Macrocatchment Systems

Macrocatchment 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 microcatchments, ranging from a few percent to 50 percent 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 macrocatchment systems are discussed below.

14.3.2.2.1 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 important 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.

14.3.2.2.2 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 dike 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.

14.3.2.2.3 Jessour

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 spillway—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.

14.3.2.2.4 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.

14.3.2.2.5 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 m and a height of 1-2 m. 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 not only support trees, shrubs, and annual crops but also support sorghum and millet in sub-Saharan Africa.

14.3.2.2.6 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 stone walls. 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.

14.3.2.2.7 Cisterns

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.

14.3.2.2.8 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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