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

Humid tropical regions are often characterized by extreme variability of fl uvial processes. The Rio Terraba drains the largest river basin, covering 4 767 km(2), in Costa Rica. Mean annual rainfall is 3139 +/- 419(sd) mm and m ean annual discharge is 2168 +/- 492(sd) mm (1971-88). Loss of forest cover , high rainfall erosivity and geomorphologic instability all have led to co nsiderable degradation of soil and water resources at local to basin scales . Parametric and nonparametric statistical methods were used to estimate se diment yields. In the Terraba basin, sediment yields per unit area increase from the headw aters 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 vi ew that deposition typically exceeds sediment delivery with increase in bas in 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 in dicate a strong land use effect related to intensive agriculture besides hy dro-climatology. The best explanation for the observed pattern in the Terra ba 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 observ ed 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 so il protection. It is also associated with steep, unstable slopes near the b asin mouth. Improvements in land use and soil management in the lower-lying regions exp osed to highly erosive rainfall are recommended, and are especially importa nt to basins in which sediment delivery ratio increases downstream with inc reasing basin area. Copyright (C) 2001 John Wiley & Sons, Ltd.