EVALUATION OF THE COMPARATIVE INFLUENCE OF SOIL HYDRAULIC-PROPERTIES AND ROUGHNESS ON OVERLAND-FLOW AT THE LOCAL SCALE

A mathematical model is used to interpret border irrigation tests carr ied out on plots sown with clover and maize, in order to evaluate the influence of the roughness coefficient compared with the parameters de scribing the hydraulic properties of the soil. The model simulates the surface runoff by integrating the unsteady motion equations of a grad ually varied free surface flow according to a finite difference scheme of explicit type. In the momentum equation the convective inertia ter m is ignored. The infiltration is schematized through the physically b ased equation of Green and Ampt. Preliminary validation of the model i s made on the basis of field data available in the literature. Border irrigation tests were carried out on 40 and 100 m long plots and measu rements were made of advance times of the wave front and surface runof f volumes. The initial conditions were defined by measurement of the s oil water content just before watering. The changes of the ground cove r and the decrease of porosity along the soil profile owing to ploughi ng and to the formation of a plough sole are found to: exert an import ant influence on the runoff. The sensitivity analysis of the model, ca rried out taking into account the wave front celerity, the discharge h ydrograph at the end of the plot and the outflow volume, confirms the prevailing influence of roughness coefficient and natural hydraulic co nductivity compared with the wetting front suction. According to the s ingle-parameter perturbation technique, decreasing the values of rough ness coefficient and hydraulic conductivity individually, compared wit h the calibration values, increases the computed outflow volumes by ne arly 400% compared with experimentally measured outflows. (C) 1995 Sil soe Research Institute