The study of the effect of climate variability/ climate change on rainfall-runoff modeling is limited in humid tropical regions. Climate change has implications beyond water resources sector such as agriculture and fisheries. Hence, such studies are becoming increasingly important. This study makes use of both historical data acquired from the field as well as future forecasts of climate model data derived from GCM HADCM3. This data is further downscaled using HADRM3 with PRECIS software under the three QUMP (Quantifying Uncertainties in Model Projections) with a horizontal resolution of 25 X 25 sq. km for the state of Tamil Nadu by the Centre for Climate Change and Adaptation Research, Anna University Chennai. This downscaled data is used to study runoff changes due to climate change for the Kosasthaliyar sub basin situated in South India. A trend analysis of the hydro-meteorological data for the sub basin indicates that future rainfall is expected to decrease by about 10% while mean temperatures are going to increase by the year 2100. Runoff changes for 2011 to 2040 appear to indicate no change when compared to the historical period 1971 to 2000. This study is one of the first attempts to provide information on climate variability and its impacts on runoff for Kosasthaliyar sub basin.