequencies at the catchment outlet (fflow). For the reference case, fflow = 0.25. As it was expected (Section 2), runoff rate limitations never improve the reference situation, while runoff volume ones always do. In detail, runoff rate regulations do not affect the indicator for q * >2 l s -1 ha -1 , while for lower values they significantly worsen the situation (up to fflow = 0.42). In these last cases, the "flattening" of flow-frequency curves is marked enough to increase frequency of really small flow-rates (e.g. see the 1 l s -1 ha -1 curve in Fig. 12).

Physically, this phenomenon occurs when the emptying time of reservoirs is longer than the undisturbed hydrograph's duration. Thus, flow-rate regulations distort low-flow regimes only if they leads to reservoirs' emptying time longer than catchment-scale lag-times.

Volume regulations, on the contrary, rapidly improve the indicator value for i * <20 mm. fflow is positively affected even for extremely small storage values (i * = 0.01 mm), because of the infiltration effect of filter strips covering 1% of previously impervious areas, even without storage (see Section 4.1.5).

Volumes exceeding the treatment unit flow capacity (indicator V0.8) are plotted in Fig. 15. Also for this indicator, volume regulations show a monotonous improving behavior, slowing down with increasing constraint values. As in the previous case, a small constraint (e.g. i * = 6 mm) is enough to half (from 1.93e5 m 3 to 0.82e5 m 3 ) the reference value of the indicator.

Flow-rate regulations have a more complex behavior, showing four different ranges:

• q * < 5 l s -1 ha -1 . The flattening effect causes the flow-rate to be always below 0.8 m 3 s -1 , and thus V0.8 = 0 m 3 .

• 5 l s -1 ha -1 < q * < 15 l s -1 ha -1 . In this range the regulation shows a "good" behavior: strengthening the constraint improves values of the indicator.

• 15 l s -1 ha -1 < q * < 30 l s -1 ha -1 . In this range, regulation produces worse performances than the reference case. The reason is that, during reservoirs emptying, the catchment flow-rate is above 0.8 m 3 s -1 for longer durations than in the reference case.

• q * > 30 l s -1 ha -1 . No significant variation from the reference case. Obviously, these thresholds and the existence of degradation ranges are catchment specific: for example, regulations do not affect the fflow indicator if the emptying time of the reservoirs for a rain event is shorter than the duration of the corresponding hydrograph in the reference situation. In the same way, we find no aggravation ranges for qpeak while other researchers did, for other catchments (Section 2).

This complex behavior of the flow-rate limitations appears to be linked to the flattening effect observed on the flow-frequency curves. The three indicators, even if with different formulations, are all linked to some specific part of these curves: qpeak to high flow-rates, fflow to small ones, V0.8 to intermediate ones. Flow-rates limitations do not reduce these curves systematically like volume ones, but filter high flow-rates increasing smaller ones. This is why substantial improvements of V0.8 occur for values of q * worsening fflow. Improving this last indicator is impossible with this form of regulation.

With regard to avoiding sewer overflows, we observed that the same qpeak values that can be obtained through flow-rate regulations can also be obtained through volume ones. This remark can be discussed in the light of current practices. Current values of q * range from 0.5 l s -1 ha -1 to 10 l s -1 ha -1 , while i * from 4 mm to 19 mm (Section 4.1.5). Comparing these ranges on Fig. 13 shows differing qpeak values: current values of q * lead to qpeak values ranging from 10 l s -1 ha -1 to 25 l s -1 ha -1 , while current values of i * to qpeak values ranging from 33 l s -1 ha -1 to 44 l s -1 ha -1 .

If these results were confirmed for other catchments, it would be possible to infer that, according to the current practices, low peak flow-rates can be obtained only with flow-rate regulations. An authority facing problems of sewer overflows, and without the resources or the motivation to adopt "experimental" policies, can only limit flowrates. However, the evolution of flow-rate regulations in the last 30 years showed that, once the regulation principles are set, the constraint can be rapidly strengthened because the main impediments to SC regulations are more institutional barriers than technical difficulties. Today, a few authorities already apply runoff volume regulations with moderate constraints. It is possible that in a few years current practices will include stronger runoff volume limitations, able to cope also with sewer overflows.

Conclusions

SC policies have to be analyzed from a temporal perspective. This paper identifies, analyzing development of current policies, some actual trends in policy-making logics. The hydrological analysis assesses what will be the consequence of these trends if they continue to be applied in the long term.

The regulations' analysis revealed some shortcuts, that many regulators developing SC policies take instead of following a more hydrologically-sound reasoning. Two of the three shortcuts identified seem residuals of old approaches to SC: a persistence of the flow-rate approach and, more deeply, of a pioneer-approach oriented to develop as much and as larger BMPs as possible.

