Agricultural activity can significantly affect the environment. The rising global demand for agricultural products, largely as a result of population growth, only increases the pressure on soil resources. In fact, soil erosion and water pollution with sediment are considered among the most critical environmental threats worldwide. This thesis aims to obtain new insights into the phenomenon of soil erosion and sediment export in cultivated areas, of its processes, and to the factors that control them, and evaluate our capacity to predict and simulate some of these processes. The spatial and temporal scale of the study consists of multiple levels, starting from the analysis of at the "watershed scale" using the Experimental Agricultural Watershed Network of the Government of Navarre, both at the multiannual and event-level, and going further into the analysis at the micro-watershed level by studying ephemeral gully erosion and its modeling using information from very detailed observations in Iowa. The following specific objectives addressed this general objective: (i) analyze the hydrological and sediment export behavior of small agricultural watersheds in Navarre; (ii) explore the use of high-resolution measurements of hydrological variables (turbidity, discharge and precipitation) to characterize sediment concentration in agricultural watersheds during storm events; (iii) obtain new insights about internal watershed processes by investigating the factors that condition the occurrence and growth of ephemeral gullies in agricultural micro-watersheds in Iowa; and (iv) to evaluate the capability of the AnnAGNPS-TIEGEM (Tillage lnduced Ephemeral Gully Erosion Model) model to simulate ephemeral gully erosion in Iowa, and propose improvements to the model in light of the results obtained.

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