The Development and Use of Spatially Explicit Erodible Soil Indices for Nebraska

Transcription

1 The Development and Use of Spatially Explicit Erodible Soil Indices for Nebraska By The Rainwater Basin Joint Venture and The U.S. Department of Agriculture Natural Resources Conservation Service May 2014 Contributing Authors: Andrew Bishop, Rainwater Basin Joint Venture Coordinator, Rainwater Basin Joint Venture, Grand Island, NE Neil Dominy, State Soil Scientist, U.S. Department of Agriculture s Natural Resources Conservation Service, Lincoln, NE Roger Grosse, GIS Specialist, Rainwater Basin Joint Venture, Grand Island, NE Christopher Jorgensen, Contracting Wildlife Biologist, Rainwater Basin Joint Venture, Grand Island, NE Kelly Klenke, Resource Assessment/GIS Specialist, U.S. Department of Agriculture s Natural Resources Conservation Service, Lincoln, NE Daniel Shurtliff, Assistant State Soil Scientist, U.S. Department of Agriculture s Natural Resources Conservation Service, Lincoln, NE -i-

2 Suggested Citation: Rainwater Basin Joint Venture The development and use of spatially explicit erodible soil indices for Nebraska. Rainwater Basin Joint Venture Report, Grand Island, NE, U.S.A. Table of Contents Introduction... 3 Methods... 5 Data Procurement and Processing... 5 Calculating and Developing Erosion Indices... 5 Results and Discussion... 7 References Appendix A: Visual Basic Script for calculating LS (Combined Effects of Slope Length and Steepness) Appendix B: The R and C Factor Scores by County used to Calculate Water and Wind Erodibility Indices for Nebraska ii-

3 INTRODUCTION Globally, anthropogenically accelerated soil erosion has significantly increased over the past 60 years and is widely attributed to land degradation throughout the world (Oldeman et al. 1990). Unlike the slow geologic erosion process, which is caused by displacement of soil over time by a variety of mechanisms (e.g., flooding, runoff, wind, gravity, etc.) and can be beneficial for soil fertility renewal, human-induced accelerated soil loss can be extremely destructive (i.e., United States 1930s Dust Bowl; Schubert et al. 2004). Wind and water erosion can remove fine organic particles from the substrate, leaving behind a coarse, nutrient-depleted soil. Severely nutrient-depleted soils can stunt plant growth and reduce crop yields up to 30% compared to uneroded soils (Follet & Stewart 1985). And even more alarming, continuous soil loss can lead to arable land becoming unproductive. As land is rendered infertile, grassland and forests are continually being cut, plowed, and converted to fulfill agricultural demands (Myers 1989). Additional costs to society, including sedimentation and pollution of waterways, property damage, and impairment to fisheries, are some of the many additive effects of culturally accelerated soil erosion and are externalized to local communities, states, and regions. In the United States, anthropogenically accelerated soil erosion has consistently been an issue in regions that use traditional cropping practices. Consequently, in the last century various federal and state programs have been established to offer landowners incentives to reduce soil erosion by taking lands containing highly erodible soils out of production and putting them into various soil conservation practices (e.g., 1956 Soil Bank program, the 1985 Conservation Reserve Program; Cain & Lovejoy 2004; U.S. Department of Agriculture, Natural Resources -3-

