Choé Sauvage

Clélia Robert

Béatrice Sorrente

Frederic Grillot

Didier Erasme

Fréderic Grillot

Study of short and mid-infrared telecom links performance for different climatic conditions

Keywords: Free Space Optics, Link budget, Infrared transmittance, Scintillation

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INTRODUCTION

Free Space Optics (FSO) is a growing up technology offering a higher bandwidth with fast and cost-effective deployment compared to fiber technology associated to a lack of regulation unlike radio frequencies [START_REF] Khalighi | Survey on Free Space Optical Communication: A Communication Theory Perspective[END_REF] and more directivity induced discretion. Multiple applications are envisioned including but not limited to campusscaled network, substitution for a fiber network after a disaster (e.g., earthquake, attack, etc.) or connecting a drone as a relay in white spot. Despite that, FSO performances are restricted by atmospheric phenomena [START_REF] Sunilkumar | Performance of free-space optical communication systems: effect of aerosol-induced lower atmospheric warming[END_REF] (e.g., turbulence [START_REF] Stotts | Optical communications in atmospheric turbulence[END_REF] , fog or scattering [START_REF] Esmail | Outdoor FSO Communications Under Fog: Attenuation Modeling and Performance Evaluation[END_REF]). Therefore, the operating wavelength is an important parameter that has to be chosen wisely so as to reduce the impact of the environmental parameters. In order to improve the FSO availability, performance and range, the investigation of the cross relation between the climatic conditions and the wavelength is highly required.

The main scope of this study is to evaluate and compare the performance and the availability of a FSO link of a few kilometers in urban environment for two infrared wavelengths matching the appropriate atmospheric windows [START_REF] Wallace | Atmospheric Science -An Introductory Survey[END_REF].

We will compare the standard telecom wavelengths λ = 1.55 µm and the mid infrared λ = 4 µm. The wavelength λ = 1.55 µm is very interesting because it is a well known technology with low cost and dedicated telecom components. It would be very easy to make a transition with the current fiber network. The drawbacks of this wavelength are the hazard of retinal damage, its high level of scintillation in turbulent air and poor transmittance compared to 4 µm [START_REF] Ghassemlooy | Terrestrial Free-Space Optical Communications[END_REF]. The 4 µm does not show such problems, indeed the use of λ = 4 µm for a FSO link is very interesting and emerging [START_REF] Delga | Free-space optical communications with quantum cascade lasers[END_REF]. This paper is organized as follows: Section 2 presents the system architecture of the FSO link, Section 3 is dedicated to the computing of the link budget of our system. In Section 4 we discuss about the BER performance and the availability of our link, and Section 5 summarizes conclusions and prospects.

For more informations send correspondence to C.Sauvage (chloe.sauvage@onera.fr) or C.Robert (clelia.robert@onera.fr)

METHODOLOGY : SYSTEM ARCHITECTURE

To make the comparative study of these two wavelengths, we design a simple and shared optical system. The Figure 1 depicts the optical system, where laser is used as a transmitter. The beam is then enlarged and launched into the atmosphere with a diameter lens D E = 10 cm whose coefficient of transmission is l E = 0.95. The beam crosses the atmosphere on a Z = 4 km horizontal path with a t(λ) atmospheric transmittance. The flux is collected by an other D R = 10 cm diameter lens of coefficient transmission l R = 0.95, which focuses the beam on the detector. The laser light is directly modulated whith an On Off Keying Return to Zero (OOK-RZ) format. The emitted power is 0 mW for bit 0 and 70 mW for bit 1. An emitted power of 70 mW is easily reached by laser diodes and quantum cascade lasers which are found on the market [START_REF] Patel | High power infrared QCLs: advances and applications[END_REF]. The binary rate of the modulation is 666 Mbits/s which corresponds to a bandwidth ∆f = 532 MHz. This rate is determined by our signal clock recovery. The characteristics of the laser and the detectors come from commercial components. Their specifications are displayed on Table 1 π × 0, 05 2 0, 5 × 0, 5

Table 1. Specifications of components for each wavelength.

LINK BUDGET

Atmospheric transmittance

The first step is to compute the transmittance versus the visibility for the two wavelengths.

For this study, two weather cases are compared with MATISSE [START_REF]MATISSE-V3.0 | Matisse[END_REF], our radiative transfer software. The first simulation, on Figure 2a, is made with a winter profile, troposperic aerosols and advection fog. Fog is defined to have a visibility lower or equal to 1 km. The second simulation, on Figure 2b, is similar but without fog. In the two cases which are shown on Figure 2 the transmittance is better at 4 µm than 1.55 µm. These values of transmittance are essential in order to compute the received power for a 4 km path.

Received power

The link between the emitted signal flux P E (in W) and the received signal flux P R (in W) in case of atmospheric attenuation is given by the Beer-Lambert law:

P R = P E t(λ) , (1)
where Z is the link path (in m) and t(λ) the atmospheric transmittance coefficient.

Our system is a non diffracted limited system so the Beer Lambert law follow this equation [START_REF] El-Wakeel | Free space optical communications system performance under atmospheric scattering and turbulence for 850 and 1550 nm operation[END_REF] :

P R = P E D 2 R (D E + φ E Z) 2 t(λ) l E l R , (2)
reminding D E,R (in m) and l E,R are respectively the diameters and lens losses of the transmitter and emitter. We define φ E as the beam divergence angle which is given by φ E = λ πω0 (using the usual truncation equation

ω 0 = D E
3 ) . On Figure 3 we compare the evolution of the received power as a function of visibility for an emitted power P E of 70 mW, which corresponds to bit 1. Figure 3a presents the case with advection fog, showing that λ = 4 µm is the most powerful link. However Figure 3b, which corresponds to the case without fog, shows that the 4 µm is more interesting in terms of power up to 2.4 km of visibility, whereas beyond, the 1.55 µm wavelength is better. This effect is due to the divergence of the beam and the geometric losses.

