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International audience; The hydrological dynamics of a small rural river catchment in southern Brazil have been used to understand the impact of agricultural activity on soil and water resource degradation. The monitoring and modeling strategies employed to better understand the integrated phenomena focus on surface flow dynamics and the associated processes, such as erosion, sediment yield (SY), and chemical element transfer. In this context, monitoring and modeling techniques are combined in order to include the main hydrological processes involved in soil and water resource degradation. The project began in 2002 and over 500 rainfall-runoff-sediment events and suspended sediment concentration make up the hydrological database under different land use and soil management conditions. The 1.2 km2 catchment of the study is characterized by the presence of shallow soils, high slope, intense agricultural activity, and high SY (∼150 t.km-2.y-1). Techniques that enable the identification of the origin and redistribution of sediments (e.g fingerprinting approach and Cs137) and hydrograph analysis are being used to further understand the dynamics of SY in the catchment. The comprehension of the factors involved in the SY dynamics has allowed the improvement and application of different mathematical models (e.g., LISEM and WATERSHED) to simulate surface flow and SY. These models are used to represent the influence of soil and water conservation practices while considering the interaction between the different landscape components (crops, roads, drainage network, etc.). From this set of data and tools, the capacity of the agricultural production system, drainage network, wetlands and riparian forests in accelerating or retarding water flow, sediment, and chemical elements in the landscape are evaluated. All of these studies have been used to improve soil and water conservation practices at catchment scale, meeting the expectations of farmers in erosion control and water storage as well as of the society interested in maintaining the hydrological functions of soils in catchment scale.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES The Limburg Soil Erosion Model (LISEM) erosion prediction model is an environmental planning and diagnostic tool that, through the simulation of the processes of sediment detachment, transport and deposition provided by runoff, calculates erosion and sediment yield (SY) in watersheds on an event scale. Currently the model presents options for adjusting the detachmentprocess through the use of efficiency coefficients in the form of equations that act as reducers of the detachment level provided by the main disaggregation equation of the model, which obeys the principle of the capacity of transport by the power of the flow adapted by Govers (1990). In simulations for catchments with cohesive soils and rough reliefs, these coefficients may be limiting for the good estimation of SY in erosion modeling. In this work, we tested different efficiency equations by the water flow in a set of 20 events on a small, intensively monitored rural catchment, characterized by shallow and cohesive soils, marked relief and intense agricultural activity. From the three equations tested, one which is used in the EUROSEM model (Morgan et al., 1998) represented by y = 0.79 * exp (-0.85 * COH) satisfactorily simulated SY for 17 of the 20 rainfall-runoff events, compared to the one originally used in the LISEM model that was developed for non-cohesive materials , and also in relation to that used in the MMF model (Morgan et al., 1984), maintaining during the calibration the soil cohesion values measured in the field. For three simulated events the adjustment was not satisfactory due to the particular characteristics of the SY of these events, which do not follow the predictive logic of the erosive process proposed by the model. Thus, the availability of better equations to represent the conditions of the disaggregation for cohesive soils opens up new perspectives for the use of the model in studies of the erosive processes in catchment scale, and the estimation of the SY as a reflection of the conservation management practices adopted in the slopes. O modelo preditivo de erosão Limburg Soil Erosion Model (LISEM), é uma opção de ferramenta de diagnóstico e planejamento ambiental que por intermédio da simulação dos processos de desagregação, transporte e deposição de sedimentos proporcionados pelo escoamento superficial, calcula a erosão e a produção de sedimentos (PS) em bacias hidrográficas em escala de eventos. Atualmente o modelo apresenta opções de ajuste do processo de desagregação através do uso de coeficientes de eficiência na forma de equações que atuam como redutores da desagregação proporcionados pela equação principal de desagregação do modelo, que obedece ao princípio da capacidade de transporte pela potência do escoamento adaptado por Govers (1990). Em simulações para bacias com solos de características coesivas e relevos acidentados, estes coeficientes podem ser limitantes para a boa estimativa da PS na modelagem da erosão. Nesse trabalho testamos diferentes equações de eficiência de desagregação pelo fluxo em um conjunto de 20 eventos sobre uma pequena bacia hidrográfica rural intensivamente monitorada, caracterizada por solos rasos e coesivos, relevo acentuado e intensa atividade agrícola. Das três equações testadas, a que é utilizada no modelo EUROSEM (MORGAN et al., 1998) representada por y = 0,79 * exp (-0,85 * COH) simulou satisfatoriamente a PS para 17 dos 20 eventos simulados, em relação à originalmente usada no modelo LISEM que foi desenvolvida para materiais não coesivos, e também em relação à utilizada no modelo MMF (MORGAN et al., 1984), mantendo durante a calibração os valores de coesão do solo mensurados a campo. Para três eventos simulados o ajuste não foi satisfatório devido às características particulares da PS destes eventos, que não seguem a lógica de previsão do processo erosivo proposta pelo modelo. Assim, a disponibilidade de melhores equações para representar as condições da desagregação para solos coesivos abrem novas perspectivas para o uso do modelo em estudos dos processos erosivos em escala de bacia, e da estimativa da PS como reflexo das práticas de manejo conservacionistas adotadas nas encostas.