DEFINING MONITORING STRATEGIES FOR RUNOFF AND EROSION STUDIES IN AGRICULTURAL CATCHMENTS - A SIMULATION APPROACH

The number of runoff and erosion studies in cultivated catchments in n orth west Europe is increasing. Measurements are usually costly and ti me-consuming, in particular of soil hydrological properties because ag riculture causes considerable spatial and temporal variation. A subdiv ision of the catchment is necessary to optimize the choice of sampling sites and to define extrapolation procedures. Four sampling strategie s were examined, consisting of subdividing the catchment according to the crops (1), or according to the surface structure (2), followed by increasing the number of sampled fields by adding fields that are pres umed to have maximum runoff activity (3) or intermediate activity (4). These strategies were compared by modelling the runoff and erosion in a virtual catchment, containing 15 arable fields with various crop se quences, at three times during a winter season. Strategy 2 gave better results than 1, because of the large variability of surface structure of the bare fields, which was not correlated with the crop type. Runo ff prediction benefited from extra sampling only when groups of fields with a large heterogeneity in hydrological behaviour were included. T he quality of erosion prediction increased when the steeper fields wer e included in the sampling scheme. A monitoring strategy is proposed t hat consists of: (i) repeated, fast and exhaustive descriptions of the soil surface using surface structure classes, (ii) subdivision of the catchment into groups of fields with identical surface characteristic s before measurements are made, and (iii) sampling and extrapolation a ccording to these groups.