The hydrological analysis showed that SC policies based on these two shortcuts, applied together, can produce negative effects when widely applied on a catchment. In fact, we simulated "traditional" flow-rate regulations (i.e.

flow-rate approach) and observed that strengthening the constraint (i.e. larger BMPs) aggravates the distortion of low-flow regimes.

While these two shortcuts are linked to decision-making inertia, the third one-a too simplistic concept of scaletransition-seems to be caused by the absence of appropriate hydrological tools to account for this transition.

Today, the only available tool to evaluate SC policies on a specific catchment is detailed modeling, requiring conspicuous quantities of data, time, know-how and financial resources. Few local authorities, among all those that could be interested in adopting SC policies in the next years, are able to provide this effort.

Volume regulations, even if constrained by local infiltration possibilities, appear "safer" than flow-rate regulations for the authorities unable or reluctant to provide modeling effort: according to the analyses available and to our results, it seems that, for them, the approach "the more the better" is valid, as they do not seem to aggravate the situation in any case. Studying their application, consequences and practical limits on the catchments where they are already applied appears to be a priority. In the same time, their diffusion could be encouraged. More in general, studying SC policies effect at the catchment-scale seems a good approach to better understand the potential mitigations of the social and environmental impacts of urban growth, and to support regulators to reach a right balance between policy-making necessities and urban hydrology rationality.

Fig. 2 .

 2 Fig. 2. Geographical position of the three case-studies.

Fig. 3 .

 3 Fig. 3. Plan of the ZOH catchment and main collector's profile.

Fig. 4 .

 4 Fig. 4. Example of the model of a subcatchment.

1.

  Calibration and validation of all the global parameters and of the local parameters for the subcatchments upstream of point B. Calibration was performed on two thirds of the data from the 2009 campaign; validation on the remaining third. 2. Validation of global parameters on data from the 2010 campaign (point A). In this validation, also local parameters of subcatchments upstream of point A are validated. 3. Calibration and validation of local parameters for subcatchments between point B and C, on data from the 2010 campaign (point C). Two-thirds of data were used for calibration, the remaining third for validation.

Fig. 5 .

 5 Fig. 5. Model of a subcatchment with flow-rate limitation. (a) reservoir, (b) reservoir's outfall, and (c) reservoir's spillway.

  m 3 ha -1 ) 658 565 485 447 377 323 299 262 252 231 205 184 168 155 145 SWMM allows to use stage-discharge curves in the form:

Fig. 7 .

 7 Fig. 7. Scheme of a subcatchment's model with volume regulation.

  1.4): calibration of global parameters and of local parameters of the upstream part of the catchment (2009 data); validation on 2010 upstream (point A) data; calibration and validation of local parameters for the downstream part of the catchment (2010 data, point C).

Figs. 9

 9 Figs. 9 and 10 shows samples of the simulated hydrographs, plotted against data. The values of the Nash criterion obtained are:

Fig. 10 .

 10 Fig. 10. Comparison between simulated and measured flow-rate. Point C, calibration.

  The values of the three indicators, for all the simulated SC scenarios, are plotted in Figs. 13-15. According to the final remark of Section 1, they are plotted on inverse axes to show results in the same order: in the positive xdirection regulations' "strength" increases.

Fig. 11 .

 11 Fig. 11. Flow-frequency curves for a selection of volume regulation scenarios.

Fig. 12 .

 12 Fig. 12. Flow-frequency curves for a selection of flow-rate regulation scenarios.

Fig. 13 .

 13 Fig. 13. qpeak values as a function of flow-rate and volume constraints.

Fig. 14 .

 14 Fig.14. fflow values as a function of flow-rate and volume constraints. For q * ⩾15 l s -1 ha -1 , fflow is not computed.

  4.3.3. Flow-rate versus volume regulationsGlobally, results for the three indicators show that volume regulations have a simpler behavior than runoff rate ones. Indeed, volume regulations exhibit always a monotonous trend: strengthening the constraint generates improvements in the indicators' value. On the contrary, flow-rate regulations show threshold-effects and, for some ranges, can produce degradation of indicators value.

Acknowledgments

The authors wish to express their gratitude to the many technicians who shared their competences and experience to help this research, and to their institutions. We particularly thank the SIVOA for making available the information and data on the ZOH catchment.


