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Water Resources Colorado. The Colorado River also loses water as it crosses the desert, so that the undepleted runoff at the Mexican boundary is only about 0.9 million acre-feet greater than at Lee Ferry. Groundwater Supplies Although groundwater is a major source of supply in the Pacific Southwest, the data needed for a reliable estimate of the yield that could practically be sustained on a permanent basis are not available for some areas. Large volumes of water are stored in aquifers, chiefly in the unconsolidated fill of valleys and plains. The largest volumes occur where the deposits are deep and extensive; such deposits occur in some of the most arid parts of the area. While estimates of the groundwater reserves in the Pacific Southwest area appear huge, the amount that can be practicably withdrawn from any given region or area may be only a small fraction of the total and depends on many physical, technological, legal, and economic factors. Individual groundwater estimates were developed in each region. The relevance of these estimates to the quantities that may be practicably withdrawn is uncertain. Among the problems encountered are encroach- ment of sea water or other water of poor quality, deterioration of quality with depth, or with recircula- tion of used water, low yields from wells in some of the aquifers, remoteness of some aquifers from areas of present and future use, increasing cost of with- drawal with increasing depth to water, land subsi' dence associated with the dewatering of aquifers, the limited period during which a large overdraft can be maintained, and the legal rights of owners of over- lying land. Many of these problems have not been adequately evaluated and will require future study. In many arid areas of the Pacific Southwest it has taken millions of years to fill these groundwater reservoirs and they are subject to depletion or exhaustion ifthe water is not replaced. These groundwater reserves need careful management. If other sources of water are made available to these areas, a coordinated use of groundwater with other sources may be possible. Some States' law permit groundwater mining, while some do not. The total groundwater storage capacity in the California Region is more than I ,000 million acre, feet. Of this total, 570 million is in the San Joaquin Valley above a depth of 1,000 feet, with about 93 million acre.feet above a depth of 200 feet. The usable stored groundwater for the entire California Region is estimated to be more than 250 million acre.feet. In the Utah and Nevada portion of the Great Basin Region, groundwater is developed on a continuing or sustained yield concept. The sustained or perennial 22 .~:': ,~' .... yield of a groundwater reservoir may be defined as the maximum amount of natural groundwater that can be used each year over the long term by pumping without depleting the groundwater reservoir. Natural groundwater discharge in the Region is 2.3 million acre.feet annually; however, development usually occurs in localized areas and these rarely are dis. tributed so that full recovery of the natural discharge can be obtained. Also, the groundwater in parts of these areas may have relatively high concentrations of dissolved constituents. Groundwater in storage is many times the average annual recharge to and discharge from the ground. water reservoirs. Much of the stored groundwater obviously is beyond any reasonable extension of economic pumping limits and it must be considered a time-limited supply. The estimated quantity of one. time groundwater reserve stored in the upper 100 feet of saturated deposits in the region is 194 million acre-feet. In the Lower Colorado Region several estimates of groundwater reserves were made. Above a depth of 200 feet below ground surface, about 100 million acre-feet of water are in storage. The alluvial aquifers in the first 700 feet below ground surface contain approximately 990 million acre.feet; an additional 400 million acre.feet is estimated to be found between 700 and 1,200 feet. The tight nonalluvial formations found generally throughout the Little Colorado River subregion contain additional stored groundwater and it is estimated that 250 million acre.feet of water exists in a 100.foot-thick section of the formation. The water contained in the tight formations of the Uttle Colorado subregion is con- sidered generally not usable because of very low well yields and the high salinity. The estimated stored groundwater in the Upper Colorado Region above a depth .of 100 feet of saturated thickness of aquifers is 83 million acre.feet (range of estimates, 50.116 million). The withdrawals have been relatively minor and probably will remain so. Water Quality Before extensive development the quality of most surface water was good, but even then a few streams were too saline or too muddy for many uses. The quality tended to degrade from the headwaters to the mouth, especially where streams flow long distances through the desert. The quality also tends to degrade with use. The quality of groundwater varies widely, depend. ing on the source of recharge and the materials through which it has percolated. It commonly con- tains more dissolved solids than surface water.