Acacia trees modify soil water dynamics and the potential groundwater recharge in savanna ecosystems.
The effect of increasing tree density on groundwater resources of semiarid landscapes is a topic of controversy. Since 2007, we have registered the soil water dynamics with field monitoring techniques on a commercial rangeland farm in the central Namibian thorn-bush savanna. Monitoring profiles are located below Acacia mellifera canopies, in the intercanopy area, and on a de-bushed grassland. Here we demonstrate (1) an increase in soil moisture larger than precipitation at some rain events, interpreted as water run-on resulting from surface ponding; (2) an overall reduction in water infiltration in the below-canopy area of A. mellifera compared to the intercanopy space; and (3) a faster drying of the soil in the below-canopy space because of root water uptake. These processes resulted in a potential for deep drainage about threefold larger in the intercanopy space than in the area below the canopy. Thus, increasing bush encroachment is likely to reduce groundwater recharge and should be validated by an interdisciplinary analysis of hydrogeologists, soil scientists, botanists, and farm managers.