Towards an improved understanding of the soil water balance of sloping land under pasture

Soil water content and surface runoff data for a hill country soil under pasture in the Tararua district are reported, showing significant water extraction down to at least 350 mm depth, and suggesting repellency-induced runoff at certain times of the year. This paper provides a soil water balance model, including a simplified version of the innovative way in which previous workers modelled repellency-induced runoff. That work provides the only other detailed analysis of the water balance of New Zealand hill country under pasture. The model presented here suggests that between 65 and 80% of rainfall contributed to available soil moisture and was evaporated by pasture. These results contrast with those of the other researchers in that their water balance model assumed a much shallower rooting depth of 150 mm, implying that hill country soils have a small moisture storage capacity and that probably less than 50% of annual rainfall replenishes root zone soil moisture.