Christine Michel
email: christine.michel@insa-lyon.fr

Élise Garrot-Lavoué

MESHAT: MONITORING AND EXPERIENCE SHARING TOOL FOR PROJECT-BASED LEARNING

Keywords: Project-based learning, monitoring tools, metacognition, experience sharing, acquisition of expertise, Web 2.0

MEShaT: Monitoring and experience sharing tool for project-based learning.

INTRODUCTION

Project-based learning is often applied in the case of complex learning (i.e. which aims at making learners acquire various linked skills or develop their behaviours). In comparison to traditional learning, this type of learning relies on co-development, collective responsibility and co-operation [START_REF] Huber | Apprendre en projet 2[END_REF]. Learners are the principal actors of their learning. A significant enrichment arises from their activity, both for them and all the other learners. A consequence of this approach is the segmentation of the class into sub-grouped projects, monitored by tutors. We generally observe that the coordination and harmonisation of tutors' activities are extremely difficult to operate when each group works autonomously, on different subjects and in real and varied environments (for example enterprises). It is even more difficult when the project is conducted over a long period (more than four weeks). In this context, the perception of individual's and group's activity is also very difficult, especially if no technical support for information and communication is used. Finally, the implementation of project-based learning in engineering schools, universities or professional training do not benefit from all its capacities [START_REF] Thomas | Preparing project managers to deal with complexity -Advanced project management education[END_REF]. Indeed, this learning should implement an educational model based on the Kolb's cycle [START_REF] Cortez | Learning to collaborate by collaborating: a face-to-face collaborative activity for measuring and learning basics about teamwork[END_REF], composed of four phases (personal experience, reflexive observation, conceptualisation, experiment). But it is often action (via the articulation conceptualisation-experiment) which is favoured to the detriment of reflection and of personal experience [START_REF] Thomas | Preparing project managers to deal with complexity -Advanced project management education[END_REF].

To understand better the type of tool necessary to improve these trainings, we have studied a project management training course [START_REF] Michel | Knowledge management concepts for training by project : An observation of the case of project management education[END_REF]. This course is supported by a rich and complex organisation, especially for tutors that we detail in part 2. We have used KM methods to identify all the problems encountered by students and tutors. We mainly identify three problems: (1) difficulties for students to acquire some skills (e.g. project management organisation, use of monitoring tools and group work) and autonomy, (2) lack of information so that tutors can monitor and evaluate students individually and by group, and (3) lack of tutors' communication and coordination so that they develop their expertise, knowledge and competences. In part 3, we study existing tools which can help to solve these problems, especially monitoring and experience sharing tools. We then observe that no existing tool could solve all these problems on its own. We thus propose a new tool named MEShaT (for Monitoring and Experience Sharing Tool). We finally conclude by all the futures directions offered by this work.

CASE STUDY: A PROJECT MANAGEMENT TRAINING COURSE

The course organisation

The course is composed of a theoretical presentation of the principles and methods of project management and their practical application to a project (called 'PCo' for 'Collective Project') carried out by groups (12 groups of 8 students which answer to different industrial needs). Envisaged by Patrick Prévôt [START_REF] Michel | Knowledge management concepts for training by project : An observation of the case of project management education[END_REF], the project management course lasts six months and corresponds to an investment of approximately 3000 students' working hours per project. The instructional objectives are to acquire hard competences (e.g. knowing how to plan the project (Gantt's chart), to organise the project management, to manage resources, to control quality) and soft competences (e.g. social competences of collaboration and communication, empathy, consideration of the others, leadership). The pedagogical team (see figure 1) is composed of 24 tutors (a technical and a management tutor per group), 2 managers (technical and management) in charge of the coordination of the technical and management tutors' activities, 1 teacher who presents the theoretical concepts and 1 director responsible for the organisation of the training of all groups. The project is composed of four phases: -( 1) November: answer to the call for tender (formalisation of the client's requirements). -(2) December: elaboration of a master plan (means, tools and organisation of the team project), definition of tools to drive the project (dashboard) and rules to test the deliverables quality (rules of receipt).

-(3) January to March: development of a product or a study.

-(4) Until mid-April: delivery of a technical report which describes the product and management report (a project closure report which is an analysis, from the student's point of view, of the flow and problems of the project). The project is closed by one dramatised presentation in front of all the actors of the project.

The tutors play various roles which depend on the type of skills that students have to acquire [START_REF] Michel | Dispositif de supervision pour les tuteurs impliqués dans un apprentissage à la gestion de projets[END_REF]. According to Garrot's taxonomy [START_REF] Garrot | Supporting a Virtual Community of Tutors in Experience Capitalizing[END_REF], for the acquisition of soft skills, tutors are social catalysts (create a friendly environment to incite students to participate), intellectual catalysts (ask questions and incite students to discuss, to criticise), 'individualisers' (help every student to overcome the difficulties, to estimate his/her needs, difficulties, preferences) and 'autonomisers' (help students to regulate their learning and to acquire autonomy). For the acquisition of hard skills, tutors are relational coaches (help students to learn how to work in group and to become a leader), educationalists (redirect groups' activities in a productive way, clarify points of methodology, supply resources), content experts (answer to questions on the course contents), evaluators (evaluate students and groups' productions and participation) and 'qualimetrors' (measure and give feedback on the quality of the course).

Tutors monitor a unique and non reproducible project. They work with students most of time in face-toface and no organisation, communication or capitalisation tool is proposed. For example, no specific tool is currently proposed to the tutors for the monitoring of students' activities or for their evaluation. The appreciation of students' activity is made in an implicit way, according to the number and the quality of faceto-face students-tutor interactions. In terms of communication and coordination, each tutor works individually with his/her group and does not communicate much with the other tutor of the same group (management or technical) in order to have a complete vision of the group's activity.

The observed problems

The observation methodology is adapted from the MASK (Method for Knowledge System Management) method (Benmahamed1 et al, 2005), completely described in [START_REF] Michel | Knowledge management concepts for training by project : An observation of the case of project management education[END_REF]. The observation data are various experience feedbacks from students and tutors and were collected by 62 students in the 5th year of the engineer school. The observed students are 23 males and 18 females and are between 22 to 25 years old. 38 of them have carried out the project management course the previous year, 3 of them are currently 'project leader' in a project. Observation consists in direct feedbacks (called REX) made by interview of the course director, of 6 tutors and of 3 students currently 'project leader' and by self-observation for the other 38 students. Indirect feedbacks are based on various groups' experience and analyses expressed in there 'management report', which is one of the projects deliverables. 24 management reports have been considered (each one relating to the experience of a group). Each identified problem has been described on a RISE (Reuse, Improve and Share Experiment) card. We have observed 36 different types of problems.

The majority of cards (57%) relate to a problem with the management of the team work by the team itself. More precisely, 29% relate to a lack of project management skills, 18% relate to difficulties working in group, 10% relate to problems with some students who think they are not responsible enough. 31% of the problems concern tutors' activity and impact on teaching organisation of the project. Indeed, 13% concern a lack of coherence, coordination and communication between tutors, which involve problems of information diffusion. For example, the instructions given to the project groups were described as ambiguous or contradictory. About 5% concern a lack of communication between tutors and students or a lack of presence of some tutors. 13% concern a lack of information for tutors on the teaching objectives or on the knowledge and skills they have to teach to students. Indeed, students feel them alone when they have to learn using some tools or when they have to apply theoretical project management concepts. Students sometime do not understand the role tutors play and the help they can bring them. Moreover, 8% of the problems concern failure in the teaching design of the course (not enough time to work, a not adapted calendar and a too short timing for the deliverables). Finally, many groups and tutors express a same problem concerning the monitoring of individuals' or groups' activity and students evaluation (4% of the problems). The students express a feeling of injustice concerning the individual evaluation because the notation is the same for all the members of a project (with about + or -2 points according to their investment), even if the students involved more or less than the others. All the tutors also express their difficulties to evaluate the students individually. These difficulties are explained by the intuitive and tacit character of the evaluations, by the lack of traceability of students' actions, and by the lack of discussion with their colleagues.

It is possible to partially solve problems concerning the course design and the course organisation by changing the timing and the teacher's and coordinator's responsibilities. Nevertheless many problems remain and most of them are directly or indirectly bound to tutors' activity. That is why we aim at helping tutors, on the one hand, to monitor and to evaluate the students and the groups and, on the other hand, to exchange information, coordinate and develop their skills and expertise. Although the pedagogical context is not distance learning, we hope to benefit from using tools to support this activity. In next part, we study CSCL and Web 2.0 tools which are suitable for our case. We focus on monitoring tools and expertise sharing tools.

TOOLS TO SUPPORT LEARNING ACTIVITIES

In this part, we detail existing tools to help tutors to monitor students' activities and to communicate with the other tutors. We study how these tools can help tutors and solve the problems identified in the previous part. We finally show that none of them answer to all the needs for which we develop our own tool.

