Managing surface water requires special attention if productivity on highly erodible land is to be increased. Generally, there are four reasons for surface water management:
All these reasons are important; however, the following overview of surface water management practices for erodible land emphasizes water conservation, only mentioning the other factors as necessary.
Terraces. The main purpose of terracing is to reduce runoff velocity and soil erosion by breaking the effective length of slopes. There are two types of terraces: graded and level. Graded terraces, combined with suitable sod or subsurface outlets, are used in areas of abundant rainfall to safely divert excess surface water. Although these terraces reduce runoff velocity, they do little to reduce runoff volume. Graded terraces do not provide any practical water conservation, but will reduce on-site erosion damage.
Level terraces are designed for areas with permeable soils and low to moderate rainfall. They can hold runoff and increase infiltration from normal rainfall, but need supplementary waterways and a collection arrangement to handle rainfall that exceeds terrace storage capacity. Level terraces may conserve water for crop use along the channel, as well as providing on-site erosion protection.
The principal advantages of terracing include controlling on-site erosion by reducing runoff velocity and controlling off-site damage by holding sediment in terrace channels. Disadvantages include construction costs, long-term commitment to maintenance, and little water conservation effect on highly erodible land in areas of low to moderate rainfall.
Diversions and sediment basins. These structures are designed to reduce on-site damage by redirecting runoff to decrease velocity. Some off-site damage control may also be provided.
Sod waterways. In humid regions waterways are essential for good water management, serving as a supporting component to terraces, diversions, and contoured rows. They contribute to reducing on-site and off-site damages, but offer no water conservation benefit per se.
Some disadvantages of waterways include construction costs, land area removed from crop production, and variable maintenance costs.
Contour tillage and planting, crop rotation, strip-cropping, and conservation tillage are the most important cultural practices for water management.
Contour farming. Contour rows serve as mini-terraces that slow runoff and facilitate water infiltration. Contour farming not only reduces on-site and off-site damages like the structural practices do, but also conserves water for crop production. This practice is inexpensive and doesn't usually require added equipment investment.
However, effective contour farming is limited to slopes of three to seven percent; equipment operating time would increase with shorter row lengths. Also, contouring may be ineffective in controlling erosion during heavy rainfall if water breaks over rows of slowly permeable soils.
Crop rotation. Both soil and water conservation benefits can be attributed to crop rotation. When sod crops are included in the rotation for one or more years, the average erosion rate is sharply reduced over time. Sod crop production protects the soil, improves soil structure, increases water infiltration, and reduces clean tillage practices. After production of a sod crop, soil erodibility is reduced.
Rotating crops without sod production increases yields in corn/soybean or corn/tobacco/peanut vs. corn following corn.
The major disadvantage of crop rotation is a reduction in crop production acreage when using sod-based rotations. The main advantage is that additional long-term investment is rarely required.
Strip-cropping. Alternating strips of close-growing crops with cultivated wide-row crops reduces runoff velocity, improving infiltration and water conservation. Strips are laid out across slope, but benefits increase as their configuration approaches the contour.
Conservation tillage. Successful conservation tillage controls soil erosion well, reduces runoff, and increases infiltration. Generally, conservation tillage includes any practice that reduces erosion and runoff in comparison to conventional tillage, for example, planting with no-till, ridge-till, or strip-till practices. Surface mulch can reduce evaporative losses. However, if the soil becomes smooth, compacted, or crusted, water-storage capacity will be reduced, leading to increased runoff.
The advantages of conservation tillage include lower labor and fuel costs due to reduced tillage operations, relatively small long-term investment, substantial erosion control, and increased yields on some soils due to the soil conservation effect. Some disadvantages of conservation tillage include inconsistent performance, for example, soil compaction and poor weed control; increased use of pesticides; and greater managerial and operational skills are required.
In conclusion, the benefits of surface water management will likely outweigh the disadvantages over the long term, especially where local or national programs offer subsidies to farmers for implementing these improvements.
For more information:
Dr. George C. Naderman
Extension Soil Specialist
North Carolina State University
Raleigh, NC 27695-7619, USA