ASSESSING THE CHANGE IN RUNOFF RESPONSE IN A DEVELOPING SE PIEDMONT WATERSHED THROUGH THE UNIT HYDROGRAPH, UNIT IMPULSE RESPONSE, AND MANN-KENDAL STATISTICAL TREND TEST

In this study, the change in the runoff hydrology of five developing SE Piedmont sub-watersheds (10% to 54% land use change) with a combination of engineered stormwater control measures and stream restoration was evaluated with the unit hydrograph, unit impulse response, and Mann-Kendall trend test approaches. The runoff response from the first sub-watershed (10% developed) demonstrated a decreasing average baseflow and streamflow trends from the Mann-Kendall trend test approach and decline in peakflow discharge (-13.2% to -28.4%) from the unit hydrograph and unit impulse response analyses. The second sub-watershed with similar development but a 55% restored mainstem tributary indicated increase in peakflow discharge (3.1% to 9.5%) and direct runoff coefficient (3.9%) from the above analyses methods. The third sub-watershed with 50% development, constructed stormwater detention basins, and 70% restored stream reach demonstrated a decreasing trend in average baseflow and streamflow and decline in peakflow discharges (-12.0% to -7.8%). The forth sub-watershed with similar development and stormwater control detention indicated an increasing trend in average baseflow streamflow and decline in peakflow (-15.8% to -4.4%). The fifth sub-watershed, which drains the first three sub-watersheds, demonstrated a significant increase in the peakflow discharge (25.7% to 24.8) attributed to now more coincidental peakflow from the larger contributing sub-watersheds and increase in peakflow from the second sub-watershed. The three approaches applied in this study indicated similar runoff responses from the land development, stormwater detentions, and stream restoration. The decline in peakflow discharge and increase in direct runoff coefficient and runoff duration is attributed to the stormwater dentition basins in the most developed sub-watersheds whereas, little difference in runoff response could be attributed to the stream restoration projects.