Erosion in Walla Walla County
Cultivation of the Walla Walla area began in the 1860s. Since then, agriculture has grown to become a major part of the Walla Walla County economy and culture. A detailed, but not comprehensive, list of the products grown includes the major crops of wheat, barley, corn, potatoes and the minor crops of canary peas, dry peas, winter peas, garbanzo beans, lentils, mustard, canola, buckwheat, safflower, sunflower, carrots, winter onions, summer onions, asparagus, dry beans, lima beans, strawberries, apples, peaches, apricots, plums/prunes, nectarines, wine grapes, juice grapes, mint, alfalfa hay, and alfalfa seed (Hooker).
Currently, Walla Walla County has about 604,000 acres of cropland in production. 490,000 acres of this cropland are in dry cropland systems, while the rest are irrigated. Of the dry cropland area, 148,000 acres are part of the Conservation Reserve Program sponsored by the Natural Resources Conservation Service, which requires that these acres have a perennial plant cover during their enrollment of 10+ years (Hooker).
While this agricultural development is an important part of Walla Walla County, the current use of the land is leading to severe problems in the quality and quantity of the topsoil. According to statistics on soil erosion in southeastern Washington, somewhere between 2 to 10 tons/acre/year of topsoil could be lost in Walla Walla County as a result of both wind and water erosion (www.wa.nrcs.usda.gov). For the croplands enrolled in the Conservation Reserve program, that number is much lower at <1 ton/acre/year (Hooker).
Some soil erosion is a natural phenomenon, however agricultural development expedites the process by the alteration of the natural vegetative cover. Often, exotic species are brought into the area as agricultural crops. Since they have not adapted with the soil in the area, these species are unable to protect the soil and support the soil with their root structures.
The main reason why agricultural development increases soil erosion is that a continuous vegetative cover does not exist throughout the year. Traditionally, after crops are harvested in the fall, farmers till under all old vegetative cover to help prepare the soil for the next spring’s planting. Without a vegetative cover and root structure throughout the winter and early spring, the soil becomes vulnerable to erosion.
When soil does not have a cover to protect it or roots to hold it together, it becomes vulnerable to soil erosion (Figure 1) Soil erosion occurs in two main ways, with either water or wind. When rain falls on uncovered soil, the surface absorbs all of the energy from the raindrops, resulting in detachment of soil particles. Surface water builds up and transports the soil particles down slope. Wind transportation is also a major factor in soil erosion, particularly in the arid regions of Washington. (Gebhardt). Soil erosion by water can result in rills (Figure 2), which can develop into larger gullies (Figure 3), or result in a sheetwash (Figure 4) or sheet flood erosion (Figure 5).
In addition to the loss of quality topsoil for agriculture, soil erosion as a result of agriculture development also has a negative impact on the water quantity and quality of the region. The lack of vegetative cover means that water is not held in one area after it falls, so it does not have the time to infiltrate into the soil, therefore depleting the water table. The increased runoff picks up loosened soil particles, stripping the surface and causing gullying of the soil surface. This runoff water can be highly polluted, with high sediment content, dissolved nutrients, and pesticides from the soil surface (Gebhardt).
Buffers are necessary to decrease the impacts of the erosion, particularly to streams or rivers that are adjacent to cultivated fields. These buffers can be made of earth and used to trap sediment and encourage infiltration, or they can be a vegetative strip that parallels the stream and protects it from harmful products of the agricultural field.
Conventional tillage was developed in the 1800s as a means to provide a suitable seedbed for crops, improve surface drainage and reduce the competition from other plants. This method uses a plow to bury the residue from the previous year’s crop (Gebhardt). Conventional tillage is considered to be tillage practices that result in <15% surface residue after seeding. Surface residue is considered to be any crop organic material remaining from the preceding crop (Hooker).
Over the last century, concern increased about the loss of soil and the
pollution of water as a result of agriculture.
This concern has led to the development of alternative tillage practices
to reduce the loss of soil, while keeping the benefits of tilling.
The following tilling practices are used in Walla Walla County to help to
reduce soil erosion by leaving the residue from previous crops to protect the
soil from raindrop impact and wind erosion.
