Use of Watershed Modeling to Target Management Practices in an Agricultural Watershed

View the project final report

Commodities

• Animal Products: dairy

Practices

• Crop Production: conservation tillage, crop rotation
• Sustainable Communities: sustainability measures

Identification of areas contributing disproportionately to high amount of pollutants (i.e., critical source areas (CSAs)) to streams is important to efficiently and effectively target best management practices (BMPs). Process-based models are commonly used to identify CSAs and evaluate the impact of alternative management practices on pollutant load reductions. The objective of this study was to use the Soil and Watershed Assessment Tool (SWAT) to identify CSAs at the subwatershed level and evaluate the impact of alternative BMPs on sediment and total phosphorus (TP) load reductions in the Pleasant Valley watershed in South Central Wisconsin (USA). The Nash-Sutcliffe efficiency, percent bias, and coefficient of determination ranged from 0.58 to 0.71, -12.87 to 38.33, and 0.67 to 0.79, respectively, indicating that SWAT was able to predict stream flow, sediment and TP loadings at a monthly time-step with sufficient accuracy. Average annual (2006-2012) subwatershed yield for sediment and TP ranged from 0.06 to 3.14 tons ha^-1 yr^-1 and 0.04 to 1.9 kg ha^-1 yr^-1, respectively. The croplands were the major source of sediment and TP in this watershed (≥ 84%). Reduction in sediment and TP loading ranged from 66 to 99% at the subwatershed level after conversion of croplands to CRP grasslands in subwatersheds identified as CSAs. On the other hand, reduction in sediment and TP loading with implementation of no-till practices ranged from only 14 to 25%. At the watershed outlet, sediment and TP loading reduction was less than ≤ 15% after conversion of croplands to CRP grasslands and implementation of no-till practices. The results of this study suggest that targeting the croplands in critical subwatersheds for BMPs can help to reduce sediment and P delivery to streams.

Introduction:

Sediment and phosphorus (P) are important non-point source pollutants causing impairment of surface waters. Excessive P and sediment delivery to streams results in problems, such as, toxic algal blooms, oxygen deficiency, fish kills and loss of biodiversity (Carpenter et al., 2008). To reduce excessive sediment and P delivery from agricultural landscapes to streams, best management practices (BMPs) need to be implemented at sensitive areas contributing non-point source pollutants to streams. Implementation of BMPs at a watershed scale is expensive and time consuming. Therefore, BMPs should be targeted to areas capable of disproportionate contributions of non-point source pollutants to streams and other water bodies to achieve desired water quality improvement (Nowak et al., 2006).

Source area contributions to streams can vary within a watershed due to variability in topography, hydrology, soil type, land-use, and management practices. Several studies have shown that only a small portion of a watershed can contribute significant amount of pollutants. For example, White et al. (2009) found that in six Oklahoma watersheds, 5% of the land area contributed 50% of the sediment and 34% of the total P (TP) load. Ghebremichael et al. (2010) reported that only 24% of the total watershed area contributed to 80% of TP losses in a predominantly agricultural Rock River watershed in Vermont. Similarly, Busteed et al. (2009) showed that 85 to 90% of the sediment and TP load originated from 10% of the Wister Lake watershed located in southeastern Oklahoma and southwest Arkansas.

Different methods are used to identify areas contributing disproportionately high amount of pollutants (i.e., critical source areas, CSAs) to streams within a watershed. One approach is to monitor pollutant load to prioritize CSAs. However, measuring pollutant loading from individual fields in a watershed is expensive, labor intensive and impractical (Giri et al., 2012). These experiments also require several years to account for climate fluctuations (Veith et al., 2005). Tools such as the P Index can be used on single or multiple fields within a watershed to prioritize fields for BMP implementation. Conservation planners can also identify CSAs via qualitative evaluation based on their professional judgment (White et al., 2009). However, these methods also face limitation when employed at the watershed scale where the data requirements are extensive. Off-site movement of sediment and P is controlled by both source and transport factors. Therefore, approaches which can capture the hydrologic complexities within a watershed drainage system and incorporate important variables influencing sediment and P transport are needed for effective delineation of CSAs.

Process-based models, such as Soil and Watershed Assessment Tool (SWAT), can simulate complex processes involved in water, sediment and P movement (Ghebremichael et al., 2010; Gitau et al., 2008) and, therefore, are often used to identify CSAs for BMP implementation at a watershed scale (e.g., Nirula et al., 2013; Shang et al., 2012; Srinavasan et al., 2005; Tripathi et al., 2005; White et al., 2009). SWAT has proven to be an effective tool to identify CSAs, since it incorporates land cover, topography, soil characteristics, rainfall and land-use management, which influence sediment and P transport from source areas to the watershed outlet (White et al., 2009). The SWAT model has been often used to evaluate the impact of BMPs to obtain intended water quality benefits (Arabi et al., 2008; Ulrich and Volk, 2009). For example, Mankin et al. (2013) used SWAT to simulate the effect of different tillage practices on sediment loss in the Black Kettle Creek watershed in south central Kansas. Similarly, Kirsch et al. (2002) used SWAT to assess the impact of tillage and nutrient management on sediment and P losses in the Rock River watershed in south-central and eastern Wisconsin. Previous studies indicate that SWAT can be successfully used to identify CSAs and simulate the impact of BMPs on reduction in sediment and TP load.

(1) Identify CSAs for sediment and TP at the subwatershed level; (2) Evaluate the impact of cropland conversion to conservation reserve program (CRP) grasslands and effect of implementing no-till practices on sediment and TP loads at the subwatershed and watershed scale.