Monitoring tools

Many tools have been developed to support tutors in the monitoring of distant and synchronous students' individual activities. ESSAIM [START_REF] Després | Synchronous Tutoring in Distance Learning[END_REF] gives a global view of a student's progress in the course and tutors have a perception of the activity with referring to the path, the actions and the productions of each student. FORMID [START_REF] Guéraud | Automatic Semantic Activity Monitoring of Distance Learners Guided by Pedagogical Scenarios[END_REF]) offers a tutor interface with a global view of a class during a session (e.g. students' login, their progress in the course) or a zoom in on a precise course stage (successfully validated or not by the class, by a student or by a group of students so as to identify their difficulties). These tools work in a synchronous environment with automatically generated tracks. They are thus only meant for tutors and do not offer the possibility to students to regulate their learning for a long period. Furthermore, they are not meant for asynchronous learning situations for which tutors need information on learners' activities on a long period.

Other tools are meant for helping tutors to monitor asynchronous activities and allow to go with the learners towards their autonomy or to regulate their learning by determine themselves the state of their progress in the course. Croisières [START_REF] Gueye | Instrumentation des activités du tuteur en ligne le cas de CROISIERES, dispositif de Formation Ouverte et à Distance en langues[END_REF] offers services which support individually learners in their learning progress and assist them in autonomy situation. Learners select their learning activities according to their objectives and learning strategies. Reflet [START_REF] Després | Reflet, un miroir sur la formation[END_REF]) is a tool meant for showing the state of progress of a student or a class. It supplies information to the tutors who monitor the students in distance training and to the students who have a feedback on their progress with regard to the learning objectives and the other students. Learners determine their state of progress in the course with regard to the tasks they have to carry out and tutors can deny learners the validation of some of their tasks.

There also are tools to monitor the activities of groups, not simply individuals. SIGFAD [START_REF] Mbala | Supporting Distributed Collaborative Learning with Usage Analysis Based Systems[END_REF] offers a support for actors' interactions in restricted groups (8-15 persons) in distance learning. It helps tutors to hold the groups, to boost them and indeed to conduct well the course. The interaction statistics allow to model and to show the collaboration into groups, to estimate the group's life and evolution. SIGFAD supplies three main categories of estimations: at the level of the group (present, absent or still persons, the state of the group with regard to the realisation of the activities), at the level of individuals (their productivity in term of realisation of the activities and their sociability which indicates their level of communication with the other members of the group) and at the level of the activity (level of realisation of an activity by all the participants). TACSI [START_REF] Laperrousaz | Analyzing a collaborative writing activity in order to improve tutor's perception of individual contributions of learners[END_REF] offers more specifically a perception of the individual learners' activity into the activity of their group. It distinguishes the perception of learners' activity in an individual task (individual productions), the perception of learners' activity in a collective task (their contributions in the collective activities and their contributions to the discussions) and the perception of learners' situation in the group dynamics (social behaviour and sociometric status). The LCC (Learning to Collaborate by Collaborating) collaborative activity software [START_REF] Cortez | Learning to collaborate by collaborating: a face-to-face collaborative activity for measuring and learning basics about teamwork[END_REF] is used for teaching and measuring teamwork skills using technologically supported face-to-face collaborative activities. LCC allows measuring seven variables : first ones measure the Activity score (i.e. the group's efficiency in performing the task assigned), last ones measure Teamwork variables (corresponding to core components (skills) of teamwork like team orientation (TO), team leadership (TL), monitoring (MO), feedback (FE), back-up (BA) and coordination (CO)). Communication has not been included in the measurable variables.

The individual and collective indicators for the monitoring of learners and project groups offered by these tools are relatively well adapted to our context. We especially adopt those proposed within the LCC framework [START_REF] Cortez | Learning to collaborate by collaborating: a face-to-face collaborative activity for measuring and learning basics about teamwork[END_REF] for the development of our own monitoring tool. However, the course which interests us does not use instrumented activity and does not thus allow using automatically collected tracks of learners' activity. That is why we have to think about other ways of collecting information on their activities.

The tools which help learners to acquire autonomy incite them to evaluate their progress in the course, according to the tasks they have achieved and those they have to achieve. However, these tools are not adapted because they do not help learners to build an individual reflection neither on the relevance of the knowledge they acquire and the modalities of this acquisition nor on their behavioural changes. These selfregulatory processes are individual and mainly result from the activities carried out with the tutors. We think useful [START_REF] Michel | Dispositif de supervision pour les tuteurs impliqués dans un apprentissage à la gestion de projets[END_REF] to support these processes by using a metacognitive tool [START_REF] Azevedo | Understanding the complex nature of self-regulatory in learning with computer-based learning, an introduction[END_REF] which takes into account learners' point of view of: cognition (e.g. activating prior knowledge, planning, creating subgoals, learning strategies), metacognition (e.g. feeling of knowing, judgment of learning, content evaluation), motivation (e.g. self-efficacy, task value, interest, effort) and behaviour (e.g. engaging in help-seeking behaviour, modifying learning conditions, handling task difficulties and demands).

All the tools studied in this part are exclusively centred on learners' activity and help neither learners nor tutors to have reflections on their activity. In our context, in which the roles played by tutors are extremely varied, it is essential to have a base structuring this reflection. For example, [START_REF] Berggren | Rethinking project management education: Social twists and knowledge coproduction[END_REF] propose to use a 'learning contract' defined as 'a number of fairly simple questions, such as: What do I want to learn? How will I learn this? Who can give support? When can I start? How will I know that I have learned? How will others realize that I have learned?'. This contract could be useful not only for students but also for tutors.

Furthermore, all the tools do not brought help to tutors to understand or interpret what they observe. They supply useful information for tutors but these information are rather quantitative than qualitative and do not thus allow to evaluate the quality of the contributions or productions, or to explain learners' behaviour neither individually nor inside the group. These tools can be useful for tutors only if they know how to use it, how to interpret the supplied information and how to react effectively and in an adapted way. Finally, these tools address every tutor individually and do not allow them to coordinate at the level of the monitoring of a same project group and to exchange on their activity so as to acquire more expertise. That is why we study in next part the tools which support exchanges between tutors to bring them to help each other and to develop their skills.

Experience sharing tools

The results of a previous study [START_REF] Michel | Instrumented Collective Learning Situations (ICLS): the gap between theoretical research and observed practices[END_REF] about tools supplied to the tutors show that they do not have adapted tools to exchange or formalise their experience, as allowed for example by Knowledge Based Systems (KBS) or experience booklets [START_REF] Kamsu Foguem | Knowledge formalization in experience feedback processes: An ontology-based approach[END_REF]. Furthermore, we observed that tutors are rather structured in a hierarchical way within the organisation and do not have coordination tools or dedicated spaces for meeting between peers.

To compensate for a lack of training and formal help, Communities of Practice (CoPs) of tutors emerge. Web technologies (e.g. forums, blogs, wikis) have allowed the emergence of online CoPs [START_REF] Cuthell | The role of a web-based community in teacher professional development[END_REF][START_REF] Pashnyak | What and Why do Classroom Teachers Blog[END_REF]. CoPs gather tutors together in an informal way because of the fact that they have common practices, interests and purposes (i.e. to share ideas and experiences, build common tools, and develop relations between peers). Members exchange information, help each other to develop their skills and expertise and solve problems in an innovative way. They develop a community identity around shared knowledge, common approaches and established practices and create a shared directory of common resources [START_REF] Wenger | Communities of practice: Learning, meaning, and identity[END_REF][START_REF] Garrot-Lavoué | Interconnnection of Communities of Practice: a Web Platform for Knowledge Management[END_REF]. The use of technology does allow the accumulation of exchanges, but they are relatively unstructured and not contextualised. Web tools such as blogs, mailing lists, chat and email, allow discussion without building concrete knowledge (only forums bring a slightly higher degree of explicit emergence, thanks to the spatial representation as discussion threads which highlights relations between messages).

Numerous works aim at answering the question by supplying tutors with tools to support specific activities. Some tools work through member participation and sociability, for example by offering a virtual 'home' like the Tapped In environment [START_REF] Schlager | Teacher professional development, technology, and communities of practice: Are we putting the cart before the horse?[END_REF], others by supporting collaboration between members like CoPe_it! [START_REF] Karacapilidis | Web-based collaboration and decision making support: A multi-disciplinary approach[END_REF]. Other tools favour the creation of contextualised resources and contextual search facilities such the learning environment doceNet [START_REF] Brito Mírian | Integrating Instructional Material and Teaching Experience into a Teachers Collaborative Learning Environment[END_REF]. However, all these environments either favour sociability (engaging members to participate) to the detriment of the reification of the produced resources, or they favour the accumulation and indexation of contextualised resources, but to the detriment of sociability and member participation.

We have developed the TE-Cap platform [START_REF] Garrot-Lavoué | Interconnnection of Communities of Practice: a Web Platform for Knowledge Management[END_REF]) so as to support a good structuralisation of the information without decreasing the member participation (for example communication). Indeed, the tutors have discussions by way of contextualised forums: they associate tags with the discussions to describe the context. These tags are subjects of a tutoring taxonomy, showed in an interactive and evolutionary way (the tutors can propose new subjects for the taxonomy). This platform, associated with a monitoring tool, could answers our needs of knowledge and skills acquisition and capitalisation about the realisation of tutors' activity and about the use of the monitoring tools.