Lavinia Slabu

Carles Escera

Sabine Grimm

Jordi Costa-Faidella

Early change detection in humans as revealed by auditory brainstem and middle-latency evoked potentials

Keywords: ABR, change detection, mismatch negativity (MMN), MLR, sensory memory

The ability to detect unexpected novel stimuli is crucial for survival, as it might urge a prompt adaptive response. Human auditory novelty detection has been associated to the mismatch negativity long-latency auditory-evoked potential, peaking at 100-200 ms. Yet, recent animal studies showing novelty responses at a very short latency (about 20-30 ms) in individual neurons already at the level of the midbrain and thalamus suggest that novelty detection might be a basic principle of the functional organization of the auditory system, expanding from lower levels in the brainstem along the auditory pathway up to higher-order areas of the cerebral cortex. To test this suggestion, we here measured auditory brainstem and middle latency response (MLR) to frequency novel stimuli embedded in an oddball sequence. To oversee refractoriness confounds a 'control block' was used. The results showed that occasional changes in auditory frequency information were detected as early as 30 ms (Pa waveform of the MLR) after stimulus onset. The control block precluded these effects as resulting merely from refractoriness, altogether supporting the notion of 'true' early auditory change detection in humans, matching the latency range of auditory novelty responses described in individual neurons of subhuman species. Our results suggest that auditory change detection of frequency information is a multistage process that occurs at the primary auditory cortex and is transmitted to the higher levels of the auditory pathway.

Introduction

The detection of unexpected events in the acoustic environment is crucial for survival, as preparing the organism for rapidly changing surrounding conditions. Change detection has been associated to a particular brain response, the mismatch negativity (MMN;Naatanen et al., 1978), derived from the auditory-evoked potential (AEP) as recorded with the oddball paradigm. In this paradigm, rare novel (deviant) stimuli are embedded within a sequence of repeating homogenous (standard) stimuli. Subtracting the AEP elicited to the standard stimuli from that to the deviant stimuli, a negative deflection, the MMN, is obtained at approximately 100-200 ms after the stimulus onset. According to the accepted account, the MMN is thought to reflect that the acoustic regularity embedded in the standard stimulus repetition has been extracted and encoded into a memory trace [START_REF] Winkler | Interpreting the mismatch negativity[END_REF].

Recent results from single-unit recordings in animals using an oddball paradigm establish the premises for new questions in humans. Indeed, [START_REF] Ulanovsky | Processing of low-probability sounds by cortical neurons[END_REF] described the existence of neurons in the primary auditory cortex of the cat that show strong stimulus-specific adaptation (SSA) to a repeating stimulus, but restore their firing rates at the occurrence of any change in the stimulus features. Additional primary cortical contribution has been reported by von der [START_REF] Von Der Behrens | Correlating stimulus-specific adaptation of cortical neurons and local field potentials in the awake rat[END_REF]. Further data have shown the existence of SSA along the subcortical auditory pathway, including the thalamus [START_REF] Anderson | Stimulus-specific adaptation occurs in the auditory thalamus[END_REF][START_REF] Antunes | Is there stimulusspecific adaptation in the auditory thalamus?[END_REF] and the inferior colliculi [START_REF] Perez-Gonzalez | Novelty detector neurons in the mammalian auditory midbrain[END_REF][START_REF] Reches | Stimulus-specific adaptations in the gaze control system of the barn owl[END_REF][START_REF] Malmierca | Stimulus-specific adaptation in the inferior colliculus of the anesthetized rat[END_REF]. Taken together, these results suggest that the MMN recorded from the human scalp might be preceded by earlier noveltyrelated activity, and that the detection of novel sounds in humans might involve different levels of the auditory system. Consequently, the detection of rare deviant stimuli can be considered as a multistage process, implemented in the auditory pathway from the brainstem up to higher-order areas of the cortex.

In contrast to the large amount of research in humans dealing with this neurophysiological phenomenon, limited data exist on early processing stages involved in the detection of novel sounds. One study in humans obtained deviance-related responses as early as 25 ms from stimulus onset using an oddball paradigm [START_REF] Sonnadara | Occasional changes in sound location enhance middle latency evoked responses[END_REF]. The lack of a proper control condition in this study, however, does not allow relating this neural response to 'true' novelty detection. One way to overcome this limitation is to implement a 'control condition' to rule out the refractoriness effects by presenting the same physical stimulus used as deviant in the oddball block with the same probability, but embedded in a context of other rare stimuli (Schroger [START_REF] Schro¨ger | Mismatch response of the human brain to changes in sound location[END_REF].

The present study aimed at testing whether the detection of frequency changes was already reflected in early stages of human auditory processing. For that purpose, a typical frequency oddball paradigm and the corresponding control conditions were used, while the parameters of electroencephalogram (EEG) acquisition were adjusted to record simultaneously the auditory brainstem (ABRs) and the middle latency responses (MLRs) in a sample of healthy human subjects.

Materials and methods

Eighteen volunteers (aged 18-29 years, 11 females, three lefthanded) with audiometric thresholds below 25 dB SPL for the five different frequency bands used in the experiment were included in the study. Threshold values were obtained for each experimental auditory stimulus from 0 to 60 dB SPL in 5-dB increments, for each ear. All values were measured three times and the threshold was determined as the average of the three measurements. The experimental protocol was approved by the Ethical Committee of University of Barcelona, and was in accordance with The Code of Ethics of the World Medical Association (Declaration of Helsinki). Subjects were paid for their participation and written informed consent was obtained from each participant prior to the experiment.

