4. Water harvesting.
In: Proc. of the FAO Expert Consultations on the Role of Forestry in Combating Desertification, Saltillo, Mexico, 1985; FAO Conservation Guide No. 21, 1989; ISBN92-5-102802-8
The earliest evidence of the use of water harvesting are the well publicized systems used by the people of the Negev Desert perhaps 4000 years ago. Hillsides were cleared of vegetation and smoothed in order to provide as much run off as possible; the water was then channeled in contour ditches to agricultural fields and/or to cisterns. By the time the Roman Empire extended into the region, this method of farming encompassed more than 250,000 hectares.
Water harvesting is a technique of developing surface water resources that can be used in dry regions to provide water for livestock, for domestic use, and for agroforestry and small scale subsistence farming.
Water harvesting systems may be defined as methods whereby precipitation can be collected and stored until it is beneficially used. The system includes a catchment area, usually prepared in some manner to improve run off efficiency and a storage facility for the harvested water, unless the water is to be immediately concentrated in the soil profile of a smaller area for growing drought-hardy plants. A water distribution scheme is also required for the systems devoted to subsistence farming for irrigation during dry periods.
A successful system must be:
- Technically sound, properly designed and maintained.
Water harvesting offers methods of effectively developing the scarce water resources of arid regions. As contrasted to the development of groundwater, which is usually a finite water resource in arid zone, the method allows use of the renewable rainfall which occurs, even though in limited amounts. It is also a relatively inexpensive method of water supply that can be adapted to the resources and needs of the rural poor.
It is necessarily small scale, and as such it can provide stability and improve the quality of life in small rural communities and that of small land holders who are several stages removed from the benefits of large scale development projects. It involves some risk, dependent upon the vagaries of climate. New skills, though simple, are required, maintenance is a constant necessity, and good design is imperative.
There is no universally "best" system of water harvesting. However, there will be some type of system that can be designed to best fit within the constraints of a given location. Each site has unique characteristics that will influence the design of the most optimum system. All factors, technical, social, physical and economic must be considered.
During the past two decades, there have been many water harvesting systems constructed and evaluated at a number of different places in the world. Some of the systems have been outstanding successes, while others were complete failures. Some of the systems failed, despite extensive effort, because of poor design or the materials used. Other systems failed despite good design and proper materials because social factors were not integrated into the systems. These systems failed because of poor communication and lack of commitment by the local people both in planning and financing the projects.
Sufficient knowledge and experience has been accumulated to put into operation water harvesting projects throughout the arid lands of the world. Empirical information and documentation is needed from successes as well as failures on which to build a more exact technology.
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Case study, USA, irrigation systems, costs and benefits, cotton production
LETEY, J. et al.
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