Water Harvesting in Arid Lands

Abstract

The use of water harvesting systems in arid lands offers the potential of making lands productive that are now largely unusable due to lack of water for domestic livestock or agricultural use. As long as there is rainfall a water harvesting system can be designed to collect that rainfall and store it until it can be used for beneficial use. The water harvesting system consists of a catchment and a storage facility. If the water is to be used for agriculture it would also include an agricultural area. The agricultural area could be located within the catchment area or in a separate nearby area. Many different treatments have been tested for use in catchment construction. These treatments increase the runoff by decreasing the permeability of the surface and or reducing the time the water stays on the surface or amount of water trapped on the surface. A list of the more promising treatments in order of their increasing cost, are: (1) Shaped compacted-earth; (2) sodium-treated shaped compacted-earth; (3) wax-treated shaped compacted-earth; (4) gravel-covered plastic; (5) fiberglass-asphalt chipcoated; (6) asphalt-plastic-asphalt chipcoated; (7) rubberized-asphalt chipcoated; and (8) reinforced-mortar-covered plastic. The use of compartmented reservoirs make storage of water more efficient. Evaporation and in some cases seepage losses are reduced using the compartmented reservoir by keeping the water concentrated into a volume with as small a surface area as possible. This method of storage when combined with the collection of runoff from a natural surface or with one that is inexpensively treated makes it practical to provide water for supplemental irrigation. This combination is called a water harvesting agrisystem. Concentration of water in a compartmented reservoir can be accomplished in flat terrain using a pump. If the water is being used at a fast enough rate concentration can also be accomplished by selective removal. Alternatively with topography of a sufficient grade, concentration can be accomplished by gravity. Evaporation control on the compartmented reservoir can be improved by placing an evaporation control barrier on the "last" compartment, the one in which water is concentrated and has water in it the longest time. This enhances the value of the evaporation control barrier and increases the dependable water supply. A computer model has been developed to help in the design of the water harvesting systems including agrisystems with compartmented reservoirs. This program fits on portable personal computers and can thus be taken by the designer to a field location to develop an optimum design at a minimum cost. The model can be improved through calibration in a given area as systems are installed and data collected.