Stimuli and procedure

The auditory stimuli consisted of short bursts of band-pass-filtered broadband noises in the range 500-3000 Hz in steps of 500 Hz, generated with the Neurosoft (El Paso, TX, USA). They had a duration of 40 ms and were delivered monaurally via Beyerdynamic DT 48 A headphone (Heilbronn, Germany) to the right ear at an intensity of 80 dB SPL with a stimulus-onset asynchrony of 96 ms. The left ear was masked with white noise at an intensity of 60 dB SPL to prevent the early brain responses to be contaminated by a cross-heard response from the opposite cochlea [START_REF] Gorga | The choice of stimuli for ABR measurements[END_REF]. Three different conditions were used. First, an oddball condition was presented with a deviant probability of P = 0.2, in which the standard stimulus was a broadband noise band-pass-filtered from 500 to 1000 Hz (termed S1), and the deviant frequency was a broadband noise filtered from 1000 to 1500 Hz (termed S2); the stimulus sequence in this condition was constrained in a way that at least two standard stimuli followed each deviant stimulus. Second, a reverse oddball condition was applied, in which the deviant and standard stimuli used in the oddball condition switched their roles. Finally, a control condition was presented in which stimuli of five different band-passfiltered broadband noises, 500-1000 Hz (S1), 1000-1500 Hz (S2), 1500-2000 Hz (S3), 2000-2500 Hz (S4) and 2500-3000 Hz (S5), were presented randomly, each with a probability of P = 0.2, as introduced by [START_REF] Schro¨ger | Mismatch response of the human brain to changes in sound location[END_REF]. A total of 60 000 stimuli were delivered in the three conditions, including 4000 deviants per oddball condition and corresponding relevant stimuli in the control one. Conditions were split into a total number of six blocks of 10 000 stimuli, each lasting approximately 16 min, which were arranged in a randomized order. During the experiment, subjects sat comfortably in a recliner chair in an acoustically and electrically shielded cabin. They were instructed to relax and to watch a silent movie with subtitles, while ignoring the auditory stimulation. After each run, subjects had a short break allowing for movements.

Data acquisition

ABRs and MLRs were both extracted from the same continuous EEG recording, which was acquired with a Neuroscan SynAmps amplifier and Scan software (NeuroScan, Herndon, VA, USA) from the Cz electrode. Electrodes placed at the left ear lobe and forehead served as reference and ground, respectively. All electrode impedances were maintained below 5 kX. The continuous EEG was online band-passfiltered from 0.05 to 1500 Hz. The data were collected at a sampling rate of 20 000 Hz and a gain of 5000.

Statistical analysis

Epochs for deviant, standard and the control stimuli were averaged separately. In order to obtain a similar number of trails for each type of stimuli, the average for standard stimuli only included those standard stimuli that strictly preceded the deviant stimuli.

Epochs for deviant stimuli, standard stimuli preceding immediately the deviant ones, and the control stimuli were averaged separately. To better isolate the different frequency characteristics of the correspond-ing brain responses in the two latency ranges of interest (ABR and MLR), different epoch duration, filtering and rejection criteria were used and therefore distinct averages were extracted from the raw EEG, one optimized to identify the ABR and one to determine the MLR. For the analysis of the ABR, data were filtered off-line with a band-pass filter from 100 to 1500 Hz. Averaged evoked potentials were baseline-corrected for a 30-ms interval before sound onset for the deviant, standard preceding the deviant and control stimuli. Trials with artefacts exceeding 35 lV were rejected off-line [START_REF] Russo | Deficient brainstem encoding of pitch in children with Autism Spectrum Disorders[END_REF]. For MLR analysis, data were filtered off-line with a band-pass filter from 15 to 250 Hz [START_REF] Baess | Attenuated human auditory middle latency response and evoked 40-Hz response to self-initiated sounds[END_REF]. Averages were baseline-corrected for 50 ms before sound onset for the deviant, standard preceding the deviant and control stimuli. Trials with artefacts exceeding 80 lV were rejected from further analysis. The waveforms V of the ABR and Na, Pa, and Nb of the MLR were identified for the three stimulus types and the two frequency ranges.

Individual peak latencies were measured as the maximum peak in the time windows 7-9 ms for waveform V of ABR, and 28-32 ms for waveform Pa of MLR, and as the minimum peak in the time windows 18-22 ms for the Na, and 41-43 ms for the Nb of the MLR. Mean amplitude instead of peak amplitude measurements were chosen in the present study to allow for a better signal-to-noise ratio while determining these minuscule neuroelectric responses [START_REF] Picton | Guidelines for using human event-related potentials to study cognition: recording standards and publication criteria[END_REF]. Thus, the mean amplitude of the wave V of the ABR was calculated in a 2-ms time window, and that of the Na, Pa and Nb waves of the MLR in a 4-ms time window, centred in both cases around the peak latency identified in the respective grand-averages.

