A DERIVED-DISTRIBUTION APPROACH TO DAILY LOADS OF SEDIMENT IN COASTAL PLAIN STREAMFLOW

Authors: Hubbard, Robert; Sheridan, Joseph; Bosch, David - Dave

Submitted to: First Interagency Conference on Research in the Watersheds
Publication Type: Proceedings
Publication Acceptance Date: 7/1/2003
Publication Date: 11/1/2003
Citation: Hubbard, R.K., Sheridan, J.M., Bosch, D.D. 2003. A derived-distribution approach to daily loads of sediment in coastal plain streamflow. First Interagency Conference on Research in the Watersheds. In: Renard, Kenneth G., McElroy, Stephen A., Bburek, William J., Canfield, H. Evan and Scott, Russell L., eds. First Interagency Conference on Research in the watersheds. October 27-30, 2003, United States Department of Agriculture, Agricultural Research Service, p.. 595-600.

Interpretive Summary: Excessive amounts of dissolved and particulate matter in streamflow degrade water quality. The amount of dissolved and particulate matter transported in stream systems depends on geology, soils, vegetative cover, topography, and agricultural management practices, as well as climate. Total maximum daily loads (TMDLs) of sediment must be assigned to watersheds across the US to quantify total assimilative daily loads and maintain water quality, to identify streams with impaired water quality, and for formulating plans to improve water quality. Mathematical methods are needed to take limited information on actual sediment concentrations and stream flow rates to predict TMDLs for streams. The USDA-ARS, Southeast Watershed Research Laboratory in Tifton, GA has collected over 30 years of hydrologic and climatic data from the 334 km2 Little River Watershed (LRW). In addition, short term studies were conducted to determine total solids and suspended sediment in LRW streamflow. Using a distribution of flow rates calculated from the 30+ years of hydrology records plus mean concentrations for total solids and suspended sediments predictions were made of TMDL. TMDLs of suspended sediments were predicted to range from 125 kg day-1 for flows with a 99% probability of occurrence, to 11,845 kg day-1 for flows with a 50 % probability of occurrence. This research is important because it provides land managers and regulators with a tool for predicting TMDLs based on risk probabilities calculated from actual flow rates and sediment concentrations.

Technical Abstract: Estimates of allowable total maximum daily loads (TMDLs) of sediment must be developed for selected streams and rivers across the US. The sediment TMDLs are required to quantify total daily assimilative loads 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. A 'derived distribution' (DD) is the frequency distribution of the dependent variable that is 'derived' from the distribution of independent variables through a monotonic functional relationship between the dependent and independent variables. 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 DD approach is being used to estimate total maximum daily total solids and suspended sediment loads in LRW streamflow. This paper presents results of coupling a derived flow distribution with mean total solids and suspended sediment concentrations to estimate TMDLs for the LRW of the southeastern Coastal Plain.