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The Tri-County Agricultural Area (TCAA) project area contains approximately 154,000 hectares of which 15,569 hectares, or roughly 10 percent, is irrigated vegetable cropland comprised of predominantly potato and cabbage farms. A diagnostic assessment was conducted from 1991–1993 to quantify and qualify nutrient loading from 10 representative area farms. The results of this study revealed that nutrient loading was primarily associated with the growing season (Jan.–June) and storm events. The study showed that nonpoint source (NPS) loading rates associated with agricultural row crop landuse are 4 to 5 times greater than other land use activities. Subsequently, costeffective BMPs were identified and their ability to reduce nutrient and sediment loading to receiving waters evaluated (1994–1997).

The TCAA Watershed Assessment Model (TCAA WAM), a GIS-based tool for determining the spatial influence of land uses and soils on water quality and quantity in a watershed (SWET, Inc., 1998), was employed to simulate the cumulative effects that BMP implementation would have on watershed nutrient and sediment loads. These results were imported into ArcView and used to generate maps and summary tables. ArcView was then used to identify and rank watersheds, or subbasins, as high, medium and low priority based upon nitrogen and phosphorous loadings. Those subbasins with high N and P loads were ranked as high priority for water quality treatment.

Both in-field BMPs and wetland treatment were evaluated for their ability to remove nitrogen and phosphorous. A spreadsheet model, RtMod, was used to calculate the wetland sizing and performance efficiencies.

The effects of implementing both in-field BMPs and RST on the total nitrogen and phosphorus loads conveyed from the TCAA project area were determined and their efficiency evaluated at three spatial scales: 1) within 18 high priority subbasins; 2) within 41 agricultural subbasins and 3) TCAA-wide (60 subbasins). While the TCAA project area consists of 60 individual hydrological subbasins, row crop agriculture is only present in 41 of them and occupies only 10% of the total project area. Eighteen of these subbasins were identified as high priority and contain 51% of the TCAA row crop agriculture. The maximum achievable load reduction after in-field BMPs are implemented on all row crop land in the 41 basins is estimated to be 26% for N and 18% for P. By targeting only the high priority subbasins within the TCAA for in-field BMP implementation, nearly half of the maximum potential load reduction can be achieved. Implementation of RST in the 18 high priority subbasins was estimated to reduce the TCAA agricultural TN load by approximately 22% and the TP load by approximately 50%. Total reductions following the implementation of both in-field BMPs and RST in the high priority subbasins, though not expected to equal the sum of the reductions achieved individually, are expected to be greater than those achievable by either methodology employed separately.

While targeting treatment in high priority subbasins optimizes load reduction, limiting treatment to only these subbasins may not reduce pollution loads sufficiently to meet restoration goals. Results demonstrate the benefits of employing a planned approach for treating agricultural stormwater runoff compared to random implementation. Priority consideration should be given to those drainage basins that include a greater percentage of agricultural cropland in order to maximize the performance efficiency of the treatment methods. These drainage basins will generally be characterized as high priority because of the higher loading rates typically associated with agricultural land use.

Document Type: Research Article


Publication date: 2002-01-01

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