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Sheet and Rill (Water) Erosion Findings

State of Oregon
1997 National Resources Inventory Results
(Revised December 2000)
 

See Water Erosion Topic List for detailed results.
 

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Background

Sheet and rill erosion is the removal of layers of soil from the land surface by the action of rainfall and runoff. It is the first stage in water erosion. NRI estimates of sheet and rill erosion do not include loss of soil from concentrated sources, such as gully and streambank erosion.

NRI estimates for soil loss from sheet and rill (water) erosion on non-federal lands are calculated using the Universal Soil Loss Equation (USLE) model. The USLE erosion model is designed to predict the long-term average soil losses in runoff from specific field areas in specified cropping and management systems.

USLE estimates for sheet and rill erosion are calculated for cropland, pastureland, and Conservation Reserve Program (CRP) land only. Estimates are not made for rangeland, forestland or other land uses.

The equation is: A = RKLSCP where:
A = Computed soil loss per unit area
R = Rainfall and runoff factor
K = Soil erodibility factor
L = Slope-length factor
S = Slope-steepness factor
C = Cover and management factor
P = Support practice factor

The NRI calculations use location-specific data for the field in which the NRI sample point falls or that portion of the field surrounding the point that would be considered in conservation planning.

One goal of USDA-NRCS conservation planning efforts with private landowners is to reduce sheet and rill erosion to a rate of annual soil loss that permits crop productivity to be sustained economically and indefinitely on a given soil. This maximum rate of sustainable soil loss is called the soil loss tolerance factor (T Factor). T factors range from 1 to 5 tons per acre per year.

 

Reduction in Sheet and Rill Erosion Rates

The NRI estimates indicate a significant decrease in average sheet and rill erosion rates on agricultural lands in Oregon from 1982 to 1997. For example, rates on cultivated cropland fell 35% from 4.6 to 3.0 tons per acre per year.
 

Estimated Annual Rates of Sheet and Rill Erosion in Oregon Tons/Acre/Year
Year Cultivated
Cropland		 Non-Cultivated
Cropland		 Pastureland CRP Land
1982 4.6 0.7 0.6 No Data
1987 3.4 0.5 0.5 2.9
1992 3.2 0.4 0.5 0.4
1997 3.0 0.4 0.4 0.4
Percent Change
1982 to 1997		 - 35% - 43% - 33% No Data

Adobe Acrobat DocumentChart for Water Erosion Rate on Cropland, Pastureland, & CRP Land (23 KB).
Adobe Acrobat DocumentTable for Water Erosion Rate on Cropland, Pastureland, & CRP Land (9 KB).

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    Reduction in Total Tons of Soil Lost Due to Sheet and Rill Erosion by Water

    The total amount of soil being lost from agricultural lands may be more important to the health of Oregon’s environment and watersheds than the rate of soil loss. From 1982 to 1997, NRI estimates indicate a 46% reduction of total tons of soil lost per year from sheet and rill erosion on cropland, pastureland, and CRP land in Oregon. This amounts to 8.1 million tons of soil saved on Oregon’s agricultural lands every year.
     

    Total Estimated Tons Per Year of Soil Lost From Sheet and Rill Erosion in Oregon
    Year Cultivated
    Cropland    		 Non-
    Cultivated
    Cropland    		 Pastureland CRP Land Total Tons Per Year
    1982 15,958,600 595,000 1,228,800 No Data 17,782,400
    1987 9,996,700 479,100 1,020,900 1,125,600 12,622,300
    1992 8,908,700 425,700 992,800 202,300 10,529,500
    1997 8,157,600 467,800 877,400 175,900 9,678,700
    1982 to 1997
    Reduction
    (Tons/Yr)    		 - 7,801,000 - 127,200 - 351,400 No Data - 8,103,700
    Percent
    Reduction
    1982 to 1997    		 - 49% - 21% - 29% No Data - 46%

    Adobe Acrobat DocumentChart for Tons/Year Lost on Cropland, Pastureland, & CRP Land (25 KB).
    Adobe Acrobat DocumentTable for Tons/Year Lost on Cropland, Pastureland, & CRP Land (7 KB).

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    In 1997, about 84 percent of the sheet and rill erosion occurred on cultivated cropland, followed by pastureland (9%), non-cultivated cropland (5%), and CRP land (2%).

     

    Sheet and Rill Erosion in Relation to the Soil Loss Tolerance Factor (T)

    The acres of Oregon cropland, pastureland, and CRP land eroding at a rate above the sustainable soil loss level were significantly reduced between 1982 and 1997. The estimated 1.6 million acres of agricultural land eroding above T in 1982 were reduced to less than a million acres eroding above T in 1997. The percentage of agricultural lands eroding above T was reduced from 25% in 1982 to 15% in 1997.
     

    Sheet and Rill Erosion Rates in Relation to the T Factor Acres of Cropland, Pastureland, and CRP Land in Oregon
    Year Less Than
    or = T    		 Greater
    Than T    		 % Greater
    Than T    		 Total
    Estimated
     Acres
    1982 4,827,900 1,582,200 25% 6,410,100
    1987 5,114,700 1,259,100 20% 6,373,800
    1992 5,312,300 987,800 16% 6,300,100
    1997 5,245,000 960,000 15% 6,205,000

    Adobe Acrobat DocumentChart for Soil Loss in Relation to T on Cropland, Pastureland, & CRP Land (33 KB).
    Adobe Acrobat DocumentTable for Soil Loss in Relation to T on Cropland, Pastureland, & CRP Land (10 KB).

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    The fact that there were an estimated 960,000 acres eroding above the sustainable sheet and rill erosion level in 1997 indicates there is still conservation work to be accomplished in Oregon.

     

    Discussion

    The total soil savings from reduced sheet and rill erosion (1982 to 1997) on all agricultural lands amounted to 8.1 million tons per year. Much of the reduction in Oregon was due to the adoption of conservation cropping systems that left more residue on the surface and to the installation of physical erosion treatment measures such as terraces. For example, average sheet and rill erosion rates on all cultivated cropland was reduced from 4.6 tons/acre/year in 1982 to 3.0 tons/acre/year in 1997.

    Another significant reason for lower erosion rates was highly erodible and environmentally sensitive cropland being converted to permanent vegetative cover through enrollment in the voluntary Conservation Reserve Program (CRP). Estimated erosion rates on cultivated croplands that were enrolled in the CRP were dramatically reduced.

    These cultivated croplands, which were eroding at an average rate of 7.2 tons/acres/year in 1982, before they were enrolled in the CRP, were eroding at an average rate of .4 tons/acre/year in 1997, after establishment of permanent cover. This is a 94% decrease in the erosion rate. Statewide results for all agricultural lands indicate that these CRP lands accounted for 37% of the total tons of erosion reduction from 1982 to 1997 in Oregon.

     

    To see all results, select Geographic Area.

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    Last Modification: Monday, July 28, 2003

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