Using a physically based model to conduct a sensitivity analysis of subsurface lateral flow in south-east Australia

Break of slope tree plantations are intended to intercept surface and subsurface lateral flow (SLF) on hillslopes and help alleviate the water imbalance in the agricultural landscapes of south-east Australia. More information on the occurrence of SLF in this region is required to assist the efficient identification of potential plantation sites. In this article, HILLS, a two-dimensional physically based model, is used to examine the sensitivity of SLF to rainfall, soil, and topographic attributes. The most influential soil property was the depth of the impeding layer. Gradient within 33 m above where SLF is determined was the topographic criteria that most effected SLF, regardless of the overall topographic shape. It was clear that as the amount of annual rainfall increased, higher hillslope gradients were required to move the excess water as SLF. Cumulative errors in the model water balance created uncertainty about the accuracy of the results, however, the general trends seem reliable. SLF did not account for a significant proportion of rainfall on the hypothetical hillslope considered. Field evidence suggests that more SLF occurs in the study region than is suggested from these results. Soil, topographic and rainfall properties alone may not be adequate to explain the occurrence of this type of flow, and the presence of a watertable may also be critical.