COMPARISON OF LOAD ESTIMATION METHODS FOR CALCULATING TOTAL MAXIMUM DAILY LOAD (TMDL) IN AGRICULTURAL WATERSHEDSResearch Day
IntroductionWaterbodies that are too polluted to meet established water quality standards are designated specific maximum amounts of pollutant that the waterbody can receive and still be considered safe for designated uses, known as Total Maximum Daily Loads (TMDL). Regulating TMDLs requires estimating pollution load which can be difficult since pollutant concentration can often only be measured on a biweekly or monthly basis. Some of the most established methods for calculating load use an averaging technique; however, this method relies on a normal distribution of flow, which is often not the case for agricultural watersheds where flows consist primarily of irrigation runoff and are determined by human activity.
PurposeThe objective of this study was to compare the accuracy of four established load estimation methods (volume-load, mean-load, daily mean-load, and instantaneous-load) and one novel method (median load) in calculating pollutant loads from agricultural watersheds.
MethodLoad of total dissolved solids (TDS) in ten agricultural watersheds located in the California central valley was determined using discrete concentration measurements and each of the five estimation methods. These results were compared with the true-load, computed using in situ continuous monitoring measurements.
ResultsThe average percent error compared to the true-load for TDS was 6.4% for the volume-load, 9.0% for the median-load, 11.0% for the mean-load, 14.7% for the instantaneous-load, and 21.2% for the daily-load methods of calculation. The results of this study support the use of the volume-load method when complete or nearly complete continuous flow data sets are available. If there are some data gaps, then the mean-load or median-load estimates should be used.
SignificanceThe results of this study are being used by the Ecological Engineering Research Program at the University of the Pacific to calculate nutrient load to the San Joaquin River (SJR) as part of an SJR Dissolved Oxygen TMDL Project, the purpose of which is to identify sources of oxygen consuming materials. The nutrient load calculations are currently being used to perform a nutrient mass balance on the San Joaquin River and are being compared to watershed analysis risk management framework (WARMF) model outputs.
LocationDeRosa University Center, Stockton campus, University of the Pacific
Citation InformationAshley A. Stubblefield, Shelly Gulati, Jeremy S. Hanlon, Chelsea L. Spier, et al.. "COMPARISON OF LOAD ESTIMATION METHODS FOR CALCULATING TOTAL MAXIMUM DAILY LOAD (TMDL) IN AGRICULTURAL WATERSHEDS" (2013)
Available at: http://works.bepress.com/shelly-gulati/16/