Dynamics of sediment discharge in relation to land-use and hydro-climatology in a humid tropical watershed in Costa Rica

Hydrology in humid tropical regions is often characterized by considerable natural variability and uncertainty. Hydrologic and land-use data from the Terraba basin in Costa Rica are used to analyze dynamics in sediment discha rge processes during the period 1971-1993. Time-series of log transformed sediment concentration and flow are analyzed with a Bayesian dynamic linear regression model (DLM) to detect changes in the sediment-flow relationship over time. Annual time-series of estimated sediment discharge based on the DLMs were regressed against various hydro-c limatic variables based on the Southern Oscillation Index (Sol), rainfall, stream flow and rainfall erosivity. Hydro-climatic variables such as rainfa ll, stream flow and rainfall erosivity were also regressed against Sol. The results from the regression models, the presence of trends in the DLM slop e and other hydro-climatic parameters for each sub-basin were used to compa re the hydro-climatic and sediment response characteristics of the sub-basi ns. Over 60% of the variability in hydro-climatic variables like rainfall, rain fall erosivity and stream flow is explained by Sol but less than 40% for se diment discharge measured at the basin mouth. The clustering of stations an d sub-basins with respect to sediment discharge responses is different from that based on hydro-climatic characteristics. Stations or sub-basins corre sponding to areas with large rates of deforestation and/or agricultural int ensification and earthquake related soil disturbance tend to fall in one cl uster. These sub-basins are characterized by an increase in the sediment-fl ow regression slope over time, and weaker relationship between sediment dis charge and hydro-climatic variables such as SOI, rainfall and flow. In sub- basins undergoing rapid land-surface changes, variability in sediment suppl ies accounted for an estimated 50-90% of the variability in annual sediment discharge. The methods described in this paper can be used to analyze many existing hy drologic time-series to detect land-use effects on watershed hydrology. (C) 2001 Elsevier Science B.V. All rights reserved.