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AVGWLF Facilitates Evaluation of Northeast NPS Pollution
by Tom Mulcahy, NEIWPCC
We’ve long known about the problem of nonpoint source (NPS) pollution, which few would deny has a significantly negative impact on the quality of watersheds throughout the country. The challenge has been to find a good way to identify and quantify NPS pollutant loads. In recent years, researchers have begun to effectively tackle that challenge by developing models that simulate NPS loadings and allow water quality managers to determine the best ways to reduce the pollution to acceptable levels. In the Northeast, a new such model, crafted especially for the region with the help of NEIWPCC, is about to become available. It goes by the abbreviation AVGWLF or “average wolf” as some have chosen to pronounce it. It’s not a simple name, but the software is relatively simple to use—and that should make it a popular new tool in the quest to control NPS pollution.
A Persistent Problem
EPA defines NPS pollution as “pollution caused by rainfall or snowmelt moving over and through the ground. As the runoff moves, it picks up and carries away natural and human-made pollutants, finally depositing them into lakes, rivers, wetlands, coastal waters, and even our underground sources of drinking water.” In the Northeast, the bulk of NPS pollution can be attributed to stormwater and agricultural runoff.
Stormwater is generated when precipitation runs through storm drains, along streets and highways, or flows directly from land into our lakes, rivers, and streams. However it’s generated, stormwater and all of the pollutants it carries enter our waterways untreated when it rains.
Agricultural runoff deposits nutrients, pesticides, pathogens, and organic materials directly into our waterways. The most recent EPA National Water Quality Inventory reports that “agricultural NPS is the leading source of water quality impacts to surveyed rivers and lakes, the third largest source of impairments to surveyed estuaries, and also a major contributor to groundwater contamination and wetlands degradation.”
A Model Solution
The options for determining the impact of NPS pollution problems include long-term surface water monitoring, which can be effective but has an undeniable downside—it takes a lot of people, a lot of time, and a lot of money. As a result, computer-based simulation modeling has increasingly been relied upon to provide the information needed to develop and implement NPS control programs.
Using a watershed model isn’t without its shortcomings. It can be a difficult, tedious task because of the broad spatial and temporal scales that must be considered, as well as the large amount of data that must be compiled, formatted, and analyzed. Fortunately, the last two decades of model development have coincided with rapid advancements in the development and use of geographic information systems (GIS) technology. Due to its many inherent benefits, GIS software has been relied upon to support recent watershed modeling efforts.
One such effort is the Generalized Watershed Loading Function with an ArcView GIS interface, or AVGWLF for short. It’s based on the original GWLF model, which Cornell University researchers created in 1987. The GWLF model provides the ability to simulate runoff, sediment, and nutrient (nitrogen and phosphorous) loadings from a watershed given variable-size source areas. It has algorithms for calculating septic system loads and allows for the inclusion of point source discharge data. It is a continuous simulation model that uses daily time steps for weather data and water balance calculations. Monthly calculations are made for sediment and nutrient loads, based on the daily water balance accumulated to monthly values.
The AVGWLF tool takes all that a step further by incorporating ArcView software, which allows users to visualize, analyze, create, and manage data with a geographic component. Dr. Barry Evans of Penn State University developed the first AVGWLF software package, which was used to quantify nutrient and sediment loads and develop EPA-approved TMDLs for 66 NPS-impaired segments of the Neshaminy Creek Watershed. In January 2005, NEIWPCC began collaborating with Dr. Evans to build a version of the model that would reflect the range of landscape characteristics in the New England states and New York State, and help support NPS and Total Maximum Daily Load (TMDL) programs in the region.
Custom Version
According to Dr. Evans, the general approach in most applications of AVGWLF is to 1) derive input data for GWLF for use in an impaired watershed, 2) simulate nutrient and sediment loads within the impaired watershed, 3) compare simulated loads within the impaired watershed against loads simulated for a nearby reference watershed that exhibits similar landscape, development and agricultural patterns, but has been deemed to be unimpaired, and 4) identify and evaluate pollution mitigation strategies that could be applied in the impaired watershed to achieve pollutant loads similar to those in the reference watershed. The primary bases of comparison between impaired and reference watersheds are the estimates of their respective average annual nutrient and sediment loads.
That, however, is the general approach. In order for a simulation model to adequately represent the processes of a specific region, the model algorithms must be calibrated to reflect the region’s distinctive environmental conditions. The calibrated model algorithms then must be verified by comparing the calibrated model results to actual water quality data collected from watersheds in the region.
NEIWPCC, with assistance from our member states, EPA, and Dr. Evans, selected 22 watersheds in the Northeast for the purpose of calibrating and validating a model tailored for the region. We selected watersheds that represented the assortment of landscape characteristics in the region; each watershed also met a minimum threshold for available historic water quality and flow data. We used half of the 22 watersheds to calibrate the model, and the other half to validate the calibrated model results.
Ready for Rollout
The customization process resulted in calibrated and verified model algorithms that can now be applied to watersheds throughout the region. With AVGWLF, sound estimates of sediment and nutrient concentrations can efficiently be established using a user-friendly GIS interface that includes a comprehensive data set covering the entire region. The regional GIS data set will also allow for the evaluation of interstate waters by employing GIS data from all six New England states and New York State in one standardized format. AVGWLF offers the northeastern states an effective alternative to costly monitoring programs and complicated model applications.
A companion software tool for evaluating and implementing agricultural and non-agricultural pollution reduction strategies at the watershed level will be included with the AVGWLF software package. This tool, called PRedICT (Pollution Reduction Impact Comparison Tool), allows the user to create scenarios in which current landscape conditions and pollutant loads (both point and nonpoint) can be compared against future conditions that reflect the use of best management practices. It includes pollutant reduction coefficients for nitrogen, phosphorus and sediment, and has built-in cost information for an assortment of pollution mitigation techniques.
The Northeast AVGWLF software package is nearing completion and will be available to water quality managers early this year. It will be distributed by both NEIWPCC and Penn State. For more information, contact NEIWPCC’s Becky Weidman. More information about AVGWLF is also available online at www.avgwlf.psu.edu/ or www.neiwpcc.org/avgwlf.