Effects of forest conversion on soils and hydrologic processes in the Terraba Basin of Costa Rica

How forest conversion and land-use change affects soil and hydrologic processes in the humid tropics are poorly quantified compared with changes in temperate ecosystems. The effects of land-use changes on soil properties, basin sedimentation, and hydrologic processes in the Terraba River basin of Costa Rica was the focus of this study.; Soils under forest, pasture and agricultural crops were sampled to a depth of 120 cm on alluvial fans and terraces in an incomplete block design. Forest conversion to pasture decreased soil carbon by 10--20 Mg ha --1, and in the upper 30 cm, increased surface (0--30 cm) bulk densities from 0.81 to 1.04 g cm--3 and decreased water-stable aggregates >2 mm by 36.5%. Compared to forest, pasture soils elevated pH (4.60 to 4.96) and effective cation exchange capacity or ECEC (2.82 to 4.20 cmolc kg--1) throughout the upper 120 cm. Forest clearing and burning of forest biomass followed by conversion to pasture reduced soil carbon but increased the ability of these soils to retain cationic nutrients due to the effect of pH-dependent ECEC.; To evaluate spatial patterns of controls on sedimentation several GIS and statistical approaches were used to describe sediment yield, rainfall erosivity, and land use. In contrast to many descriptions of spatial patterns of sediment transport, specific sediment yield in the Terraba basin increases with basin area, progressing from 112 +/- 11.4sd tons km --2 yr--1 to 404 +/- 141.7sd tons km--2 yr--1 from upper stream lengths to the basin mouth. This trend is attributed to a spatial interaction of land use and rainfall erosivity, and not to spatial patterns in soils, geology, topography or rainfall amount.; To evaluate the dynamic effects of roles of changing land-use and hydro-climatology on hydrologic and sediment response, Bayesian dynamic linear regression models were used together with remote sensing land-use data. Overall, 32% of the variability in annual sediment discharge from the basin in 1963--88 was attributed to changes in sediment supply related to land-use changes. The remaining 68% was attributed to ENSO related hydro-climatologic fluctuations. Inspite of a decreasing annual trend in rainfall, in sub-basins undergoing rapid land-surface changes, changes in sediment supplies accounted for up to 90% of the variability in annual sediment discharge in 1971--88. Increases in frequency and magnitude of peak flows (>90th percentile), and the monthly flow-rainfall coefficient were also observed. In sub-basins with small to moderate land-use changes, up to 85% of the annual sediment discharge variability in 1971--88 was controlled by hydro-climatology.; The ability to detect land-use effects on hydrology and sedimentation in the humid tropics against a background of high natural hydro-climatic variability requires the use of statistical techniques that explicitly account for these controlling factors.