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Submitted to: Water Management to Meet Emerging TMDL Environmental Regulations Conference
Publication Type: Proceedings Publication Acceptance Date: 3/11/2002 Publication Date: 3/11/2002 Citation: Bonta, J.V. Framework for estimating TMDLs with minimal data. Proceedings of the ASAE Conference on Watershed Management to Meet Emerging TMDL Environmental Regulations, Fort Worth, Texas. March 11-13, 2002. p. 6-12. Interpretive Summary: Current environmental regulations specify allowable chemical and sediment loads for surface waters called "total maximum daily loads" (TMDLs). In some cases these quantities are developed for watersheds for which there are little data. Furthermore, these loads are established in terms of a load/day (e.g., kg/day) when only in-stream concentration data are available (mg/L). They are often assigned under conservative conditions, with no estimation of risk or how often a particular load will be exceeded and of the uncertainty of the estimate. A method that has historically been used to compute total chemical and sediment loads in water-quality studies to determine water-quality trends has intermediate calculations that are not often used, but may be useful for TMDL estimation. The intermediate steps provide information on the percent of time that a concentration and load (TMDL) will be exceeded, the duration of concentrations/loads, etc. The method can be used by itself or as a supplement to more complex watershed modeling. It is useful for determining the range of concentrations expected from a watershed, for characterizing actual in-stream conditions, and for tracking actual in-stream conditions after a best-management practice in a watershed is implemented. The method is simple to use, and is promising for areas where there are no flow data, and for which there are only a few samples. The concepts of the method are presented, along with those for using this method when data are scanty and for determining uncertainty in the TMDL estimates. The paper provides information a method that is useful to scientists and environmental-regulating agencies (e.g., the USEPA). Technical Abstract: Current regulations specify the derivation of total maximum daily loads (TMDLs) for surface waters. Yet these standards are often derived from incomplete information. In some cases these quantities are assigned to watersheds for which there are little data, and are established in terms of loads when only concentration data are available. Furthermore, they are often assigned under conservative conditions with no estimation of risk an uncertainty of the estimate. Flow-duration curves and regressions between flow rate and constituent concentration have historically been used to compute average total loads to determine water-quality trends. However, intermediate calculations in this methodology, not often used, have utility for TMDL estimation. From these intermediate calculations, one can determine the percent of time that a concentration and load (TMDL) will be exceeded, the duration of concentrations/loads, etc. The method can be used by itself or as a supplement to more complex watershed modeling. It is useful for determining the range of concentrations expected from a watershed, for characterizing actual in-stream conditions, and for tracking actual in-stream conditions after implementation of best-management practices. The method is simple to use, and is promising for areas where there are no flow data, and for which there are only a few samples. The concepts of the method are presented, along with those for using this method when data are scanty and for determining uncertainty in the TMDL estimates. |
