Managing Arid and Semi-Arid Watersheds

Training Course in Watershed Management

3. Water Resource Management

Water limits much of what people can do. Stability of water supplies is critical to programs of development. Emphasis in water resource management in dryland regions generally is placed on developing or conserving water supplies. The usefulness of water supplies to people also depends largely upon their physical, chemical, and biological characteristics. It is important, therefore, that both quantity and quality be considered in the management of water resources.

Developing Water Supplies

Numerous methods have been used in developing water supplies in the dryland regions of the world. Water harvesting is one example of note. Water harvesting systems were used by people in the Negev Desert over 4,000 years ago, with applications of this technology continuing to the present. Not all of the systems have been successful, although some form of water harvesting has been necessary to sustain livestock and agricultural crop production and forestry, in many instances.

Water harvesting methods involve the collection and, in many instances, storage of rainfall until the water can be used beneficially. Components of water harvesting systems include:

A catchment area, the surface of which is often treated to improve runoff efficiency.
A storage facility for collected water, unless the water is to be utilized immediately, in which case a water spreading system is necessary.
A distribution system when the stored water is to be used later for irrigation purposes.

Other methods of developing water supplies have little to do with the management of watershed lands directly. However, their applications in increasing the amounts of water available for irrigation, livestock production, or human use provide a means of increasing land productivity that, in turn, reduce the pressures of overgrazing by livestock, improper agricultural cultivation, and deforestation. Irrigation with saline water, reuse of irrigation and waste water, and construction of wells and development of springs are some of these methods. Still other methods, including cloud seeding, desalinization of sea and other saltwater, and transfers of water from water-rich areas to water-poor areas, have applications only under special conditions.

Conserving Water Supplies

Water conservation, where available water supplies are conserved for use at a later date, involves a variety of methods to reduce evaporation, transpiration, and seepage losses. Some of these methods pertain to treatments of water, soil, and plant surfaces, while others methods consist of manipulations of vegetative surfaces.

Reducing evaporation can lead to significant savings of water. Among the methods of reducing evaporation from small ponds and livestock tanks are covering these water bodies with blocks of wax, plastic, or rubber sheeting and floating blocks of concrete, polystyrene, or other materials. Liquid chemicals that form monomolecular layers on a water surface (for example, aliphatic alcohols) have been used on larger bodies of water, although their effectiveness can often be limited because of wind and deterioration in the sun. The use of evaporative retardants might be restricted by adverse environmental effects of aquatic organisms in natural lakes or reservoirs.

Transpiration losses from plants can be reduced in many ways, including:

Replacing plant species that have high transpiration rates with species that have lower transpiration rates.
Removing phreatophytes, plants with deep rooting systems that can extract water from shallow water tables, from stream banks.
Planting windbreaks of trees or shrubs to reduce wind velocities.
Applying antitranspirant compounds that either close stomata or form a film on leaf surfaces.

Antitranspirants, although effective in experimental investigations, have not been used widely on a large scale in natural vegetative communities.

Earthen canals and reservoirs that are constructed in pervious soils can lose considerable amounts of water through seepage. Methods of reducing the seepage losses from these structures include the compaction of the soil, treatment of the soil surfaces with sodium salts to break up aggregates, and lining canals and bottoms of small reservoirs with various impervious materials. This latter method is expensive for large reservoirs, although it usually is effective for a long period of time.

Water Quality

Water of low quality can present as much of a limitation to available water supplies as deficient quantities. In many instances, there can be an abundance of water, but its quality is such that it cannot be used safely for irrigation, domestic consumption, or other uses. Water supplies, therefore, must be considered in the context of usable water, or water that is suitable for a specified use.

The quality of water is affected by natural geologic-soil-plant-atmospheric systems and land uses practices. Water quality characteristics of concern to people can be grouped into physical, chemical, and biological characteristics.

Physical Characteristics

Physical characteristics that determine water quality include suspended sediments, turbidity, thermal pollution, dissolved oxygen, biochemical oxygen demand, pH, acidity, and alkalinity. Rainfall events frequently produce large amounts of runoff in short periods of time. These events often promote erosion and transportation of sediments. Streams in the dryland regions commonly transport high levels of suspended sediment and exhibit high turbidity, the latter indicating that light penetration into water is reduced severely.

Dissolved oxygen and biochemical oxygen demands are a concern in perennial streams, lakes, and reservoirs where biodegradable materials enter these bodies of water. The temperatures of water in dryland environments are usually high. In upland areas with cooler water temperatures, care is needed to prevent temperatures from increasing in water bodies where cold-water fish are found. In general, surface water and groundwater resources in dryland regions tend to be alkaline with high pH, due largely to the typically high levels of calcium and salts in the water.

Chemical Characteristics

Dissolved chemical constituents in surface water and groundwater systems reflect the characteristics of the drainage area. Processes such as the weathering of rock, physical-biological processes occurring on watershed lands, and atmospheric deposition all affect chemical compositions of water. Because it is an excellent solvent, when water comes into contact with rock surfaces and other soil materials, its chemical composition changes. The longer the contact is, the greater the change, such as the case with groundwater. Rock and soil substrates generally control ionic concentrations, including calcium, magnesium, potassium, and sodium, on watersheds with little human disturbance. Nitrogen and phosphorus concentrations are affected by biological activities, although much of the nitrate, chloride, and sulfate anions are added through atmospheric inputs.

High concentrations of dissolved solids (salts) can be one of the greatest limitations to the use of water. Salts tend to concentrate because of high evaporative rates and limited amounts of water in dryland environments. As a result, salinity frequently exceeds 100 parts per million (ppm). People can tolerate salinity levels of 2,500 to 4,000 ppm, at least temporarily. Livestock can tolerate 3,000 ppm. Irrigation with high salinity water, when feasible, is often restricted to salt tolerant plants, and it is necessary to flush salts through the soils so that levels of salt accumulations do not become toxic.

Biological Characteristics

Biological characteristics are determined largely by the organisms that impact the use of water for drinking and other forms of human contact. Disease organisms are associated with situations in which human and animal wastes are treated improperly, or the deposition of these wastes has been in close proximity to bodies of water.

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19 March 2001

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