Effects of mean annual rainfall on landscape-level distributions of soil moisture and soil development in a semi-arid ecosystem

Landscape-level distribution of soil moisture is fundamental to vegetation pattern and pedogenic development in semi-arid ecosystems. We examine the temporal dynamics of soil moisture specific to under-canopy and inter-canopy areas along the Kalahari Transect (KT) in southern Africa, which spans a mean annual rainfall gradient from ~1000 mm/yr to ~250 mm/yr, all on an aeolian sand formation. At four sites along the KT, under- canopy and inter-canopy portions of the landscape were instrumented with capacitance probes at four separate depths for continuous measurements of volumetric soil moisture. The influence of tree canopies on surface water balance, through interception, and on energy balance, through radiation attenuation, had the net effect of leaving inter-canopy areas with more moisture shortly following rain events. After this initial period, however, the below-canopy areas were found to have relatively higher soil moisture due to reduced bare-soil evaporation. The degree of contrast between the long-term averages of inter- and below-canopy soil moisture was found to be dependent upon rainfall frequency. These long-term effects were manifest in the observed depths to the carbonate-rich layers, which were consistently deeper for the below-canopy positions on the landscape. This contrast, however, was found to decrease with increasing mean annual precipitation along the transect. A one-dimensional Richard's equation model was developed and applied to the separate landscape classes. Long-term simulations examine the effects of mean rainfall on vertical and lateral soil water distribution, and the results are used to interpret observations of both soil horizon development and tree clustering in this semi-arid environment.