Having a long record of stream flow is very valuable in planning water resources development projects.
However, in many occasions, stream flow records are available for very short periods though very long rainfall
records are available. Therefore, possibility to relate rainfall over a catchment to the stream flow at its outlet will
enable having a long record of stream flow. Besides prediction of stream flow using already available predicted
rainfall will permit taking precautionary measures in water related disaster situations such as floods and
droughts. This paper presents a research carried out to find a model to predict daily stream flow of Kalu River at
Ratnapura. The model, a non-linear regression model based on Marquardt’s procedure, was developed using
measured daily stream flow at Kalu River at Ratnapura and daily rainfall at eight rainfall gauging stations within
the catchment above Ratnapura. Data for the period 1987-1994 were used for the calibration of the model while
data for the period 1995-2000 were used for verifying it. The model was validated using Nush-Sutcliffe
efficiency and pseudo R2 . Nush-Sutcliffe efficiency (78%) and pseudo R2 (85%) show the possibility of the
fitted model in predicting daily stream flow of Kalu River at Ratnapura
