Water availability is a critical limiting element in semi-arid ecosystem productivity and presents particular challenges in Sahelian countries such as Senegal. The landscapes are characterized by the presence of two common semi-arid shrubs (Piliostigma reticulatum and Guiera senegalensis), both of which may be important hydraulic regulators in these water-limited ecosystems. Dry season observations revealed higher moisture levels in soil surrounding the shrub shallower roots relative to bare soil. This observation led us to hypothesize that these shrubs may participate in a natural irrigation phenomenon termed as "hydraulic redistribution" (HR). This dissertation reports on three studies performed to ascertain the existence of and investigate the characteristics of this hypothesized phenomenon. The first study investigated seasonal variability in soil water and shrub root patterns. Soil moisture content declined steadily in the 0.9-1.2 m depth range and increased in the 0.2-0.4 m depth range, which supports the HR hypothesis. The second study quantified plot scale water balance fluxes. Notably, shrub water uptake from the water table served as a crucial contribution to the system water balance. The third study investigated the magnitude and dynamics of HR during the dry season. Crop-shrub associations were evaluated from measurements of soil moisture and potential, root sap flow and plant physiological measurements. This study proved the existence of HR and quantified the HR magnitude (~0.1 mm d⁻¹). For shrubs and annual crops in close association with shrubs, HR clearly provides a mechanism for drought-stress avoidance and maintenance of plant physiological functions. At the landscape scale, the interplay between shrub root morphology and HR may play a vital role in ecosystem function with practical implications for nutrient cycling and water balance in arid ecosystems. / Graduation date: 2006