Conservation Implications of Climate Change: Soil Erosion and Runoff from Cropland
Soil and Water Conservation Society, 2003
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Soil Erosion, Runoff, and Precipitation
Simulated and Observed Changes in Precipitation
Estimated Effects on Soil Erosion and Runoff
Soil and Water Conservation Society. 2003. Conservation Implications of Climate Change: Soil Erosion and Runoff from Cropland. Ankeny, IA: Soil and Water Conservation Society.
A changing climate will affect soil and water resources on agricultural land in many ways, but will the effect of climate change on soil and water resources on agricultural land be large enough to warrant changes in U.S. conservation policy or practice? To answer this question, the Soil and Water Conservation Society (SWCS) reviewed the literature and engaged members of an expert panel in a discussion of quantitative estimates of the effects of climate change on soil and water resources in agricultural landscapes. We chose to focus on one climatic variable, precipitation; two primary conservation effects, soil erosion and runoff; and one type of agricultural land, cropland.
In the end, our answer to this question was an emphatic yes. Conservationists should be seriously concerned about the implications of climate change–as expressed by changes in precipitation patterns–for the conservation of soil and water resources in the United States. The magnitude and extent of increased rates of soil erosion and runoff that could occur under simulated future precipitation regimes are large. More importantly, analyses of the climate record in the United States show that changes in precipitation patterns are occurring now. In fact, the magnitude of observed trends in precipitation and the bias toward more extreme precipitation events are, in some cases, larger than simulated by global climate change models, particularly since 1970. Extrapolating those relationships to the changes in precipitation observed over the past century suggests increases in soil erosion ranging from 4 percent to 95 percent and increases in runoff from 6 percent to 100 percent could already be evident on cropland in some locations.
Unless additional protective measures are taken, such increases in soil erosion and runoff from cropland–if widespread–could reverse much of the progress that has been made in reducing soil degradation and water pollution from cropland in the United States.
The potential for climate change–as expressed in changed precipitation regimes–to increase the risk of soil erosion, surface runoff, and related environmental consequences is clear. The actual damage that would result from such a change is unclear. Regional, seasonal, and temporal variability in precipitation is large both in simulated climate regimes and in the existing climate record. Different landscapes vary greatly in their vulnerability to soil erosion and runoff. Timing of agricultural production practices creates even greater vulnerabilities to soil erosion and runoff during certain seasons. The effect of a particular storm event depends on the moisture content of the soil before the storm starts. These interactions between precipitation, landscape, and management mean the actual outcomes of any particular change in precipitation regime will be complex.
In sum, a change in precipitation regime also produces a change in the level of risk to which agricultural land is exposed. In general, a regime with greater annual precipitation–particularly if increased storm intensity changes more than storm frequency–heightens the risk of soil erosion, runoff, and related environmental and ecological damages. In general, the risk increases at a greater rate than precipitation amount or intensity increases. Whether that new, more risky baseline condition translates into greater soil degradation, pollution of surface water, pollution of groundwater, or a combination of all three outcomes is highly dependent on other factors.
We identified three particularly promising approaches to begin adapting soil and water conservation policies and practices to a changing precipitation regime.
1. Immediately update climatic parameters in critical conservation planning tools. Two key efforts already are underway. The Natural Resources Conservation Service (NRCS) is currently updating the climate components of its primary erosion prediction and conservation planning tool using precipitation and temperature data covering the period 1971-1999. The National Oceanic and Atmospheric Administration’s National Weather Service (NOAA-NWS) Hydrometerological Design Studies Center is updating precipitation frequency studies used in engineering applications, with support from NRCS, the U.S. Army Corps of Engineers, and others. These two efforts should be completed as soon as possible and then updated more frequently in the future.
2. Undertake targeted investigations to firm up estimates of the damage that would likely occur under simulated or observed climate regimes. Those investigations should include the following: (1) replicating studies at more locations and more land uses to develop better estimates of the variability in soil erosion and runoff responses to an altered precipitation regime; (2) utilizing the NRCS National Resources Inventory framework to update national and regional estimates of soil erosion, runoff, and related environmental outcomes based on updated climate parameters in the Universal Soil Loss Equation or the Revised Universal Soil Loss Equation; (3) utilizing the NRCS National Resources Inventory framework to conduct sensitivity analyses and simulate the effect of simulated climate scenarios on national and regional estimates of soil erosion, runoff, and related environmental outcomes; and (4) completing targeted evaluations and studies to better understand the causes of and future trends in changes in precipitation regime, particularly those portions of the regime of most concern for soil erosion, runoff, and related environmental effects.
3. Evaluate the benefits of building the risk of damage from severe rainstorms into the conservation planning process through risk-based assessments targeted to key conservation systems and environmental outcomes. In current practice, conservation systems applied to agricultural land are usually designed, planned, and implemented to address soil erosion, runoff, and related effects under estimates of expected average annual climate regimes. Conservation planning approaches based on annual average precipitation should be reevaluated in light of the evidence that future–or current–climate regimes are characterized by increased frequency of extreme events. Such a change in conservation planning would require (1) identifying and quantifying thresholds of soil erosion, runoff, or transport of pollutants above which damage at some specified frequency is unacceptable; (2) identifying critical times during the year when the nature and timing of agricultural production practices increase the risk of damage; (3) determining the probability of occurrence of storms of sufficient intensity and timing to cause damage that exceeds threshold levels; and (4) designing conservation systems to be protective at vulnerable times and during particular events.
Conservationists should take these three foregoing steps quickly. Policymakers and program managers need answers soon so they can begin to make the investment decisions and changes in operational policy needed to ensure U.S. soil and water resources are protected under a changing climate.