Soil water storage and groundwater behaviour in a catenary sequence beneath forest in central Amazonia. II. Floodplain water table behaviour and implications for streamflow generation

Valley floor groundwater level data collected during the ABRACOS project (Gash et al. 1996), and published streamflow data from small forested catchments in geomorphologically similar areas nearby have been analysed to improve the understanding of the processes of streamflow generation. Early in the wet season, the floodplain water table is typically at 0.8 m depth, or less, and receives only local, vertical recharge. Large storms may create a groundwater ridge beneath the floodplain, temporarily creating a gradient in the direction of the hillslope. Later in the wet season, floodplain water levels are controlled primarily by the discharge of groundwater which maintains the dry season streamflow. The groundwater is recharged by deep drainage from beneath the plateau and slope areas once the dry season soil water deficit has been overcome. In the late wet season, the water level is almost at the floodplain surface and may create seeps on the lower slopes in very wet years. For the period 1966-1989, the recharge was estimated to range from 290 mm to 1601 mm with a mean of 1087 mm. Published data show that baseflow is 91% of annual runoff. Stormflow is generated on the floodplain, and water table recessions after rainfall events show that the runoff response depends on the depth to the water table. These results are from areas with deeply weathered and permeable soils; in areas of Amazonia with shallower soils, the predominant flow generation processes will differ (Elsenbeer and Lack, 1996).