Runoff and soil loss from bench terraces. 1. An event‐based model of rainfall infiltration and surface runoff

Summary Overland flow resulting from an excess of rain over infiltration is an essential component of many models of runoff and erosion from fields or catchments. The spatially variable infiltration (SVI) model and a set of associated equations relating depth of runoff and maximum rate of ‘effective’ runoff (as used in the GUEST erosion model) to storm depth, depth-averaged intensity of rainfall, average maximum infiltration capacity and an additional amount of initial infiltration were validated and tested on back-sloping bench terraces in volcanic upland West Java, Indonesia. Data used were runoff rate and depth from 31 small (1.0–8.2 m2) bounded plots representing sections of terrace beds or risers and from six larger (53–231 m2) terrace units with hydrologically defined boundaries. Modelled runoff rates using rainfall intensity data corresponded well with observed patterns and the storm-based equations were used successfully to model runoff depths and maximum effective runoff rates for individual events. Resulting values for maximum average infiltration rate (Im) varied between 18 and 443 mm hour−1 and reflected effects of vegetation or mulch cover and soil compaction. We conclude that the SVI model and the derived equations provide a robust and accurate method for predicting runoff at the investigated scale.