A soil water balance model for sloping land

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Abstract
Simulation models of soil water balance for flat land are not applicable to hill soils principally because on the latter sites significant runoff can occur before the soil reaches field capacity. Part of the data from regular measurements of topsoil (0-75 mm depth) moisture content over a 3-year period on the north aspect of a steepland yellow-brown earth soil was used to construct a simulation model which described changes soil moisture to 150 mm depth during the year. SImIlar data collected on a south aspect of the soil and also on north and south aspects of a yellowbrown loam hill soil were used to evaluate the model. A 4-layer model was developed in which the rate of soil rewetting was empirically limited according to soil moisture content and evapotranspiration rate was primarily soil-controlled. Predicted topsoil moisture levels provided similar, but generally lower values compared to actual levels especially during the summer to late winter period: The greater discrepancy during early spring could be attributed to the role of unaccounted-for subsurface flow downslope and/or rapid infiltration bypassing surface layers but contributing to actual moisture levels below the soil surface. Despite these limitations the modelling exercise enabled 2 major conclusIOns to be drawn which were not previously apparent. First, because of the low storage capacity of these soils the availability of moisture to pasture was highly dependent on rewetting frequency rather than total rainfall and, second, as a result of this, probably less than 50% of the total annual rainfall was involved in replenishing soil moisture at plantavailable depths.

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