Semi-arid pastoralist societies are strongly coupled to the dynamics of vegetation and water by frequent herd relocation to areas with “green grass”. These societies are increasingly sedentary, which limits herd migration. Reduced ranges, especially when coupled with climate change, could have serious impacts on local vegetation and the sustainability of pastoralist lifestyles. Differences in available water and nutrients between canopy and intercanopy spaces have long been recognized in semi-arid landscapes. The current experimental study uses a suite of soil moisture TDR probes, runoff plots and rain gages to characterize general differences in water balance and soil moisture in four specific patch types: bare soil, open grass, Acacia tortilis, and A. mellifera. We quantify patch-specific dynamics using the total soil moisture response to individual precipitation events and subsequent decay rates over time.
In general, bare patches are drier and have higher runoff than vegetated patches. The patch-scale differences in soil moisture are strongest in the upper 15cm and attenuate with depth. Bare patches exhibit the largest dynamics in terms of soil moisture increase after precipitation and faster decay rates during dry down, and open grass patches are typically drier than tree patches. Unexpectedly, the estimated dynamics between open grass and A. mellifera were very similar, which may be due to consistent fine root distributions observed for each of these patch types. The initial soil moisture response of A. tortilis is greater than A. mellifera at 15cm; however, the rate of water use inferred from soil moisture decay rates is smaller in A. tortilis than A. melifera.. The roots of A. tortilis are shallow wide spreading tubes with a deep tap root. Various water use strategies between species may affect spatial and temporal niches with changing climates and remains to be studied. Mass balance of individual storms and lagged responses of several days in the response of deeper soil moisture to rainfall events indicate that subsurface flows may be significant despite low infiltration rates. A preliminary modeling activity suggests that macropores (termites) could have a substantial effect on subsurface soil moisture redistribution. The spatial and temporal extent of macropore impacts on plant available water and overall impact on landscape structure and function remains an area of ongoing research.