The effect of geomorphology on the main parameters of Rainfall- runoff models from a given watershed was considered in many studies. Drainage network analysis can be useful for determine these relations. This case study tries to determine the best-fitted probability distribution models to the Strahler stream lengths. Theoretical Strahler stream length distribution is derived based on Poisson process. Then, for Chehel Chai basin, the stream network, stream order and stream length were extracted from a digital elevation model by Arc GIS 9.3. Suitability of the derived length distribution was performed by Statistical analysis. The number, mean and standard deviations of First and second order streams were 1227 and 289 respectively, mean of these were 456.2 and 634 m and standard deviations were 270.7 and 509.5 respectively. Results show that the length distribution of the first order streams in Chehel Chai basin can be modeled well by Erlang distribution (3p) and for second or higher order streams, gamma distribution recognized as the better model. The results of this study can be useful for runoff modeling, determine the time parameters, in ungaged watershed.
