Soil and Water Assessment Tool (SWAT)

SWAT is a process model that integrates field-scale BMPs within a watershed and evaluates the water quality benefits that can be expected from their implementation at sub-watershed- and watershed-scale over a long period of time. It uses the continuous, daily time-steps to simulates the water, nutrient, chemical and sediment movement in a watershed resulting from the interaction of weather, soil properties, stream channel characteristics, land management practice and crop growth. The ArcGIS interface with SWAT2005, ArcSWAT 2.3.4 was used for the watershed modeling in Neshanic River Watershed. The following data were used to build the baseline SWAT model:

Additional data used in the SWAT modeling including the 2 rounds of agricultural land use inventories  in the watershed in 2007 and 2008 and agricultural field operation and agricultural BMP data collected through farmers and agricultural extension specialists. The watershed was delineated into 25 subbasins and 625 hydrological response units for SWAT modeling. The baseline SWAT model was successfully calibrated for crop yields, streamflow, total suspended solids, and total nitrogen, total phosphorus, fecal coliform and E. coli for the time period 1997-2008 using the observed county average crop yields, the monitoring streamflow and water quality collected by the USGS, NJDEP and the project team.

The well calibrated SWAT model was used to: (1) evaluate the baseline water quality condition, (2) analyze the sources and causes of the water quality degradation, (3) evaluate the pollutant load reduction targets for achieving the water quality standards, and (4) evaluate the potential of BMPs scenarios for water quality improvement in the watershed.

Neshanic River Watershed
Neshanic River Watershed and Subbasin Delineation for SWAT Modeling

Progress and Status

  • Completed in September 2010

Results and Outcomes

The SWAT modeling results indicated the Neshanic River was severely impaired by pathogens including both fecal coliform and E. coli., moderately by phosphorus and slightly by sediments measured by total suspended solids. Nitrogen contamination did not appear to be a water quality issue. The SWAT modeling results were consistent with the water quality monitoring data by USGS, NJDEP and the project team. SWAT Modeling results indicate that the phosphorus loads to the Neshanic streams primarily originated from the land sources in the watershed. The primary source of phosphorus is agriculture including row-crop and other agriculture, and accounts for 61 percent of phosphorus loads to the streams. Lawn care in urban lands contributes 29 percent of the phosphorus loads. Human and animal wastes are major sources of pathogens to the Neshanic streams. While the failing on-site wastewater disposal systems contribute 46 percent of the pathogen loads, the manure being applied to the agricultural field accounts for 31 percent of the pathogen loads and the livestock access to streams contributes 19 percent of annual pathogen loads to the Neshanic streams in the watershed. The streams themselves including stream bank erosion and sediment resurfacing are the primary sediment source in the watershed and accounts for 60 percent of the total annual sediment load. The remaining 40 percent of sediments come from various land uses in the watershed.

Distribution of annual sediment loads from streams
Distribution of Annual Sediment Loads from Streams
Distribution of Annual Phosphorus Loads from Land Sources
Distribution of Annual Phosphorus Loads from Land Sources

NJDEP (2010) designated the Neshanic River and its tributaries as FW2-NT.  “FW2” refers to the freshwater bodies that are used for primary and secondary contact recreation; industrial and agricultural water supply; maintenance, migration, and propagation of natural and established biota; public potable water supply after conventional filtration treatment and disinfection; and any other reasonable uses. “NT” means those freshwaters may support other fish species, but are not suitable for trout production or trout maintenance due to their physical, chemical, or biological characteristics. At the watershed outlet, the SWAT modeling results indicate that the frequencies of exceeding the water quality standards for the FW2-NT streams are 12, 38, 61 and 63 percent for the total suspended solids, total phosphorus, fecal coliform and E. coli, respectively. In order to meet the water quality goals at the watershed outlet, the load reduction target should be 9 percent for the total suspended solids, 49 percent for total phosphorus, and 89 percent for pathogens including both fecal coliform and E. coli.

The SWAT modeling results showed that no tillage, cover crop, filter strips and stream channel protection would be effective BMPs for reducing sediment loads. Reducing manure application, phosphorus fertilizer management, cover crop and filter strips are effective BMPs for phosphorus load reduction. Eliminating failing on-site wastewater disposal systems, reducing manure application, livestock exclusion fencing, filter strips, and channel protection are effective in reducing pathogen loads to the streams. It is relatively easier to achieve the desired reductions for the total suspended solids and phosphorus than for the pathogens. Various BMPs are applied together to achieve the desired reduction for phosphorus and pathogens.

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