Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

The Indicator

This indicator presents the percentage of U.S. cropland (minus pastures, but including Conservation Reserve Program [CRP] acreage) in each of three categories of land condition (least prone, moderately prone, and most prone to erosion), based on both inherent soil properties and management practices, for 1982, 1992, and 1997, for both wind and water erosion. Also, those lands most prone to wind and water erosion are mapped.

Soil erosion is affected both by the inherent properties of the soil, landscape, and region (e.g., slope, soil type, rainfall) and by management factors that may change more rapidly (specifically, the use of terracing, wind barriers, and the type, amount, and duration of vegetative cover). Soils with higher inherent likelihood of eroding and with high vulnerability due to the way they are managed are likely to erode the most. (Enrollment of these acres into the CRP, which requires steps toward reducing erosion (e.g., planting perennial grasses), will lead to improvement of this indicator.) Conversely, soils with low inherent likelihood of eroding and low vulnerability because of good management are likely to erode least.

Categories for this indicator were developed using parameters measured for use in the Universal Soil Loss Equation (USLE) and Wind Erosion Equation (WEQ). These equations were developed to predict long-term average erosion based on measurements of the inherent soil and plot features and management and surface treatment factors. For water erosion (USLE), inherent soil and plot factors are R, rainfall and runoff; K, soil erodibility; and L and S, topographic factors related to slope steepness and length of slope. Management and surface treatment factors included C, cover management, which essentially measures whether and how much vegetative cover is left on the soil surface, and P, support practice factor, which measures whether there are features such as terraces. The equation form is A (annual soil erosion per unit area) = C*P*R*K*L*S. For wind, the inherent soil and plot factors are I, soil erodibility index, and C, climatic factor. Management and surface treatment factors are K, ridge roughness; L, unsheltered distance along the prevailing wind direction; and V, vegetative cover. Wind erosion, E (annual soil erosion per unit area), is a function of I, K, C, L, and V (see references for more details).

This report uses the underlying principles of these equations to identify cropland area with combinations of inherent soil properties and management practices that are likely to erode most and least. Though inherent soil properties change slowly or not at all, management practices can significantly reduce erosion. Thus, reductions in acreage with high propensity to erode result primarily from application of management practices that reduce erosion, including removal of acreage from cultivation, such as CRP.

Areas with the least susceptibility to both wind and water erosion (“least prone”) are generally those with a predicted erosion rate of less than 1 ton per acre per year. Areas with the greatest susceptibility to erosion (“most prone”) are those with a predicted erosion rate of 3 tons per acre or more. Areas with moderate susceptibility to erosion have predicted values between about 1 and 3 tons per acre per year.

Standard application of both USLE and WEQ uses the equations to predict total erosion, in tons per acre. In this report, we have chosen not to take this last step in the process. We do so because we believe taking this step overstates actual erosion, as the USLE does not account for deposition, only the initiation of soil movement. Some soil particles move only very short distances, and when erosion is reported in units of “tons per acre” there is a strong implication (and sometimes an explicit statement) that these tons of soil are lost from the farm field.

The WEQ estimates how much eroding soil leaves the downwind edge of the field, in tons per acre per year.

The Data

References

Bondy, E., Lyles, L., and Hayes, W.A. 1980. Computing soil erosion by periods using wind energy distribution. Jour. Soil and Water Conserv. 35(4):173–176.

Skidmore, E.L., and N.P. Woodruff. 1968. Wind erosion forces in the United States and their use in predicting soil loss. Agriculture Handbook No. 346.

Woodruff, N.P., and Siddoway, F.H. 1965. A wind erosion equation. Soil Sci. Soc. Amer. Proc. 29(5):602–608.