A PLATFORM FOR TUTORS AND STUDENTS

We have designed a customised platform called MEShaT (see figure 2). It proposes different interfaces according to the learning actor: a project group, a student or a tutor. Every interface consists of (1) a monitoring tool (on the form of a dashboard) which helps the concerned actor to have a global view of his/her activity and (2) a publication tool which allows spreading his/her experience. Three dashboards are offered; two for students (one to monitor the progress of their project and the other one to monitor their own learning process) and one for tutors (to monitor students' and groups' activities and students' learning). The project monitoring dashboard is meant for the project group and shows various indicators: the group's frame of mind, the skills to build for the project realisation, the working time of each member, tasks to realise, the deliverables to produce, the delays, etc. The metacognitive tool is meant for a student and shows reflexive indicators. It aims at improving the learning in the case of complex domain or when a behavioural evolution is expected. According to [START_REF] Azevedo | Understanding the complex nature of self-regulatory in learning with computer-based learning, an introduction[END_REF] (see part 3.1), we consider that it is important to reflect the students' cognition, metacognition, motivation and behaviour. The learning monitoring tool is meant for tutor and shows information on the individual students' activity and the groups' activity thanks to indicators such as the group orientation, leadership, monitoring, feedback and coordination [START_REF] Cortez | Learning to collaborate by collaborating: a face-to-face collaborative activity for measuring and learning basics about teamwork[END_REF].

The publication tools are blogs and TE-Cap. Blogs (one per student and one per group) are spaces where students can freely describe for example the realisation contexts of their actions and their frame of mind. These blogs help the group members and the tutors to understand the project context, to explain the value of some indicators (as delays or the group's frame of mind) and so to anticipate or to solve more quickly the problems. TE-Cap is offered to tutors to allow the emergence of a Community of Practice composed of all the tutors who monitor a project. The indexation model is built on three main subjects, corresponding to the different types of expertise required for tutors: (1) their roles and tasks, (2) the project calendar (so as to coordinate) and (3) the specific progress of every group. By exchanging, tutors will acquire expertise on their roles and knowledge on their application ground. TE-Cap can be considered as an expertise transfer tool.

A fixed section shows information accessible by all the actors: the learning contract (see part 3.1). The learning contract [START_REF] Berggren | Rethinking project management education: Social twists and knowledge coproduction[END_REF] helps every actor (tutor and student) to evaluate him/herself in regards to the educational model and to better reach his/her objectives.

We detail the modalities of use (represented by arrows on figure 2). The information on the dashboards can be modified by their owner(s) and are not visible for everybody. Students can modify their blog and their individual dashboard by means of a data entry interface. The groups' dashboard is updated by the project leader, using individual information. Leaders confirm the data and decide what is published on the blog. The tutors' dashboard is directly updated by them and automatically updated according to the information entry on the groups' and students' interfaces. Tutors also contribute directly to the CoP. Tutors have access to the groups' and students' interfaces. The project leaders have not access to the individual dashboards of their group members. The learning contract cannot be modified during the course progress. It is updated at the end of the project, according to the events which were related on blogs and on TE-Cap.

MEShaT is meant for the complex educational context of project-based learning, using the Kolb's learning process. The metacognitive tool, the blogs, TE-Cap and the learning contract, favour the reflection and personal experience phases of the Kolb's cycle, the monitoring tools help action phases (conceptualisation and experiment). Moreover, MEShaT solves some of the problems identified in section 2.2. Monitoring tools and blogs facilitate the group work, the group cohesion, the professionalism of students by making more tangible the consequences of their acts and by informing them. Metacognitive tool and blogs help the students to acquire knowledge and reinforce their motivation (by a better understanding of what they have to do and why they do it). If these phenomena do not naturally appear, tools will help the tutors to make them emerge. Indeed, MEShat reinforces the tutor-student link by allowing a continuous monitoring of the knowledge acquisition process. It also helps tutors to assume some of their roles, like their roles of relational coach and social catalyst (concerning the group work or the leadership), their role of intellectual catalyst (by asking precise and conceptualised questions to incite students to discuss or ask critical questions) and their roles of expert and pedagogue. Moreover, the association of Te-Cap with the learning contract offers to tutors a space for refining or developing their expertise.

CONCLUSION AND FUTURES DIRECTIONS

Our work aims at studying the monitoring and expertise transfer tools proposed to tutors and students involved in face-to-face or blended project-based learning activities. To understand better the needs and expectations of each actor, we are especially interested in the case of project management training. Indeed, this type of learning is complex since it has for objective the acquisitions of soft and hard knowledge and relies on rich and varied social organisations. In the first part of this article we described a course which is based on these principles. The lack of monitoring and expertise transfer tools involves important dysfunctions in the course organisation and therefore dissatisfaction for tutors and students (in particular about the acquisition of knowledge and expertise). The study of existing tools highlights two points: (1) there is no tool which help both tutors and students, (2) there are not clear strategies proposed to acquire, transfer and capitalise the actors' experience. Indeed, studied tools do not offer metacognitive functions, formal or informal publication tools (as knowledge books or blogs) and tools to support Communities of Practice.

So as to solve this problem, we propose to associate personalised monitoring tools (one for the project group, one for the student and one for the tutor) with tools for the transfer of experience and the acquisition of knowledge. Regarding the monitoring: the 'Team feedback' is a dashboard for the project management, the 'Student feedback' is a metacognitive tool and the 'Tutor feedback' is a monitoring tool for individuals' and groups' activity. The tool for the acquisition of knowledge considers two types of knowledge: the acquired experience is formalised in a kind of knowledge book called 'learning contract', the experience being acquired is revealed and capitalised in blogs (for students and project groups) and within a CoP supported by TE-Cap (for tutors). We describe their articulation in a platform: MEShaT. This platform is dedicated to project management education but can also be used to support different type of face-to-face project-based learning activities. Indeed, all the phases of the Kolb's cycle are well taken into account. Furthermore, it supports the acquisition of various experiences: those of the individuals (students and tutors) and those of the social organisations (project group, CoP of tutors). Our future work will consist in testing this platform on a long time so as to validate experimentally our hypotheses. We will also observe how the actors (students, tutors and course designer) appropriate this type of technologies and how these last ones participate in the redefinition of their roles.

Figure 1 .

 1 Figure 1. Pedagogical team and course organisation

Figure 2 .

 2 Figure 2. MEShaT : Monitoring and Experience Sharing Tool for project-based learning


Stephane Bonnet
email: stephane.bonnet@univ-rennes1.fr

Shrinking and Splitting of drainage basins in orogenic landscapes from the migration of the main drainage divide

Climate

, and in particular the spatial pattern of precipitation, is thought to affect the topographic and tectonic evolution of mountain belts through erosion 1-5 . Numerical model simulations of landscape erosion controlled by horizontal tectonic motion 6 or orographic precipitation 7,8 result in the asymmetric topography that characterizes most natural mountain belts6, and in a continuous migration of the main drainage divide. The effects of such a migration have, however, been challenging to observe in natural settings6. Here I document the effects of a lateral precipitation gradient on a landscape undergoing constant uplift in a laboratory modelling experiment. In the experiment, the drainage divide migrates towards the drier, leeward side of the mountain range, causing the drainage basins on the leeward side to shrink and split into smaller basins. This mechanism results in a progressively increasing number of drainage basins on the leeward side of the mountain range as the divide migrates, such that the expected relationship between the spacing of drainage basins and the location of the main drainage divide 9 is maintained. I propose that this mechanism could clarify the

In separating the water flux coming from precipitation between the drainage basins located over the two opposite flanks of topography, the main drainage divide is an important physiographic element of orogen topography. When associated with an orographic effect, drainage divide delineates domains where differences in precipitation and hydrologic regimes may directly influence erosional processes that shape topography and thus, landscape forms [START_REF] Roe | Orographic precipitation and the relief of mountain ranges[END_REF][START_REF] Anders | Influence of precipitation phase on the form of mountain ranges[END_REF] , erosion rates [START_REF] Reiners | Coupled spatial variations in precipitation and long-term erosion rates across the Washington Cascades[END_REF] and, over a geological time-scale, the rates and patterns of exhumation of metamorphic rocks [START_REF] Montgomery | Climate, tectonics, and the morphology of the Andes[END_REF] and internal strain [START_REF] Willett | Orogeny and orography: The effects of erosion on the structure of mountain belts[END_REF] within the orogens. The continuous migration of the drainage divide in orogens, as observed in the numerical modelling of surface processes when erosion is forced by tectonic advection [START_REF] Willett | Uplift, shortening, and steady-state in active mountain belts[END_REF] or orographic precipitations [START_REF] Roe | Orographic precipitation and the relief of mountain ranges[END_REF][START_REF] Anders | Influence of precipitation phase on the form of mountain ranges[END_REF] , is hardly demonstrable in natural settings. Only the observation that natural orogens usually exhibit asymmetric topography [START_REF] Willett | Uplift, shortening, and steady-state in active mountain belts[END_REF] supports numerical results. In natural settings, divide dynamics have only been inferred in local and timely-discontinuous shifts in their location consecutive to river capture events [START_REF] Craw | Structure and topographic evolution of the Main Divide in the Landsborough-Hopkins area of the Southern Alps, New Zealand[END_REF] . They have also occasionally been inferred from changes in sediment content at the drainage basin outlets [START_REF] Kuhlemann | Miocene shifts of the drainage divide in the Alps and their foreland basin[END_REF] . This emphasizes the need to find criteria that can be used to better investigate divide migration in orogens.

