Practice English Speaking&Listening with: Controlling Water Erosion

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Water erosion is a natural process by which soil is washed away by running water. It can

be relatively minor such as when topsoil is shifted around by rainfall, or it can literally

reshape a landscape and cause flooding and landslides. This can cause a lot of problems

for agriculture, but there are things that can be done to prevent at least some of the

effects of water erosion. Im Dr. DeBusk and in this video, Ill discuss methods

for reducing water erosion. All methods of controlling water erosion are

based on one of four actions. First, reducing raindrop impact to lessen detachment. This

can be done by growing vigorous crops that fill in the canopy quickly, leaving crop residues

on the surface, mulching, or by growing a total vegetative cover. Second, reducing or

slowing runoff. This lessens detachment by scouring and reduces the amount of soil that

can be transported. Avoiding compaction, maintaining organic matter levels, and subsoiling help

water infiltrate the soil. Contour practices and conservation tillage both reduce runoff.

Third, carrying excess water off the field safely by use of grass waterways or tile outlets.

Fourth, filtering soil particles out of running water. One could also distinguish conservation practices,

things we do regularly to preserve soil, like conservation tillage, from conservation structures,

permanent modifications of the land, like terraces. Most of these measures reduce erosion

by several of the actions listed above, and many of these practices can be combined. While

these conservation measures apply most directly to agricultural settings or production horticulture

like vegetable or nursery production, water erosion can also occur in the grounds around

homes and buildings. However, landscaping, properly designed, executed, and maintained,

is itself a soil conservation practice. It covers the soil with vegetation, mulches,

and other materials, and retaining walls are even a form of terrace.

Practices that improve infiltration of water into the soil, thus reducing runoff, diminish

erosion rates. Foremost among these are practices that increase soil organic matter content

and improve aggregate stability. Organic agriculture, conservation tillage, cover cropping, and

other practices can improve, or at least preserve, soil organic matter. Meadow crops

most effectively improve aggregate stability, so crop rotations including meadow crops save

soil, as shown in table. Avoiding or treating compaction by subsoiling also improves infiltration,

as do the biopores common to no-till soils. One might even consider lawn aeration a form

of soil improvement. A newer version is the use of polyacrylamide (PAM), a synthetic

water-soluble compound that can be sprayed on clay soils. At the molecular level PAM

consists of strands that bind clay particles together to stabilize soil aggregates, improving

infiltration and reducing surface sealing and crusting.

Vegetative cover protects soil from raindrop impact, standing vegetation impedes overland

flow, and root systems help bind the soil together. A standard erosion control practice

is simply selecting a crop suitable for a sites erodibility. Nursery and vegetable

crops offer the least vegetative cover, and therefore are least suitable for sloping ground

without protective measures. Small grains offer greater protection than common row crops,

so including them in a rotation with row crops helps save soil. Permanent and complete vegetative

cover such as native prairie grasses or pasture offers the best soil protection, so is the

most suitable treatment for highly erodible land.  Other forms of vegetative cover include

cover crops, roadside vegetation, and turf-grass lawns. A well-maintained lawn with dense turf

cover suffers negligible erosion, but a thin lawn may indeed erode.

Conservation tillage sharply reduces sheet and rill erosion. It is the lowest-cost conservation

method per ton of soil saved. For these reasons, conservation tillage is the most widely accepted

Best Management Practice for controlling soil losses. The effectiveness of conservation

tillage depends on a rough soil surface and surface residues, especially during the critical

period before the crop canopy fills in. The definition of conservation tillage states

that residue coverage be adequate to bring erosion below T30 percent coverage is

often taken as a minimum amount. Coverage can be measured directly by stretching a 50-foot

cord, marked every 6 inches, diagonally across several rows. The number of marks touching

a piece of crop residue is the percentage of coverage. For instance, if 35 marks touch

crop residues, the coverage is 35 percent. This procedure should be repeated in several

parts of the field and the results averagedThe figure shows the effect of several residue

levels according to the RUSLE. In some settingssuch as stabilizing slopes during new constructionmulching

serves the same purpose of covering the soil to protect it.

Contour practices include tillage and planting across slopes, on the contour, rather than

up and down slopes. A contour is an imaginary line across a slope that remains at constant

elevation. Water runs downhill perpendicular to these contour lines, so these practices

prevent or impede that flow. Rainfall ponds behind the small ridges created by tillage,

providing more time for water to infiltrate and impeding downhill flow. However, high-intensity

rainfalls reduce the ridges, and high rainfall amounts can cause ponded water to overtop

the ridges, which can be erased by water flowing over themTherefore, areas of moderate slopes

and low-intensity rainfall benefit most from contour tillage as seen in the graph. Combining

contour tillage with conservation tillage layers two technologies for greater protection.

