Determining sediment source areas in a tropical river basin, Costa Rica

Four hydrologic events were studied at three gauging stations in the Pejibaye river basin, Costa Rica. The sediment sources were determined by comparing the timing of the sedigraph at the gauging station and the timing of rainfall in the catchment with addition of time for the new water to flow from different land use districts along the main rivers to the gauging stations. The rainfall was recorded in the basin upstream of Pejibaye gauging station, which was, therefore, the only station where the sedigraph/rainfall method could be properly used for determining sediment sources. On three occasions, the sediment discharges at the Pejibaye station were of about equal height. The sedigraph/rainfall method indicated that, on two of these occasions, the sediment came mainly from the pasture and sugar districts, and in one case from the forested middle basin. On the fourth occasion, rainfall amounts and intensities were locally high in the forested upper basin, but water and sediment discharges were small. sion of material deposited in the channel increased the width of clockwise sediment concentration/discharge loops. In one storm, the concentration/discharge loop changed from an eight-shaped loop at the upper station to a counter-clockwise loop at the downstream station, explained by the lagging of the sediment after the water.