HILLSLOPE DYNAMICS OF ON-FARM GENERATION OF SURFACE-WATER FLOWS - THECASE OF RAIN-FED CULTIVATION OF PEARL-MILLET ON SANDY SOIL IN THE SAHEL

Results from on-farm measurements of surface overland flow of both run on (R-on) and runoff (R-off) are presented for a continuously cultivat ed pearl millet (Pennisetum glaucum (L.) R.Br.) field, along a charact eristic catena, in the Sahel (Niger). Despite the deep sandy soil alon g the studied hillslope, classified as a Typic Haplustult, non-negligi ble depths of R-off were measured during two rainy seasons: 12-13% of total rainfall for the upslope position and 6-8% for the downslope pos ition. This was explained by the presence of surface crusts with low i nfiltrability. Upslope, more than 30% of the soil surface was covered with erosion crusts (runoff coefficient (K-r) = 60-70% for moist soil conditions) during 40% of the rainy season. The mean coverage of erosi on crust for two rainy seasons was 26% upslope, compared with 11% and 15% midslope and downslope, respectively. Weeding, carried out manuall y with hand hoes, had a very positive effect on soil infiltrability. T he destruction of surface crusts resulted in zero R-off (K-r = 0) for rainstorms directly following weeding operations. However, surface cru sts were quickly re-established as a result of the erosive effect of t he following storms. Large volumes of runon (R-on) water were measured at the upslope limit of the field (14.4 m(3) m(-1) in 1995, correspon ding to a rainfall depth of 47.5 mm if redistributed over the 8.5 ha m illet field). Small volumes were registered downslope (R-on = 0.75 m(3 ) m(-1)), indicating redistribution of R-on along the hillslope. The R -on originated from the upstream degraded fallow zone and the adjacent sparsely vegetated plateau, which in effect functioned as water-harve sting zones.