Evaluation of the Comparative Influence of Soil Hydraulic Properties and Roughness on Overland Flow at the Local Scale

Abstract

A mathematical model is used to interpret border irrigation tests carried out on plots sown with clover and maize, in order to evaluate the influence of the roughness coefficient compared with the parameters describing the hydraulic properties of the soil. The model simulates the surface runoff by integrating the unsteady motion equations of a gradually varied free surface flow according to a finite difference scheme of explicit type. In the momentum equation the convective inertia term is ignored. The infiltration is schematized through the physically based equation of Green and Ampt. Preliminary validation of the model is 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 measurements were made of advance times of the wave front and surface runoff volumes. The initial conditions were defined by measurement of the soil water content just before watering. The changes of the ground cover and the decrease of porosity along the soil profile owing to ploughing and to the formation of a plough sole are found to exert an important influence on the runoff. The sensitivity analysis of the model, carried out taking into account the wave front celerity, the discharge hydrograph at the end of the plot and the outflow volume, confirms the prevailing influence of roughness coefficient and natural hydraulic conductivity compared with the wetting front suction. According to the single-parameter perturbation technique, decreasing the values of roughness coefficient and hydraulic conductivity individually, compared with the calibration values, increases the computed outflow volumes by nearly 400% compared with experimentally measured outflows.