PERFORMANCE 4.1 Bit Error Rate

The BER enables to assess the performance of a digital communication. For a OKK-RZ modulation, the BER expression is given by [START_REF] Agrawal | Fiber-Optic Communication Systems[END_REF] :

BER = 1 2 erf c Q √ 2 , (3)
with the Q factor following this equation [START_REF] Agrawal | Fiber-Optic Communication Systems[END_REF] : In Equation (4) I 0,1 is the photocurrent for the bits {0, 1}, and σ 0,1 are their respective total noise. When the bit is 0 the photocurrent I 0 = 0 and when the bit is 1 the photocurrent I 1 = I max . The photocurrent is given by this equation :

Q = I 1 -I 0 σ 1 + σ 0 . ( 4
)
I = R • P R , (5)
with R the responsivity (in A/W) of the detector.

By replacing Q (Equation ( 4)) in the BER (Equation ( 3)), it turns to :

BER = 1 2 erf c 1 √ 2 • I 1 -I 0 σ 1 + σ 0 . (6)
In section 3.2 we computed the power received, these values are used in the determination of the BER. Now we can focus on the detection noise of our system, and study its composition.

Noises

Various noises affect the performance of a telecommunication link. The first one is the shot noise defined by :

σ 2 Sh = 2eI∆f , (7)
where e is the elementary charge of the electron, I the photocurrent (in A) and ∆f the bandwidth (in Hz).

The second noise of the study is the detector thermal noise :

σ 2 T h = 4kT room R L ∆f , (8)
with k the Boltzmann constant, T room the room temperature (in K), R L the load resistance (in Ω).

The third noise is the thermal background noise which comes from the thermal radiation of the optics and of the atmosphere. This noise has the following expression :

σ 2 Back = 2eP Back R∆f , (9)
with P Back (in W) the power of the background which is given by :

P Back = S Detec E Back , (10)
and S detec (E Back ) is the detector area (in m 2 ) (background irradiance (in W/m 2 )) .

The total background irradiance is given by :

E Back = π sin θ D 2 N L medium , (11)
where θ D is the field of view of the detector, N the number of crossed medium (atmosphere and lenses) by the beam and L medium the luminance for each medium.

Assuming that each medium produces a black body radiation, the luminance L medium (in W.m -2 .sr -1 ) follows this expression :

L medium = t medium × λ2 λ1 L Ω,λ (λ, T medium )dλ , (12)
with t medium a combination between the emissivity of the regarded medium and the transmission coefficient of crossed path by the background radiation from the emitted medium to the detector. L Ω,λ (λ, T medium ) (in W.m -2 .m -1 .sr -1 ) is computed by using the Planck law that describes the distribution of the spectral energetic luminance emitted by a black body at the thermodynamic equilibrium for a given temperature. Here T medium is the temperature of the medium for which the background radiation is created. The spectral range of the detector is delimited by the two wavelengths λ 1 and λ 2 .

In our system we have N = 4 media, they are indexed with their contribution to the luminance in the Table 2 The last noise comes from the intensity fluctuations of the laser, named Relative Intensity Noise (RIN). The RIN (dB/Hz) is expressed like in Ref. [START_REF] Hashemi | Relative Intensity Noise (RIN) in High-Speed VCSELs for Short Reach Communication[END_REF] :

RIN = 10 log < δP (t) 2 > P 2 R -10 log(∆f ) , (13)
with < δP (t) 2 > the temporal square average of intensity fluctuation. So the noise of the laser is :

σ Laser = R • P R ∆f • 10 RIN/10 , (14)
σ 2 Laser = R 2 • P 2 R • ∆f • 10 RIN/10 . ( 15
)
Finally we sum quadratically the noise variances to obtain the total noise on the detector, so that :

σ 2 T OT AL = σ 2 Sh + σ 2 T h + σ 2 Back + σ 2 Laser . (16)
The RIN and the shot noise depend directly of the laser source, contrary to the termal and background noises.

We focus on the scenario with advection fog that is indeed the most restrictive case because of the weakness of atmospheric coefficient transmission. On Figure 4 we investigate the contribution of the noises versus wavelength and visibility. Figure 4a shows the noises contribution for λ = 1.55 µm. We can observe that the thermal noise dominates until 700 m of visibility and the RIN overlooks after 700 m of visibility. On Figure 4b we focus on noises contribution for λ = 4 µm, the thermal noise dominates until a visibility close to 1 km, then for a visibility of 1 km the RIN overlooks. The thermal background noise seems to have no variation on both parts of the Figure 4, because of the large scale which use in our case. Between 100 m and 1 km of visibility the variation of this noise is 0.22% for λ = 1.55 µm and 3.2% for λ = 4 µm, that is negligible and so not visible.

Moreover we can notice that the total level of noise is very close for λ = 1.55 µm and λ = 4 µm up to 800 m of visibility, then the total level noise is higher at λ = 1.55 µm than at λ = 4 µm. To study this effect we focus on the evolution of atmospheric transmittance as a function of visibility (Cf. Figure 2a). Indeed from 700 m of visibility t(λ = 1.55 µm) increases non linearly, that induces a quadratic increase of σ 2

Laser (λ = 1.55 µm) as a function of visibility. This increase also appers for t(λ = 4 µm) from 500 m of visibility. However we can observe on Figure 4b that σ 2

Laser (λ = 4 µm) increases less higher than σ 2 Laser (λ = 1.55 µm), this is due to the fact that the RIN for a λ = 1.55 µm diode laser is less larger than the RIN for a λ = 4 µm QCL (Cf. Table 1).