The data were analysed by means of repeated-measures analyses of variance (anova) on the mean amplitudes of the V and Na, Pa, Nb waveforms separately, including the factors Frequency (S1, S2) and Stimulus Type (deviant, standard, control). Bonferroni correction was applied to adjust for multiple testing, with the statistical significance being defined for values of P < 0.016. Post hoc pairwise differences between single levels of Stimulus Type were tested applying repeated measures contrasts. All significant effects for the post hoc evaluations were for values of P < 0.05. In addition, similar analyses were performed for the peak latencies of the V, Na, Pa and Nb waveforms elicited by deviant, standard and control stimuli.

Results

The control stimuli elicited robust ABR and MLR responses for the five different band-pass-filtered broadband noises used in the exper-iment (Fig. 1), with similar waveforms obtained for the control stimuli of interest, S1 and S2. Grand-average ABRs and MLRs to deviant, standard and control stimuli are shown in Fig. 2. In the grand-average V, Na, Pa and Nb components, elicited by the deviant, control and standard stimuli. AEPs for all conditions we could clearly identify the waveforms V in the ABR, and Na, Pa and Nb in the MLR, with the corresponding mean amplitudes and mean peak latencies presented in Tables 1 and2, respectively.

For the V component of the ABR, the repeated-measures anova revealed no significant effects of Frequency (F 1,17 = 0.794, n.s.) or Stimulus Type effect (F 2,34 = 0.071, n.s.) on its mean amplitudes. No significant difference in peak latency was observed either (F 1.17 = 0.486, n.s.; F 2,34 = 0.337, n.s).

As for the MLR components, the repeated-measures anova showed that there were no significant effects of Frequency for the time windows of Na (F 1,17 = 0.237, n.s.), Pa (F 1,17 = 0.726, n.s.) and Nb (F 1,17 = 0.013, n.s.). Remarkably, however, a Stimulus Type effect was observed for the time window of the Pa component (F 2,34 = 4.792, P = 0.015), which did not become evident for any of the other components Na (F 2,34 = 0.920, n.s.) and Nb (F 2,34 = 0.649, n.s.). Similar analysis of the peak latency showed that there were no significant effects of Frequency (Na: F 1,17 = 0.177, n.s.; Pa: F 1,17 = 1.510, n.s.; and Nb: F 1,17 = 0.270, n.s.) or of the Stimulus Type (Na: F 2,34 = 2.572, n.s.; Pa: F 2,34 = 0.312, n.s.; and Nb: F 2,34 = 0.355, n.s.). Post hoc repeated-measures contrasts confirmed statistical significant differences on the mean amplitude between deviant and standard AEPs (F 1,17 = 5.313, P = 0.034), and between deviant and control AEPs (F 1,17 = 7.297, P = 0.015) in the Pa latency window. These effects resulted from the deviant stimuli eliciting larger amplitudes compared with the standard and the control stimuli for the Pa component of the MLR (Fig. 2). For an easier visualization of the effect, Fig. 3 shows the bar-graph of the grand-average mean amplitudes, including the standard errors of the

Discussion

The results of the present study have revealed that auditory novelty (deviance) detection can take place in humans as early as 30 ms from the onset of the stimulus feature that is novel compared with a repetitive background (i.e. the standard stimuli). This was supported by the fact that the Pa component of the MLR, peaking at 30 ms from change onset, was larger for the deviant stimulus than for the responses elicited both to the standard and the control stimuli. It is worth noting that all three responses used for comparison (to deviant, to standard, and to control stimuli) were elicited by the same physical stimulus occurring, in the three different experimental conditions, with a different probability and different regularity role.

Auditory novelty detection has been studied so far mainly in humans by means of the MMN, which has been obtained to simple features changes in frequency [START_REF] Sams | Auditory frequency discrimination and event-related potentials[END_REF][START_REF] Mu¨ Ller | Cortical activation to auditory mismatch elicited by frequency deviant and complex novel sounds: a PET study[END_REF], intensity ( Naatanen et al., 1987), duration ( Naatanen et al., 1989) and spatial location [START_REF] Paavilainen | Mismatch negativity to change in spatial location of an auditory stimulus[END_REF]. Also, MMN has been elicited by changes in more complex levels of regularity, such as the frequency relationship of two tones within a pair [START_REF] Saarinen | Representation of abstract attributes of auditory stimuli in the human brain[END_REF][START_REF] Carral | Auditory event-related potentials as a function of abstract change magnitude[END_REF]. The nature of the MMN as a 'genuine' novelty detector was not, however, supported until the use of the control block introduced by Schroger & Wolff (1996). With the use of this block, the responses to the deviant stimuli are compared with the responses to the very same stimuli occurring in a context of other different stimuli of the same low probability to oversee for refractoriness. Using this type of control condition, 'true' novelty detection in humans reflecting regularity extraction and encoding [START_REF] Winkler | Interpreting the mismatch negativity[END_REF] has been shown for location [START_REF] Schro¨ger | Mismatch response of the human brain to changes in sound location[END_REF], pitch [START_REF] Jacobsen | Is there pre-attentive memory-based comparison of pitch?[END_REF], intensity (Jacobsen et al., 2003a) and duration (Jacobsen & Schro¨ ger, 2003b) deviant stimuli. Although previous studies on humans have shown frequency novelty detection in the long latency range, no work has focused on the early latency range so far.