The experimental modelling of erosion is a powerful tool to investigate landscape dynamics. It indicates that landscapes may be more dynamic than the numerical models suggest [START_REF] Hasbargen | Landscape instability in an experimental drainage basin[END_REF] , e.g. when considering the evolution of ridge crests, and it has already been used to demonstrate that some landscape features, such as long narrow perched drainages, form in areas of actively migrating divides [START_REF] Hasbargen | Some Characteristics of Drainage Basin Realignment[END_REF] . Figure 1 shows an example of divide migration in the laboratory modelling of landscape dynamics [START_REF] Bonnet | Landscape response to climate change: Insights from experimental modeling and implications for tectonic versus climatic uplift of topography[END_REF][START_REF] Lague | Laboratory experiments simulating the geomorphic response to tectonic uplift[END_REF] forced by uniform uplifting of the eroded material but with a precipitation gradient (see the Methods section). In this experiment, the precipitation gradient was applied after a first phase of uniform precipitation and the attainment of a steady state between erosion and uplift [START_REF] Bonnet | Landscape response to climate change: Insights from experimental modeling and implications for tectonic versus climatic uplift of topography[END_REF][START_REF] Lague | Laboratory experiments simulating the geomorphic response to tectonic uplift[END_REF] (Fig. 1). During this initial phase, the topography is symmetric overall (Fig. 1a, Fig. 1c), and the elevation of the divide remains constant with time (Fig. 1a, Fig. 1b). Application of the precipitation gradient induces migration of the drainage divide toward the drier side of the landscape and development of an asymmetric topography (Fig. 1), as also observed numerically [START_REF] Roe | Orographic precipitation and the relief of mountain ranges[END_REF][START_REF] Anders | Influence of precipitation phase on the form of mountain ranges[END_REF] . The divide is simultaneously uplifted (Fig. 1b) such that the mean topographic slope of the wetter side of the landscape remains constant during its elongation (Fig. 1a). At no time is the establishment of a new steady-state observed in this experiment (Fig. 1b), nor is the divide position pinned (Fig. 1c). The progressive shortening of the drier side of the landscape consecutive to divide migration results in an increase in the roughness of the surface and in a very unstable landscape (Fig. 2).This leads to an original mechanism that splits the drainage networks: each initial drainage network splits into two individual networks that become progressively separated by the growth of a new hillcrest (Figure 2). Through this mechanism, the numbers of drainage basins extending to the main divide increases during divide migration.

Given the pattern of rainfall and once the precipitation gradient is applied, the migration of the drainage divide induces a continuous decrease in the mean runoff within the drainage basins located on the drier side of the landscape (Fig. 3b). There, the area of the drainage basins decreases because of the combination of two processes: a continuous size reduction (a direct consequence of the divide migration), and an abrupt size reduction consecutive to the split of the drainage network and the individuation of two drainage basins from a previous single one. Overall, the decrease of the drainage basins' size correlates with a steepening of their channels (Fig. 3c). The detailed analysis of the experimental drainage basins' response to the rainfall gradient documents a two-step evolution of the channels, before and after splitting occurred (Fig. 3). In a first phase, from the establishment of the precipitation gradient up to the splitting, an erosion wave propagates upward within the former steady-state channels (Fig. 3c). It generates the upstream migration of a knickpoint, defined as an abrupt change in the channel gradient, which separates an upstream segment passively uplifted from a downstream segment steepened to a new steady-state gradient (Fig. 3c). This mechanism has already been described analytically [START_REF] Whipple | Dynamics of the stream-power river incision model: Implications for height limits of mountain ranges, landscape response timescales, and research needs[END_REF][START_REF] Whipple | Fluvial landscape response time: How plausible is steady-state denudation?[END_REF] and experimentally [START_REF] Bonnet | Landscape response to climate change: Insights from experimental modeling and implications for tectonic versus climatic uplift of topography[END_REF] . After the erosion wave has swept the entire channels, channel steepening drives a temporary steady-state between uplift and erosion (Fig. 3 c,d). This temporary steady-state only concerns channels and not the whole landscape (Fig. 1), as hillcrests are passively uplifted at the same time (Fig. 3d). In a second phase, after the split of drainage networks, a profound disruption of the temporary steady-state of the channels occurs, preventing the establishment of new steady-state conditions. From splitting onward, the channels continuously steepen as they shorten because of divide migration (Fig. 3c). Figure 3d shows the elevation history of three geographically-fixed spatial points of the model, illustrating how complex elevation histories can be during this sequence of landscape changes. Specifically, it illustrates how some parts of the floodplains are uplifted and transformed into a divide separating the two newlyformed drainage basins after splitting has occurred.

As for natural channels [START_REF] Leopold | The hydraulic geometry of stream channels and some physiographic implications[END_REF] , the hydraulic properties, width, depth, cross-sectional area, mean flow velocity, hydraulic radius and wet perimeter all increase with water discharge in the laboratory [START_REF] Turowski | Experimental channel response to tectonic uplift[END_REF] . By continually reducing the discharge within drainage basins, divide migration consequently drives a narrowing of the channels so that erosion is progressively localized within the floodplains during divide migration, resulting in their abandoned parts being uplifted (Fig. 3d). Channels have also been shown to narrow as they steepen, theoretically [START_REF] Wobus | Self-formed bedrock channels[END_REF] and in the field [START_REF] Whittaker | Bedrock channel adjustment to tectonic forcing: Implications for predicting river incision rates[END_REF] , so that narrowing may also be driven by the steepening of the channels described here (Fig. 3c). Even if channels cannot be observed directly during experiments because of opacity during rainfall, a careful examination at the time-step evolution of the experimental landscape (Fig. 2) shows that the location of the newly-formed trunk channels after splitting is intimately linked to the former geometry of the upstream tributaries of the channel network. As illustrated in Figure 4, the network splitting mechanism most likely occurs at the tributaries' junction and lies in the combination of channel narrowing associated with ongoing uplift and reduced erosion rates in the interfluves area (Fig 3d). This enables flows coming from the tributaries to disconnect in place of a former single channel.

The mechanism of network splitting proposed here led to transient dynamics that are exemplified by the Sierra Aconquija range in the Sierras Pampeanas province of NW Argentina 10 (Fig. 5). The Aconquija is an uplifted basement range, bounded by active high-angle reverse faults on one or both sides against Neogene sedimentary basins 10,24-25 (Fig. 5). Thermochronological data [START_REF] Sobel | exhumation and precipitation: Tectonic and climatic control of late Cenozoic landscape evolution in the northern Sierras Pampeanas, Argentina[END_REF] indicates the start of the rapid exhumation of the range ~6 Ma ago and a total rock uplift of at least 6.4-11.1 km over the last 6 Myr. It presently forms a prominent landscape above its adjacent foreland plains, reaching elevations > 5 km. Because of its location on the eastern front of the Andes, representing a major topographic barrier to the moisture flux coming from the Atlantic Ocean [START_REF] Bookhagen | Orographic barriers, high-resolution TRMM rainfall, and relief variations along the eastern Andes[END_REF] , the Sierra Aconquija is an orographic barrier: its eastern flank receives much more precipitation (> 2 m yr -1 ) than its western one [START_REF] Sobel | exhumation and precipitation: Tectonic and climatic control of late Cenozoic landscape evolution in the northern Sierras Pampeanas, Argentina[END_REF][START_REF] Strecker | Tectonics and climate of the Southern Central Andes[END_REF][START_REF] Bookhagen | Orographic barriers, high-resolution TRMM rainfall, and relief variations along the eastern Andes[END_REF] (Fig. 5), where an arid climate prevails. To the west of the Aconquija, climate proxies indicate that aridification initiated 3 Myr ago [START_REF] Kleinert | Climate change in response to orographic barrier uplift: Paleosol and stable isotope evidence from the late Neogene Santa Maria basin, northwestern Argentina[END_REF] . It is interpreted as reflecting the onset of the orographic barrier via a surface uplift of the Aconquija 27 , which occurred when the topography reached elevations of 2-2.5 km in the Andes [START_REF] Sobel | exhumation and precipitation: Tectonic and climatic control of late Cenozoic landscape evolution in the northern Sierras Pampeanas, Argentina[END_REF] . Compared to modern maximum elevations of the range, this suggests that the divide of the Aconquija has been uplifted (surface uplift) by 3-3.5 km during the last 3 Myr 10 . The Aconquija presently shows a jagged topography characterized by deeply-incised, regularly-spaced, transverse rivers. Overall, its topography is asymmetric and its drainage divide shows an offset position toward the drier side of the range (Fig. 5), with a fractional divide position 6 of 0.6-0.7. The drainage networks on the drier leeward flank of the range show multiple examples of unusual landscape configurations (Fig. 5c), indicating that the split of drainage networks likely occurred following the mechanism described experimentally (Fig. 4). It implies the migration of the main divide of the Aconquija toward the drier flank of the range and the progressive development of its topographic asymmetry. The asymmetric development of topography would not exist in the absence of asymmetry of at least one forcing parameter 6 : rock erodability, tectonic forcing or climatic conditions. Differences in erodability are not likely to be the cause of the asymmetry because variations in topography do not coincide with lithologic ones. Horizontal tectonic motions are likely negligible in the Aconquija case because of the high-angle of the bounding faults [START_REF] Sobel | exhumation and precipitation: Tectonic and climatic control of late Cenozoic landscape evolution in the northern Sierras Pampeanas, Argentina[END_REF][START_REF] Strecker | late Cenozoic tectonism and landscape development in the foreland of the Andes: Northern Sierras Pampeanas (26°S-28°S)[END_REF] and because the range is bounded by opposite reverse faults on both sides in the area considered (Fig. 5). The development of the topographic asymmetry of the Aconquija is most likely the direct result of climate, through the establishment of the orographic rainfall gradient, following a mechanism of orographic influence on asymmetry development also observed numerically [START_REF] Roe | Orographic precipitation and the relief of mountain ranges[END_REF][START_REF] Anders | Influence of precipitation phase on the form of mountain ranges[END_REF] . Indirectly [START_REF] Sobel | Formation of internally drained contractional basins by aridity-limited bedrock incision[END_REF] , climatic variations led to different sequences of aggradation and degradation events in sedimentary basins flanking the range and resulted in a base-level more elevated on the leeward side than on the windward one [START_REF] Sobel | exhumation and precipitation: Tectonic and climatic control of late Cenozoic landscape evolution in the northern Sierras Pampeanas, Argentina[END_REF] , a phenomenon that may also have influenced topographic asymmetry development. Assuming a symmetric topography at the onset of orographic barrier development 3 Myr ago [START_REF] Kleinert | Climate change in response to orographic barrier uplift: Paleosol and stable isotope evidence from the late Neogene Santa Maria basin, northwestern Argentina[END_REF] , the subsequent estimated rate of divide migration is ~0.8-1.5 mm yr -1 , of the same order of magnitude as uplift rates.

