A Simple Model for Flow on Hillslopes

A simple hillslope hydrological model predicting water movement on sloping shallow soils is developed. Unlike existing hillslope models, the new model can be used to predict flow for long records of climatological data and it can be easily extended to include preferential flow. It compares favorably to other models with more restrictive input requirements. Its application is illustrated for experimental data collected for an homogeneous hillslope at the Cooweeta Hydrological Laboratory. INTRODUCTION Hillslopes in forest and nature reservations have an interconnecting network of macropores, animal burrows and other pores which can conduct large amounts of water. For this reason the Hortonian surface runoff mechanism (i.e. runoff occurs when the rainfall intensity exceeds the infiltration capacity of the soil surface) has seldom been observed (Hewlett and Hibbert, 1965; Dunne, 1983; Ward, 1984). Subsurface flow, especially piping, quick return flow or interflow, is particularly significant in undisturbed forest watersheds as a source of water for streams (Corbett, 1979). The surface organic matter maintains high surface permeabilities (not allowing surface crusting by rain drop impact) and the upper soil profile contains numerous pathways for rapid water flow. If an impermeable layer is reached by percolating water, water moves downhill towards the hillslope bottom. There are also many other observations of preferential flow on hillslopes (Whipkey, 1965; Retezer, 1963; Hewlett and Hibbert, 1965; Dunne and Black, 1970; and last but not least Lawles et al., 1882). Various algorithms, ranging from simple analytical models to complex finite element models which may include macropore flow, have been developed for describing the saturated~unsaturated flow on hillslopes. Analytical models were developed by Stagnitti et al., (1986) and Hurley and Pantelis (1985). These do not include a mechanism to assess the effect of macropores and only can be carried out for one storm at a time. Finite difference and finite element models developed by Nieber and Walter (1981), Nieber (1982), Beven (1981), Sloan and Moore (1984) in theory can be run for an infinite time but the practicality of computer time and cost usually reduces their applicability to a storm period. The purpose of this paper is to present an alternative hydrologic model which, unlike others, can determine the drainage and moisture 0378-3774/88/$03.50 © 1988 Elsevier Science Publishers B.V.