Mu¨ ller et al. ( 2001) provided empirical evidence that the earlier components of MLR (Na, Pa, Nb with a window for MLR mean amplitude between 15 and 23 ms, 25 and 30 ms, and 35 and 41 ms, respectively) exhibit amplitude suppression to a repeated sound in a paired-click paradigm, reflecting the primary cortical ability to distinguish repetitive stimuli. This phenomenon was proposed to reflect the existence of a neural inhibition that contributes to the suppression of the response for the second identical stimulus or by an increased refractoriness and or habituation of the specific neurons [START_REF] Mu¨ Ller | Suppression of the auditory middle-latency response and evoked gamma-band response in a paired-click paradigm[END_REF]. Although showing the sensitivity of the MLR to stimulus repetition, this study did not address whether the MLR could exhibit novelty responses when a stimulus parameter is changed. On the other hand, [START_REF] Sonnadara | Occasional changes in sound location enhance middle latency evoked responses[END_REF] observed an enhancement of the Na component (25 ms) elicited by location deviant stimuli compared with standard stimuli in an oddball paradigm with stimuli consisting of brief noise bursts. In lack of a proper control, however, the increase in amplitude between deviant and standard stimuli in this study could have resulted from a different state of refractoriness of location-specific neurons (Schroger & Wolff, 1996) responding to the two types of sounds presented within the oddball sequence.

A step forward in understanding the neural mechanisms of novelty detection in the auditory system has been recently provided by studies of single-unit recordings in anaesthetized animals. For instance, in the primary auditory cortex of the cat, [START_REF] Ulanovsky | Processing of low-probability sounds by cortical neurons[END_REF] have A B Fig. 2. Grand-average AEPs of N = 18 subjects for the frequencies S1 and S2. (A) The auditory brainstem (ABR) for the standard stimuli that preceded the deviant stimuli (solid grey line) and the control stimuli (solid black line) compared with the deviant stimuli (dashed black line). (B) The middle latency responses (MLR) for the same stimulus type as specified in (A). The grey bar denotes the latency window for Pa waveform measurement, for which a significant deviant enhancement was observed compared with both standard and control stimuli (*P < 0.05, Bonferroni corrected). Notice that the responses corresponding to the three types of stimuli are in fact elicited to the same physical stimulus (either S1 or S2) that had different contextual roles in the three different experimental conditions. described the existence of the neurons that show strong SSA to repeating stimuli, but restore their firing rates when a stimulus parameter is changed. Therefore these neurons were proposed as the neural correlate of MMN. To a greater extent, however, [START_REF] Antunes | Is there stimulusspecific adaptation in the auditory thalamus?[END_REF] could identify novelty neurons in the thalamus of rat, supporting the seminal observations by [START_REF] Kraus | Discrimination of speech-like contrasts in the auditory thalamus and cortex[END_REF] 2009) have found units in the inferior colliculus of the rat that exhibit similar novelty responses to those found by [START_REF] Ulanovsky | Processing of low-probability sounds by cortical neurons[END_REF] in the cat's primary auditory cortex, supporting the subcortical origin of some novelty responses in the auditory system. Up to now, the inferior colliculi (IC) novelty neurons expressed a response to novel stimuli that depended on stimulus repetition rate, the frequency contrast between standard and deviant stimuli, and the probability of the deviant stimulus occurring [START_REF] Ulanovsky | Processing of low-probability sounds by cortical neurons[END_REF][START_REF] Malmierca | Stimulus-specific adaptation in the inferior colliculus of the anesthetized rat[END_REF]. For individual neurons, the response was greatest at the largest frequency contrast, the lowest probability of the oddball stimulus, and a repetition rate of 4 Hz. At a higher stimulus rate, many IC neurons failed to respond entirely.

