Rainfall-runoff induced soil erosion causes important environmental degradation by reducing soil fertility and impacting on water availability as a consequence of sediment deposition in surface reservoirs used for water supply, particularly in semi-arid areas. However, erosion models developed on experimental plots cannot be directly applied to estimate sediment yield at the catchment scale, since sediment redistribution is also controlled by the transport conditions along the landscape. In particular, representation of landscape connectivity relating to sediment transfer from upslope areas to the river network is required. In this study, the WASA-SED model is used to assess the spatial and temporal patterns of water and sediment connectivity for a semi-arid meso-scale catchment (933 km 2 ) in Brazil. It is shown how spatial and temporal patterns of sediment connectivity within the catchment change as a function of landscape and event characteristics. This explains the nonlinear catchment response in terms of sediment yield at the outlet.Key words sediment yield; watershed connectivity; catchment scale; semi-arid; WASA-SED model Modélisation des patrons spatio-temporels de la connectivité et de la production de sédiments dans un bassin versant semi-aride à l'aide du modèle WASA-SED Résumé L'érosion des sols induite par le ruissellement cause d'importantes dégradations de l'environnement en réduisant la fertilité des sols et en ayant un impact sur la disponibilité de l'eau suite au dépôt de sédiments dans les réservoirs, particulièrement dans les régions semi-arides. Cependant, des modèles d'érosion développés pour des parcelles expérimentales ne peuvent pas être appliqués à l'évaluation de la production de sédiments à l'échelle des bassins hydrographiques, parce que la redistribution des sédiments est aussi contrôlée par les conditions de transport dans le paysage. En particulier il est nécessaire de représenter la connectivité du paysage en relation avec le transfert des sédiments des versants vers le réseau hydrographique. Dans cette étude, on utilise le modèle WASA-SED pour évaluer les patrons spatio-temporels de la connectivité de l'eau et des sédiments dans un bassin versant semi-aride de méso-échelle (933 km 2 ) au Brésil. Les résultats montrent comment les patrons spatio-temporels de la connectivité des sédiments dans le bassin versant changent en fonction des caractéristiques du paysage et des événements. Ceci explique la réponse non-linéaire du bassin versant en termes de production de sédiments à l'exutoire.Mots clefs production des sédiments; connectivité; échelle du basin versant; semi-aride; modèle WASA-SED

Sustainability of communities, agriculture, and industry is strongly dependent on an effective storage and supply of water resources. In some regions the economic growth has led to a level of water demand that can only be accomplished through efficient reservoir networks. Such infrastructures are not always planned at larger scale but rather made by farmers according to their local needs of irrigation during droughts. Based on extensive data from the upper Jaguaribe basin, one of the world's largest system of reservoirs, located in the Brazilian semiarid northeast, we reveal that surprisingly it self-organizes into a scale-free network exhibiting also a power-law in the distribution of the lakes and avalanches of discharges. With a new self-organized-criticality-type model we manage to explain the novel critical exponents. Implementing a flow model we are able to reproduce the measured overspill evolution providing a tool for catastrophe mitigation and future planning.floods | scarcity | time series | water transport W ater is simultaneously the most relevant commodity for life and the primary medium through which people and the environment are affected by natural disasters (1-6). According to the Food and Agriculture Organization of the United Nations, the water demand in the last century has grown more than twice the population increase and as forecast, by 2050, about 2 billion people will be living in water scarcity (7). This is not only related to the availability of freshwater but also with the lack of proper storage and distribution infrastructures. It is thus of paramount interest to understand how to guarantee an even spatial and temporal distribution of water, avoiding the imbalance between supply and demand. The first step in this direction, which we address here, is to analyze how the interplay between the rainfall and evaporation affects the filling level in the reservoirs and the network.In semiarid environments like the Brazilian northeast it is common to have droughts drastically affecting the crops. The weather in the region is characterized by intermittent rains and long periods of water scarcity. The water supply, in the dry season, is assured by a dense set of surface reservoirs, which are essential for the sustainability of the region, because during the rainy season water is stored to be distributed afterward. A proper reservoir size is crucial: While smaller reservoirs are typically unable to supply enough water throughout the entire dry season, due to the low storing capacity and high losses through evaporation and infiltration, too large reservoirs can have serious consequences for life sustainability. For example, if the storage capacity in a basin exceeds the volume equivalent to three times the average annual runoff, its water will not be sufficiently renewed (8). Not only the properties of each reservoir are relevant but also the way they are interconnected. For example, upstream reservoirs may retain a significant part of the collected water, which enables an efficient spatial distri...

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Abstract. Current soil erosion and reservoir sedimentation modelling at the meso-scale is still faced with intrinsic problems with regard to open scaling questions, data demand, computational efficiency and deficient implementations of retention and re-mobilisation processes for the river and reservoir networks. To overcome some limitations of current modelling approaches, the semi-process-based, spatially semi-distributed modelling framework WASA-SED (Vers. 1) was developed for water and sediment transport in large dryland catchments. The WASA-SED model simulates the runoff and erosion processes at the hillslope scale, the transport and retention processes of suspended and bedload fluxes in the river reaches and the retention and remobilisation processes of sediments in reservoirs. The modelling tool enables the evaluation of management options both for sustainable land-use change scenarios to reduce erosion in the headwater catchments as well as adequate reservoir management options to lessen sedimentation in large reservoirs and reservoir networks. The model concept, its spatial discretisation scheme and the numerical components of the hillslope, river and reservoir processes are described and a model application for the meso-scale dryland catchment Isábena in the Spanish Pre-Pyrenees (445 km 2 ) is presented to demonstrate the capabilities, strengths and limits of the model framework. The example application showed that the model was able to reproduce runoff and sediment transport dynamics of highly erodible headwater badlands, the transient storage of sediments in the dryland river system, the bed elevation changes of the 93 hm 3 Barasona reservoir due to sedimentation as well as the life expectancy of the reservoir under different management options.

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