Strip-cropping takes advantage of the fact that close-growing small grains and forage

provide a dense vegetative cover, so protect the soil better than row cropsThe photo shows

strips of alfalfa alternating with corn on the contour. The distance of the less protected

soil will be short, so less soil will be transported before entering the denser vegetative cover

of the alfalfa strip, where water slows and soil particles are filtered out. Strip-cropping

adds another layer of protection to contour tillage, and can be more effective in areas

of moderate rainfall. Adding conservation tillage adds yet another layer of protection.

A grassed waterway is a shallow, sodded, wide ditch that runs down a slope. As shown

in top photo, water flow concentrates where opposing slopes meet; it is precisely in such

places where a grassed waterway may be installed. One of the waterways in the bottom photo illustrates

this nicely. Since ephemeral gullies and gullies form here, grassed waterways are a BMP for

prevention of such gullying. Waterways are also designed to carry excess water off the

field safely, and may be used to collect water from tillage contours or terraces. Grassed

waterways are also a version of a conservation buffer, described next.

Strips of permanent vegetation that retard overland flow and act as living filters to

remove sediment, nutrients, and pesticides are called conservation buffers. Conservation

buffers reduce erosion and preserve water quality of nearby bodies of water. Conservation

buffers also enhance wildlife habitat for ground-nesting birds such as pheasants and

other wildlife. Conservation buffers work by slowing runoff while improving soil physical

properties so infiltration is improved. Contour buffer strips are strips of permanent vegetation

planted on the contour between strips of cultivated crops. Filter strips lie along the downhill

edge of a field, and work primarily to filter sediment out of runoff from a field before

it can enter a receiving area, such as a nearby streamRiparian buffers are strips of permanent

vegetation along streams, wetlands, and other bodies of water, to protect water quality

from field runoff. Riparian buffers include versions with herbaceous plants like stiff

grasses, and woody plant buffers with trees and shrubs or some combinationConservation

buffers, like all other erosion control procedures, must be properly designed to function properly,

taking into account slope length and other factors. An example of a design element is

the width of a vegetative strip, which must be wider if more water can be expected to

enter itWhere other measures fail to reduce erosion

adequately, terraces may be built. Long or steep slopes on impermeable soil, for example,

require terraces. Terraces are costly to install and are used most commonly for valuable crops

or where there is a shortage of good land. In general, two kinds of terraces are used:

Level terraces are parallel to the slope and do not empty into a waterway. This type of

terrace is used where soil is permeable enough so that water can seep in once captured in

a terrace. Graded terraces are needed where water cannot soak in enough. These may slope

gently toward a waterway or be drained by an underground tile outlet. Several terrace

designs are shown in the bottom figure. Of these designs, the broad-based terrace is

most common. Terrace construction begins by designing them to fit conservation needs without

overly hampering farming. The land is surveyed and the terraces are marked on the slope.

Terraces must be properly maintained to ensure effectiveness. Terraces break up a slope into

several shorter slopesDiversions are large-capacity terraces that divert runoff from higher elevations.

Diversions are not farmed but are covered with grass.

Our erosion-prevention technologies were developed in response to conditions of a mid-twentieth-century

climate. Since climate is changing rapidly, many soil scientists are concerned that those

technologies may be overwhelmed by climate change. Indeed, there is already some evidence

of a quickening of erosion rates in some locations. Climate change models predict increases in

average temperature; greater frequency of extreme weather events such as prolonged drought,

heat waves, or higher-intensity storms; and changing rainfall patterns across the globe.

We observe these climatic changes happening already, and they are likely to become more

pronounced over time. For water erosion, a predicted increase in the number of highly

erosive rainfalls in some locations will impact water erosion. Indeed, many of us have already

experienced a marked increase in extreme rainfall events in recent years. Where contour tillage

had been a satisfactory way to control erosion, heavier rainfalls can erase the low-contour

ridges that tillage creates and render the practice ineffective. There may also be a

shift from sheet and rill erosion to concentrated flow. A 2003 report from the Soil and Water

Conservation Society predicted erosion increases as high as 90 percent on some cropland. Our

erosion-prediction models like RUSLE may no longer predict erosion as accurately. The

isoerodent map we use to determine the rainfall (R) factor will likely become inaccurate,

as could other factors, such as crop management factors (C). The report mentioned above suggests

that climatic factors be updated immediately. Growers may need to alter their cropping practices

in response to climate changeIn conclusion, growing vigorous crops, maintaining organic

matter, and avoiding overtillage and compaction help control erosionOther strategies include

conservation tillage, contour tillage, terraces, and buffers.

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