INFILTRATION EXPERIMENTS ON LOESS SOILS AND THEIR IMPLICATIONS FOR MODELING SURFACE RUNOFF AND SOIL-EROSION

Infiltration parameters such as infiltration rate, the infiltration control zone depth and the initial soil moisture content, used in the Holtan/Overton equation, have a strong impact on the output of the erosion model 'ANSWERS'. Therefore, infiltration was measured accurately and spatial and temporal variability were taken into account. 72 infiltration experiments on three types of loess soils under simulated rain show that the Holtan/Overton equation describes infiltration correctly. Both Philip's equation and the Green-Ampt model yield similar results. Because the infiltration parameters show a "within-unit" variance which is larger than the "between-unit" variance, no differences in infiltration characteristics between the three loess soil types could be detected statistically. To evaluate the consequences of the large spatial varability of infiltration, stochastic methods must be used in combination with a distributed hydrologic model. Consecutive simulated rainstorms applied to the same samples show the influence of temporal variability in infiltration behaviour. This type of variability is attributed to the formation of surface crusts. The influence of crust formation on Holtan infiltration can be expressed in terms of a change in infiltration control zone depth. Assuming a relation between crusting and cumulative kinetic energy of rain, the temporal variability of infiltration behaviour was successfully modelled, yielding an empirical relation between control zone depth and cumulative kinetic energy of rain. However, a more physically based hydrologic submodel of the influence of crusting on infiltration during the growing season is needed.