4 Conservation Service 2012). Although we are more aware of the negative effects and mitigation of soil erosion in today s agricultural practices, soil erosion continues to be an issue. In order to help pinpoint areas with potentially elevated wind and water erosion, various erodibility models have been developed (e.g., Universal Soil Loss Equation [USLE], Revised Universal Soil Loss Equation [RUSLE], Wind Erosion Equation [WEQ], etc.; Woodruff & Siddoway 1965; U.S. Department of Agriculture 1978; Renard et al. 1991, Renard et al. 1997). Although the USLE was originally intended to assist soil conservationists in planning farming practices (Renard et al. 1991), today soil erosion indices are useful to professionals in a broad set of disciplines, including agricultural economists, wildlife biologists, and foresters. Ecologists and conservationists use soil erosion indices to strategically target land parcels for conservation or farming practices that can reduce soil loss and increase wildlife habitat (e.g., establish prairie for grassland birds), while still meeting the requirements of the land manager or farmer. Moreover, since their inception, various soil indices have been employed to estimate the regional and national impacts of anthropogenically accelerated soil erosion, and to develop public policy. To help pinpoint specific regions in Nebraska, U.S.A. that contain highly erodible soils and are well suited for establishing soil conservation practices, the Rainwater Basin Joint Venture worked in cooperation with the U.S. Department of Agriculture s Natural Resources Conservation Service to construct a Wind and Water Erodibility Index using the Soil Survey Geographic (SSURGO) database (U.S. Department of Agriculture Natural Resources Conservation Service 2011). Our objectives were to produce statewide, spatially explicit Geographic Information System (GIS) layers indicating highly erodible soils to aid in conservation planning and design. -4-

5 METHODS Data Procurement and Processing We obtained statewide spatially explicit gridded soil survey data for Nebraska from the Soil Survey Geographic (SSURGO) database (U.S. Department of Agriculture Natural Resources Conservation Service 2011). The chorizon, chtexture, chtexturegrp, mapunit, and mutext tables in the Gridded SSURGO database were joined together using the mukey attribute field in a geographic information system (GIS). The representative values for slope (rvslope) and slope length (rvslopelenusle), the susceptibility of the soil to water erosion (Kw), and the soil loss tolerance (t_fact) values were obtained from the set of joined tables and were included in the Water Erosion Index calculation. We acquired county-specific rainfall and runoff factor values (R) from the U.S. Department of Agriculture s Natural Resources Conservation Service (NRCS). Fields used in calculating the Wind Erosion Index calculation (Woodruff & Siddoway 1965), such as the susceptibility of soils to wind erosion (wei), and the county-specific climate characterization of wind speed and surface soil moisture values (C), were obtained from the joined SSURGO tables or from the NRCS, respectively. Calculating and Developing Erosion Indices We used a modified version of the Universal Soil Loss Equation (USLE) to create the Water Erosion Index for Nebraska (U.S. Department of Agriculture 1978). In order to calculate the combined effects of Slope Length and Steepness (LS) on water erosion, we first adjusted any null representative slope length values (rvslopelenusle) using a predefined criterion (see Appendix A; U.S. Department of Agriculture 1978). Furthermore, we adjusted the LS equation -5-

6 to more accurately compute soil loss on irregular slopes (i.e., appreciably convex, concave, or complex) such as those found in cropland, ranging from 3 to 18 percent steepness (U.S. Department of Agriculture 1978). We added an exponent (m) to the LS equation (Appendix A), where m is 0.5 if the percent slope is 5 or more, 0.4 on slopes of 3.5 to 4.5 percent, 0.3 on slopes of 1 to 3 percent, and 0.2 on uniform gradients of less than 1 percent. Based on the LS equation for irregular slopes (see U.S. Department of Agriculture 1978), we calculated out LS using: (( (0.0456* rvslope)+ ( *(rvslope^2)))*((rvslopelenusle / 22.1) ^m)) We combined the remaining factors used in the USLE with the LS values, and the final index for sheet and rill erosion was calculated using the formula: R* Kw* LS t _ fact In order to estimate the potential erodibility caused by wind, we used a modified version of the Woodruff and Siddoway (1965) Wind Erosion Equation (WEQ). We reduced the climate characterization of wind speed and surface soil moisture values (C) by a factor of 10. We then multiplied the values representing the susceptibility of soils to wind erosion (wei) with the reduced C values and divided the product by the potential maximum annual rate of soil erosion that can take place without reducing long-term productivity (t_fact). The potential Wind Erodibility Index calculation is represented by the formula: ( C*0.1)* wei t _ fact -6-

