Dry season water use patterns under Guiera senegalensis L. shrubs in a tropical savanna

The extent of rooting and the water use of Guiera senegalensis bushes at a fallow savanna site in south-west Niger were investigated. Low root length densities were found up to the maximum sampling distance (8 m horizontally and 2 m vertically) from the base of the bushes. Changes in soil water content of the soil profile were monitored using a neutron probe at seven locations over a 20 m transect between two G. senegalensis bushes. During the 1993/94 dry season, a progressive drying front was observed moving away from a G. senegalensis bush both horizontally (up to 10 m) and vertically (up to 4 m). Mean water loss from the top 4 m of the soil profile over the 8-month dry season was 151 mm. This was attributed to water use by G. senegalensis, and comprised 28% of the 1993 total annual rainfall. Clearing fallow savanna land for millet production will result in increased deep drainage partly through reduced dry season water use by the deep rooted bushes