Yearly availability

We choose to present here the variation of the BER as function of visibility with advection fog (Cf. Figure 5), by using the results previously obtained. The red line on Figure 5 corresponds to a BER limit of 10 -4 , assuming an error correction code that brings the BER down to a low value of 10 -9 . This is the limit above which the FSO link is strongly disrupted and does not operate.

Thus, looking at Figure 5, the minimum visibility required to get an effective FSO link is respectively 600 m at 1.55 µm and 300 m at 4 µm. The different values of BER for visibility larger than 900 m for λ = 1.55 µm and higher than 400 m for λ = 4 µm, are equal to 0, so they are not plotted.

In order to prepare a future telecom link at ONERA-Chatillon laboratory, we compute a practical availability. We created a weather visibility database which indexes the visibility for each hour at Velizy-Villacoublay (France), a station near our laboratory. The statistics of the database is depicted by the histogram on Figure 6 that represents the visibility variation in 2017. We show in green the visibility lower or equal to 1 km and in blue the visibility larger to 1 km to discriminate fog conditions. On the left part of the graph, we make a focus on low visibility, typical for fog, for the January month. By combining this statistics and the visibility limits it is easy to estimate an availability for each wavelength. Table 3 indexes the theoretical availability for each month and each wavelength. We can observe with the histogram on Figure 6 that in January 2017, fog occurs 15,1 % of the time, so it is an interesting month to make an experiment at low visibility. Hence for January 2017, Table 3 shows that the availability is 87.9 % for the 1.55 µm against 92 % for 4 µm, which represents an increase of 30 hours of operation.

However between April and July, the availability are the same for both wavelengths, but the 4 µm will have an advantage against scintillation : making an experimentation at this period of the year, through hot air could be interesting to lower the impact of the scintillation on the received signal. Indeed the scintillation index σ 2 I is proportional to λ -7/6 so scintillation decreases as a function of wavelength.

CONCLUSION AND PROSPECTS

The choice of wavelength has an impact on the telecom channel availability especially in the case of fog : the availability of a FSO link will be increased in winter by using a mid-infrared wavelength (4 µm) instead of a short-infrared wavelength (1.55 µm). We saw on this paper that a telecom link at 4 µm wins 30 hours of operation in a foggy month (January 2017 at Velizy-Villacoublay). This result shows the very interesting perspectives of using the mid-infrared wavelength for future FSO links.

In this paper we have presented the transmission simulation on a 4-km horizontal line of sight in a urban environment. We developed a link budget calculation by specifying the received power. Then we described the different detection noises and estimated the BER of our system. Finally we discussed about the availability of our FSO link for the two wavelengths.

This study could be extented to make spatial and temporal (many years) statistics from database assimilation in order to assess on the FSO availability for many conditions (clouds, fog, rain and turbulence).

In the coming months we wish to establish a first FSO link at 4 µm at ONERA-Chatillon, at a bit rate around 600 MHz and for a 4-5 km horizontal path at 40 m height. We plan to do experimentation in winter with low visibility and in summer with strong turbulence. We propose to use our Cn 2 profiler, SCINDAR (SCINtillation Detection And Ranging), working at 4 µm to build this future experiment to get a simultaneously diagnostic of the channel. . The SCINDAR will make the channel diagnostic, by gathering intensity, phase at its telescope aperture and estimating Cn 2 profiles series. This instrument is composed by an infrared Shack-Hartmann wavefront sensor (λ = 3.4 to 4.2 µm) which aims at two halogen lamps 1 m-apart located at several kilometers from the receiver [START_REF] Nguyen | Measurement of the spatial distribution of atmospheric turbulence with SCINDAR on a mosaic of urban surfaces[END_REF]. We have already modified the instrument in order to record the telecom signal on a splitted arm of this bench just before the infrared wavefront sensor. The 4 µm QCL source will be launched by a telescope next to the halogen lamps in a distant building.

Figure 1 .

 1 Figure 1. System architecture

Figure 2 .

 2 Figure 2. Transmittance as a function of visibility for the two wavelengths. For a winter profile and tropospheric aerosols a) with advection fog b) without fog.

Figure 3 .

 3 Figure 3. Received power as a function of visibility for the two wavelengths. With an emitted power of 70 mW for a winter profile and tropospheric aerosols a) with advection fog b) without fog.

Figure 4 .

 4 Figure 4. Contribution of noises as a function of visibility for an emitted power of 70 mW and advection fog for a) λ = 1.55 µm and for b) λ = 4 µm.

Figure 5 .

 5 Figure 5. Bit Error Rate versus visibility for two wavelengths and for winter profile with advection fog.

Figure 6 .

 6 Figure 6. Histogram which depicts the visibility variation in 2017 at Velizy-Villacoublay (France). Focus on visibility ≤ 1 km on January (at left).

  for both wavelengths.

	λ (µm)	1,55	4
	R L (Ω)	50	50
	R (A/W)	1	1,3
	RIN (dB/Hz)	-120	-150
	λ 1 (µm)	0,8	2,5
	λ 2 (µm)	1,75	5
	θ M ( o )	20	18
	Area (mm 2 )

Table 2 .

 2 . Contribution of each medium in luminance calculation.

	Medium	t medium	T medium (in o C)
	Transmitter lens	0.05 × 0.95 × t(λ)	20
	Atmosphere	0.95 × t(λ)	10
	Received lens	0.05	20
	Room	1	20

λ

  Mos. Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. 2017

	1.55 µm	87.9 95.2 99.7	100	100	100 99.9 98.9 98.3 96.8 99.4 94.4 97.6
	4 µm	92.0 96.7 99.9	100	100	100 99.9 99.2 98.9 97.7 99.7 95.3 98.3

Table 3 .

