Lab #7 – Soil Erosion
(50 points)
In order to estimate current water erosion rates, and to prescribe best management practices (BMPs) that reduce erosion rates, an accurate method to estimate soil erosion was needed. The first model that was used to estimate erosion is known as the Universal Soil Loss Equation (USLE). It is an empirical model, meaning that it was developed from experimentation. It was later revised to form the Revised Universal Soil Loss Equation (RUSLE). For both models, the following equation was used:
? = ? × ? × ?? × ? × ?
Where A is the annual soil loss (tons/ac), R is the rainfall erosivity, K is the soil erodibility, LS is the combined factor of the length of slope and the slope gradient, C is for cover, and P is erosion control factors. The R and K factors cannot be changed through management. The LS factor can be changed through terracing. The C factor can be changed with crop residue management, such as through conservation tillage or cover crops. The P factor can be changed through the use of strip-cropping, contour tillage, or terracing.
The information required for this calculation can be found from a variety of sources. The R factor can be found on maps made available from the Natural Resources Conservation Service (NRCS) or through the extension service. The K factor can be found in the Web Soil Survey. The LS factor is determined from a table of factors (assuming that you have collected the length of the slope and the slope gradient from your field site) or from a topo map. The C factor can be estimated using tables, often from the extension service. Lastly, P can be found from tables provided by the NRCS.
You are going to estimate the erosion rate for the Cushman loam, 1 to 5 percent slopes map unit located in El Paso County, Colorado. The erosivity factor (R) for the NE corner of El Paso County is 100.
You will find the K value using the Web Soil Survey. Using your computer, navigate to the Web Soil Survey. Navigate to El Paso County, Colorado, then make your AOI (small box) around the town of Calhan (located in NE El Paso County). Then click on the “Soil Map” tab. You will see several map units in the “Map Unit Legend”. Scroll down and make sure you have Cushman loam, 1 to 5 percent slopes (map unit symbol 22) in your legend.
Click on the “Soil Data Explorer” tab, then click on the “Soil Properties and Qualities” sub-tab. Expand “Soil Erosion Factors, and click on “K Factor, Whole Soil”. Click on “View Rating” to load that data onto the map. Scroll down, and note the “Rating” for the 22 Map Unit Symbol. Record it below.
Next you will determine the topographic (LS) factor. The percent slope is needed. Typically, this is measured in the field using a clinometer. However, today we will assume that it is a 4% slope (the soil mapping unit description is 1 to 5% slope). Using Table 8.1, determine the LS factor given the length of slope of 150 feet with a 4% slope.
Note that if your value falls between two listed values in the table, it is recommended that you use either the higher value for slope gradient and/or length of slope, so as to calculate a more conservative erosion estimation (i.e. higher erosion rate).
Table 8.1. Values for Topographic Factor (LS). (Adapted from Jones et al. (1988) with permission).
Slope Gradient Slope Length (ft)
(%) 50 100 150 200 300 400 600
_______________________________________________________________________
2 0.16 0.20 0.23 0.25 0.28 0.30 0.34
4 0.30 0.40 0.47 0.53 0.62 0.70 0.82
6 0.49 0.67 0.82 0.95 1.17 1.35 1.65
8 0.70 0.99 1.21 1.41 1.72 1.98 2.43
10 0.97 1.37 1.68 1.94 2.37 2.74 3.36
12 1.28 1.80 2.21 2.55 3.13 3.61 4.42
14 1.62 2.30 2.81 3.25 3.98 4.59 5.62
16 2.01 2.84 3.48 4.01 4.92 5.68 6.95
18 2.43 3.43 4.21 4.86 5.95 6.87 8.41
20 2.88 4.08 5.00 5.77 7.07 8.16 10.00
_________________________________________________________________________
?1 = ? × ? × ?? =
?1 =
The farmer wants to reduce the annual erosion rate from this slope and needs your help to compare the impacts of potential conservation practices. One possibility is to switch to a conservation tillage practice in a wheat-on-wheat (W-W) rotation that leaves at least 30% of the soil surface covered at the time of planting the next crop. Use Table 8.2 to determine the C factor for the wheat-on-wheat rotation if 30% of the soil surface is covered with residue at the time of planting the following crop under conservation tillage.
Table 8.2. Cover and management (C) values for combinations of tillage and residue cover after planting and crop sequence for wheat rotations. (Adapted from Jones et al. (1988) with permission)
| Tillage | No Tillage | |||||||||||
| <5% | <5% | |||||||||||
| Residue | Fall | Spring | 20% | 30% | 40% | 50% | 20% | 30% | 40% | 50% | 60% | 70% |
| Rotation | ||||||||||||
| W-B | – | – | – | – | – | – | – | – | – | 0.07 | 0.06 | – |
| W-M | – | – | – | – | – | – | – | – | – | – | – | – |
| W-W | 0.2 | 0.2 | 0.1 | 0.09 | 0.08 | 0.07 | – | – | – | – | – | 0.04 |
| W-O | 0.23 | 0.23 | 0.12 | 0.11 | 0.09 | 0.08 | – | – | – | – | 0.05 | 0.04 |
| W-Co | 0.16 | 0.16 | 0.13 | 0.11 | 0.1 | 0.08 | – | – | – | – | – | – |
| Co-W | 0.34 | 0.3 | 0.14 | 0.11 | 0.1 | 0.09 | – | – | – | – | – | – |
| B-W | 0.3 | 0.28 | 0.18 | 0.16 | 0.12 | – | – | – | – | – | – | 0.03 |
† Sorghum (milo) may be substituted for corn; all C values are for wide row plantings.
‡ Crop abbreviations are as follows: Co, corn; B, soybeans; W, winter wheat; M, meadow (alfalfa, clover, grass, etc.); Fl, fallow.
Using the values from the previous RUSLE calculation (?1) above, incorporate the C factor to determine the resulting erosion rate (tons/acre) after the implementation of conservation tillage.
?2 = ? × ? × ?? × ? =
?2 =
How does the erosion rate under conservation tillage compare to the tolerable erosion rate?
How does the erosion rate under conservation tillage compare to the erosion rate under conventional tillage?
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