ENERGY AND WATER BUDGETS OF AN AREA OF PATTERNED WOODLAND IN THE SAHEL

Measurements of radiation, energy and water budgets were made in an ar ea of patterned woodland in the Sahel. This area consists of strips of dense vegetation separated by areas of completely bare soil. Details are given of the instrumentation and of the procedure used to weight t he radiation measurements to the fetch of the flux measurements. Durin g the wet season the total evaporation was 3-4 mm day-1. At the end of the wet season the total evaporation declined from about 4 mm day-1 t o 1.5 mm day-1 over an 8 week period. Analysis of the entire 1990 rain y season suggests that, during that relatively low rainfall year, most of the rainfall was re-evaporated, leaving little for runoff or groun dwater recharge. An estimate of an upper limit on the soil evaporation suggests that the measured rate of evaporation from the whole system can only be maintained if the vegetation, covering only about 40% of t he surface, is able to use at least 70% of the rainfall. It appears, t herefore, that the bare soil areas act as rainfall collectors for the vegetation and are essential if the vegetation is to survive in its pr esent vigorous form.