 3 Availability for each months of 2017 at Velizy-Villacoublay for the two wavelengths.


Yann Kerbrat

Sandrine Maljean-Dubois

Precautionary Principle.. Gabriela G B Carina Costa De Oliveira

Fabrício Lima Moraes

Ramos

Keywords:

est destinée au dépôt

The Stockholm Declaration (1972) principles, taken up and developed in the Rio Declaration on Environment and Development (1992), have played a major role in the development of international environmental law in the last forty years. Independently from their binding nature, unequally admitted, they have inspired negotiations of new conventional rules. They have also spread to most national laws and they have influenced, although sometimes only implicitly, decisions of courts and tribunals. The structuring role of these principles has been particularly important in the context of a rapid and continuous law-making process. Among these principles, the precautionary principle is certainly one of the most fundamental. Enshrined in many international declaration and treaties, it is now recognised and to some extent promoted by international courts, with concrete consequences for States and -indirectly but surely-for non-states actors, particularly for corporations acting under their jurisdiction.

(*) This article is based on : "Les juridictions internationales face au principe de précaution, entre grande prudence et petites audaces", Yann Kerbrat, Sandrine Maljean-Dubois, published in Unity and diversity of international law, Essays in Honour of Professor Pierre-Marie Dupuy, ed. Denis Alland, Vincent Chetail, Olivier de Frouville, Jorge E. Vinuales, Martinus Nijhoff, 2014, pp. 929-948. The authors would like to thank Dr. Isabell Buschel for her kind assistance in translating this article.

I. The precautionary principle in texts

Symbol of what some have called the "technical democracy", this principle has been one of the cornerstones of the Rio Declaration. It is formulated in principle 15 as follows: "In order to protect the environment, the precautionary approach shall be widely applied by States according to their capabilities. Where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation." This proclamation was the starting point of a broad and rapid development of the principle -or the approach-of precaution which today underpins many legal systems.

The precautionary principle is laid down, notably, in the United Nations Framework However, despite of its reiteration in numerous treaties, the status of the precautionary principle has long been uncertain in international law. That was due to hesitations of international tribunals on the customary authority of the principle. These are becoming increasingly blurred in case-law (II). That was also due to the uncertainty of the content of the principle, which still remains, at least partially (III).

II. The precautionary principle in international case-law

The precautionary principle has, for a couple of years, been dealt with like a "hot potatoe" between international jurisdictions, having been passed on from one to another, each one refusing to decide on its authority, on the pretext that the others had not stated either.1

The phenomenon begun in 2009 with the European Court of Human Rights decision in the case of Tatar vs. Romania. Basing its reasoning on the Rio Declaration, the Court first stated that "the precautionary principle recommends that States do not delay the adoption of effective and proportionate measures aimed at preventing the risk of serious and irreversible damage to the environment in the absence of scientific or technical certainty".2 Referring to European Union law, it then recalled "the importance of the precautionary principle (laid down for the first time by the Rio Declaration), which 'may be applicable in order to ensure a high level of protection of health, of the consumer safety and the environment, relating to the entire range of activities of the Community'".3 It highlighted that its introduction in the Rome Treaty at the time of the revision of the Maastricht Treaty (art. 130 R transformed into 191 TFEU) "maps out, on the European level, the evolution of the principle from a philosophical concept to a legal norm" whereas "

[t]he Court of Justice of the European Communities ("ECJ") considers this principle, at the light of article 17, paragraph 2, first subparagraph, EC, as one of the cornerstones of the policy aiming at a high level of protection by the Community in the field of the environment".4

The European Court of Human Rights also quoted the case law of the International Court of Justice when analyzing the applicable law. It refered more precisely to the excerpt of the decision of the International Court in the case of the Dam on the Danube, in which the ICJ "recognizes (…) the need to take environmental concerns seriously and to take the required precautionary measures (…)".5 It should be noted, at this point, that the European Court of Human Rights has taken some liberties compared to precedents by using a shortened quotation. The ICJ has recognized that "both Parties agree on the need to take environmental concerns seriously and to take the required precautionary measures",6 which constitutes an important detail when searching whether or not it has recognized a customary authority to the principle.

The year following the decision of the European Court of Human Rights, in 2010, in the case of the Pulp Mills on the River Uruguay, the International Court of Justice refered again to precaution, considering that a "precautionary approach" "may be relevant in the interpretation and 1 S. Maljean-Dubois, « Le spectre de l'isolation clinique: quelle articulation entre les règles de l'OMC et les autres instruments et principes internationaux? », Revue européenne de droit de application of the provisions of the Statute" of the River Uruguay.7 The judgment of the Court did not, however, tell anything about the customary character of the principle. Discussion on this aspect was unnecessary, as parties had agreed to consider that the Court should interpret the statute according to a precautionary approach, even though they disagreed on the consequences which judges should draw from this.