While these practices reduce soil erosion for the farmer, they may also
make it more difficult to prevent weeds growing with the crops.
Conservation) Till – A tilling practice that leaves less than <30%
surface residue after seeding (Hooker).
Mulch Till – A
larger percentage of residual crop material is left on or near the soil surface
to act as a protective mulch (Morgan) (Figure 10).
Special equipment is used to plant seeds in existing residue. Tillage is confined to narrow strips that have just enough
loose soil to cover the seeds (Gebhardt) (Figure 6).
This method is the most costly because it involves different equipment to
Table 1. Acreage of
tilling techniques in Walla Walla County (Hooker)
# of acres
% of total acreage
In addition to using different tilling practices, other
conservation farming techniques can be used to discourage soil erosion.
– Plowing perpendicular to the slope to discourage soil and water erosion down
the slope (Figure 7) .
– This practice uses minimal disturbance to prepare the seedbed for planting.
Chemicals are used to kill existing vegetation, and then tillage is used
to open a narrow seedbed to plant the seed.
Herbicides are used to control weeds (Morgan).
– Alternating strips are made of row crops and protection-effective crops.
These strips are aligned on the contour or are perpendicular to the wind.
Erosion mostly occurs in the row crops, and the majority of the eroded
sediment is trapped in the protection-effective strip that is downslope or
downwind. The protection-effective
crops are likely to be either from the legume or the grass family (wheat, rye,
alfalfa) (Morgan) (Figure 8 and 9). Strip
cropping can also be done with the same crop, except the rows this time are made
of strips ready to harvest and strips that merely have ground cover left on them
from last year’s crop. This
method also works well to trap sediment that is eroded by water.
Crop residue use
– Crop residue is left on the fields after harvest to provide cover from
raindrop impact and supporting root structure to the soil (Figure 10)
Berms – Earth
bank berms are built perpendicular to the slope and serve several purposes.
In addition to being a barrier to runoff, they form a water storage area
on the upslope side (which encourages infiltration), they break the slope into
shorter segments (which discourages rills from forming), and they also trap
sediment on the upslope side and prevent it from being lost downslope (Morgan).
Conservation Reserve Program – This program was started to encourage farmers to convert highly erodible cropland to vegetative cover, such as tame or native grasses, wildlife plantings, trees, filterstrips, or riparian buffers. To help cover the cost of establishing vegetative cover practices, farmers sign a multi-year contract in which they receive an annual rental payment. The CRP has seen dramatic results from its work. Before the program was enacted in the arid regions of Washington in 1982, only 57% of agricultural land was classified as eroding less than 2 tons/acre/year. In 1997, 95% of arid cropland in Washington was eroding at less than 2 tons/acre/year (www.nrcs.usda.gov).
Gebhardt, M.R., Daniel, T.C., Schweizer, E.E., and Allmaras,
R.R. “Conservation Tillage”.
Science, vol. 230, 8 Nov 1985, pp. 625-630.
Hooker, Larry. National Resource Conservation Service,
Walla Walla, Washington. Interview.
October 15, 2001.
Morgan, R.P.C. Topics in Applied Geography: Soil Erosion.
London: Longman Group Limited,
Additional information was found on the National Resource
Conservation Service website at
<www.nrcs.usda.gov> and at the Washington site at <www.wa.nrcs.usda.gov>.
Figure 1. Wind erosion in a 1935 dust bowl, eastern Colorado
Figure 2. Rills in a wheat field, Palouse Hills, southeast Washington
Figure 3. Large gully as a result of water erosion, southeast Washington
Figure 4. Rills transitional to sheetwash, southeast Washington
Figure 5. Sheetflood erosion can be very destructive, southeast Washington
Figure 6. No-till wheat field, southeast Washington
Figure 7. Contour plowing in the Palouse Hills, southeast Washington
Figure 8. An aerial view of strip cropping, southeast Washington
Figure 9. Strip cropping, southeast Washington
Figure 10. Ground cover left in the Palouse Hills, southeast Washington