Effects of Uncertainty of Drainage Area on Low-Gradient Watershed Hydrology

An accurate estimate of a watershed drainage area contributing to stream flow is critical for reliable estimates of storm event hydrograph characteristics e.g., peak flow rate, outflow depth, runoff coefficient (proportion of rainfall as runoff) as well as water balance and pollutant load estimates. Peak flow rate, a hydrologic parameter widely used in stormwater management, design of channels and cross drainage structures and other BMPs, is calculated as a function of watershed drainage area and other parameters. However, in contrast with the upland watersheds with high gradient, currently available topographic maps and DEMs used to estimate the watershed drainage area are not adequate for flat, low-gradient landscape, which may even likely result in “imbalance” of water. Furthermore, with the construction of new roads and culverts (after the DEM is developed) in the watersheds, the runoff flow path is often changed and thus inaccurate runoff calculation. Above all, the low resolution digital elevation models (DEMs) are not accurate and they do not reflect the present ground reality as they are developed earlier and through other sources like Shuttle Radar Thematic Mapper (SRTM) data and surveying information. However, Light Detection and Ranging (LiDAR) based DEMs are more accurate for identifying actual topographic conditions. For coastal flat watersheds, recent high resolution, and accurate DEMs are very essential for hydrologic analysis. In this presentation, we explore a case study of a 3 order Lower Coastal Plain (LCP) watershed (Turkey Creek) at US Forest Service Santee Experimental Forest at headwaters of east branch of Cooper River draining to Charleston Harbor, SC to address potential errors in estimating the streamflow as area-based depth and water balance as a result of uncertainty in watershed drainage area estimated based on various methods since it was established in 1963. The results demonstrate a need for very high resolution DEMs developed with high resolution LiDAR data, with field verification to reduce errors and uncertainties in hydrologic studies of the LCP systems. INTRODUCTION Watersheds in lower coastal plain (LCP) region are generally characterized by flat, low-gradient landscapes with less than 1% slope. The region is dominated by a humid climate with long-term average precipitation exceeding potential evapotranspiration (PET) and thus the surplus soil moisture. About 60 percent (%) of the region is covered by forest ecosystems, including wetlands (Sun et al., 2002), where the regional long-term water balance includes 70-75% of average annual precipitation lost as evapotranspiration (ET). The remaining balance primarily as runoff occurs due to shallow surface and subsurface drainage. Water yield of this region generally depends on the precipitation and ET. Reliable and sustainable water yield from watersheds in the Southeastern Coastal Plain have become an area of concern in recent years because of changing population growth, land use, and potential climate change. To address this concern, there is a need for a reliable understanding of hydrologic processes and water balance of less disturbed, forested watersheds on low-gradient coastal plain (LCP) lands (Williams et al., 2012; Amatya and Skaggs, 2011; Callahan et al., 2011; Garrett et al., 2011; La Torre Torres et al., 2011; Amatya et al., 2009; Bosch et al., 1999). This reference water balance could be used to quantify the magnitude and potential change to water balance in the LCP due to the impacts of human and natural disturbances, which is important for economic development and land management practices. Monitoring and modeling approaches are often used to understand the processes and quantify the runoff, water balance, and pollutant loads (Amatya and Jha, 2011). However, there are challenges in accurately quantifying the runoff and subsequently the water balance of these LCP systems, which are often impacted by hurricanes and tropical storms, tidal surges, submergence of flow measuring structures, and backwater flows. Furthermore, much of the freshwater runoff carrying the pollutant loads in the LCP may occur during the summer-fall tropical storms potentially impacting the nearby nutrientsensitive estuarine ecosystem. Such problems will be exacerbated by potential impacts of climate change and