7 To identify areas in Nebraska containing highly erodible soils, we modified the wind erosion and water erosion indices by reclassifying raster values greater than 8 to a value of 1, and everything else to a value of 0, using a GIS. The two reclassified erosion indices were combined into a single GIS raster dataset. Soils in the resulting raster dataset are considered highly erodible if either the wind erosion or water erosion index has an erosion index value greater than 8. The final Highly Erodible Soils Index raster dataset has a resolution of 10 x 10m. RESULTS AND DISCUSSION Water Erosion Index values ranged from 0 314, with higher values representing greater erosion potential. The resulting Water Erosion Index for Nebraska pointed to a number of regions throughout the state as having an elevated potential for severe rill and sheet erosion. Parts of the Missouri River valley, Panhandle (i.e., Pine Ridge), the southwest (i.e., Loess Canyons), and central (i.e., Loess Hills) regions of the state are all indicated as having high water erosion potential (Figure 1). Even more alarming, many of these regions are considered Biologically Unique Landscapes in the Nebraska s State Wildlife Action Plan (Schneider et al. 2011) and represent biodiversity hotspots in the state. These regions are prime examples of areas where soil conservation practices may help reduce soil loss due to water erosion and may provide additional benefits to wildlife by improving or even increasing habitat. -7-

8 Figure 1. The spatially explicit Water Erosion Index for Nebraska. Index values range from 0 314, withe larger values (dark red) indicating higher soil erodibility potential caused by rill and sheet erosion. The Wind Erosion Index values ranged from 0 66, with higher values representing more erosion potential. The resulting model highlighted regions containing loose, sandy soils as being susceptible to erosion (Figure 2). Not surprisingly, a large portion of the Sandhills region in central Nebraska was indicated as being at risk for wind erosion (Figure 2). Additional regions, including parts of the Panhandle and the southwestern corner of the state, are also indicated as having high potential for wind erosion. -8-

9 Figure 2. The spatially explicit Wind Erosion Index for Nebraska. Index values range from 0 66, with larger values (dark red) indicating higher soil erodibility potential caused by wind. Based on the soil properties and the wind and water erosion calculations, the resulting Highly Erodible Soils Index for Nebraska highlights a large portion of the state as having high soil erodibility potential (Figure 3). Given the low erodibility threshold value of eight, which was the cut-off value used to reclassify the wind and water erosion indices (Figures 1 & 2) as highly erodible or not highly erodible, we suspect the model is a conservative estimate of what is truly at risk for soil erosion. However, given the significant amounts of land in the state containing loess or sandy soils, it is not surprising that much of the soil in the state is considered as highly erodible. Farmers, land operators and managers may want to take special precautions when planning farming or other land-use practices in these at-risk regions of the state. -9-

10 The resulting wind, water, and highly erodible soil indices may be an effective means to pinpoint regions in need of special conservation attention, and certainly have been used in similar fashion in the past (Woodruff & Siddoway 1965; U.S. Department of Agriculture 1978; Renard et al. 1991; Renard et al. 1997). Furthermore, these GIS layers can be used in conjunction with other spatially explicit datasets for Nebraska, such as Groundwater Management Areas (established by Nebraska s Natural Resources Districts) or species distribution models, to better inform decision makers as to what conservation practices are most appropriate to meet multiple objectives, and where to implement conservation programs in the landscape to gain the most benefit from each conservation dollar. Decision support systems of this nature allow policy makers and managers a means to compromise and meet multiple objectives established by stakeholders from a variety of conservation disciplines. -10-

11 Figure 3. The Highly Erodible Soils Index was constructed by reclassifying Water Erodibility and Wind Erodibility Indices for Nebraska, where any water or wind erosion values over 8 were reclassified as being highly erodible. -11-

