Climate change impacts on soil erosion in Midwest United States with changes in crop management
Introduction
The consensus of atmospheric scientists is that climate change is occurring, both in terms of air temperature and precipitation. For instance, the year 1998 was likely the warmest of the last 1000 years in the Northern Hemisphere (IPCC, 2001), the year 2001 was second warmest on record (NCDC, 2002), and globally 9 of the 10 warmest years since 1860 have occurred since 1990 (WMO, 2001). Karl and Knight (1998) found that from 1910 to 1996, total precipitation over the contiguous U.S. increased, and 53% of the increase came from the upper 10% of precipitation events (the most intense precipitation). The percent of precipitation coming from 50-mm-or-more rain days also increased. Mean streamflow in U.S. watersheds also increased by approximately 1/6 from 1939 to 1999, and has been related to increasing precipitation (Groisman et al., 2001). Summarizing from over 30 climate and soil erosion related studies for the U.S., SWCS (2003) determined that the research pointed to increasing soil erosion and runoff in the future. They determined that the potential impacts were serious enough to warrant increased attention by conservationists on changing policies to prepare for the anticipated impacts of more severe erosion and runoff on soil and water resources.
Increasing air temperatures affect soil erosion indirectly in several ways. Warmer temperatures mean faster accumulation of the necessary growing degree-days for crop maturity, which can increase biomass production rates. In other cases warmer temperatures can limit crop production because of excessive temperatures (Pruski and Nearing, 2002a, Pruski and Nearing, 2002b). Temperature also impacts microbial activity levels, and hence residue decomposition rates. The level of carbon dioxide in the air also has a direct impact on the amount of biomass produced by various crops via direct CO2 fertilization effects (Stockle et al., 1992). Such biomass changes affect canopy and ground residue cover, which affect erosion rates. Increased CO2 can also enhance stomatal resistance, suppress transpiration, and lead to a moister soil, conducive to greater runoff-induced erosion (Schulze, 2000). Temperature can also influence evapo-transpiration rates, which impact soil moisture, which in turn may influence infiltration and runoff amounts and rates (Pruski and Nearing, 2002b).
Climate changes are also likely to be accompanied by changes in crop management, as farmers adapt their management practices to the new climate (Southworth et al., 2000, Southworth et al., 2002b, Southworth et al., 2002a, Southworth et al., 2002c, Pfeifer and Habeck, 2002, Pfeifer et al., 2002). For instance, decreased crop yields may lead the farmer to plant a new crop, or farmers may change planting dates of maize to take advantage of increased warmth or to avoid high temperatures during silking. Farmers may also plant crop varieties of different maturity type, thus affecting the timing and duration of soil cover. All of these changes in management affect the impacts of climate change on erosion, but have so far received little attention in the literature.
Several researchers have examined erosion under climate change without taking into account farmer adaptation. Favis-Mortlock and Boardman (1995), using the Erosion Productivity Impact Calculator (EPIC) model (Williams and Sharpley, 1989), found a 7% increase in precipitation could lead to a 26% increase in erosion in the United Kingdom. Lee et al. (1996), also applying EPIC, found that for the U.S. Corn Belt, a 20% precipitation increase gave a predicted 37% increase in erosion and a 40% increase in runoff. Panagoulia and Dimou (1997) predicted increases in both the length and frequency of flood episodes (double and triple average streamflow) in Greece, based on precipitation outputs from the GISS climate change model, which they linked to possible increased bed and bank erosion. Schulze (2000), using the CERES-Maize and ACRU models, predicted a 10% increase in precipitation would lead to a 20–40% increase in runoff in South Africa. With continuous soybeans in Brazil, Favis-Mortlock and Guerra (1999) predicted a − 9% to + 55% change in sediment yield for the year 2050 from three climate models, with the Hadley Centre climate model (HadCM2) showing a 22–33% increase in mean annual sediment yield with a 2% increase in annual precipitation, and monthly sediment yield increasing by up to 103%. Nearing (2001) predicted significant changes in mean annual erosivity over the U.S. for the 21st century using output from both the Canadian global coupled climate model (CGCM1) and the revised Hadley Centre climate model (HadCM3).
Pruski and Nearing (2002a) used HadCM3 model predictions coupled with the Water Erosion Prediction Project-Carbon Dioxide (WEPP-CO2) model (Flanagan and Nearing, 1995, Favis-Mortlock and Savabi, 1996, Nearing et al., 1989), and determined soil loss and runoff rates for the 21st century for eight locations in the United States. Their results indicated that in every case where precipitation was predicted to increase significantly, erosion increased significantly. In the locations where decreases in precipitation were predicted, erosion decreased in some cases and increased in others. Cases of predicted erosion increasing where precipitation decreased were attributed to large reductions in crop biomass production levels. These are prime examples where farmer adaptation should be accounted for. It is unlikely that a farmer will continue to grow a crop if production levels decrease greatly.
