Changes the watershed unit hydrograph response due to constructed small detention structures

Watershed management structures affect the hydrological outflow hydrograph components such as peak discharge, time to peak, base time and runoff volume. The unit hydrograph approach can be used to evaluate the effects of watershed management practices on hydrologic response, through comparison of unit hydrograph derived from recorded observed events before and after the management practices in the watershed. The hydrologic evaluation reports of watershed measures can be helpful in design improvement and implementation of flood control structures. The aim of this study was to evaluate the effect of small detention dams on watershed unit hydrograph response in Jafar-abad watershed (with an area of 10900 ha), Golestan Province. The 43 recorded rainfall-runoff events were analyzed before and after the mechanical measures and the Tc-duration unit hydrograph were derived. In this regard, the D-hr hydrographs were converted to Tc-hr hydrographs using S-curve method. Then the unit hydrograph components were obtained for pre and post construction of structural measures and hydrograph components were analyzed using t-test analysis and Box-and-whisker plots. The results showed an increase in the peak flow and base time of index unit hydrograph and also the time to peak of the unit response decreased due to constructed measures. While, the statistical analysis showed a significant difference (p-value<0.05) due to the effect of structural measures on unit hydrograph base-time component and increasing it up to 6 hours (on average) in the post construction period. Although the mean value of time to peak has reduced from 15.9 to 14.8 hrs, which was not statistically significant. In general, the watershed response and resultant hydrograph component of the watershed are affected due to the implementation and allocation of detention structures in the watershed.