RUNOFF GENERATION PROCESSES - RESULTS AND ANALYSIS OF FIELD DATA COLLECTED AT THE EAST CENTRAL SUPERSITE OF THE HAPEX-SAHEL EXPERIMENT

Within the scope of the HAPEX-Sahel experiment, the hydrological funct ioning of two small nested catchments was studied at two different sca les: the plot scale (of the order of 100 m(2)) and the catchment scale (0.2 km(2)). At local scale, four runoff plots were set-up on the typ ical soil surface conditions observed on the catchments (plateau bare soil, two plots on fallow grassland) and an additional one was install ed on a millet field. Soil moisture investigations at the plot scale h ave shown that infiltration was limited between 0.6 to 2 m deep on thr ee sites, but was deeper than 3.4 m on the most pervious site (millet) . The maximum water storage on all the sites was found to be reached a t the maximum activity of the rainy season (late August), and not at t he end of the season. During the dry months, the soil was fully dried off by evapotranspiration, resulting in the absence of inter-annual so il water storage. No influence of vegetation cover on runoff was obser ved on the fallow sites, but runoff generation was found to be very se nsitive to tillage on the millet field. The parameter P-u, calculated from a rainfall hyetograph and defined as the rainfall depth that can actually produce runoff, is shown to be relevant to compute runoff on untilled soils, as it explains more than 87% of the variance in runoff depth. On tilled soils, it is necessary to take into account addition ally the temporal evolution of the soil surface, especially the days a fter weeding operations. Simple linear relationships were derived to c ompute runoff depth from P-u on the plots for the most typical soil mo isture conditions observed, and modified SCS equations have been deriv ed for the catchments. Using the linear equations derived at the plot scale in a simple, empirical, semi-distributed model lead to formulate the assumption that the partial source area concept applied on the ca tchments. Analysis of discharge data at the catchment scale highlights that seepage through the bottom of a gully between two gauging statio ns leads to the abstraction of non negligible volumes of water. Moreov er, the water totally infiltrates in a spreading zone downstream from the outlet of the largest catchment showing that discontinuities occur in the surface water transmission within a catena. Such discontinuiti es constitute a major problem for the concern of aggregation of hydrol ogic precesses. (C) 1997 Elsevier Science B.V.