Spatial patterns of suspended sediment yields in a humid tropical watershed in Costa Rica

Humid tropical regions are often characterized by extreme variability of fluvial processes. The Rio Terraba drains the largest river basin, covering 4767 km(2), in Costa Rica. Mean annual rainfall is 3139 +/- 419(sd) mm and mean annual discharge is 2168 +/- 492(sd) mm (1971-88). Loss of forest cover, high rainfall erosivity and geomorphologic instability all have led to considerable degradation of soil and water resources at local to basin scales. Parametric and nonparametric statistical methods were used to estimate sediment yields. In the Terraba basin, sediment yields per unit area increase from the headwaters to the basin mouth, and the trend is generally robust towards choice of methods (parametric and LOESS) used. This is in contrast to a general view that deposition typically exceeds sediment delivery with increase in basin size. The specific sediment yield increases from 112 +/- 11.4(sd) t km(-2) year(-1) (at 317.9 km(2) on a major headwater tributary) to 404 +/- 141.7(sd) t km(-2) year(-1) (at 4766.7 km(2)) at the basin mouth (1971-92). The analyses of relationships between sediment yields and basin parameters for the Terraba sub-basins and for a total of 29 basins all over Costa Rica indicate a strong land use effect related to intensive agriculture besides hydro-climatology. The best explanation for the observed pattern in the Terraba basin is a combined spatial pattern of land use and rainfall erosivity. These were integrated in a soil erosion index that is related to the observed patterns of sediment yield. Estimated sediment delivery ratios increase with basin area. Intensive agriculture in lower-lying alluvial fans exposed to highly erosive rainfall contributes a large part of the sediment load. The higher elevation regions, although steep in slope, largely remain under forest, pasture, or tree-crops. High rainfall erosivity (> 7400 MJ mm ha(-1) h(-1) year (-1)) is associated with land uses that provide inadequate soil protection. It is also associated with steep, unstable slopes near the basin mouth. Improvements in land use and soil management in the lower-lying regions exposed to highly erosive rainfall are recommended, and are especially important to basins in which sediment delivery ratio increases downstream with increasing basin area. Copyright (C) 2001 John Wiley & Sons, Ltd.