Dynamics of sediment discharge in relation to land-use and hydro-climatology in a humid tropical watershed in Costa Rica Jagdish Krishnaswamy * , Patrick N. Halpin, Daniel D. Richter Nicholas School of the Environment, Duke University, Durham, NC 27708, USA Received 9 August 1999; revised 18 May 2001; accepted 11 June 2001 Abstract Hydrology in humid tropical regions is often characterized by considerable natural variability and uncertainty. Hydrologic andland-usedatafromtheTerrababasininCostaRicaareusedtoanalyzedynamicsinsedimentdischargeprocessesduringthe period 1971±1993. Time-series of log transformed sediment concentration and ¯ow are analyzed with a Bayesian dynamic linear regression model DLM) to detect changes in the sediment±¯ow relationship over time. Annual time-series of estimated sediment discharge based on the DLMs were regressed against various hydro-climatic variables based on the Southern Oscillation Index SOI), rainfall, stream ¯ow and rainfall erosivity. Hydro-climatic variables such as rainfall, stream ¯ow and rainfall erosivitywerealsoregressedagainstSOI.Theresultsfromtheregressionmodels,thepresenceoftrendsintheDLMslopeand otherhydro-climaticparametersforeachsub-basinwereusedtocomparethehydro-climaticandsedimentresponsecharacter- istics of the sub-basins. Over60%ofthevariabilityinhydro-climaticvariableslikerainfall,rainfallerosivityandstream¯owisexplainedbySOIbut less than 40% for sediment discharge measured at the basin mouth. The clustering of stations and sub-basins with respect to sedimentdischargeresponsesisdifferentfromthatbasedonhydro-climaticcharacteristics.Stationsorsub-basinscorrespond- ing to areas with large rates of deforestation and/or agricultural intensi®cation and earthquake related soil disturbance tend to fall in one cluster. These sub-basins are characterized by an increase in the sediment±¯ow regression slope over time, and weaker relationship between sediment discharge and hydro-climatic variables such as SOI, rainfall and ¯ow. In sub-basins undergoingrapidland-surfacechanges,variabilityinsedimentsuppliesaccountedforanestimated50±90%ofthevariabilityin annual sediment discharge. Themethodsdescribedinthispapercanbeusedtoanalyzemanyexistinghydrologictime-seriestodetectland-useeffectson watershed hydrology. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Hydrology; Sediment discharge; Dynamic Linear model; ENSO; Land-use change; Costa Rica 1. Introduction The sediment response of catchments is controlled by a complex function of ecological, climatic, and geomorphic processes. The ¯uvial regime of seasonally dry tropical basins is very dynamic due to intra-annual and inter-annual variability. The task of detecting and analyzing the impact of recent Journal of Hydrology 253 2001) 91±109 0022-1694/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII:S0022-169401)00474-7 www.elsevier.com/locate/jhydrol * Corresponding author. Present address: Ashoka Trust for Research in Ecology and the Environment ATREE), Post Box 2404, HA Farm Post, Hebbal, Bangalore 560024, India. Fax: 191-80-3530070. E-mail addresses: jug@duke.edu J. Krishnaswamy), jagdish@ atree.org J. Krishnaswamy).