Changes in soil hydraulic properties, soil moisture and water balance in Acacia senegal plantations of varying age in Sudan

The effects of Acacia senegal trees on soil moisture (SM) and hydraulic properties in relation to plantations age and associated changes in soil organic carbon (SOC) were investigated and compared to grasslands at two sites in Sudan. Soil hydraulic properties were computed using pedotransfer functions based on texture and SOC, and SM measured using TDR. The measured SM data was used to parametrize a simple daily water balance model in which Hortonian runoff was calculated using the SCS runoff curve number (CN) method and evapotranspiration calculated using crop coefficients, Kc, adjusted to seasonal values using NDVI. Measured SM was higher in the grasslands than plantations, but increased with plantation age, reflecting a similar trend in plantation SOC and plant available water capacities. The modelling resulted in lower runoff from the plantations, increased infiltration, evapotranspiration, reduced drainage and lower SM, as shown by measurements. Greater SM contents in the grasslands were attributed to lower evapotranspiration and resulted in greater drainage fluxes compared to the plantations. The study highlighted the need for more empirical studies on the effect of tree density and cover on rainfall-runoff relationships, infiltration, evapotranspiration and drainage in drylands, especially those of the drier parts of semi-arid Africa.