Assessing cultivated cropland inherent vulnerability to sediment and nutrient losses with the Soil Vulnerability Index

GOAL AND PURPOSE OF THE SOIL VULNERABILITY INDEX

The National Assessment for Cropland (Conservation Effects Assessment Project [CEAP] Cropland) intends to report the effects of conservation practices applied to cropland with respect to societal benefits, such as cleaner water and soil quality improvements that will result in more sustainable and profitable production over time. One outcome of CEAP is the need for a consistent approach to assessing where conservation systems would be beneficial so that the USDA Natural Resources Conservation Service (NRCS) and other conservation managers can target efforts to those areas. This approach requires the development of tools to assess cropland soil vulnerability to cultivation. Soil vulnerability describes the ability of soil resources to withstand potentially hazardous impacts of cultivation that allow losses of sediment or excess nutrients from the farmer's field into surface and ground waters. Such losses can lead to reduction in surface and ground water quality in agroecosystems. The Soil Vulnerability Index (SVI) was developed to rank a soil for vulnerability to loss of sediment and/or excess nutrients from cultivation. Such ranking will increase the speed of interpretation for conservation planning and allow planners and resource managers to more efficiently assess a field's needs and to narrow the suite of conservation solutions viable for meeting resource concerns.

Guiding Principles. Three general principles guided the design and development of the SVI:

1. Conservation managers, producers, and the public should easily be able to understand the meaning of the SVI. NRCS designed the SVI as a categorical index that classifies vulnerability into four categories from low to very high.

2. The SVI should be regionally relevant, meaning that within a specific region, cropland areas with a lower vulnerability experienced less sediment and nutrient losses than cropland with a greater vulnerability. A region was defined as an area equivalent in size to a hydrologic unit of level two (HUC2), i.e., the drainage area of a major river (e.g., the Missouri River) or a water body (e.g., the Chesapeake Bay region or the Great Lakes region).

3. The SVI should be based on soil behavior without any conservation or management consideration. Nutrient and sediment losses from cropland were assessed for a no-conservation practice scenario, which included nonconservation tillage, preventive pesticide applications, and uniform fertilization at rates that exceed crop removal (Thompson et al. 2020).

With these three guiding principles in mind, NRCS used the results of Agricultural Policy/Environmental eXtender model (APEX) simulations conducted during the Cropland CEAP assessment in the Upper Mississippi, Tennessee, and Ohio River basins (USDA NRCS 2012) to select the variables that drive the SVI and develop vulnerability classes based on those variables. Even though the guiding principles precluded management from affecting the SVI, the leaching component ultimately included consideration for artificial drainage because it significantly affects the subsurface movement of water and associated loss of dissolved constituents. Application and evaluation of the SVI in several USDA landscape-scale water quality initiatives and feedback from watershed resource managers led to the development of new versions of the SVI: the Soil Vulnerability Index for Cultivated Cropland (SVI-cc [USDA NRCS 2018]) and the Soil Vulnerability Index for Cultivated Cropland 2 (SVI-cc2 [USDA NRCS 2019]). In addition to reviews and evaluations by NRCS soil scientists, an independent review of the 2012 SVI version—presented in the Upper Mississippi Cropland CEAP report (USDA NRCS 2012)—started in 2012, first in the Goodwater Creek Experimental Watershed (Chan et al. 2017), and then later across 13 CEAP watersheds. These watersheds were selected among the 23 watersheds of the CEAP Watershed Assessment Studies, based on percentage of cropland and the range of precipitation and physiographic characteristics. This evaluation across multiple regions resulted in a special collection of papers presented in this issue of the Journal of Soil and Water Conservation. Among those, Thompson et al. (2020) include a presentation of the methods used to develop the SVI, a definition of the SVI evaluated in this special collection, and a synthesis of the strengths and limitations of the index. Table 1 provides an overview of the development and review of the SVI.

USES OF THE SOIL VULNERABILITY INDEX

The SVI helped assess land resources during previous landscape-scale water quality initiatives. As early as 2009, SVI maps in combination with model results helped identify the most vulnerable cropland. When combined with spatial information about conservation practices already implemented, these results helped inform the need for additional conservation practices (USDA NRCS 2011). These assessments can help NRCS, states, or other local agencies complete needed resource assessments at field, landscape, or watershed scale, at the planning or evaluation stage.

In Tennessee, the SVI helped assess the extent to which erosion from agricultural land can affect stream and river water quality in the different regions of the Tennessee River basin (University of Maryland Center for Environmental Science, Integration and Application Network 2018a). The SVI was calculated, reclassified by calculating the percentage of high vulnerability cropland in each hydrologic unit of level twelve (HUC12), and rescaled by calculating the Z-score to classify each HUC12 in one of five possible vulnerability categories (University of Maryland Center for Environmental Science, Integration and Application Network 2018b). This reclassification was necessary to be consistent with the other resource vulnerability assessments conducted in the river basin.

For future evaluations and conservation planning, the new NRCS Conservation Assessment and Ranking Tool (CART), designed to streamline the assessment of conservation needs and delivery of conservation services, uses the SVI as an underlying layer for cultivated croplands. The SVI is used as part of the site risk assessment aspect of this new NRCS tool. It is also used as one of the data sources to inform the conservation planning thresholds aspect of CART.