In natural settings, the spacing of drainage basin outlets along mountain fronts is remarkably regular, regardless of their tectonic and climatic settings [START_REF] Hovius | Regular spacing of drainage outlets from linear mountain belts[END_REF][START_REF] Talling | Regular spacing of drainage outlets from linear faults blocks[END_REF] . With the exception of some Himalayan catchments [START_REF] Hovius | Regular spacing of drainage outlets from linear mountain belts[END_REF] , most landscapes obey a single empirical scaling law, which relates outlet spacing to half the distance between the main divide and the range front [START_REF] Hovius | Regular spacing of drainage outlets from linear mountain belts[END_REF][START_REF] Talling | Regular spacing of drainage outlets from linear faults blocks[END_REF] . As studied here, the experimental landscapes follow a similar law (see Supplementary Figure). In the context of widening mountain belts, the preservation of the spacing ratio implies that processes such as river capture or drainage divide collapse decrease the number of outlets during the lengthening of drainage basins [START_REF] Hovius | Regular spacing of drainage outlets from linear mountain belts[END_REF][START_REF] Talling | Regular spacing of drainage outlets from linear faults blocks[END_REF] . The present study demonstrates that the reverse occurs during the shortening of drainage basins induced by divide migration, and the split of drainage networks described here represents the only existing mechanism that allows to increase the number of drainage basins at mountain fronts and to maintain their spacing ratio. Experiments illustrate how complex the elevation history of a spatial point of a landscape can be. Many issues must still be investigated to better understand the mechanism of drainage splitting, both in experiments and nature; the channel behaviour at the tributaries' junction in the context of reducing discharge is likely the most important. However, the identification of the splitting mechanism provides the first opportunity to investigate drainage divide migration in active orogens through the coeval dynamics of the associated drainage networks.

METHODS

Experiments were performed in the Modelling Laboratory at Geosciences Rennes/University of Rennes1. I used a paste of pure silica grains (mean grain size of 20 µm) mixed with water. The water content was chosen such that the paste has a vertical angle of rest and water infiltration was negligible. The paste was introduced into a box with a vertically adjustable base, whose movements were driven by a screw and a computer-controlled stepping motor. The internal area of the box was 60 X 40 cm and 50 cm deep. During an experimental run, the base of the box was raised at a constant rate and pushed the paste outside the top of the box at a rate defined as the uplift rate. Precipitation was generated by a system of four sprinklers that delivered water droplets with diameter of ~10 µm, which was small enough to avoid any splash dispersion at the surface of the model. The precipitation rate at the surface of the model could be controlled by changing the water pressure and the configuration of the sprinklers. Precipitation was measured by collecting water in 20 pans at the location of the model before and after each experimental run. The coefficient of variation (standard deviation/mean) of rainfall rates for measurement intervals of 10 minutes is less than 5 % for the experiment performed here. The surface of the model was eroded by running water at its surface and grain detachment and transport occurred mainly by shear detachment through surface runoff. The topography was measured by using a commercial stereogrammetric camera system, which has a precision of ~20 µm. The raw data were gridded to produce DEMs with a pixel size of 0.5 mm.

Inherently to all laboratory modellings of landscape dynamics (at University of Rennes 1: see refs. 16, 17, 21 and at University of Minnesota: see ref. 14 and 15), experiments such as those developed here are oversimplifications of natural systems. Oversimplification is imposed by the difficulty to model some particular processes (vegetation dynamics, weathering processes and chemical erosion, atmospheric processes, etc.) but it is also a choice that is motivated by the necessity to understand the influence of each forcing parameter before investigating more complex systems. More importantly, experiments cannot be scaled to nature because of the impossibility to downscale natural conditions to the laboratory (for examples, see refs. 15, 17, 21). Because of these scale distortions, modelling of landscape dynamics is only experimental and not analog [START_REF] Hasbargen | Some Characteristics of Drainage Basin Realignment[END_REF][START_REF] Bonnet | Landscape response to climate change: Insights from experimental modeling and implications for tectonic versus climatic uplift of topography[END_REF][START_REF] Lague | Laboratory experiments simulating the geomorphic response to tectonic uplift[END_REF] . However, there is a consensus about the qualitative relevance of these models [START_REF] Hasbargen | Some Characteristics of Drainage Basin Realignment[END_REF][START_REF] Bonnet | Landscape response to climate change: Insights from experimental modeling and implications for tectonic versus climatic uplift of topography[END_REF][START_REF] Lague | Laboratory experiments simulating the geomorphic response to tectonic uplift[END_REF] , which permits a much more dynamic view of landscape evolution than the usual numerical models (see ref. 15). 30. Moyano, M. S. Análisis regional del emplazamiento, sedimentación y evolución sintectónica de los abanicos aluviales cuaternarios de las Sierras Pampeanas Nordoccidentales. Catamarca y Tucumán. Unpublished PhD thesis, Facultad de Ciencias Naturales e IML -Universidad Nacional de Tucumán, 209 p. (2009). Correspondence and requests for materials should be addressed to S.B.

Figure captions

Figure 1 Laboratory modelling of landscape dynamics under uniform uplift of the eroded material and lateral precipitation gradient. The uplift rate is 12 mm h -1 . a, Evolution of topographic profiles. Each line is the mean elevation along a 20-cm-wide transverse swath calculated from DEMs. The patterns of rainfall forcing are also shown. Note the superimposition of the topographic profiles at the end of the first step, which implies a steady-state between uplift and erosion [START_REF] Bonnet | Landscape response to climate change: Insights from experimental modeling and implications for tectonic versus climatic uplift of topography[END_REF][START_REF] Lague | Laboratory experiments simulating the geomorphic response to tectonic uplift[END_REF] . b, Evolution of mean and maximum (drainage divide) elevations of the model. Error bars give the standard deviation of the mean. The solid line indicates the amount of applied uplift, i.e. elevation of the model if no erosion occurred. The line is only interrupted upward for graphical convenience; uplift forcing was applied during the entire experiment at a constant rate. Note that the constancy of mean and maximum elevations during the first evolution step implies a steady-state between uplift and erosion [START_REF] Bonnet | Landscape response to climate change: Insights from experimental modeling and implications for tectonic versus climatic uplift of topography[END_REF][START_REF] Lague | Laboratory experiments simulating the geomorphic response to tectonic uplift[END_REF] and the absence of a steady-state from the application of the rainfall gradient. c, Evolution of the normalized divide position [START_REF] Willett | Uplift, shortening, and steady-state in active mountain belts[END_REF] , highlighting the asymmetry development from the application of the rainfall gradient. d, Photographs of three stages of landscape evolution, taken at steady-state (600') and during the subsequent divide migration (940 and 1300'). The model width on the view is 400 mm. ) and of some selected points of the landscape whose elevation history is detailed. b, Timeevolution of mean runoff within the selected drainage basin, water flux at outlet and basin size. c, Time-evolution of the longitudinal profile of the main trunk channel. The stack of longitudinal profiles until 600 minutes corresponds to the steady-state between erosion and uplift during the first phase of uniform rainfall. Note the progressive shortening and steepening of the channel once the rainfall gradient is applied and the existence of a temporary steady-state before splitting occurred. d, Detailed elevation history of three selected points of the landscape. Point A is located in a channel and it remains in the channel during the entire experiment. After the application of the rainfall gradient, its elevation first increases but rapidly stabilizes during the temporary steady-state; it is finally uplifted after splitting occurred. Point B is located on a permanent hillcrest that separates two main drainage basins. It is continuously uplifted after the application of the rainfall gradient. The evolution of point C is a combination of histories of points A and B. It corresponds to a channel with a similar evolution than A until splitting occurred. It is then uplifted and ends on a hillcrest that divides up a former single drainage basin into two individual ones. Dotted lines show the trend of applied uplift ("rock uplift") and therefore indicate the elevation of points that are passively uplifted, i.e. where no erosion occurred.

