Sourcebook of Alternative Technologies for Freshwater Augmentation in West Asia


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United Nations Environment Programme
Division of Technology, Industry and Economics

Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augmentation
in West Asia>

1.1.3 Rainwater Harvesting In The Field

One of the major concerns of farmers in arid and semi-arid regions is to secure irrigation water, particularly during the periods of plant growth that coincide with the end of the rain season. Consequently, farmers do their best to secure water by construction of water structures for rainfall and surface runoff harvesting, to be used when the water is needed. The several technologies of rainwater harvesting can be distinguished on the basis of the area of the harvesting region and the cultivated land utilizing the water. For neighboring harvesting and cultivated regions, the appropriate system is known as the micro-catchment water harvesting system. In contrast, the system is known as the macro-catchment water harvesting when the harvesting areas are located far from the cultivated areas.

In the case of field harvesting, the harvesting region ranges between 1-30 m². The small area allows for a design suitable for plant needs. Evaporation and filtration losses are reduced because of the short length of water flow. The ratio between the area of the harvesting region and the cultivated region is difficult to determine for fields with trees. Some mathematical relations and runoff coefficients that must be considered include the harvesting area, surface runoff factor, annual water demand of the plants, and the area occupied by the plant roots.

The micro-catchment area for each tree generally ranges between 10-100 m², based on the degree of aridity and the type of plant. Although these technologies date back to ancient times, they have proven their feasibility for all regions. For the technologies used, probably the most important factor is the terraces which are created at the mountain foothills to limit surface runoff, and to increase water filtration into the soil in order to increase its moisture content. At the same time, they help reduce soil erosion at the sites where they are created. These structures are used in countries throughout the region, including Jordan, Lebanon, Palestine, Syria, Iraq, Oman, Saudi Arabia and Yemen.

Technical Description

The names of the water structures used for rainwater harvesting vary according to the country and location where they are to be used. In general, terraces are the structures constructed on the foothills of mountainous areas, slopes or river valley courses for capturing surface water runoff and conveying it to irrigate fruits and cultivate fields. In fact, construction of terraces is the best technology for harvesting surface water runoff. It also helps to transform steep areas, with loose and shallow soils, into fertile agricultural lands.

Terraces (or stepped platforms) are created on lands with slopes ranging between 10-35%, and which have sandy loam soil texture (i.e., not coarse or clayey). The width of the platforms is influenced by such factors as slope, soil depth and type of water flow.

The general principle is that the steeper the slope, the smaller the platform step width (Figures 7 and 8). The depth of the soil be removed from the higher part of the slope also determines the dimensions of the platform. The water flow velocity also plays a role in dictating the platform dimensions – the higher the velocity, the longer the platform. The platform is usually sloped to approximately 5-10%, facilitating the surface runoff flow to the lower platforms (Figures 9 and 10), whereas flat platforms increase the water infiltration into the soil.