Water harvesting techniques may be classified into two major types, based on the size of the catchment (Figure 6): micro-catchment systems and macro-catchment systems (Oweis et al., 2001).
Surface runoff in micro-catchment 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 pro-
ductive land it is practiced only in the drier environments, 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 macro-catchment 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 micro-catchment water harvesting systems in the dry areas are described below.
a. 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%.
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.
b. 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 on an even, flat slope, but also on slopes up to 15%. 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.
c. 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, down slope 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.
d. 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% slopes), but on slopes above 5%, 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.
e. 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.
f. Contour Bench Terraces
Contour bench terraces are constructed on very steep sloping lands and combine soil-and-water conservation and 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.
g. 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.
Macro-catchment systems collect runoff water from relatively large catchments, such as natural rangeland or a mountainous area, mostly outside farm boundaries, where individual farmers have little or no control. Water flows in temporary (ephemeral) streams called wadi and is stored in surface or subsurface reservoirs, but it can also be stored in the soil profile for direct use by crops. Sometimes water is stored in aquifers as a recharge system. Generally, runoff capture, per unit area of catchment, is much lower than for micro-catchments, ranging from a few percent to 50% of annual rainfall.
One of the most important problems associated with these systems involves water rights and the distribution of water, both between the catchment and cultivated areas and between various users in the upstream and downstream areas of the watershed. An integrated watershed development approach may overcome this problem. The most common macro-catchment systems are discussed below.
a. Small Farm Reservoirs
Farmers who have a wadi passing through their lands can build a small dam to store runoff water. The water can subsequently be used to irrigate crops or for domestic and animal consumption. These reservoirs are usually small, but may range in capacity from 1,000 to 500,000 m3. The most impor tant aspect of this system is the provision of a spillway with sufficient capacity to allow for the excessive peak flows. Most of the small farm reservoirs built by farmers in the rangelands (badia) have been washed away because they lacked spillway facilities or because their spillway capacity was insufficient. Small farm reservoirs are very effective in the badia environment. They can supply water to all crops, thus improving and stabilizing production. Moreover, the benefits to the environment are substantial.
b. Wadi-Bed Cultivation
Cultivation is very common in wadi beds with slight slopes. Because of slow water velocity, eroded sediment usually settles in the wadi bed and creates good agricultural lands. This may occur naturally or result from the construction of a small dam or dyke across the wadi. This technique is commonly used with fruit trees and other high-value crops. It can also be helpful for improving rangelands on marginal soils. The main problems associated with this type of water harvesting system are the costs and the maintenance of the walls.
Jessour is an Arabic term given to a widespread indigenous system in southern Tunisia. Cross-wadi walls are made of either earth or stones, or both, and always have a spill-way—usually made of stone. Over a period of years, while water is stopped behind these walls, sediment settles and accumulates, creating new land that is planted with figs and olives, but which may also be used for other crops. Usually, a series of Jessour are placed along the wadi, which originates from a mountainous catchment. These systems require maintenance to keep them in good repair. Because the importance of these systems for food production has declined recently, maintenance has also been reduced and many systems are losing their ability to function.
d. Water-Spreading Systems
The water-spreading technique is also called floodwater diversion. It entails forcing part of the wadi flow to leave its natural course and go to nearby areas, where it is applied to support crops. This water is stored solely in the root zone of the crops to supplement rainfall. The water is usually diverted by building a structure across a stream to raise the water level above the areas to be irrigated. Water can then be directed by a levee to spread to farms at one or both sides of the wadi.
e. Large Bunds
Also called tabia, the large bund system consists of large, semicircular, trapezoidal or open V-shaped earthen bunds with a length of 10 to 100 meters and a height of one to two meters. These structures are often aligned in long staggered rows facing up the slope. The distance between adjacent bunds on the contour is usually half the length of each bund. Large bunds are usually constructed using machinery. They support trees, shrubs, and annual crops but also support sorghum and millet in sub-Saharan Africa.
f. Tanks and Hafaer
Tanks and hafaer usually consist of earthen reservoirs, dug into the ground in gently sloping areas that receive runoff water either as a result of diversion from wadi or from a large catchment area. The so-called "Roman ponds" are indigenous tanks usually built with stonewalls. The capacity of these ponds ranges from a few thousand cubic meters in the case of the hafaer to tens of thousands of cubic meters in the case of tanks. Tanks are very common in India, where they support more than 3 million hectare of cultivated lands. Hafaer are mostly used to store water for human and animal consumption. They are common in West Asia and North Africa.
Cisterns are indigenous subsurface reservoirs with a capacity ranging from 10 to 500 m3. They are basically used for human and animal water consumption. In many areas they are dug into the rock and have a small capacity. In northwest Egypt, farmers dig large cisterns (200-300 m3) in earth deposits, underneath a layer of solid rock. The rock layer forms the ceiling of the cistern, whereas the walls are covered by impermeable plaster materials. Modern concrete cisterns are being constructed in areas where a rocky layer does not exist. In this system, runoff water is collected from an adjacent catchment or is channeled in from a more remote one. The first rainwater runoff of the season is usually diverted from the cistern to reduce pollution. Settling basins are sometimes constructed to reduce the amount of sediment. A bucket and rope are used to draw water from the cistern.
Cisterns remain the only source of drinking water for humans and animals in many dry areas, and the role they play in maintaining rural populations in these areas is vital. In addition to their more usual domestic purposes, cisterns are now also used to support domestic gardens. The problems associated with this system include the cost of construction, the cistern's limited capacity, and influx of sediment and pollutants from the catchment.
h. Hillside-Runoff Systems
In Pakistan, this technique is also called sylaba or sailaba. Runoff water flowing downhill is directed, before joining wadi by small conduits, to flat fields at the foot of the hill. Fields are leveled and surrounded by levees. A spillway is used to drain excess water from one field to another farther downstream. When all the fields in a series are filled, water is allowed to flow into the wadi. When several feeder canals are to be constructed, distribution basins are useful. This is an ideal system with which to utilize runoff from bare or sparsely vegetated hilly or mountainous areas.
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