On February 1st, 2011, a third, even more decisive step was taken with the adoption of the very bold advisory opinion of the Seabed Disputes Chamber of the International Tribunal for the Law of the Sea.8 The Chamber was called upon notably to determine whether States have the obligation to adopt a precautionary approach when they sponsor a company which is engaged in the exploration or exploitation of the Area, even if these activities fall outside the scope of the two Regulations on prospecting and exploration for polymetallic nodules and polymetallic sulphides which refer explicitly to it. The Chamber noted that "it is appropriate to point out that the precautionary approach is also an integral part of the general obligation of due diligence of sponsoring States, which is applicable even outside the scope of the Regulations".9 Due diligence being a customary obligation which was laid down in particular by the ICJ in its judgment on the Pulp Mills case and the precautionary approach forming part of due diligence, it logically follows that precaution is an obligation which takes its source in a customary rule. Which was to be demonstrated… The consequence is obvious: "[t]he due diligence obligation of the sponsoring States requires them to take all appropriate measures to prevent damage that might result from the activities of contractors that they sponsor", including in "situations where scientific evidence concerning the scope and potential negative impact of the activity in question is insufficient but where there are plausible indications of potential risks".10

It appears however that the Chamber tried to circumscribe its reasoning to the marine environment, when adding afterwards that a "a trend towards making this approach part of customary international law" has been initiated by the incorporation of the precautionary principle "into a growing number of international treaties and other instruments, many of which reflect the formulation of Principle 15 of the Rio Declaration".11 However, if we accept the premise, its reasoning could be extended and apply to the whole environment. The advisory opinion of the ITLOS could map out a new step; international jurisdictions which are called upon to state on the precautionary principle may build upon this recognition and continue the dialogue between judges, which has only started on this matter. Moreover, this change could be facilitated by the fact that the Chamber used the precautionary approach and principle as synonyms: "the precautionary approach (called "principle" in the French text of the standard clause just mentioned) is a contractual obligation of the sponsored contractors whose compliance the sponsoring State has the responsibility to ensure".12 The Chamber itself used both expressions indistinctly. As a matter of fact, this distinction is to be relativized. In that regard, it should be noted that certain instruments, be they national or international, refer to the "precautionary principle", others to the "precautionary approach", the "precautionary method", or "precautionary measures". Some authors have regretted the semantic uncertainty. [START_REF]Conseil national de l'alimentation (National Food Council of France), Rapport et avis sur le principe de précaution et la responsabilité dans le domaine alimentaire[END_REF] The reality is that these expressions are generally chosen on purpose.14

Those who refuse to recognize a customary and general character to the precautionary principle may accept that in certain cases, it is mandatory to follow a precautionary approach or to adopt precautionary measures… In practice, however, the distinction reveals to be essentially a symbolic one.

III. What obligations for States?

The International Court of Justice has not specified the concrete consequences of the precautionary principle in the Pulp Mills case, even though it recognized that it could be useful for interpreting and applying the Convention concerned (a treaty adopted in 1975). Wouldn't it be, referring to the expression of Vaughan Lowe, a mere "interstitial norm", a meta-principle which participates in ensuring the coherence of other norms, that is, a principle which does not address rights-bearing subjects, but which might be used by the judge when interpreting primary rules?15

This would amount to limiting the principle to "a purely hermeneutic tool at the disposal of the interpreter and conferring the latter a large margin of appreciation when carrying out his or her task, but lacking normative independence and thus not being able to frame, as such, the behaviour of rights-bearing subjects". [START_REF] Freestone | Le développement durable en droit international : essai sur les incidences juridiques d'un concept évolutif[END_REF] If the precautionary principle also aims at prescribing, the normative threshold (because of its general and abstract character) may only be reached through further implementation, notably through a judicial process or the adoption of implementing rules. This is what the European Court of Human Rights seems to highlight when it stated that the principle "recommends that States do not delay the adoption of effective and proportionate measures aimed at preventing the risk of serious and irreversible damage to the environment in the absence of scientific or technical certainty".17 The Court of Justice of the European Union also infers very concrete consequences from it when it states, for example, that the principle "requires" "the authorities in question, in the particular context of the exercise of the powers conferred on them by the relevant rules, to take appropriate measures to prevent specific potential risks to public health, safety and the environment, by giving precedence to the requirements related to the protection of those interests over economic interests".18 The principle thus brings with it positive obligations for Member States and European Union institutions.19

At the universal level, the path for implementing the precautionary principle has also recently been opened by the Seabed Disputes Chamber of the International Tribunal for the Law of the Sea, which, in its advisory opinion of 2011, focused in particular on clarifying several practical consequences. First of all, the Chamber considers that the Authority will be called upon to include or develop the provisions related to this approach in its regulation of exploitation activities or activities concerning minerals of different types, in order to complete the reference to the precautionary approach in the two Regulations, which concerns more particularly the activities envisaged therein, namely prospecting and exploration for polymetallic nodules and sulphides.20

The Chamber notes that for States, the respect of a precautionary approach brings with it several positive obligations. First, the precautionary approach, as far as it forms part of the due diligence obligation, imposes, first of all, on each State to proceed to a potential impact assessment on the environment of the activities which are being carried out under its jurisdiction,21 which supposes that it establishes adequate evaluation procedures and mechanisms. Secondly, it follows from the precautionary approach that States are required to anticipate even uncertain risks by establishing a legislative and regulatory framework which puts constrain not only on public authorities but also on private persons within their jurisdiction in order to comply with this approach. Thus, concerning the exploration and exploitation of the seabed, "the sponsoring State has to take measures within the framework of its own legal system in order to oblige sponsored entities to adopt such an approach".22 Here lies a twofold obligation: the international obligation (of the State), in turn, has an impact on private persons through the national regulations of each State. Vertical in the beginning, the obligation in this way transforms into a horizontal obligation extending to the relationships between internal subjects. With regard to the scope of the precautionary approach, the Chamber, by referring at several moments to Principle 15 of the Rio Declaration, classically limited its scope to the "risk of serious and irreversible damage". Additionally, it specified that the precautionary measures must be applied by States "according to their capabilities". For the Chamber, "the first sentence of Principle 15 introduces the possibility of differences in application of the precautionary approach in light of the different capabilities of each State".24 This criterion of the capability of States introduces a subjective factor in the assessment of the precautionary principle, related to the level of scientific knowledge and technical capabilities of each State. By so doing, the Tribunal accepted "a dose of rationality and of reason"25 inherent to the idea of precaution and necessary in order to -implement the principle. However, the fact remains that each State is bound, albeit only according to its capability. This obligation applies to all States but its content may vary, depending on the level of development of each individual State. To summarize, the precautionary principle flirts with the albeit somewhat different principle of prevention, because of its common ground in due diligence, but also with the principle of common but differentiated responsibilities, from which it differs because of its binding character.

