Estimated impacts of soil degradation on the African water balance and climate

This study uses a well-established water balance methodology, the Thornthwaite-Mather approach, to evaluate the effects of soil water holding capacity assumptions on estimates of African evapotranspiration rates, moisture deficit, and moisture surplus conditions. Under constant climate conditions, the model tests the impact of using a constant 150 mm soil water holding capacity compared to using a newly derived soil water holding capacity data set (the Dunne-Willmott data set). The study also uses a worldwide survey of soil degradation between 1950 and 1980 (GLASOD: Global Assess- ment of Soil Degradation) to evaluate the impacts of human-induced soil degradation on local water balances. The GLASOD data are used to alter local soil water holding capacities based on the Dunne- Willmott data to simulate human soil degradation patterns in Africa. Results indicate that the use of simplified soil water holding capacities can lead to significant errors in estimated evapotranspiration rates and water surplus and deficit conditions in Africa. Regions most affected are those with seasonal wet and dry climates, which are also those locations with the greatest climate variability. Because these are often the climates most studied to detect and model environmental change, it is important that accurate soil moisture estimates be used to simulate climate conditions in these regions. Results also indicate that soil degradation occurring over a 30 yr period (1950 to 1980) has had a significant impact on local water resources. The greatest impacts of these changes are in some of the more productive agricultural areas in the wetter sub-humid climates. Changes include increased runoff during wet seasons and an extended drought period during the dry seasons. Given that these agricultural systems have less flexibility to respond to long-term desiccation as compared to pastoral systems, this could lead to significant changes in local growing seasons and perhaps overall productivity in the future.