low concentrations of these substances, because it has limited contact with rocks containing them. Stormwater, however, often contains a high sediment load and, particularly when generated over urban or agricultural areas, may contain high concentrations of nitrates, phosphorus and other nutrients, as well as metals and organic material from fertilizers, pesticides, animal wastes, and trash.

Surface water generally requires more treatment prior to human consumption than does ground water. However, in the natural environment, plants and animals are in contact with untreated surface water, which often contains urban, agricultural, or industrial contaminants. Even relatively low concentrations of some of these contaminants in surface water can stress the biological integrity of lakes, reservoirs, and other receiving waters through various physical and chemical processes. Metals can be toxic to aquatic life and are often accumulated and concentrated in the food chain. Excess nutrients commonly result in the growth of undesirable aquatic plants and contribute to eutrophication. Suspended material in water causes turbidity, which affects light penetration and inhibits the growth of desirable aquatic plants. Biodegradable organic material (as measured by biochemical and chemical oxygen demand) can reduce the level of oxygen dissolved in water, killing fish.

An intrinsic problem of water management in arid areas is salinity. Salts are continually added to soils through water dissolution of rocks and minerals and degradation by plant roots (for example, the salt calcium carbonate), input of aerosols and recharge (for example, the salt sodium chloride), as well as by anthropogenic activity (for example, various salts of nitrogen and phosphorus as a result of fertilizer use). Where rainfall is sufficient, such salination is balanced by salts leaching out of the soil by infiltrating water (recharge). The salts are incorporated in groundwater and eventually discharged to streams or directly to the ocean, seas, or other terminal discharge points. In this way, humid areas are net exporters of salts and salt accumulation in soils is virtually nonexistent.

Arid regions, on the other hand, are generally net importers of salts, and salt accumulates in soils. Within the study area, experience has shown that salt accumulates in soils where annual precipitation is less than about 300 mm. Where precipitation is higher than about 400 mm, winter rains are effective in leaching out salts that have accumulated in the root zone during the previous summer. From the mean annual precipitation shown in Figure 2.3, it can be seen that only a very small part of the study area is able to sustain long-term agriculture without a management scheme that artificially removes salt accumulated in the soil.

Even within the humid parts of the study area where precipitation exceeds 400 mm/yr, the natural seasonal balance of salt accumulation