Runoff Management - 2 | Agriculture Optional Notes for UPSC PDF Download

Practices that reduce runoff by controlling water movement over the surface

The fundamental idea behind these methods is to reduce the amount of runoff and decrease its speed, thus providing more time for water to penetrate the soil. This approach helps limit the runoff's capability to carry soil particles and reduces its potential to cause scour erosion.

Strip Cropping

Strip cropping involves cultivating sloping terrain in a pattern of alternating contoured strips. These strips consist of row crops that are intertilled with closely spaced crops such as cover crops or grass. The strips are oriented perpendicular to the natural direction of runoff. The close-growing crops in the strips slow down the runoff and help filter out soil that might be washed away from the intertilled crops. Typically, the close-growing and row crops are planted in a rotational fashion. Strip cropping is an effective method for controlling erosion caused by runoff on well-drained, erodible soils with slopes ranging from 6% to 15%. The width of the strips is adjusted based on factors like soil erodibility and slope steepness, with recommended widths available in reference tables.

Table 4: Soil loss and runoff from plots and watersheds under different management systems

1. Kowal (1970);
2. Fournier (1967);
3. Lal (1976; 1983);
4. Millington (1985)

Contour Farming

Contour farming is a method that involves planting crops and tilling the soil in a way that aligns with the natural flow of water runoff. This approach creates storage space on the soil's surface, which helps to slow down the runoff and allows the water to seep into the soil. The effectiveness of contour farming for conserving water and soil depends on various factors, including the system's design, soil type, climate, slope direction, and how the land is used. It's important to note that contour farming has a more significant positive impact on highly permeable soils as they allow for better water infiltration, while compact or slowly permeable soils tend to saturate more quickly.

Contour bunds are another crucial physical measure in soil and water conservation. These are earth banks that are 1.5 to 2 meters wide and are constructed at intervals of 10 to 20 meters across the slope. While contour bunds may not significantly reduce runoff, they are effective in decreasing soil erosion, as demonstrated by Roose in West Africa in 1967.

Ridge and Mound Tillage

The ridge-furrow system is a widely employed technique for soil and water conservation. When ridge-and-furrow systems are aligned parallel to the contour lines, they serve a dual purpose of preventing erosion and managing surface drainage. The effectiveness of these systems is most pronounced on less steep terrain and in soils with good permeability. Research by Fournier in 1967 showed a significant 7 to 13-fold reduction in erosion and runoff in certain areas of West Africa due to ridging.

Ridge-tying is also an effective method for conserving soil and water, particularly in arid and semi-arid regions. Mounds and tied mounds are additional strategies that prove effective in preserving soil and water resources. However, the success of tied ridges depends on factors such as soil type, slope, rainfall patterns, and design specifics. It's important to note that on clay soils, tied ridging can lead to waterlogging and potential mass movement, as observed by Gray and Brenner in 1970. In severe storms, poorly designed ridge-furrow systems may fail, leading to water flowing downhill without obstruction, posing a risk of soil erosion.

In general, for small-scale farmers using manual or animal-powered implements and growing low-value subsistence crops, employing the ridge-furrow system along contour lines is a suitable method for improving water infiltration and reducing runoff. Tying ridges or mounds to allow more rainwater to seep into the soil is effective in drier regions with annual rainfall below 1000 mm and gentle slopes under 7%. However, this method may not be as effective in wet years or more humid areas. Other beneficial practices for small farmers include contour planting and strip cropping. Large-scale mechanized farming may require additional measures to manage significant runoff, such as terraces and contour bunds to break up long slopes and diversion channels and watercourses to safely manage excess runoff.

Terrace Farming

Terrace farming involves creating earthworks perpendicular to the steepest slopes. These earthworks typically consist of a dugout channel on the uphill side, with the excavated soil forming a bank on the downhill side. This technique transforms steep slopes into a series of stepped levels with relatively flat ledges and vertical walls constructed using materials like stone, brick, or timber. There are various methods for building terraces, including bench terraces, diversion terraces with mangum, Nichols terraces, broad-based and narrow-based types, channel terraces, and retention terraces.

