Modeling conservation practices in APEX: from the field to the watershed
Introduction
With the goal of improving water quality by reducing sediments, nutrients and pesticides transported from agricultural fields, agricultural programs promote conservation practices [also referred to as Best Management Practices (BMPs)]. Conservation agricultural programs are designed by the United States Department of Agriculture, Natural Resources Conservation Service (USDA-NRCS), and implemented through local Soil and Water Conservation Districts (SWCDs). Driven by public concerns of nonpoint source environmental and water quality degradation, several conservation programs have been developed as a consequence of additional funding stipulated in the 2002 Farm Bill. To evaluate the environmental impact of such programs at the watershed scale, the Conservation Effects Assessment Project (CEAP) was established (Richardson et al., 2008). Within CEAP's Watershed Assessment Studies, the St. Joseph River watershed in northeastern Indiana has been targeted by the Agricultural Research Service (ARS) to provide information on the environmental effects of conservation practices. So far, the evaluation of a few conservation practices at the field scale has been completed for this watershed (Francesconi et al., 2014, Smith et al., 2008, Smith et al., 2015a, Smith et al., 2015b, Pappas et al., 2008). However, more research is required on the impact of single and combined conservation practices at the field and the watershed scale.
Monitoring and modeling the potential benefits of conservation practices at the watershed scale is challenging (Tomer and Locke, 2011). While monitoring provides empirical data, it is also time consuming and costly which limits the number of practices to be evaluated. On the other hand, modeling can simulate multiple conservation practices. However, modeling of large watersheds makes the evaluation of distinct conservation practices difficult, as landscape components are merged into single units (e.g., hydrologic response units in the Soil and Water Assessment Tool — SWAT) (O'Donnell, 2010). Furthermore, detailed management information of agricultural practices at the watershed scale is difficult to collect, and several years of monitoring data are usually required for the modeling analyses to be robust. So far, various studies have been conducted to provide some accountability for the incorporation of conservation practices at CEAP's targeted watersheds. The review by Richardson et al. (2008) summarizes some of these findings. Among them, monitoring results have shown the significant benefits of Conservation Reserve Programs (CRP), fertilizer management techniques, reduced tillage, and wetlands for mitigating sediment and nutrient losses. In addition, modeling is proving to be a useful tool for the evaluation of scenario simulations to identify successful practices. In conjunction, the extrapolation of monitoring and modeling results has been demonstrated to be a valuable research approach for evaluating the effectiveness of conservation practices at the watershed level.
The primary hydrological models used for the evaluation of CEAP's conservation practices have been SWAT and the Annualized Agricultural Non-Point Source (AnnAGNPS) Pollution Model (Richardson et al., 2008, Tomer and Locke, 2011). The application of these physically-based models has been favored as they can include soil, land-use, and topographic variability in the watershed (Rossi et al., 2008, Yuan et al., 2001). In the St. Joseph River watershed, SWAT has been shown to perform better than AnnAGNPS when predicting stream flow and pesticide losses (Heathman et al., 2008). Yet, the authors concluded that it is difficult to include detailed environmental and management information when modeling at the watershed scale. Modeling of large watersheds results in an increase in the model's input uncertainty and consequently in the loss of predictive power. In contrast, the use of field-scale prediction models can help evaluate the distinct contributions of selected BMPs to water quality.
The Agricultural Policy/Environmental eXtender (APEX) model has been proposed as a tool for evaluating conservation practices in CEAP watersheds (Gassman et al., 2010). APEX was developed to predict water flow, sediment, and nutrient transport in agricultural fields and small watershed (Williams and Izaurralde, 2006). In addition to simulating structural conservation practices, the model is capable of incorporating detailed environmental and management information at the field scale. Hence, it is currently promoted as being more flexible and having a broader range for evaluating agricultural practice scenarios (Gassman et al., 2010). Yet, so far APEX has only been applied to test the effectiveness of a handful of conservation practices in the USA (Francesconi et al., 2014, Tuppad et al., 2010, Wang et al., 2008, Wang et al., 2009), and some of these are specific to particular regions in the country. The study by Tuppad et al. (2010), for example, provides a review of the effect of conservation practices tailored to agricultural watersheds in Texas. The implementation of APEX modeling for the evaluation of BMPs in other agricultural regions in the US can help identify practices that can address pressing environmental issues in those regions.
In the Midwest where the primary agriculture is corn (Zea mays) and soybeans (Glycine max), farmers can choose from a variety of conservation practices available through government-supported cost-share programs. In fact, some of these programs work synergistically, and participant farmers often bundle two or three conservation practices within a single agricultural field. Given the persistent eutrophication problems in surface water bodies in the Midwest, the effectiveness of these single and combined practices adopted to improve water quality needs to be tested. One way to evaluate the effects of single and multiple conservation practices is through APEX modeling. This would allow for a quantitative evaluation of conservation practices and serve as a tool for ranking their effectiveness when dealing with specific conservation goals.
The overall purpose of this study was to evaluate different government-promoted cost-share conservation practices and their combined effect on water quality in the St. Joseph River watershed. More specifically, the objectives were: 1) to model conservation practices particular to the Midwest region and compare their relative effectiveness at reducing nutrient transport from agricultural fields, and 2) to extrapolate the APEX modeled edge-of-field outputs by their incorporation areas at the watershed scale to provide a preliminary estimation of their sediment and nutrient reduction potential.
Section snippets
Study site
The St. Joseph River watershed is an 8 digit Hydrologic Unit Code (HUC 04100003) catchment located at the intersection of the states of Michigan, Indiana, and Ohio (Fig. 1). The St. Joseph River is the main source of drinking water for the city of Fort Wayne and surrounding areas in Indiana. The watershed is composed of nine 11-digit HUC sub-watersheds. Among them, the upper Cedar Creek (HUC 04100003080) has been monitored by the National Soil Erosion Research Laboratory for water quality for
Single practice effects on water quality
Predicted sediment and nutrient loss values with edge-of-field modeling for the different conservation practices were compared (Table 3). The benefits of no-till farming practices for reducing soil erosion compared to more intensive tillage practices are well known and have been long acknowledged (Wendt and Burwell, 1985, Hobbs et al., 2008). According to the APEX simulated results, no-till practices reduced the amount of sediment lost by 56% compared to the tillage scenario. Similar or greater
Conclusions
The results suggest that single conservation practices may be effective at targeting specific sediment and nutrient transport problems in agriculture. Among the single conservation practices, cover crops and forage were the most successful at reducing sediment and nutrient losses (by 56–88% and 28–91%, respectively). Results for the combined conservation practices suggest that a systems approach to addressing water quality problems in agriculture would result in a more comprehensive nutrient
Acknowledgments
We would like to thank Allen W. Haynes and the staff at the DeKalb County Soil and Water Conservation District for all their assistance describing the conservation practices incorporated at the St. Joseph River watershed. We would like to thank NRCS for the survey data provided that made this study possible. The mentioning of a trade name or property product of specific equipment does not constitute a guarantee or warranty by the USDA and does not imply its approval to the exclusion of other
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