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The actual evapotranspiration (E(a)) was followed from 1989 to 1992 in a thorn scrub stand in the northern Ferlo region (North Senegal, West Africa) for an Acacia tortilis grove, a Balanites aegyptiaca grove (every grove possessed its characteristic herbaceous layer) and a herbaceous zone outside the tree crown shade (annuals). E(a) was derived from the water balance equation method (neutron probe measurements). The soil water profiles (soil water content and potential plotted to soil depth) were established, giving an account of preferential water uptake zones and the rooting depths of trees and annuals.

The mean seasonal actual evapotranspiration of the thorn scrub was 1.92 mm day(-1) during the rainy season (period 1), 1.62 mm day(-1) during the 'deferred' season (period 2), 0.62 mm day(-1) during the cool dry season (period 3) and 0.09 mm day(-1) during the hot dry season (period 4). The daily maximal actual evapotranspiration of the groves was higher than that of the herbaceous zone and the Acacia tortilis grove's maximal daily actual evapotranspiration was higher than that of the Balanites aegyptiaca grove: 5.04, 4.15 and 3.98 mm day(-1) (period 1), 4.28, 3.76 and 2.89 mm day(-1) (period 2), 1.73, 1.79 and 1.34 mm day(-1) (period 3), 0.93, 0.46 and 0.36 mm day(-1) (period 4) for the Acacia tortilis and Balanites aegyptiaca groves and the herbaceous zone, respectively.

For soil depth to 3.5 m taken into account in the water balance equation, by the end of the growing year, soil moisture differed by 20.8 mm between tree and grassland zones in favor of grassland. Had this water been used by trees, this thorn scrub could have produced 13.72-0.8 kg DM ha(-1) year(-1) more wood.