Figure 4 Model of drainage basin response to drainage divide migration (see text for comments).

Blue colours show active streams, whereas orange colours show hypothetical floodplain deposits. Note that some of these deposits can be passively uplifted and observed on the hillcrest separating two newly-formed drainage basins. These deposits would be classically interpreted as resulting from relief inversion or river capture. 4 (images from OpenAerialMap.org, except 1&2: GoogleEarth). Examples 1 and 2 are cases where two tributary streams do not connect at valleys' junction, but flow separately within a single valley floor (open arrows). The same configuration is observed in example 3, except that the two parallel-flowing streams are separated by elevated fluvial deposits (solid arrow) whose mapping 30 shows that they constituted a single alluvial body that extended upstream along the two streams. Consequently, it likely represents an example where drainage splitting is associated to the nascent of a new hillcrest. Example 4 shows the case of a hillcrest (solid arrow) separating two drainage basins whose outlets are very close and whose general shapes suggest that they initially formed a single drainage basin. This assumption is reinforced by the abrupt change in the flowpath direction of one river, suggesting a former connection between the two drainage basins (white dotted line), and by the existence of a system of alluvial fans whose size suggests feeding by a basin larger than the two present ones. This last example would thus represent the ultimate case of the splitting mechanism.

Supplementary Figure. Spacing of drainage basin outlets of experimental landscapes plotted

against drainage basin lengths. The linear relationship shown here is similar to the trend observed in natural landscapes [START_REF] Hovius | Regular spacing of drainage outlets from linear mountain belts[END_REF] . The linear fit defines a spacing ratio (ratio between the spacing and the length 9 ) of 2.22, whereas the values range between 1.91 and 2.23 in natural landscapes [START_REF] Hovius | Regular spacing of drainage outlets from linear mountain belts[END_REF] .

  drainage divide migration and topographic asymmetry found in active orogenic mountain ranges, as exemplified by the Aconquija Range of Argentina 10 .

Figure 2

 2 Figure 2 Landscape response to main drainage divide (MDD) migration. The images are successive shaded surface views of the driest side of the model DEMs (views are ~200 X 300 mm) with superposition of drainage networks as extracted from DEMs using a steepest-slope flow routine. The width of individual channels on the images only depends on the pixel size of the DEMs (0.5 mm) and does not reflect the width dependency with discharge or slope. The images illustrate the migration of the MDD, the induced shortening of the drainage basins and the split of their drainage network. The splitting mechanism induced the abandonment of former parts of the landscape where tributaries

Figure 3

 3 Figure 3Detailed geomorphic evolution of a drainage basin. a, Photographs taken from the driest side of the experimental landscape showing the location of the studied drainage basin (orange arrow) and of some selected points of the landscape whose elevation history is detailed. b, Timeevolution of mean runoff within the selected drainage basin, water flux at outlet and basin size. c, Time-evolution of the longitudinal profile of the main trunk channel. The stack of longitudinal profiles until 600 minutes corresponds to the steady-state between erosion and uplift during the first phase of uniform rainfall. Note the progressive shortening and steepening of the channel once the rainfall gradient is applied and the existence of a temporary steady-state before splitting occurred. d, Detailed elevation history of three selected points of the landscape. Point A is located in a channel and it remains in the channel during the entire experiment. After the application of the rainfall gradient, its elevation first increases but rapidly stabilizes during the temporary steady-state; it is finally uplifted after splitting occurred. Point B is located on a permanent hillcrest that separates two main drainage basins. It is continuously uplifted after the application of the rainfall gradient. The evolution of point C is a combination of histories of points A and B. It corresponds to a channel with a similar evolution than A until splitting occurred. It is then uplifted and ends on a hillcrest that divides up a former single drainage basin into two individual ones. Dotted lines show the trend of applied uplift ("rock uplift") and therefore indicate the elevation of points that are passively uplifted, i.e. where no erosion occurred.

Figure 5

 5 Figure 5 Topography and drainage networks of the Sierra Aconquija, northwestern Argentina. a, Map showing the topography of the Sierra Aconquija, major tectonic elements and mean annual precipitation (dotted blue lines, in mm yr -1 ; after ref. 10). b, Perspective view highlighting the topographic asymmetry of the range. c, Images showing examples of drainage networks located on the driest side of the Sierra Aconquija where different stages of drainage splitting likely are represented, following the model shown in Fig.4(images from OpenAerialMap.org, except 1&2: GoogleEarth). Examples 1 and 2 are cases where two tributary streams do not connect at valleys' junction, but flow separately within a single valley floor (open arrows). The same configuration is observed in example 3, except that the two parallel-flowing streams are separated by elevated fluvial deposits (solid arrow) whose mapping30 shows that they constituted a single alluvial body that extended upstream along the two streams. Consequently, it likely represents an example where

I would like to thank J.-J. Kermarrec for his technical assistance with the experiments and C. Andermann and M. Gaboriau for their friendly help during the experimental runs. This work benefited from discussions with B. Bookhagen, P.R. Cobbold, I. Coutand, A. Crave, P. Davy, D. Gapais, D. Lague and S. Moyano. Special thanks to P. Davy for providing and constantly improving the GridVisual software. This work was funded by the Institut National des Sciences de l'Univers (INSU-CNRS) through the "Reliefs de la Terre" research programme.


C Adloff

J Blaha

M Chefdeville
email: chefdevi@lapp.in2p3.fr

A Dalmaz

C Drancourt

A Espargilière

R Gaglione

Y Karyotakis

J Prast

G Vouters

Beam test of a small MICROMEGAS DHCAL prototype

Keywords: ILC, DHCAL, MICROMEGAS, Shower profile, HARDROC, DIRAC

A sampling hadronic calorimeter with gaps instrumented with thin MICROMEGAS chambers of small pad size and single bit readout is a candidate for an experiment at a future linear collider. Several MICROMEGAS chambers with 1 cm2 anode pads were fabricated using the Bulk technology. Some prototypes equipped with analog readout GASSIPLEX chips were developed for characterisation at the CERN/PS facility. A stack of four chambers and stainless steal absorber plates was used to assess the behaviour of MICROMEGAS in 2 GeV electron showers. Longitudinal and transverse shower profile are shown. The Bulk fabrication process was adapted to laminate a mesh on anode PCBs with front-end chips connected on the backside. It is well suited for the construction of a 1 m3 DHCAL-MICROMEGAS prototype as large and thin chambers can be made. Such chambers with digital readout chips (HARDROC or DIRAC) were fabricated and tested in a beam. First results are presented.

Introduction 1.1 Calorimetry at a future electron collider

Several important physics measurements that could be realized at a future electron linear collider (ILC or CLIC) would require a very good jet energy resolution (3 % at 100 GeV) [1]. The Particle Flow Approach (PFA) has been proposed to reach such a resolution. It consists in measuring the energy of the each particle contained in a jet with the tracker or with the calorimeters depending on its charge. While charged particle energy is measured with the tracker (which is more precise than the calorimeters), neutral particle energy is deduced from the energy deposit in the calorimeter from which the contribution of charged particles is subtracted. This calls for finely segmented calorimeters with single shower imaging capability.

Digital hadronic calorimetry

A digital hadronic calorimeter (DHCAL) would balance the cost of an increased number of readout channels (with a cell size of 1 cm 2 , the number of channel would reach 3•10 7 ) by a simpler readout circuitry (1 bit information per channel). A DHCAL can be instrumented with scintillating or gaseous layers. In the latter case, GEMs [2,[START_REF] Yu | GEM DHCal Development[END_REF], RPCs [START_REF] Bilki | [END_REF] and MICROMEGAS [5,[START_REF] Adloff | Development of MICROMEGAS for a Digital Hadronic Calorimeter[END_REF] are being considered. Some benefits of a MICROMEGAS DHCAL are a potentially high efficiency for MIPs, a hit multiplicity close to 1, low working voltages w.r.t. GEMs and RPCs (400-500 V), a thin sensitive layer (3 mm of Ar), a very good long-term irradiation behaviour and a high rate capability.