So far, few studies of erosion under climate change have looked at changes in crop management. This is important, because the impacts of management practices on erosion can be greater than the impacts of precipitation or air temperature, and many farmers will likely change crop rotations during this century. In South Africa, Schulze (2000) found with the ACRU model that time evolution of land cover significantly changed the rainfall–runoff relationship, from a combination of agricultural and urban land use changes. Focusing on cropland specifically, Schulze noted that changes in tillage type, planting date, and plant density could have a larger influence on hydrological responses than the conversion to another crop. In Denmark, Leek and Olsen (2000) found the proportion of annual erosivity contributed in the month of September to increase from 8% to 17%, as precipitation increased by 24–78% over the period of record. Government-mandated changes in cropping to autumn cereals over this period increased the percentage of bare soil in Denmark during this month, resulting in an intensified risk of erosion from the combination of climate change and crop management change.
In Ohio, USA, West and Wali (2002) found with the U.K. Meteorological Office GCM and the REM model (calibrating its empirical soil erosion component to 15 plots) that mined areas reclaimed with grassland or hayland would benefit from decreasing sediment yield under climate change (to the year 2050), related to increased biomass and surface litter from enhanced carbon dioxide levels. In northern China, Gao et al. (2002) found that 40 years of historical climate change alone would have decreased water erosion, but land use changes from grasslands to dry crop fields more than compensated for climate, increasing water erosion by at least a factor of eight, and intensifying the already increased wind erosion associated with rising air temperature.
In this study, changes in future crop management are taken into account, in addition to changes in climate, to investigate impacts of climate change on erosion in the Midwestern United States. Crops included maize (Zea mays L.), soybeans (Glycine max Merrill), and wheat (Triticum aestivum L.). The investigation was performed using the results of the yield and market profitability studies conducted by Southworth et al., 2000, Southworth et al., 2002a, Southworth et al., 2002b, Southworth et al., 2002c, Pfeifer and Habeck (2002) and Pfeifer et al., (2002). Soil loss and runoff were then predicted with an erosion model and a climate model for 2040–2059, and results were compared to crop and climate conditions for 1990–1999. Economically viable crop rotations and optimal planting dates were used for erosion simulations under future climate scenarios.
Section snippets
Study area and time period
The study area was five states of the Midwestern U.S.: Illinois, Indiana, Michigan, Ohio, and Wisconsin. These were divided into 11 regions (Fig. 1), as used by Southworth et al. (2000), corresponding roughly to the Land Resource Regions of the Natural Resources Conservation Service (NRCS). In the simulation of soil erosion and climate, the years 1990–1999 were considered for baseline conditions, while 2040–2059 were used for future climate change.
Crop rotations
The crops accounted for in this study were
Climate modeling
Eight of the 11 regions had over 5% increased predicted annual precipitation in 2040–2059 relative to the baseline (Table 4). Every region showed a decrease in July precipitation and an increase in October precipitation for 2040–2059 relative to the baseline time period (monthly data not presented).
Erosion modeling
Runoff and soil loss increased in the future scenarios compared to the baseline. WEPP-CO2 predicted increases in 10 of the 11 regions of + 10% to + 274% in soil loss, with a wider range of increase in
Discussion
Soil properties, changes in the timing of precipitation, and changes in planting date may intensify, lessen, or reverse the general pattern of changes in soil loss and runoff. For example, soils with higher hydraulic conductivity may show a greater percentage increase in runoff as a function of increased precipitation than soils with lower conductivities. Earlier soybean planting dates provide crop cover during the spring. May and July modeled precipitation was less for 2040–2059 for east
Conclusions
Soil loss and runoff were predicted to increase throughout nearly all of the eastern U.S. Corn Belt for the period from 1990–1999 to 2040–2059 based on a series of simulations using the Water Erosion Prediction Project-Carbon Dioxide (WEPP-CO2) erosion model with climate from the Hadley Centre model (HadCM3-GGa1) and yield prediction from a previous climate change study using the Decision Support System for Agrotechnology Transfer (DSSAT). The erosion simulations included a prediction of
Acknowledgements
Thanks to Mike Habeck, for crop and economic modeling and information; Otto Doering, for facilitating information exchange between co-authors and linking to previous studies; Glenn Weesies, for providing files and access to the RUSLE database; David Viner, for climate model outputs and assistance with HadCM3; Dennis Flanagan and Jim Frankenberger, for help with running and choosing parameters for WEPP; Fernando Pruski, for help with CO2 parameters, wet-day probabilities, and HadCM3; Nathaniel
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