Application of empirical methods in estimation of surface runoff is a must in ungauged watersheds.
The runoff coefficient is one of the important input variables in empirical models. Proper calculation of
runoff coefficient and study on its variation in different conditions facilitates proper estimation of runoff
and flood peaks. So far, many studies have been conducted for estimation of runoff coefficient on
monthly, seasonal and annual bases. However, limited researches have considered the variation of runoff
coefficient during a storm event. The present study was therefore formulated to study on the intra
variation of runoff coefficient and its affecting factors in Bar Neyshabour watershed comprising an area
of 11388ha. Towards this attempt, 32 storm events, with duration of equal to or more than watershed
concentration time of 2.9h and significant changes in hydrographs were selected from 440 events
recorded during 1980 to 1989 and corresponding hydrographs and hyetographs were collected. The mean
runoff coefficient and associated quarterly and deciles were also calculated. The relationships between
different rainfall and excess rainfall with mean runoff coefficient and its components was then analyzed
with the help of bivariate and multivariate regressions and applying ordinary and transformed data. The
results of the study verified a dynamic drastic change from 0.6 to 45.7% in runoff coefficient with
respective arithmetic, median, antilogarithm mean and standard deviation of 10.65, 7.96, 7.59 and 9.35%.
In general, the results of the study also revealed the inter changes of runoff coefficient during storm
events and feasibility of its simulation by using bivariate regression with correlation coefficient from 28 p0.68). It was also found that the 2nd and 3rd quartiles could estimate the runoff
to 88% (0.003
coefficient variability better than other rainfall components in the study watershed
