EFFECTS OF TILLAGE ON RUNOFF DIRECTIONS - CONSEQUENCES ON RUNOFF CONTRIBUTING AREA WITHIN AGRICULTURAL CATCHMENTS

In areas of intensive agriculture, e,g, 'Pays de Caux' in France, whic h was the study area, field observations have shown that runoff direct ions were modified by agricultural activities. In order to account for factors responsible for modifications of the runoff direction (roughn ess, tillage direction and agricultural patterns, e,g, dead furrow or dirt tracks), we constructed a discriminant function based on field ob servations, This function enables us to decide whether flow direction for slopes of up to 15% was imposed by slope direction or tillage dire ction. It can be applied to any location, provided there are known rou ghness, known slope intensity, known aspect and known tillage azimuth. In order to examine the effects of these agricultural activities at t he catchment scale, we compared two models by analysing the same hydro logical variables: the area contributing to runoff and the flow networ k. The first model (Topo) was built according to the runoff direction derived from a Digital Elevation Model (DEM). The second model (Tillag e) was constructed by combining information from the DEM, and informat ion from rules based on field observations or resulting from statistic al analysis. For 23 basic catchments, the result of the comparison bet ween the two models (Topo and Tillage) showed that a major part of the catchments and the drainage network was affected by modifications rel ated to the introduction of man-made agricultural factors. For example , for 20 of 23 catchments, the runoff flows over more than 50% of the surface of such areas were produced along the direction imposed by til lage, The introduction of tillage effect brings about modifications of both the shape and size of catchments, (C) 1998 Elsevier Science B.V. All rights reserved.