Oct 18, 2007

Protecting the harvest

A critical production constraint on many cropland areas is the availability of soil water. Precipitation should be used for crop consumptive use; it should not be removed by runoff. Crop yields may be impacted negatively if excessive amounts of runoff are lost.

Flooding problems may be compounded by excessive runoff from agricultural areas. Sediment carried in runoff may impact the effectiveness of water conveyance and storage structures. The suitability of streams and reservoirs as biological habitats and effective water supplies can be affected by sediment, nutrients, pesticides and pathogens transported by runoff.

A large nutrient concentration in water bodies can cause excessive vegetative growth, resulting in seasonal oxygen deficiencies. The type of fertilizer that is used, application rate and nutrient content of the soil may influence nutrient transport.


Runoff Factors

Runoff is defined as the portion of rainfall that does not infiltrate or accumulate on the soil surface but moves down slope. Runoff from cropland areas is influenced by rainfall characteristics, soil factors, topography, climate and land use. Computer programs can estimate the interrelated effects of these variables.

Rainfall Characteristics. Runoff occurs only when rainfall intensity exceeds soil infiltration rate. A brief storm may not cause any runoff, while a storm of the same intensity but longer duration may cause substantial runoff.

During a high-intensity storm, infiltration capacity is exceeded by a larger margin than during a less-intense rainfall event. Thus, even though the amount of precipitation is similar for two rainfall events, a high intensity storm will produce more runoff.

Soil Factors. Soils containing large stable soil particles, or aggregates, usually have larger infiltration rates. As organic matter content increases, soil aggregates become more stable, soil structure improves and infiltration rates increase. Rain-induced sealing of the soil surface may reduce infiltration.

Pastures, meadows and cultivated soils recently removed from native vegetation usually have excellent structure and stable aggregates. The incorporation of organic materials into the soil profile helps increase aggregate stability. Cultivation without the addition of organic materials may reduce aggregate stability and organic matter content, and thus increase runoff levels.

Maintaining high infiltration rates is one of the most effective means of reducing runoff. They occur when vegetative material protects the soil surface from sealing, soil structure is preserved and compaction is minimized.

Topography. Runoff rates are influenced by slope gradient, length of slope and watershed size and shape. The velocity of water increases with slope gradient. An accumulation of overland flow on longer slopes means more runoff. Crops growing in productive areas at the bottom of a hill slope may be covered with water during extreme precipitation events.

Climate. Significant runoff may occur in regions with low rainfall and limited vegetation when high-intensity rainstorms occur. Maintaining high infiltration rates not only reduces runoff but also helps maintain soil water supplies for crops.

Significant runoff may result from the rapid melting that occurs when air temperatures quickly rise or when rain falls upon a snow-covered surface, and more runoff may result from snowmelt than from rainfall events in colder climates. In some areas, runoff may not occur during several months of the year.

Land Use. This is the only factor that can be modified to reduce runoff. Runoff potential may increase substantially as forest or rangeland is converted to cropland, but cropland areas that have a complete ground cover year-round are less susceptible. The amount of surface cover maintained on a site is influenced by cropping and management conditions. The greatest runoff potential on croplands exists after planting, when residue cover is at a minimum and intense rains frequently occur.

Cultivated land left fallow with no vegetative cover is particularly vulnerable to runoff. Areas with steep slopes on which row crops are grown continuously are of concern, but the dense cover created when row crops are planted in rotation with grasses and legumes may substantially reduce runoff. Following the cropping season, legumes may also serve as supplemental nitrogen sources.


Control Measures

Several practices are available for controlling runoff from cropland areas, including contouring, strip cropping, conservation tillage, terraces, buffer strips and grassed waterways. Depending on the severity of the problem, it may be necessary to use a combination of measures to reduce runoff to reasonable limits.

Contouring. Contour farming reduces runoff by storing rainfall behind ridges. The storage capacity of furrows is significantly increased with ridge tillage systems. Row crops are planted on the top of the same furrow each year to maintain furrow storage capacity.

