A Derived-Distribution Approach to Estimating Daily Loads of Sediment in Coastal Plain Streamflow

Standards for allowable total maximum daily loads (TMDLs) of sediment are being assigned to selected streams and rivers across the US. Estimates of total daily loads (TDLs) of sediment are required for comparison with TMDLs for those streams which are deemed to have impaired water quality due to sediment, and are required for formulating plans to improve these streams. A TMDL is defined as a daily stream loading rate (e.g. kg day-1). However, actual stream sediment data are collected in terms of concentrations (e.g. mg L-1), thus requiring estimates of streamflow to make the necessary rate and load conversions. The USDA-ARS, Southeast Watershed Research Laboratory (SEWRL) in Tifton, GA has collected over 30 years of hydrologic and climatic data from the 334 km2 Little River Watershed (LRW). The LRW is typical of heavily vegetated, slow-moving stream systems of the southeastern Coastal Plain. Field studies quantifying dissolved and suspended loads in LRW streamflow have been conducted during the 30 year period (1974-1978, 1984-1986), but a continuous record of measured loads does not exist. A “derived distribution ” (DD) approach is being tested to estimate TDLs of total solids (TS) and suspended sediment (SS) loads in LRW streamflow. A “derived distribution ” (DD) is the frequency distribution of the dependent variable that is “derived ” from the distribution of independent variables through a monotoni