DISTRIBUTED-PARAMETER HYDROLOGY MODEL (ANSWERS) APPLIED TO A RANGE OF CATCHMENT SCALES USING RAINFALL SIMULATOR DATA .1. INFILTRATION MODELING AND PARAMETER MEASUREMENT

A multi-layer, transient conductivity Green-Ampt infiltration model was evaluated and parameterised for a layered soil, using rainfall simulator data, The model simulates transient surface sealing and subsoil restrictions on infiltration under rainfall. Rainfall simulator plots (1 m(2)) were run on four soil conditions/layers of a cultivated clay soil and infiltration parameters were derived for each layer: (a) surface (freshly tilled), (b) surface seal (pre-crusted), (c) tilled layer and (d) subsoil. For this soil, runoff was generated by restricted infiltration into the subsoil when surface sealing was prevented and by surface sealing alone when there was a crust created by prior rain (even though this crust was dried and cracked). For freshly tilled bare soil, surface sealing controlled initiation of surface pending and subsequent infiltration, but at longer times infiltration was controlled by the subsoil. For freshly tilled soil, a single, consistent set of parameters were obtained for the layered soil system. These parameters gave good predictions of infiltration curves for all plots, including calibration and independent test plots, for a range of tillage depth, antecedent moisture and rainfall intensity, Surface seals reformed rapidly on the pre-crusted plots; infiltration curves were predicted well using a constant low or rapidly declining conductivity. Further research is needed on crusts formed by prior rain and their effect on infiltration parameters, as such crusts are common on bare clay soils except immediately after tillage. A large number of rainfall simulator studies are conducted for a range of purposes. These data should not be ignored as a source of parameters for hydrologic modelling.