We can add that, on the procedural level, some authors consider that the precautionary principle calls for the burden of proof to be reversed. In practice, as of today, however, none of the international jurisdictions has ever inferred from the precautionary principle or approach such consequences on for the rules governing the burden of proof. As a matter of fact, the need for the evolution of procedural law towards the reversal of the burden of proof is felt less clearly nowadays because of the development of positive obligations which weigh at present on States when implementing the precautionary principle. Following the advisory opinion of the ITLOS, the precautionary attitude forms part of the due diligence obligations and consequently, as the principle of prevention, comes with positive obligations for States: the proof thus is eased. Not concerning the burden of proof, but concerning what has to be demonstrated by the plaintiff. The proof has to be made not of the existence of a risk, but that the State has not put in place the legislative and regulatory framework which would have enabled it to become aware of the risk, to measure its probability and gravity, and to take the measures aimed at preventing it from realizing. The proof of such a failure does not reveal any particular difficulty.

****

Step by step, international courts have, through a constructive dialogue, gradually highlighted the precautionary principle, by stating on its authority or its contents and its implications. The complex and composite nature of this principle continues to be a challenge. But in nowadays societies, characterized by constant technical innovation, its introduction in positive law seems quite irresistible. Judges are expected to play an important role in the development of this "open textured" standard. In such a context, the dialogue between judges including between international, European and domestic judges, may still be helpful, and could contribute to feed and stabilize the principle and its content.

As common and shared standards on the universal level, the principles of international environmental law are very often integrated in national laws, in legislative or even constitutional provisions. Sometimes, an existing formulation from the international level is used; in other circumstances, this formula is subject to debate, adaptation and modification. Through these phenomena of imitation or transplantation, the principles of international environmental law contribute in an important manner to the globalization of environmental law, and even to a certain extent, to some form of acculturation of national environmental laws. The phenomenon reflects the power of international norms as a new common law.

The

  Role of International Law in the Promotion of the Precautionary Principle Yann KERBRAT, Professor at the Sorbonne Law School (Univ. Paris 1) Director of the Sorbonne Research Institute for International and European Law and Sandrine MALJEAN-DUBOIS, Director of research at the CNRS, Director of DICE (Aix Marseille Univ, Université de Toulon, Univ Pau & Pays Adour, CNRS, DICE, CERIC, Aix-en-Provence, France)(*)

  Convention on Climate Change as follows: "The Parties should take precautionary measures to anticipate, prevent or minimize the causes of climate change and mitigate its adverse effects. Where there are threats of serious or irreversible damage, lack of full scientific certainty should not be used as a reason for postponing such measures, taking into account that policies and measures to deal with climate change should be cost-effective so as to ensure global benefits at the lowest possible cost. (…)" (art. 3 para. 3). Without claiming to be exhaustive, the precautionary principle is also mentioned, for example, in the Convention on Biological Diversity (Preamble), in the 2000 Cartagena Protocol on Biosafety (art. 9 and 10) and in the 2010 Nagoya-Kuala Lumpur Supplementary Protocol on Liability and Redress to the Cartagena Protocol on Biosafety. It is contained in several conventions on the Marine Environment, in the 1999 Protocol on Water and Health to the Convention on the Protection and Use of Transboundary Watercourses and International Lakes (art. 5), in the 2000 Protocol on the Implementation of the Alpine Convention in the field of Transport (art. 1), or in the Convention on the Protection of the Rhine (art. 4). In short, it is largely enshrined in international environmental treaties.

  The Chamber, in its exegesis of the precautionary principle, used directly Principle[START_REF] Lowe | Sustainable Development and Unstainable Agreements[END_REF] of the Rio Declaration. This is implied by Seabed Regulations [article 31, paragraph 2 of the Regulation on the Nodules and article 33, paragraph 2 of the Regulation on the Sulphides both], which state 18 Judgment of the Court of First Instance of the European Communities of 21st October 2003, Pharmaceuticals BV vs. Council of the European Union, Case T-392/02, para. 121. 19 Council Resolution on the Precautionary Principle, Annex III to the Conclusions of the Presidency of the Council of the European Union of Nice, 7-10 December 2000.the Authority and the sponsoring State "shall apply a precautionary approach, as reflected in Principle 15 of the Rio Declaration" in order "to ensure effective protection for the marine environment from harmful effects which may arise from activities in the Area". From this viewpoint it is interesting to see which normative authority23 may arise from the Rio Declaration, even though the latter is not binding. It is a mandatory instrument which refers to it and via this explicit referral the normative suggestion contained in Principle 15 becomes mandatory.

20 Abovementioned ITLOS Advisory opinion, para. 130. 21 Ibid., para. 142 and 145. 22 Ibid., para. 134.

that

See C. Thibierge, La force normative. Naissance d'un concept, Paris, LGDJ, 2009.

Abovementioned ITLOS Advisory Opinion, para. 129.

L. Boisson de Chazournes, « Le principe de précaution : nature, contenu et limites », in Le principe de précaution, aspects de droit international et européen, Ch. Leben and J. Verhoeven (dir.), Paris, ed. Panthéon-Assas, 2002, p. 82.