In the present study, the response to deviant stimuli was enhanced in the time range of MLR, in agreement with a similar observation by our group [START_REF] Grimm | Electrophysiological evidence for the hierarchical organization of auditory change detection in the human brain[END_REF]. In particular, the Pa component peaking at about 30 ms showed significant larger amplitudes in response to the deviant as compared with the standard stimulus for the two frequency bands under study. The increased Pa amplitude might reflect that the repetition of a specific standard frequency has been represented in a form of memory, and the brain's neurophysiological response to the rare stimuli reflects the outcome of a comparison with the memory trace of the preceding repetitive frequency ( Naatanen & Winkler, 1999). Alternatively, it may suggest that the difference in frequency of the deviant stimuli, which occur with a much smaller probability than the standard ones, finds frequency-specific neural populations on a less refractory state than those responding to the standard stimuli [START_REF] Ja¨a¨skela¨inen | Human posterior auditory cortex gates novel sounds to consciousness[END_REF]. However, our research data showed an increased Pa response also to the deviant stimuli as compared with that elicited to control stimuli, which were physically identical and occurred with the same low probability. By using this control block, the AEP differences between the deviant stimuli occurring in the oddball sequence and the same stimuli occurring in a non-regular acoustic context cannot reflect the consequences of different states of refractoriness. Therefore, our results strongly support a memory comparison-based mechanism and index an early process of true novelty detection in the auditory system. Alternatively, such early effects could be explained by the involvement of an auditory predictive system that preactivates neuronal populations for the upcoming acoustic events [START_REF] Grimm | The processing of frequency deviations within sounds: evidence for the predictive nature of the Mismatch Negativity (MMN) system[END_REF][START_REF] Widmann | Binding symbols and sounds: evidence from event-related oscillatory gamma-band activity[END_REF][START_REF] Bendixen | I Heard That Coming: Event-Related Potential Evidence for Stimulus-Driven Prediction in the Auditory System[END_REF].

In the ABR time range, our results failed to show any statistical significant difference between deviant compared with standard and control stimulus responses for the V component. The amplitude of V wave decreased when the frequencies cutoff of the band-pass-filtered was raised from 15 to 250 Hz and from 250 to 1500 Hz. Considering the animal studies mentioned above, even though we can not perform a straightforward analogy with AEP in humans, the most likely reason why in this study no novelty-related activity was observed in the ABR time range was probably caused by a combination of factors, such as stimulus repetition rate, the frequency contrast between standard and deviant stimuli, and the probability of the deviant stimulus [START_REF] Malmierca | Stimulus-specific adaptation in the inferior colliculus of the anesthetized rat[END_REF]. For single-neuron recordings in subcortical structures, the repetition rate typically used to study SSA is of the order of 4 Hz, with the greatest response at the largest frequency contrast, and the lowest probability of the oddball stimulus. It should be kept in mind that establishing the present experimental design was a compromise between the repetition rate, the number and duration of stimuli for recording a reliable ABR. Moreover, it should be noted that, although possibly similar in origin to novelty units in animals, the ABR responses are much earlier, i.e. 20-30 vs. 1-10 ms, respectively. Therefore, from the present observations we may speculate that early auditory deviance detection of simple features could act through top-down mechanisms. As a matter of fact, a recent study has shown that the human IC is sensitive to the topdown modulations related to behavioural goals [START_REF] Rinne | Auditory selective attention modulates activation of human inferior colliculus[END_REF].

The anatomical neural origin of the human MLR, like the ABR, is not yet well understood, and there is still uncertainty on the neural correlates of each of its waveforms. Similar sources of ABR and MLR origin in animals were identified in humans. Thus, the anatomical neural origins of the V component are likely from lateral lemniscus and inferior colliculi [START_REF] Fischer | Auditory early-and middle-latency evoked potentials in patients with quadrigeminal plate tumors[END_REF][START_REF] Fischer | Auditory evoked potentials in a patient with a unilateral lesion of the inferior colliculus and medial geniculate body[END_REF]. In the MLR time range, recent studies could identify the earliest cortical activity (16-19 ms) to be localized in the medial portion of Heschl's sulcus and Heschl's gyrus [START_REF] Yvert | Localization of human supratemporal auditory areas from intracerebral auditory evoked potentials using distributed source models[END_REF], with the Pa component supposedly originating from the primary auditory cortex [START_REF] Yvert | Multiple supratemporal sources of magnetic and electric auditory evoked middle latency components in humans[END_REF]. However, there are findings suggesting that Na and Pa components have additional contributions from the thalamus [START_REF] Buchwald | Middle-and long-latency auditory evoked responses recorded from the vertex of normal and chronically lesioned cats[END_REF]. The Nb wave is suggested to be initiated more in the anterior part of the superior temporal gyrus, corresponding to secondary auditory cortices [START_REF] Yvert | Localization of human supratemporal auditory areas from intracerebral auditory evoked potentials using distributed source models[END_REF].

Our findings support the idea of a new electrophysiological marker of auditory novelty detection that indicates the auditory response to novel stimuli in the Pa latency range, supposedly originating in the primary auditory cortex.

