Paying for sediment: Field-scale conservation practice targeting, funding, and assessment using the Soil and Water Assessment Tool

Abstract

Watershed models have been widely used to estimate soil erosion and evaluate the effectiveness of conservation practices at different temporal and spatial scales; however, little progress has been made in applying these theoretical model results to the practical challenge of allocating conservation practice funding to meet specific soil loss objectives. Black Kettle Creek subwatershed (7,809 ha [19,295 ac]) of Little Arkansas River Watershed (360,000 ha [889,579 ac]) in south central Kansas was the focus of an innovative project to target conservation practice funding and pay directly for modeled sediment reduction. Detailed data (10 m [33 ft] digital elevation model topography, Soil Survey Geographic database soils, and a manually developed land use/land cover layer) were input into the Soil and Water Assessment Tool model, and the calibrated model was used to quantify soil erosion for each field. Effectiveness of locally relevant best management practices (BMPs) was simulated for each field. The simulated field-scale effectiveness for implemented BMPs ranged from 9% to 83% for single BMPs and 67% to 100% for selected combinations of BMPs. An in-field signup sheet was developed with field-specific sediment loss-based payments calculated for each BMP option. BMP implementation was 16.7% of cropland area prior (preinstalled BMPs) to the project, and 30.6% of cropland area (postinstalled BMPs) was added due to project-funded implementation. Postinstalled BMP implementation (47.3% of cropland) resulted in 35.8% sediment yield reduction compared to the no-BMPs scenario and 21.9% reduction compared to preinstalled BMP conditions, which was better than initially projected for this project. Inclusion of nontargeted fields and less-than-optimal BMPs had no influence on achieving soil loss objectives because payments were based on implemented soil loss rather than implemented area. Targeting of conservation practices based on payments scaled directly by project outcome (in this case, dollars per ton of sediment reduction) using a modeling approach allowed flexibility for both adopters (farmers) and funders (project staff) while assuring the project objective (i.e., sediment reduction) was met.

Pay-for-Performance Conservation Using SWAT Highlights Need for Field-Level Agricultural Conservation

“…While SWAT is not often applied at the field scale, its base unit of calculation (the HRU) essentially provides fieldscale outputs, and previous studies have shown its usefulness at the field scale (Daggupati et al, 2011;Douglas-Mankin et al, 2013;Kalcic et al, 2015;Teshager et al 2016). HRUs are areas with the same soils, slope, and land use within a sub-basin and are lumped together for calculations.…”

“…To our knowledge, this project is the first implementation of the Soil and Water Assessment Tool (SWAT) to assess agricultural conservation practice performance at the field scale for a P-based PFP program. While not specifically calling the program PFP, Douglas-Mankin et al (2013) described a program for "paying for sediment" reductions that was implemented in Black Kettle Creek watershed in Kansas, and it represents the most similar approach to our work with respect to modeling in a PFP system. Their work demonstrated the potential for PFP to achieve water quality improvements in a cost-effective and flexible manner.…”

“…In addition to theoretical modeling, some studies have piloted PFP programs in the U.S. (Winsten, 2009;Douglas-Mankin et al, 2013;Fales et al, 2016). However, one difficulty in implementing a PFP program, and a potential reason why they are not used more widely, is the cost and complexity of quantifying performance.…”

“…Measuring field-scale nutrient losses, for example, can be quite costly, and a number of years may be needed to demonstrate practice effectiveness due to weather variability (Santhi et al, 2014). Therefore, PFP programs can more efficiently apply indices (Winsten, 2009) or models (Douglas-Mankin et al, 2013;Fales et al, 2016) to quantify the potential conservation performance for a given system. To our knowledge, this project is the first implementation of the Soil and Water Assessment Tool (SWAT) to assess agricultural conservation practice performance at the field scale for a P-based PFP program.…”

Assessment of Agricultural Best Management Practices Using Models: Current Issues and Future Perspectives

“…Watershed models evaluate the hydrologic and water quality response to multiple BMPs at varying scopes and locations. Their application has also been expanded to the basin and regional scales [6]. For example, by incorporating Geographic Information System (GIS) techniques, cropland conversion to forest/grassland as an effective BMP can be easily evaluated by watershed models.…”