Jérémie Sage

Emmanuel Berthier

Clément Dutremble

Delphine Porcheron

Assessing the impact of rainwater harvesting on the hydrological regime of small semi-urban watersheds

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Assessing the impact of rainwater harvesting on the hydrological regime of small semi-urban watersheds

Impacts de le récupération des eaux pluviales sur le régime hydrologique de petits bassins versants partiellement urbanisés Sage J. 1* , Berthier E. 1 , Dutremble C. 1 , Porcheron D. 1 , 1 Cerema, Equipe-projet TEAM, 12 rue Teisserenc de Bort, 78197 Trappes (*jeremie.sage@cerema.fr) ;

RÉSUMÉ

La récupération des eaux de pluie suscite aujourd'hui un intérêt croissant du fait des incertitudes sur la disponibilité future des ressources en eau. Celle-ci est par ailleurs fréquemment envisagée comme un moyen de limiter à la source les volumes de ruissellement générés niveau des surfaces revêtues, participant ainsi à la gestion des eaux pluviales urbaines. La perspective d'une mise en oeuvre de politiques incitatives de récupération des eaux pluviales impose cependant de s'interroger sur l'incidence de cette pratique sur le régime hydrologique de certains bassins versants. Dans les secteurs urbanisés, pour lesquels le potentiel de développement de la récupération des eaux pluviales est important, l'interception d'une fraction significative du ruissellement pour satisfaire divers usages pourrait en effet conduire à un déséquilibre hydrologique se traduisant par une aggravation des étiages. Une méthode associant modélisation hydrologique et exploitation de données géographiques est ici introduite pour construire différents scénarios de récupération des eaux pluviales et évaluer leur incidence de sur le régime hydrologique deux bassins-versants semi-urbains. L'analyse suggère ici que cette pratique n'est à elle seule pas suffisante pour satisfaire les objectifs usuels de gestion des eaux pluviales mais indique que sa systématisation sur des têtes de bassins versant présentant de faibles débits pourrait en revanche donner lieu à une aggravation des étiages.

ABSTRACT

Rainwater harvesting is a relevant and sustainable solution to reduce the pressure on conventional water resources. Rainwater harvesting techniques can as well provide stormwater management benefits through the reduction of runoff volumes. The impact of a wide implementation of these practices on the hydrological regimes of already disturbed catchments however remains unclear. The capture of significant fraction of runoff volume in urbanized areas to satisfy various uses, could in particular result in an over-extraction of water, exacerbating low streamflow issues. In this study, a method associating geodata processing and allotment-scale hydrological modeling is introduced to investigate the impact of rainwater harvesting on the hydrology of two semi-urban catchments, addressing both stormwater management benefits and low-flow effects. Results indicate that rainwater harvesting alone is unlikely to meet usual runoff-control objectives, but as well suggest that a systematic implementation of these practices on upstream catchments that already face low water flow issues might be detrimental to stream health.

KEY WORDS

Low-flow; Reuse; Runoff-reduction; Stormwater; Stream health

CONTEXT AND OBJECTIVES

Rainwater harvesting has become a relatively common technique to reduce potable water consumption and is likely to develop in the next decades due to the uncertainty regarding the availability of water resources. This practice usually relies on relatively basic systems that convey runoff originating from impervious surfaces (in most cases roof surfaces) to simple storage units such as rainwater tanks or barrels. Collected volumes can then supplement traditional domestic water supply for outdoor or indoor uses that do not require drinkable water quality standards. Because rainwater harvesting may provide non-negligible volume reduction at the local scale, it also increasingly seen as a solution that complements more conventional stormwater source-control systems (swales, rain gardens…) [START_REF] Walsh | Hydrologic modeling analysis of a passive, residential rainwater harvesting program in an urbanized, semi-arid watershed[END_REF][START_REF] Steffen | Water Supply and Stormwater Management Benefits of Residential Rainwater Harvesting in U.S. Cities[END_REF].

The hydrological functioning of rainwater harvesting (RWH) systems however significantly differs from other stormwater management solutions [START_REF] Gerolin | Rainwater harvesting systems for stormwater management: feasibility and sizing considerations for the UK[END_REF]. Volume reduction primarily depends on initial storage conditions governed by irregular rainwater inflows and drawdowns; the effect of RWH system therefore tends to be highly variable. The impact of these techniques on the local water cycle is also likely to differ significantly from other volume-reduction techniques; whereas the latter usually rely on infiltration (resulting in the release of a fraction of collected volumes as baseflow), RWH practices may simply divert harvested volume out of the catchment [START_REF] Fletcher | Is stormwater harvesting beneficial to urban waterway environmental flows?[END_REF]. This alteration of the water cycle can potentially affect surface waters, especially during low-flow periods. The growing interest in RWH and its potential development to face the effects of future climate or consumption changes on water resources therefore requires to investigate its impact on the hydrological regime of vulnerable catchments.

In this study, the consequences of RWH are illustrated for two small semi-urban watersheds experiencing recurring low-flow and flooding issues. A simple coupled GIS and hydrological modeling approach is introduced to compare different RWH scenarios in terms of uses and storage options.

METHODS

Study area

Hydrological modeling

The modeling of RWH system is conducted at the allotment scale for each parcel of two catchments (up to 12 000 computation units, depending of RWH scenario). A simple conceptual model (similar to the one presented in [START_REF] Mitchell | Urban stormwater harvestingsensitivity of a storage behaviour model[END_REF]) is here adopted to simulate runoff production on roof surfaces, storage and drawdown for indoor or outdoor uses. Hydrological processes such as runoff generation from remaining urban surfaces or agricultural areas are not represented here.