While terraces are commonly found in different parts of the world, such as Peru, Malawi, and the Philippines, and have proven effective in conserving soil and water resources, they are often considered prohibitively expensive to construct and maintain in comparison to the returns obtained from subsistence farming. Consequently, terraces are more frequently a historical legacy rather than a component of contemporary development initiatives. A design procedure for terraces that takes into account factors like soil type, slope, hydrology, and rainfall characteristics can be found in Schwab et al. (1966). Retention terraces are generally suitable for slopes up to 8%, while bench terraces can be used on slopes ranging from 12% to 40%.

Practices that permit safe disposal of runoff

When land use management alone is insufficient to control excess runoff, it becomes necessary to address the issue without causing erosion.

To manage unwanted runoff effectively, a combination of physical measures can be employed, including interception terraces, diversion ditches, stormwater drains, waterways, and terrace channels. Interception terraces and diversion ditches are strategically placed uphill from areas requiring protection, where they intercept hillside runoff. This intercepted runoff is then channeled across the slope through a gently graded terrace channel at a non-erosive speed to a suitable outlet, which can be either a natural or artificial waterway. Diversion terraces are recommended for slopes up to 12%.

Waterways are essentially smooth channels lined with sods to safely remove runoff water without causing erosion. Grass sod is the most effective and cost-efficient lining for these waterways. However, the success of grass-lined waterways depends on the design criteria and the adaptability of the sod species to the local climate and soil conditions. Grass-lined waterways have become less popular in recent years because they occupy land that could be used for crops and increase maintenance costs. In the USA, they are being replaced with tile drains.

In arid regions, an alternative approach to managing and utilizing runoff is to concentrate surplus rainfall by employing water harvesting techniques, which collect and concentrate the runoff to support plant growth. The concept of water harvesting dates back to ancient civilizations like the Nabateans in the Negev Desert of the Middle East, and there have been recent investigations into improving the efficiency of water harvesting by using thin asphaltic pavements to increase runoff.

Conclusion

In summary, many developing countries conduct insufficient research on runoff management, and the existing work often falls short in providing well-established practices for erosion control and soil moisture conservation. While some fundamental principles in this field may have broad applicability, conservation techniques developed in one region require rigorous testing and verification, particularly concerning local factors like rainfall, soil types, and cropping practices, before being adopted elsewhere. Runoff management is primarily guided by the goal of minimizing the concentration of runoff volume and reducing its velocity to prevent scour erosion. It aims to enhance surface detention storage, allowing water more time to infiltrate into the soil, and to safely manage unavoidable runoff without causing erosion. There is a wide array of runoff management techniques available, but their suitability depends on factors like soil characteristics, topography, climate, and socio-economic considerations. These techniques can be broadly categorized as biological and physical protection measures.

Biological control measures involve implementing sound agronomic and soil management practices to select land uses that shield the soil from the impact of raindrops, enhance surface storage and soil infiltration, improve soil aggregate stability to resist erosion, and increase soil surface roughness to slow down runoff. Mulch farming, especially when combined with planted cover crops, is an effective conservation method across various ecological regions. In combination with a no-tillage system, mulch farming is particularly valuable for grain crop production in humid and sub-humid areas. However, it may be less effective in arid and semi-arid regions with naturally compacted surface soils, where plowing and subsoiling have proven to be beneficial. Ridging is a widely adopted and effective technique for soil and water conservation, with contour ridging being favored in more humid regions and tied ridging in semi-arid and arid areas. These biological measures can be integrated into existing farming systems, as demonstrated by Temple in 1972.

On the other hand, physical conservation measures like terraces and contour bunds are considered of secondary importance in runoff management for small-scale farmers. However, large-scale mechanized farming may require them to manage increased exceptional runoff on steep cultivated lands. The use of terraces and contour bunds is limited in many developing nations due to the high construction costs. Therefore, alternative methods like contour hedges, straw barriers, and grass buffer strips or alley cropping are often suggested for managing runoff on steep cultivated lands. When the risk of runoff remains, an artificial drainage or waterway system, typically lined with sod, is constructed to safely dispose of runoff without causing erosion.

The most suitable strategy for runoff management depends on identifying the key factors influencing runoff and applying appropriate techniques based on local knowledge and conditions.