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Soil Fragility

About 23% (97 million acres) of cropland and rangeland in the Northern Plains has fragile soils with tolerable soil-loss levels of less than 3 tons per acre annually. About 43% of the rangelands are associated with fragile soils (Figure 2). Of the 64 million acres of forest lands, nearly 64% occur on fragile soils (Figure 3).

Figure 2. Rangelands with fragile soils in the Northern Plains region. By W.J. Waltman and B. Stephens, USDA, NRCS. Source: State Soil Geographic Database (Soil Survey Staff, 1994), Albers Equal Area Projection, Dec. 1995, Map Series No. 95-0017, Northern Plains GIS/Remote Sensing. Rangelands with fragile soils are derived from the State Soil Geographic Database and the 1:250000 Land Use and Land Cover Digital Data (USGS, 1986). Fragile soils are defined by tolerable (T) levels of erosion that are less than 3 tons/acre/year.

Figure 3. Forest lands with fragile soils in the Northern Plains region. By W.J. Waltman and B. Stephens, USDA, NRCS. Source: State Soil Geographic Database (Soil Survey Staff, 1994), Albers Equal Area Projection, Dec. 1995, Map Series No. 95-0018, Northern Plains GIS/Remote Sensing. Forest lands with fragile soils are derived from the State Soil Geographic Database and the 1:250000 Land Use and Land Cover Digital Data (USGS, 1986). Fragile soils are defined by tolerable (T) levels of erosion that are less than 3 tons/acre/year.

Conservation Needs

The 1992 NRI shows a total of 68 million acres of cropland, forest land, and pastureland needing conservation treatment. Approximately 45% of the cropland and 35% of the pastureland need conservation treatment for either wind or water erosion.

An estimated 236 million tons of soil erode annually in Montana, Colorado, and Kansas, three of the five states nationally which have the most soil erosion. About 60 million acres of cropland, excluding Conservation Reserve Program (CRP) lands, are considered highly erodible for conservation compliance (Table 1; Figure 4). About 11 million acres of cropland are eroding at rates greater than tolerable levels (T) and, of these, 4 million acres are eroding at rates exceeding 2T.

Figure 4. Distribution of Conservation Reserve Program (CRP) lands in the Northern Plains region. By W.J. Waltman and B. Stephens, USDA, NRCS. Source: National CRP Database, USDA/NRCS (1995), Albers Equal Area Projection, Dec. 1995, Map Series No. 95-0029, Northern Plains GIS/Remote Sensing. In the Northern Plains Region, CRP lands amounted to 14.7 million acres (6 million ha). The percent CRP lands derived from total croplands was derived for each county.

Erosion rates

Producers carrying out conservation plans made significant progress in reducing sheet, rill, and wind erosion. Between 1982 and 1992, sheet and rill erosion decreased by 19% across the 110 million acres of cultivated cropland in the region, annually saving more than 53 million tons of topsoil.

Similarly, average annual wind erosion on rangeland decreased 7.5%, saving nearly 47 million tons of topsoil each year. Wind erosion on cultivated cropland decreased 30%.

These successes reflect a reduction of 25 million acres needing conservation measures.

Irrigation and erosion

Irrigated cropland (Figure 5) increased by 468,000 acres during the period between 1982 and 1992. Gravity irrigation systems were used on over 6.3 million acres of the 13.6 million of irrigated cropland in 1992. Over 71% of these acres needed conservation treatment for irrigation-induced erosion.]

Figure 5. Irrigated cropland 1992, Northern Plains region. USDA, NRCS, Lambert Conformal Conic Projection, 1927 North American Datum. Source: National Cartography and GIS Center, NRCS, USDA, Ft. Worth, TX, in cooperation with the natural Resources Inventory Division, NRCS, USDA, Washington, D.C., using GRASS/MAPGEN software, 09/95. Map based on data generated by NRI Division using 1992 NRI. Because the statistical variance in some of these areas may be large, the map reader should use this map to identify broad trends and avoid making highly localized interpretations.