Scope of the study

We built several Bulk MICROMEGAS chambers with 1 cm 2 anode pads [10]. Some chambers are readout by GASSIPLEX chips [START_REF] Gassiplex | A 16 integrated channels front-end analog amplifiers with multiplexed serial readout[END_REF] which measure the charge. They are used for detector characterisation in electron and hadron showers. Other chambers are equipped with DIRAC [START_REF] Gaglione | DIRAC: a Digital Readout Asic for hAdronic Calorimeter[END_REF] or HARDROC [START_REF] Boudry | HARDROC1, Readout chip of the Digital Hadronic Calorimeter of ILC[END_REF] chips which provide a 3 or 2 bit information per channel. They are intended for constructing a 1 m 3 DHCAL prototype.

The behaviour of MICROMEGAS in electromagnetic showers has been studied at the PS facility at CERN. Because of the small number of chambers available, this study was carried out with one 12×32 cm 2 chamber with GASSIPLEX readout in front of which were placed a variable number of stainless steal absorber plates. Measurements of energy profile are presented in section 3. These are important to verify that MICROMEGAS performance is maintained in high track multiplicity showers. Finally, first tests in a beam of HARDROC and DIRAC based MICROMEGAS chambers are reported in section 4.

Experimental setup

Chamber geometry and readout electronics

The chambers consist of an anode PCB segmented into 1 cm 2 pads, a woven mesh maintained 128 µm above the PCB by insulating pillars [10], a 3 mm thick plastic frame which defines the drift region and a 2 mm thick grounded steel cover. The drift electrode is a copper foil glued on a kapton, itself glued on the cover surface. The gas is flushed in the chamber through two holes in the plastic frame. Measurements reported in this paper are performed in Ar/iC 4 H 10 95/5.

Four chambers with GASSIPLEX readout were built: three of 6×16 cm 2 and one of 12×32 cm 2 (called G 1 , G 2 , G 3 and G 4 ). The corresponding number of pads is equal to 96 and 384 respectively. GASSIPLEX is a 16 channel ASIC that amplifies and shapes the pad signals. Boards equipped with 6 chips are connected on the side of the chambers. The resulting signals are digitized by 10 bit ADCs placed in a crate.

One 8×8 cm 2 chamber read out by a DIRAC ASIC and three 8×32 cm 2 chambers read out by HARDROCs have been constructed (called D 0 , H 1 , H 2 and H 3 ). Pads of 1 cm 2 are first patterned on one side of a PCB. The ASICs are then connected on the backside. Finally, the MICROMEGAS is laminated onto the PCB. The resulting chamber thickness is smaller than 8 mm which is already compatible with the ILC HCAL specification for gap size between absorbers.

Beam test setup

The beam test took place in June 2009 in the CERN/PS/T10 zone where electrons, positrons and hadrons with momenta up to 10 GeV/c are delivered. At 2 GeV/c, the ratio between electrons/positrons and hadrons is roughly 1:1 and the momentum spread of about 1 %.

The setup consists of a stack of four GASSIPLEX (G 1 -G 4 ) and three HARDROC chambers (H 1 -H 3 ) (Figure 1). The stack position is such that the beam impinges perpendicularly on the chambers. For triggering purposes, three 8×32 cm 2 scintillators (S 1 , S 2 and S 3 ) are placed inside the stack parallel to the chamber planes. Two 2×4 cm 2 scintillators (S 4 and S 5 ) perpendicular to each other and parallel to the chamber planes are also available. They are placed in front of the stack. With an overlap area of 1 cm 2 they can be used to reduce the trigger angular acceptance. Shower profiles are measured with the chamber G 4 in front of which a variable number of absorbers are placed. Up to 12 2 cm thick stainless steel plates can be installed in front of G 4 . The gap between each absorber is equal to 8 mm. One absorber is placed a few cm behind G 4 to allow backscattering of the shower particles to the chamber.

A Čerenkov counter installed in the T10 zone ahead of the stack is filled with CO 2 at 4 bars. The threshold velocity β th is equal to 0.9983. At 2 GeV/c, protons and pions are too slow (β = 0.8831 and 0.9976) to produce Čerenkov light. Counter signals are hence used to discriminate between 2 GeV/c electrons/positrons and hadrons.

The chamber equipped with a DIRAC chip was tested in August 2008 in the SPS/H2 beam line. No trigger device was used and the test setup consisted mainly of the chamber itself.

Beam test of GASSIPLEX readout chambers

Energy deposit measurement

The number of ADC counts N (after pedestal subtraction) measured on a given channel relates to the energy deposited in the gas ε above the corresponding pad. Neglecting the transverse diffusion of the primary electrons and assuming that the amplified signal does not saturate the electronics, N relates to ε according to:

N = q e GS W ε (3.1)
with G the gas gain, S the conversion factor of the electronics (in ADC counts per unit of charge), W the mean energy per ion pair and q e the electron charge. The total energy ξ deposited in the chamber is the sum of the energy measured on all pads with signals larger than a hit threshold of 15 ADC counts (3 fC). Taking the channel to channel variations of gas gain and conversion factor, the sum can be written as:

ξ = ∑ i ε i = W q e ∑ i N i G i S i (3.2)
In the future, these measurements will compared to predictions from a GEANT4 simulation. In the latter case, the drift gap is uniform across the detector area. This is not the case in practice and therefore the measured energy should be corrected for these variations. This could be done by weighting N i with the ratio w i = d i /d with d i the gap thickness above the channel pad i and d = 3 mm. The values of d i could not be measured, however, they can be combined with G i and S i in w i :

ξ = W q e 1 G S ∑ i G S d d i G i S i N i = W q e 1 G S ∑ i w i N i (3.3)
with G and S the gas gain and conversion factor averaged over all channels. S is determined from a calibration of all channels while G is measured above one pad only by means of an 55 Fe source. The determination of the weight of each channel is called the inter-calibration and is explained in the next section. In addition, the gas gain is a function of pressure and temperature and may change with time. As will be shown in section 3.3, such gain variations during the beam test are not negligible and corrections for these will be applied.

Channel inter-calibration

The channel weights are determined by scanning the area of chamber G 4 with the beam. The acquisition is triggered by the time coincidence of the S 1 and S 2 (or S 1 and S 3 ) scintillator signals.

Neglecting variations of the incident angle of the tracks, the weights are given by the most probable value w i of the ADC count distribution on each channel:

w i = m i m (3.4)
When measuring the ADC count distribution, it is important to insure that the primary charge is collected on one pad only. This is not always the case as in typical operating conditions a particle crossing the gap activates on average 1.1 pads [START_REF] Espagilière | MICROMEGAS chambers for hadronic calorimetry at a future linear collider[END_REF]. A way to minimize this effect is to consider only events with a single hit. The ADC count distribution measured on one pad using such events is shown in Figure 2 (a). It exhibits an expected Landau shape with a peak around 120 ADC counts. At high counts a second peak is observed due to saturation of the preamplifier. The Landau function most probable value m is adjusted on the measured distribution. The error on m depends on the number of events (between 200-500 depending on the pad position) and was estimated by a Monte Carlo simulation. On most of the pads it is equal to 7 ADC counts. The Landau MPV averaged over all pads m is equal to 121 ADC counts with an r.m.s. of 12 counts (Figure 2 (b)). This corresponds to a most probable charge of 24 fC with variations of 12 %. Eventually, the number of ADC counts measured on a pad is multiplied by the correction factor:

a i = 1-| 1 -w i | for w i ≥ 1 1+ | 1 -w i | for w i < 1 (3.5)

Pressure and temperature corrections

Pressure and temperature impact on the gas number density and hence on the ionisation mean free path. Eventually P and T variations result in a change of primary ionisation and gas gain. In our experimental conditions, the first can be safely neglected and only gas gain variations are considered. The gain dependence on P and T can be inferred from a simple model. Using the Rose and Korff parametrization of the Townsend coefficient α [START_REF] Sauli | Principles of operation of Multiwire Proportional and Drift Chambers[END_REF], the gain can be written as:

G = exp(αg) = exp APg T exp(- BPg TV ) (3.6)
where A and B are constants that depend on the gas mixture, g is the amplification gap and V the mesh voltage. It follows that the gain relative sensitivity to P/T variations is given by:

C P/T = 1 G ∂ G ∂ (P/T ) = Ag 1 - Bg V P T exp - Bg V P T (3.7)
The constants A and B are adjusted on the measured G(V) trend, fixing P, T, g and V. In typical operating conditions, one obtains C P/T = -2.39 K/mbar in Ar/iC 4 H 10 95/5. A direct measurement of C P/T is detailed in [START_REF] Espagilière | MICROMEGAS chambers for hadronic calorimetry at a future linear collider[END_REF]. The time variations of the P/T ratio during the beam test are shown in Figure 3 (a). Over the full test period, the average P and T are equal to 970 mbar and 298 K with variations of 20 mbar and 5 K r.m.s.. The measured number of ADC counts is multiplied by the correction factor a t calculated as:

a t = 1 -C P/T ∆(P/T ) = 1 -C P/T P(t) T (t) - P 0 T 0 (3.8)
with P 0 , T 0 the average values quoted above. The distribution of the correction factor a t is shown in Figure 3

Longitudinal shower profile

The acquisition is triggered by the coincidence of the S 4 and S 5 signals in order to record tracks traversing the center of the chamber G 4 . In addition the Čerenkov counter signal is used to select electrons or positrons. For a given number of absorber plates, roughly 40•10 3 events were recorded.

