Water Harvesting Techniques
Water harvesting techniques may be classified into two major types, based on the size of the catchment (Figure 14.6): microcatchment systems and macrocatchment systems (Oweis et al. 2001).
Surface runoff in microcatchment systems is collected from small catchments (usually less than 1000 m2) and applied to an adjacent agricultural area, where it is stored in the root zone and used directly by plants. The target area may be planted with trees, bushes, or annual crops. The farmer has control, within the farm, over both the catchments and the target areas. All the components of the system are constructed inside the farm boundaries, which provides a maintenance and management advantage. But because of the loss of productive land, it is practiced only in the drier environments,
Classification of major rainwater harvesting systems in the dry areas. (From Oweis, T., et al.,
Water Harvesting: Indigenous Knowledge for the Future of the Drier Environments, ICARDA, Aleppo, Syria, 2001. With permission.)
where cropping is so risky that farmers are willing to allocate part of their farm to be used as a catchment. They are simple in design and may be constructed at low cost. Therefore, they are easy to replicate and adapt. They have higher runoff efficiency than the macrocatchment systems and usually do not need a water conveyance system. Soil erosion may be controlled and sediment directed to settle in the cultivated area. These systems generally require continuous maintenance, with relatively high labor input. The most important microcatchment water harvesting systems in the dry areas are described below.
22.214.171.124.1 Contour Ridges
Contour ridges consist of bunds, or ridges, constructed along the contour line at an interval of, usually, between 5 and 20 m. A 1- to 2-m strip upstream of the ridge is for cultivation, and the rest constitutes the catchment. The height of the ridges varies according to the slope and the expected depth of the runoff water retained behind it. The bunds may be reinforced by stones when necessary. This is a simple technique, which can be implemented by the farmers themselves. Bunds can be formed manually, with animal-driven equipment, or by tractors fitted with suitable implements. Ridges may be constructed on a wide range of slopes, from 1 to 50 percent.
Contour ridges are important for supporting the regeneration and new plantations of forage, grasses, and hardy trees on mild to steep slopes in the steppe (badia). In the semiarid tropics, they are used for the arable cropping of sorghum, millet, cowpeas, and beans. This system is sometimes combined with other techniques (such as the zay system) or with in situ water conservation techniques (such as the tied-ridge system) in the semiarid tropics.
126.96.36.199.2 Semicircular and Trapezoidal Bunds
Semicircular and trapezoidal bunds are earthen bunds created with spacing sufficient to provide the required runoff water for the plants. Usually, they are built in staggered rows. The technique can be used not only on an even, flat slope, but also on slopes up to 15 percent. The technique is used mainly for rangeland rehabilitation or fodder production, but can also be used for growing trees, shrubs, and, in some cases, field crops and vegetables.
188.8.131.52.3 Small Pits
The most famous pitting system is the zay system used in Burkina Faso. This form of pitting consists of digging holes 5-15 cm deep. Manure and grasses are mixed with some of the soil and put into the zay. The rest of the soil is used to form a small dike, downslope of the pit. Pits are used in combination with bunds to conserve runoff, which is slowed by the bunds. Pits are excellent for rehabilitating degraded agricultural lands. However, labor requirements for digging the zay are high and may constitute a large financial investment, year after year. This is because the pits have to be restored after each tillage operation. A special disk plow may be adjusted to create small pits for range rehabilitation.
184.108.40.206.4 Small Runoff Basins
Sometimes called negarim, these runoff basins are small and of a rectangular or elongated diamond shape; they are surrounded by low earth bunds. Negarim work best on smooth ground, and their optimal dimensions are 5-10 m wide by 10-25 m long. They can be constructed on almost any slope, including very gentle ones (1-2 percent slopes), but on slopes above 5 percent, soil erosion may occur, and the bund height should be increased. They are most suitable for growing tree crops like pistachios, apricots, olives, almonds, and pomegranates, but they may be used for other crops. When used to grow trees, the soil should be deep enough to hold sufficient water for the entire dry season.
220.127.116.11.5 Runoff Strips
This technique is applied on gentle slopes and is used to support field crops in drier environments (such as barley in the badia), where production is usually risky or has a low yield. In this technique, the farm is divided into strips following contour lines. One strip is used as a catchment and the strip downstream is cropped. The cropped strip should not be too wide (1-3 m), and the catchment width should be determined with a view to providing the required runoff water to the cropped area. The same cropped strips are cultivated every year. Clearing and compaction may be implemented to improve runoff.
18.104.22.168.6 Contour Bench Terraces
Contour bench terraces are constructed on very steep sloping lands and combine soil and water conservation with water harvesting techniques. Cropping terraces are usually built to be level. Supported by stone walls, they slow water and control erosion. Steeper, noncropped areas between the terraces supply additional runoff water. The terraces contain drains to safely release excess water. They are used to grow trees and bushes but are rarely used for field crops. Some examples of this technique can be seen in the historic bench terraces in Yemen. Because they are constructed in steep mountain areas, most of the work is done by hand.
22.214.171.124.7 Rooftop Systems
Rooftop and courtyard systems collect and store rainwater from the surfaces of houses, large buildings, greenhouses, courtyards, and similar impermeable surfaces. Farmers usually avoid storing the runoff provided by the first rains to ensure cleaner water for drinking. If water is collected from soil surfaces, the runoff has to pass through a settling basin before it is stored.
The water collected is used mainly for drinking and other domestic purposes, especially in rural areas where there is no tap water. Extra water may be used to support domestic gardens. It provides a low-cost water supply for humans and animals in remote areas.