Soil Conservation Assessing the National Resources Inventory, Volume 1 (1986) / Chapter Skim
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3. The Measures of Soil Erosion
3. The Measures of Soil Erosion
Pages 34-61
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From page 34... ...
The topography of most fields causes runoff to collect and concentrate in a few major natural waterways or swales before leaving the fields (Foster, 1982; Thorne, 1984~. These features are often ephemeral, and the erosion that occurs in them can be called ephemeral gully, concentrated flow, or megarill erosion.
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From page 35... ...
NRI Estimates of Sheet and Rill Erosion: The USLE Developed in the late 1450s, the USLE is designed to predict longterm average soil losses through sheet and rill erosion from specific land areas under specified cropping and management systems. In a sense, soil loss is a misnomer; movement or displacement are better terms.
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From page 36... ...
The values for R are generally less reliable for western regions in any given year. This occurs because data from the West were insufficient to develop the equation, and rainfall there is intermittent and often torrential, causing average annual sheet and rill erosion estimates to vary greatly in those areas.
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From page 37... ...
The value for the RKLS product thus represents a soil's inherent potential for sheet and rill erosion. Any erosion control practices reduce soil loss on a particular field by lowering either or both of the C or P values below 1.
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From page 38... ...
The USLE was developed for a single crop per year; therefore, its application to double-cropping is limited. The distribution of acreage by the RKLS product presents a pattern similar to the distribution of acreage according to sheet and rill erosion rates.
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From page 39... ...
a, · Q o 5o o cn o 5a, a' En o CD ._ CD ._ Cal ~ 5- o O ·CD ~ so LO o u En .
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From page 40... ...
For all cropland, the inherent potential for sheet and rill erosion was 21.8 tons/ acre~year, but the effect of the C- and P-factor values was to reduce the estimated USLE rate to 4.3 tonslacre~year. The resulting reductionfactor values in Table 3-1 show that potential erosion rates were reduced most significantly on cropland used for hay (from 35.2 to 0.6 tons/acre~year, a 58.7 reduction factor and a 98 percent decrease in erosion)
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From page 41... ...
Such a publication would thoroughly explain the uses of the concept of inherent erosion potential for sheet and rill erosion. Similar guidelines could be issued for wind erosion, when appropriate.
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From page 42... ...
Source: Pierce et al., 1986. factor values and erosion rates as a function of progressively rising inherenterosion potential.
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From page 43... ...
, 105 (northern Mississippi Valley loess hills) , 134 (southern Mississippi Valley silty uplands)
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From page 44... ...
Source: Pierce et al., 1986. ing suggests that management and cropping practices beneficial in reducing soil erosion have not been concentrated on those lands that are in greatest need of erosion control.
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From page 45... ...
Theoretically, the range of possible variation in C-factor values, from 0.001 for undisturbed forestland to 1.0 for clean-tilled, fallow land, makes it the most important variable controlled by human activity in determining estimated sheet and rill erosion rates. In application, however, the full range of C-factor values is primarily most useful when erosion rates are compared among land uses- for example, when land is converted from row crops (average C value of 0.28)
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From page 46... ...
Credit: U.S. Department of Agriculture, Soil Conservation Service.
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From page 47... ...
. Thus, where fairly high C-factor values are encountered in the NRIs, sheet and rill erosion rates might be overstated.
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From page 48... ...
Erosion rates under alternative cropping and conservation conditions can be simulated for any land use (see the boxed article Erosion Under Alternative Cropping, Management, and Conservation Practices)
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From page 49... ...
FIGURE 3 - Percentage of acres nationally with USLE erosion rates greater than 5 tons/acre year at assumed levels of C factors for land in row and closegrown crops in 1982. Note: High- and medium-potential curves illustrate land with a high or medium potential for conversion to cropland.
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From page 50... ...
Under a more optimistic assumption, all land in row and closegrown crops is assigned a C-factor value of 0.1, about the average value expected if all land were farmed by no-till methocis the ultimate form of conservation tilIage and if heavy crop residue levels were maintained throughout the cropping year. Under these circumstances about 93 percent of the 323 million acres would experience sheet and rill erosion rates well below 5 tons/acre~year.
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From page 51... ...
challenge than land used for row and close-grown crops in 1982. With a C-factor value of 0.30 the average conditions for land in row and close-grown crops in 1982 only 60 percent of the country's high-potential cropland and 57 percentofthe medium-potential cropland would have sheet and rill erosion rates less than 5 tons/acre year.
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From page 52... ...
of procedures for estimating erosion rates, and analysis of basic erosion processes and effects. The USLE data can also be used to test new techniques that might be incorporated in future resource inventories to estimate erosion or assess other phenomena, such as nonpoint pollution.
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From page 53... ...
Conservation tillage no-till farming systems, in particular demonstrates exceptional promise for erosion control, provided that adequate levels of crop residues are maintained. On many soils, additional soil conservation benefits can be achieved by using traditional supporting conservation practices such as contour farming systems, vegetated waterways, terracing, and stripcropping in conjunction with conservation tillage systems.
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From page 54... ...
The l-factor value is expressed as the average annual soil loss expected to occur from an isolated, level, smooth, unsheltered fielcl, barren of vegetation and with a noncrusted surface. Originally, the I factorwasclerivecl from the total annual erosion (in tons/acre year)
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From page 55... ...
. The correct value is selected from SCS technical guides based upon field inspection.
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From page 56... ...
For this reason, it is not yet possible to use NRI data directly to refine the factors or the overall equation, or to undertake analyses of alternative options described previously in reference to the USLE. There are several reasons for the present variation in estimates of sheet and rill erosion rates in contrast to those for wind.
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From page 57... ...
In practical terms, the estimated average annual erosion rates reported for humid areas in the 1982 NRI are generally low. Yet those estimates are still probably too high.
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From page 58... ...
The committee recommends that this research program include · An assessment of an alternative formulation of a WEE; · Verification of WEE estimates, basic parameters, and concepts through field-level measurements; · Study of the deposition patterns of soil eroded by wind, with a special focus on air and water pollution consequences; · Reconsideration of the need for nationwide wind erosion data. It might be possible to reliably apply certain variables of the WEE, perhaps with modifications, as indicators of relative wind erosion hazard.
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From page 59... ...
Establishing grass-based sod in these areas is a highly effective erosion control practice, and they are referred to as grassed waterways. There is no commonly accepted, practical method for estimating ephemeral gully erosion in the field; USLE estimates of sheet and rill erosion do not independently predict or quantify ephemeral gully ero
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From page 60... ...
Evidence from field observations, measurements, and water quality models suggests that ephemeral gully erosion can result in sediment removal rates comparable to those for sheet and rill erosion (Foster, 1986~. (Sheet and rill erosion can be soil displacement not necessarily soil loss from a given field.)
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From page 61... ...
· Acceleration of research and educational programs conducted by the ARS, the SCS, experiment stations, universities, the extension service, and other USDA agencies on ephemeral gully erosion in regions where this problem is widespread. · Assessment of deposition patterns associated with ephemeral gully erosion to ascertain the extent of any changes in the productivity of landscapes where soil sediment is deposited.
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