Simulations are conducted at an hourly time step using 15-year long meteorological records. Monthly volume reductions computed with the hydrological model are then compared to flow-rate values measured at the outlet of the two catchment so as to evaluate the potential impact of RWH on lowflows. Stormwater management benefits are additionally discussed on the basis of annual volume reductions and total storage volumes associated with each RWH scenario.

Construction of rainwater harvesting scenarios

A RWH scenario may be described as the combination of a storage (e.g. sizing of the RWH system) and a water demand scenarios. While different demand scenarios were considered in this study, the article focuses on a single "maximum scenario" for which RWH is implemented (i) on every individual housing lots to cover both indoor and outdoor uses and (ii) on collective housing and public buildings for outdoor uses only (parcel types are identified from public databases).

The demand associated with outdoor uses is here limited to garden and landscape irrigation. An outdoor demand time-series is computed for each parcel from geographical data (to derive irrigation area) and potential evapotranspiration measurements using the method described by [START_REF] Nouri | Water requirements of urban landscape plants: A comparison of three factor-based approaches[END_REF]. Indoor demand is deducted from water consumption statistics and an estimation of the number of inhabitants on each individual housing lot. The sizing of RWH systems is adapted for each demand scenario using the method described in de Gouvello et al. ( 2010). An optimization is therefore performed for each parcel so as to meet water demand while keeping reasonable storage volumes (here limited to 2 m 3 for 100 m 2 of roof surface).

Water demand estimates and corresponding storage volumes are strongly dependent on the assumptions adopted to compute indoor and outdoor uses (geographic data processing, hypotheses regarding domestic water consumption, parameterization of irrigation needs…). Uncertainties associated with the definition of each RWH scenario are thus accounted for by considering lower and upper bounds for the water demand. Here, the combination of outdoor and indoor demand ("maximum scenario") D TOT is found to represent between 120 and 230 mm/year at the catchment scale.

RESULTS

The table below shows the total storage volume V TOT (m 3 ), the volume reduction efficiency ε (mm) and the proportion of the annual water demand covered by RWH T R (%) for selected scenario (indoor and outdoor uses for individual housing lots and outdoor uses for public buildings and collective housing).

Table 1. Simulation results for the "maximum" RWH scenario (considering uncertainties regarding water demand)

Indicator

Ru d'Elancourt Maldroit

V TOT 17 800 to 18 000 m 3 17 900 to 18 100 m 3 T R 7 to 13 % 9 to 16 % ε 13 to 16 mm 9 to 11 mm Table 1 primarily indicates that the total storage volume V TOT computed for each catchment with the automatic sizing procedure remains mostly constant, despite large uncertainties regarding water demand D TOT . Here, the increase between lower and upper estimates of D TOT (from 120 to 230 mm/year) can believably not be satisfied through RWH unless considering unrealistic storage volumes (discarded by the automatic sizing procedure). T R hence exhibits large variations that are directly related to D TOT . T R values nevertheless remain relatively low suggesting that the total production (e.g. roof) area is not sufficient to meet irrigation requirements that represent more than 90% of D TOT (additional test clearly demonstrate that indoor demand is much easier to satisfy as it does not exhibit the same variations as watering needs that are maximized during dry periods).

V TOT represents less than 1 mm at the catchment scale and is therefore very unlikely affect highest stream flows, although it remains significant at the roof scale (approximately 19 mm) and may hence provide on-site volume attenuation for medium to large rain events. Similarly, the annual volume reduction ε does not exceed 16 mm at the catchment scale but represents more than 50% of the annual roof runoff. Despite a limited effect at the catchment scale (where roofs represent less than 5% of the total area), RWH could hence clearly complement other stormwater management systems at the allotment scale.

Monthly volume reduction ε mth associated with RWH are presented in figure 2 and compared to lowflow values measured at the outlet of the two catchments: 1)

The impact of RWH tends to be more important for the Maldroit catchment that exhibits significantly lower stream flows than the other one (cf. Table 1). For this catchment, ε mth may represent a large fraction of minimum stream-flow values (although maximum values shown in light orange are clearly not associated with dry low-flow periods). Average ε mth values computed for July and August hence reach between 10 and 15% of average stream-flow volumes whereas minimum ε mth values for instance exceed 20% of the minimum volume observed in July.

Results hence suggest that, for the Maldroit catchment, low-flow issues could potentially be exacerbated by RWH practices. It is however important to underline that the scenario tested here is based on the assumption of a systematic implementation of RWH at the catchment scale and should thus not be considered at totally realistic.

CONCLUSION AND PERSPECTIVES

A relatively simple method was developed to generate and simulate various RWH scenarios from hydrological and geographical data available from public geographical databases.

In this application, RWH was not found to provide significant benefits at the catchment scale ; volume reduction achieved for idealistic development scenarios remains very limited unless focusing roof surfaces, for which RWH should nevertheless be completed by more conventional runoff-control practices. Results also indicate that RWH could under specific conditions (upstream catchments with low stream flows) result in an excessive abstraction of runoff volumes during low-flow periods.

While the method adopted here offers perspective for a wider scale assessment of the effect of RWH, its integration in a more comprehensive modeling framework, incorporating other hydrological processes such as rainfall-runoff transformation, would believably be beneficial for a more detailed examination of the hydrological impacts of RWH practices.

  Fig. 1 -Study area and catchment characteristics

Fig. 2 -

 2 Fig. 2 -Monthly volume reductions on the two catchments. Dark orange: average volume reduction; Light orange: minimum and maximum volume reduction; Light blue: minimum monthly flow-rates measured at the outlet of the catchment ; Blue dotted-line: low flow indicator QMNA5 (cf.Table 1)