In agreement with previous animal studies reporting novelty detection on multiple stages along the auditory pathway, and with a recent human AEP study of us [START_REF] Grimm | Electrophysiological evidence for the hierarchical organization of auditory change detection in the human brain[END_REF] our results confirm the multi-stage hypothesis of human auditory novelty detection. Whether these processes are complementary, hierarchical, interactive or redundant [START_REF] Chechik | Reduction of information redundancy in the ascending auditory pathway[END_REF][START_REF] Malmierca | Stimulus-specific adaptation in the inferior colliculus of the anesthetized rat[END_REF] can not be decided on the basis of the present experimental design.

While traditionally change detection has been indexed by the MMN AEP, the present results show the existence of a deviance detection response in the MLR range of the AEP, at about 30 ms. More generally, the results support the notion that novelty detection is a basic property of the functional organization of the auditory system that acts at different hierarchical levels along the auditory pathway, and that the generation of the MMN recorded from the human scalp might involve different levels of the auditory system's hierarchy, at least when deviance detection occurs in regard to a single physical attribute of the acoustic input.

B

  Fig. 1. Grand-average ABR (A) and MLR (B) of N = 18 subjects for the control stimuli: S1 (500-1000 Hz), S2 (1000-1500 Hz), S3 (1500-2000 Hz), S4 (2000-2500 Hz) and S5 (2500-3000 Hz) broadband filtered noises.

  through multi-unit activity recordings to consonant-vowel contrasts in the medial geniculate body of the guinea pigs, questioned the cortical origin of MMN. Furthermore, the single-unit recording studies by Perez-Gonzalez et al. (2005) and Malmierca et al. (

Fig. 3 .

 3 Fig. 3. Mean amplitudes and the standard errors of the means of the grandaverage V, Na, Pa and Nb waveforms (P < 0.05) elicited by the deviant, control and standard stimuli. The '*' symbol denotes a significant deviant enhancement compared with standard or control stimuli.

Table 1 .

 1 who, Mean amplitude (in lV) of the V, Na, Pa and Nb waveforms of the AEPs elicited by the control, deviant and standard stimuli

		Mean amplitude (Standard error)
		S1 (500-1000 Hz)			S2 (1000-1500 Hz)
		Control	Deviant	Standard	Control	Deviant	Standard
	Wave forms
	V	0.091(0.018)	0.083 (0.018)	0.095 (0.013)	0.085 (0.014)	0.097 (0.013)	0.086 (0.009)
	Na	0.255 (0.035)	0.260 (0.031)	0.238 (0.038)	0.250 (0.041)	0.251 (0.036)	0.228 (0.039)
	Pa	0.055(0.032)	0.127 (0.031)	0.105 (0.027)	0.100 (0.026)	0.118 (0.034)	0.09 (0.035)
	Nb	0.081(0.040)	0.104 (0.039)	0.109 (0.032)	0.088 (0.036)	0.109 (0.028)	0.103 (0.031)
	Standard errors of mean are given in parentheses.

Table 2 .

 2 Mean values (in ms) of the peak latencies of the V, Na, Pa and Nb waveforms of the AEPs elicited by the control, deviant and standard stimuli

		Mean peak latency (Standard error)
		S1 (500-1000 Hz)			S2 (1000-1500 Hz)
		Control	Deviant	Standard	Control	Deviant	Standard
	Wave forms
	V	7.71 (0.069)	7.59 (0.073)	7.76 (0.089)	7.94 (0.077)	7.88 (0.057)	7.94 (0.078)
	Na	20.22 (0.137)	20.28 (0.166)	20.50 (0.116)	20.22 (0.125)	20.78 (0.154)	20.83 (0.142)
	Pa	30.39 (0.167)	30.22 (0.174)	30.39 (0.172)	30.17 (0.173)	30.72 (0.145)	30.56 (0.158)
	Nb	41.39 (0.172)	41.11 (0.161)	40.94 (0.164)	40.78 (0.183)	41.28 (0.127)	41.06 (0.143)

Standard errors of mean are given in parentheses.

Acknowledgements

This study was supported by PSI2009-08063 and EUI2009-04086 grants, and the Consolider-Ingenio2010 Program (CDS2007-00012) of the Spanish Ministry of Science and Innovation, and the 2009SGR11 grant of the Generalitat de Catalunya. The authors wish to thank Dr Misericordia Veciana De Las Heras from the Neurology Department, Bellvitge University Hospital, for her expert technical advice on the ABR and MLR recordings, and Marc Recasens for helping with the data acquisition.

Abbreviations

ABR, auditory brainstem; AEP, auditory-evoked potential; EEG, electroencephalogram; IC, inferior colliculi; MLR, middle latency responses; MMN, mismatch negativity; SSA, stimulus-specific adaptation.