To reject events with noise hits in G 4 , at least one hit is requested in two of the three chambers (G 1 -G 3 ). Moreover, in each chamber with at least one hit, the hit (or the hit with the largest number of ADC counts) should be centered at plus or minus one pad from the maximum of the beam profile. This insures that a track has traversed G 4 while reducing slightly the statistics (30•10 4 events).

Energy distributions as measured with various number of absorbers are plotted in Figure 4 (a). The distributions of the number of hits are also shown (Figure 4 (b)). The longitudinal energy (resp. hit) profile is then obtained from the mean energy (resp. number of hits) deposited after each number of absorbers (Figure 5). The energy and number of hit profile are very similar, showing a maximum between 2 and 3 absorbers. The hit profile maximum, however, is reached at a slightly latter stage of the shower development because at the beginning of the shower some secondary particles traverse the same pad.

A GEANT4 simulation of the beam test setup is being implemented. At the moment of writing, a threshold equal to 30 % of the most probable energy deposit is applied, the digitization of the energy is not performed and electronics saturation is neglected. The implementation of these effects in the simulation will be part of future work. The simulation predicts that the deposited energy is maximum after three absorbers. Considering the simplifying assumptions, this is in good agreement with our measurement. For more details on the simulation of the performance of a DHCAL, the reader is referred to [START_REF] Blaha | Monte carlo study of the physics performance of a digital hadronic calorimeter[END_REF].

Transverse shower profile

The transverse energy profile is the radial distribution of the energy in the chamber plane. The radial distance should be calculated as the distance between the intersection of the shower axis with the chamber plane (x 0 ,y 0 ) and the center of the hit pad. The coordinates (x 0 ,y 0 ), however, vary from one track to the other. To minimize the spread, only tracks passing through the same single pads in the three small chambers G 1 -G 3 are considered. These pads are determined as the maximum of the beam profile in each small chamber. The coordinates (x 0 ,y 0 ) are then taken as the center of the pad corresponding to the maximum of the beam profile in chamber G 4 when no absorber is present. Depending on the number of absorbers, between 1•10 3 -5•10 3 events are used.

The energy and hit radial distributions from 2 GeV electron/positron showers after various number of absorbers are shown in Figure 6 (a) and (b). Their shapes are similar, however, the energy distribution is slightly more peaked at the beginning of the shower (e.g. with two absorbers). This effect is illustrated in Figure 7 where the distribution r.m.s. is plotted as a function of the number of absorbers.

At the beginning of the shower, some secondary particles traverse the same pad in chamber G 4 . The measured energy on that pad is then significantly higher than on the neighboring pads. This is not the case when a threshold is applied: a similar number of hits is recorded on the central pad and on its neighbors. As a result the profile is flatter and its r.m.s. slightly larger. At the maximum of the longitudinal profile (2-3 absorbers), the transverse profile r.m.s. is about 1.6-1.8 pads.

Conclusion

Shower profiles from 2 GeV electrons were measured with a 12×32 cm 2 MICROMEGAS chamber equipped with 1 cm 2 pads and GASSIPLEX chips. The detector was operated during 12 days in a beam of electrons, positrons and hadrons. At a gas gain of 17•10 3 , the spark rate was very low (a few sparks per day) and no damage on the detector was observed. The energy and number of hit profile are very similar. The longitudinal profile shows a maximum between 2 and 3 absorbers. In this configuration the transverse spread of the shower is about 1.6-1.8 pads r.m.s..

Chambers with embedded electronics 4.1 ASICs for digital calorimetry at ILC

The time structure of the electron and positron beams at ILC would consist of 1 ms long bunch trains with a bunch crossing period of 300 ns, separated by 199 ms. During this idle time, it is proposed to switch off the front-end electronics of the detectors. Based on this power pulsing scheme, the HARDROC and DIRAC ASICs were designed to meet the requirements of a digital hadronic calorimeter. They are 64 channel self-triggered integrated circuits, each channel being equipped with an adjustable gain preamplifier, two (three for DIRAC) comparators and a memory.

Each channel mainly provides a digital information. An essential point of HARDROC and DIRAC-based chambers is that they can be made very thin. Thanks to the Bulk fabrication process, anode PCBs with chips connected on the backside can be equipped with a mesh. The overall thickness remains below 8 mm including the chamber cover which will be part of the absorber. This already complies with ILC constraints on the DHCAL gap size between absorbers. Moreover, the Bulk is used by several experiments with large areas to instrument [13,[START_REF] Anvar | [END_REF]. It is thus well suited for the construction of a 1 m 3 DHCAL MICROMEGAS prototype.

First chamber tests and future plans

Chambers with one DIRAC (8×8 cm 2 ) or four HARDROCs (8×32 cm 2 ) have been constructed (Figure 8 (a) and (b)). They are dubbed ASU: Active Sensor Unit. The DIRAC chamber was placed in a 200 GeV pion beam in August 2008 at the SPS (Figure 9 (a)). A stack of four chambers should be available in October 2009. It will be used for measuring efficiency and hit multiplicity at the CERN/PS. For recent measurements of the characteristics of DIRAC, the reader is referred to [START_REF] Gaglione | MICROMEGAS chamber with embedded DIRAC ASIC for hadronic calorimetry[END_REF].

Three ASUs with four HARDROCs were placed in a 2 GeV electron beam at the CERN/PS. Despite dead zones due to faulty chips, a clear image of the beam profile in the anode plane is obtained in each chamber (Figure 9 brought together in a single gas volume. Each ASU is equipped with 24 HARDROC (version 2) chips (Figure 10). Measurements of the performance of HARDROC based chambers are foreseen in the October 2009 at the CERN/PS.

Conclusion

Bulk MICROMEGAS chambers equipped with GASSIPLEX chips were used to measure the spatial characteristics of 2 GeV electron showers at the CERN/PS. During twelve days, the chambers were operated at high gas gain (17•10 3 ) in Ar/iC 4 H 10 95/5 while the spark rate was negligibly low.

Longitudinal and transverse profile of the energy and number of hits were measured. These results will soon be confronted to GEANT4 predictions. This will constitute a good test of the simulation.

In view of the construction of a MICROMEGAS DHCAL, 8 mm thin chambers with digital readout embedded on the backside of the anode PCB were fabricated. The first test was carried out in 2008 with an 8×8 cm 2 chambers equipped with a DIRAC chip at the CERN/SPS facility. In 2009, 8×32 cm 2 chambers equipped with HARDROCs were tested in the CERN/PS beam. Signals from the beam particles were recorded, demonstrating the feasibility of thin MICROMEGAS chambers. The next step is to build and test larger area chambers. For that purpose, two 32×48 cm 2 PCBs were recently equipped with twenty four HARDROCs (version 2) each and a mesh. Tests of the electronics and of their amplification properties will be carried out at LAPP in the coming months. Eventually they should be tested in a beam at the end of the year 2009 inside a 1 m 2 chamber.

Figure 1 .

 1 Figure 1. Beam test setup.

Figure 2 .

 2 Figure 2. ADC count distribution measured on one pad (a). Most probable value of the adjusted Landau function for all pads (b).

Figure 3 .

 3 Figure 3. Pressure and temperature variations during the beam test (a). Correction factor applied to the measured number of ADC counts (b).

Figure 4 .Figure 5 .

 45 Figure 4. Energy (a) and number of hit (b) distributions from 2 GeV electron showers with various number of absorber plates. The distributions are normalized to one.

Figure 6 .Figure 7 .

 67 Figure 6. Energy (a) and number of hit (b) radial distributions from 2 GeV electron showers after various number of absorber plates.

Figure 8 .

 8 Figure 8. Photograph of the PCB backside of DIRAC (a) and HARDROC (b) based chambers.

Figure 9 .

 9 Figure 9. Beam profile measured with one DIRAC (a) and three HARDROC (b) chambers. The hotter the color the larger the number of hits recorded on a given pad. Each pad has an area of 1 cm 2 .

Figure 10 .

 10 Figure 10. Photograph of two PCBs equipped with 24 HARDROCs each. The active area of a PCB is 32×48 cm 2 .

Acknowledgments

We would like to thank Didier Roy for his work on the GASSIPLEX readout software (CEN-TAURE) and Lau Gatignon for his help in the PS/T10 zone. Special thanks go to Raphael Gallet, Nicolas Geffroy and Fabrice Peltier for their decisive contribution to the assembly of the chambers and the installation of the beam test setup. Thanks also to L. Fournier and J. Jacquemier for their creative work on the reconstruction/analysis software.