Sprinkler irrigation systems were used on over 6.9 million acres, of which 60% needed conservation treatment.

An additional 395,000 acres were under a combination of sprinkler and gravity irrigation systems, and 68% of these acres also needed conservation treatment.

Conservation tillage

Across the region, conservation tillage practices are applied to approximately 37% of annual crop acres (Table 2). Nebraska has shown the greatest adoption of conservation tillage practices, while Wyoming has shown the least.

Between 1989 and 1994, no-till expanded from 2.9 to 8.0% of total planted acres. Given the estimates of conservation tillage management in the region, considerable opportunity remains to encourage more adoption of conservation tillage.

Soil Salinity

Soil salinity is more widespread in the Northern Plains than anywhere else in the U.S. (Figure 6). Salinity adversely affects crop growth in the region's most northern states. Saline conditions in the root zone severely affect nearly 10% of Northern Plains landscapes.

Figure 6. Salinity-affected landscapes of the Northern Plains region. By W.J. Waltman and B. Stephens, USDA, NRCS. Source: R. Srinivasan (1995); Blackland Research Center, Texas Agric. Expt. Station, Temple, TX 76502, Albers Equal Area Projection, Dec. 1995, Map Series No. 95-0019, Northern Plains GIS/Remote Sensing. Soils and landscapes affected by salinity problems were derived from the State Soil Geographic Database (Soil Survey Staff, 1994). The areas of the saline-affected soils were based on the presence of a horizon with greater than 4dS/m. within 50 cm of the surface. The electrical conductivity estimates of the soils followed the saturated paste method of the Soil Survey Staff (1995).

Climate Change

Even small changes in climate affect soil health and agricultural production. Given CAST (Council for Agricultural Science and Technology; 1992) and OTA (Office for Technology Assessment; 1992) projections of 3.6 to 5.4°F (2 to 3°C) increases of mean annual air temperatures by the next century, climate change and its associated greater variability may have a significant impact on soil quality, crop management systems, and conservation practices across the Northern Plains.

Temperature

According to both CAST (1992) and OTA (1993), a 5.4°F (3°C) increase in mean annual air temperature (MAAT) can be projected for the region around the year 2030.

The Newhall Simulation Model (Van Wambeke et al., 1991), which was used to estimate shifts in soil temperature regime, indicates that a 5.4°F (3°C) increase in MAAT would shift the thermic regime (warm temperate) from Kansas to Nebraska and could remove tundra and permafrost regimes from the Rocky Mountains. Frigid soils would decrease from 16% of the region to 4%. The warm-phase (54-58°F; 12-14°C) of the mesic soil temperature regime would expand from 15 to 38% and thermic soils would expand from 6 to 21%.

Evapotranspiration and growing-degree days

Associated with the temperature increase would be parallel increases in potential evapotranspiration and growing-degree days-- 13% and 36%, respectively.

Regionally, some geographic areas could benefit climatically, while the ecosystems of others could become unsustainable. Since 44% of the region has less than 2,000 growing-degree days, and 29% of the landscape has less than a 100-day frost-free period, an increase in MAAT may allow expansion of the winter wheat belt and corn belt.

Potential effects of changes

The period of 1931 to 1940 often has been used to illustrate the potential impacts of climate change in 2030. Statistics from 116 long-term weather stations in the Northern Plains therefore were compared to quantify the differences in climatic character between the "Dust Bowl" years and the "normal" years of 1961 to 1990.

Table 3 summarizes the climatic variability differences between the two periods. Notably, the mean air temperature across the region increased by only 1.7°F (~ 1°C) during the Dust Bowl years. But mean annual precipitation decreased by 15%.

Growing-degree days showed more change than frost-free period. The biological window (the time in cumulative days when soils are moist and warmer than 40°F [5°C], which quantitatively describes the period of soil microbial activity) decreased nearly 34 days during the Dust Bowl years.