12 REFERENCES Cain, Z., and Lovejoy, S History and outlook for Farm Bill conservation programs. Choices 19: Follet, R.F., and Stewart, B.A Soil Erosion and Crop Productivity. American Society of Agronomy and Crop Science Society of America. Madison, Wisconsin, U.S.A. Myers, N Deforestation Rates in Tropical Forests and their Climatic Implications. Friends of the Earth Report, London. Oldeman, L., Hakkeling, R., and Sombroek, W World map of the status of soil degradation, an explanatory note. International soil reference and information center, Wageningen, The Netherlands and the United Nations Environmental Program, Nairobi, Kenya. Renard, K. G., Foster, G. R., Weesies, G. A., and Porter, J. P RUSLE: Revised universal soil loss equation. Journal of Soil and Water Conservation 46: Renard, K. G., Foster, G. R., Weesies, G. A., McCool, D. K., and Yoder, D. C Predicting soil erosion by water: A guide to conservation planning with the revised universal soil loss equation (RUSLE). Agriculture Handbook, 703, Washington, D.C., U.S.A. Schneider, R., Stoner, K., Steinauer, G., Panella, M., and Humpert, M The Nebraska Natural Legacy Project: State wildlife action plan, 2 nd edition. Nebraska Game and Parks Commission, Lincoln, Nebraska, USA. Schubert, S. D., Suarez, M. J., Pegion, P. J., Koster, R. D., and Bacmeister, J. T On the cause of the 1930s Dust Bowl. Science 303: U.S. Department of Agriculture Predicting rainfall erosion losses: A guide to conservation planning. U.S. Department of Agriculture, Agriculture Handbook 537. U.S. Department of Agriculture, Natural Resources Conservation Service SSURGO soil data. Available online at < U.S. Department of Agriculture, Natural Resources Conservation Service Conservation Reserve Program. Available online < Woodruff, N.P. and F.H. Siddoway A Wind Erosion Equation. Soil Science Society of America Proceedings 29:

13 APPENDIX A: VISUAL BASIC SCRIPT FOR CALCULATING LS (COMBINED EFFECTS OF SLOPE LENGTH AND STEEPNESS) Comment: The representative slope length value adjustment if it is listed as null DEFINE rvslopelenusle IF NOT ISNULL(slopelenusle_r) Then (slopelenusle_r) Else If rvslope >= 0 And rvslope < 0.75 Then 31 Else If rvslope >= 0.75 And rvslope < 1.5 Then 61 Else If rvslope >= 1.5 And rvslope < 2.5 Then 91 Else If rvslope >= 2.5 And rvslope < 3.5 Then 61 Else If rvslope >= 3.5 And rvslope < 4.5 Then 55 Else If rvslope >= 4.5 And rvslope < 5.5 Then 49 Else If rvslope >= 5.5 And rvslope < 6.5 Then 46 Else If rvslope >= 6.5 And rvslope < 7.5 Then 43 Else If rvslope >= 7.5 And rvslope < 8.5 Then 40 Else If rvslope >= 8.5 And rvslope < 9.5 Then 38 Else If rvslope >= 9.5 And rvslope < 10.5 Then 37 Else If rvslope >= 10.5 And rvslope < 11.5 Then 33 Else If rvslope >= 11.5 And rvslope < 12.5 Then 31 Else If rvslope >= 12.5 And rvslope < 13.5 Then 27 Else If rvslope >= 13.5 And rvslope < 14.5 Then 24 Else If rvslope >= 14.5 And rvslope < 15.5 Then 21 Else If rvslope >= 15.5 And rvslope < 17.5 Then 18 Else 16 Comment: The calculation for the exponent used in the LS equation DEFINE m if (rvslope <1) then (0.2) else (If (rvslope <3) then (0.3) else (If (rvslope <5) then (0.4) else (0.5))) Comment: The final calculation used to produce the LS value in the USLE DEFINE ls (( ( * rvslope) + ( *( rvslope ** 2)))*((rvslopelenusle / 22.1) **m)) -13-

14 APPENDIX B: THE R AND C FACTOR SCORES BY COUNTY USED TO CALCULATE WATER AND WIND ERODIBILITY INDICES FOR NEBRASKA -14-

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