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Water: Watershed Central

Characterize the Watershed - Estimate Pollutant Loads

Estimating pollutant loads is essential to eventually satisfy element b (estimate of the load reductions expected from management measures) of the nine minimum elements. This element is the component most often missing from current and past watershed plans, although it is one of the most important. Without knowing the sources of pollutants, you cannot effectively control them and restore and protect your watershed. The loading analysis provides a more specific numeric estimate of loads from the various sources in the watershed. It also helps you plan restoration strategies, target load reduction efforts, and project future loads under new conditions.

Various approaches can be used to do the loading analysis, and the method you choose depends on several factors, including water quality parameters, time scale, source types, data needs, and user experience. Some loading analyses are focused on determining how much load is acceptable, whereas others focus on source loads that attribute loading to each category of sources in the watershed. For watershed planning purposes, source load estimates are desirable because the information can be used to support management planning and targeting of restoration resources. Different approaches have been used to estimate pollutant loads in watersheds. Table 8-1 (PDF) (44 pp, 896K, About PDF) on page 8-3 of EPA's Watershed Handbook provides brief descriptions of some of the most widely used approaches.

Check whether a previous study is available, like a Total Maximum Daily Load (TMDL), Clean Lakes study, or other watershed-based programs that might have required development of loading estimates. Such studies can often be used to provide loading estimates appropriate for developing the watershed plan.

There are two general types of techniques used to estimate pollutant loads: techniques that directly estimate loads from monitoring data or literature values, and watershed modeling techniques that indirectly estimate loads.

Your loading analysis essentially quantified your loads, but now you have to decide how to present the information for use in your watershed plan. Two factors will affect this decision ¾space and time. You need to decide the spatial resolution for your loads, as well as the time scale for their calculation. Table 8-12 (PDF) (44 pp, 896K, About PDF) on page 8-39 of EPA's Watershed Handbook summarizes typical scales for calculating and presenting loading results from watershed models. Presentations can use a combination of tables and graphical displays. Developing maps, graphs, bar charts, and pie charts can help to summarize information and facilitate interpretation of results.

The loads you have calculated will provide the basis for identifying the necessary load reductions to meet watershed goals and help guide the selection of management practices.


AVGWLF Exit EPA Disclaimer - Best Management Practices document is available.

AnnAGNPS Pollutant Loading Model - The AnnAGNPS PL model is a batch-process, continuous-simulation, pollutant loading (PL) computer model.

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