A RAINFALL SIMULATOR STUDY OF INFILTRATION INTO ARABLE SOILS

Since Hortonian surface runoff is one possible mechanism for the fast transport of agricultural chemicals from arable soils to surface water, more information is needed on its significance in agricultural areas. The present study concerns the sandy soils of the Dutch Cover Sands area, and is based on the approximate one-dimensional Mein-Larson infiltration model. This model was modified to incorporate the surface storage of water in the micro-relief, thus providing a tool to predict surface runoff on a scale of square metres. Infiltration measurements were carried out with a portable rainfall simulator. Factors which influence infiltration parameters appear to be the type of crop and tillage practices. Strong variability within fields was found due to the presence of compacted tracks caused by the passage of agricultural machinery. Comparison of infiltration characteristics to rainfall data indicates when and where small-scale surface runoff is expected. Modeling results indicate that this is mainly controlled by rainfall, and less by the variability in infiltration parameters. In spring and summer, fields growing potatoes and sugarbeets are the most sensitive to surface runoff, especially the compacted parts (tracks). Surface runoff also occurs occasionally on maize fields. Wheat appears to enhance infiltration capacities by creating cracks (preferential flowpaths) around the roots. This is especially significant after a dry period. Grasslands are also sensitive to ponding, but surface runoff is limited by the relatively large storage capacity of the sod layer. Though bare fields in the autumn and winter show the lowest infiltration rates and storage capacities, Hortonian surface runoff hardly occurs, due to the lower rainfall intensities in those seasons.