The effectiveness of ridges in trapping runoff decreases as slope gradient increases. On steep slopes, furrow storage capacity may be exceeded during high-intensity rainfall, when water previously stored in the furrow is released. To prevent the water from overtopping ridges, a small slope gradient along the row is desirable. Field boundaries should be located on the contour or moved to eliminate odd-shaped fields with short rows.

Strip Cropping. Under strip cropping conditions, alternate parcels of different crops are grown on the same field. The strips with the greatest vegetative cover typically have higher infiltration rates, and they may capture runoff from upslope areas. Strip widths are dictated by farm implement requirements.

To enhance runoff control, strips are usually planted on the contour in a rotation that shifts crops between strips annually. The most effective rotations include perennial grasses and legumes that alternate with row crops. In arid and semiarid regions, strips may be placed perpendicular to the prevailing wind direction for wind erosion control.

Conservation Tillage. Residue from the previous crop may also reduce runoff. Crop residue enhances infiltration by absorbing and dissipating raindrop energy. Any tillage or planting system that leaves at least 30 percent of the soil surface covered with residue after planting is defined as a conservation tillage system. A 30 percent cover of soybean or corn residue can reduce runoff by approximately 42 percent and 82 percent, respectively.

Small impoundments may form above residue materials during precipitation events. The impoundments found above larger diameter residue materials such as corn have greater volumes and are, therefore, more effective in storing runoff.

Furthermore, runoff velocity decreases as the amount of crop residue increases. There is an increased opportunity for infiltration on areas with relatively small runoff velocities.

Excessive tillage can destroy soil structure, resulting in surface sealing and reduced infiltration. With the increased availability of herbicides, the use of tillage for weed control has diminished. When tillage is performed, existing crop residues can be maintained by using special implements that minimally disturb the soil surface. To maintain residue cover for runoff control, no tillage is performed before planting on some areas with row crops.

Terraces. On steep land, terraces, or broad channels, are built perpendicular to the slope to temporarily store runoff. Runoff travels at relatively low velocities along the gentle grades, collects in the terraces and is transported to grassed waterways or underground pipes. Terraces increase the amount of soil water available for crops.

Important considerations in terrace design include soil characteristics, climatic conditions and cropping and management practices. Contouring is usually used on terraced fields since crop rows are typically planted parallel to the terrace channel. Terraces are expensive to construct, cause some inconvenience to the farming operation and require periodic maintenance. Therefore, they should only be used when other runoff control measures do not provide adequate protection.

Buffer Strips. Buffer strips are designed to intercept runoff using permanent vegetation. Contour buffer strips, filter strips and grassed waterways are frequently used types. Usually other runoff control practices are employed in association with buffer strips.

Perennial grasses are often planted along steep slopes within contour buffer strips. Site-specific conditions dictate the types of vegetation and spacing of contour buffer strips, which are much less expensive than terraces. Filter strips provide an increased opportunity for infiltration and do not interfere significantly with normal farming operations since they are located at the edge of fields or adjacent to streams, ponds or wetlands. Areas with gentle slopes are the best locations for filter strips. Contour buffer strips are usually relatively wide. Runoff quantities can be substantially reduced by the use of narrow grass hedges.

Grassed Waterways. Runoff from terraces or other concentrated flow areas can be conveyed using grassed waterways. A stable outlet located below a grassed waterway serves to disperse the flow before it enters a vegetative filter. The types of vegetation used in the channel are dictated by local soil and climatic conditions.

To prevent failure, the waterway should not be used as a road, stock trail or pasture, especially during wet conditions. Care should be taken when farm machinery crosses the waterway. The area should be managed to stimulate new growth and control weeds, and an annual application of fertilizer is recommended.

About the author

John E. Gilley is a research agricultural engineer for the U.S. Department of Agriculture, Agricultural Research Service, at the University of Nebraska-Lincoln. Gilley can be reached by e-mail at [email protected].