The effect of rainfall pattern on changes of time to runoff and runoff coefficient at plot scale

Determination of time to runoff and runoff volume in watershed response management against rainfall are the key parameters in watershed system management. Among different factors of effect on time to runoff and runoff volume, the rainfall intensity is one of the most important factors. In this study, the effect of rainfall pattern on variables of time to runoff, runoff volume and coefficient was studied under simulated rainfall in the southeastern Noshahr city, Mazandaran province. To achieve the study purposes, four rainfall patterns, each with three changes in rainfall intensity (I: Low-Medium-High, II: Low-High-Low, III: High-Medium-Low, IV; High-Low-High) were simulated. Then, the time to runoff, runoff volume and runoff coefficient were measured for each rainfall pattern. The measured data were analyzed in Excel and SPSS 23 softwares to determine the relationship between the rainfall pattern and variables of time to runoff, runoff volume and runoff coefficient. The results showed that there was a significant difference (P≤0.05) in variables time to runoff, runoff volume and runoff coefficient resulted rainfall patterns. Also the results showed that, in all of rainfall patterns between mean amounts were the significant differences on time to runoff, runoff volume and runoff coefficient in level of 99 percent. The results showed that the rainfall pattern I had the longest time to runoff and rainfall pattern III had the largest amounts of runoff volume and runoff coefficient. The average time to runoff in rainfall patterns of I, II, III was 5.90, 4.24, 0.71 and 1.25 min, respectively. Also, the average runoff coefficient in rainfall patterns of I, II, III and IV measured 30.03, 49.63, 88.82 and 75.16 percent, respectively.