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Lower Safford Valley, Graham County, Ariz., is an alluvial lowland plain 1 to 3 miles wide along Gila River. The valley is underlain to an average depth of about 100 feet by silt and waterbearing sand and gravel deposited by the river. The plain is occupied by farmed lands, which are irrigated from wells and from the river, and by a belt of natural vegetation in the bottom land along the river. Most of the natural vegetation consists of phreatophytes, plants that send their roots to the capillary fringe or to the water table. The principal phreatophytes in lower Safford Valley are saltcedar, baccharis, cottonwood, and mesquite, all of which are nonbeneficial users of large quantities of ground water. In order to supply the basic data from which an estimate could be made of the quantity of water that might be saved if such bottomland vegetation were destroyed, an investigation of the annual use of water by the vegetation was made in 1943 and 1944. Six methods of determining use of water were applied during the investigation: Tank method, transpiration-well method, seepage-run method, inflow-outflow method, chloride-increase method, and slope-seepage method. The tank method was based on the measurement of the quantity of water used by native vegetation growing in tanks in which a water table was maintained, so that hydrologic conditions in the tanks resembled those in the field as nearly as possible. The transpiration-well method was based on the measurement of the diurnal water-table fluctuations caused by transpiration by phreatophytes; the daily rate of use of ground water, as shown by observation wells in the bottom-land area, was determined by multiplying the amount of daily decline of the water table by the coefficient of drainage (specific yield). The seepage-run method was based on sets of discharge measurements made at about monthly intervals along Gila River to determine the seepage inflow to the river; use of water by vegetation along the river was computed as the difference in the seepage inflow between the growing and nongrowing seasons. The inflow-outflow method was based on a water inventory for which, in a given area in a given time, the quantities of water entering and leaving and the changes in stored water were determined by measurements in the field; the use of water by vegetation, which could not be determined for this method by field measurements, was computed as the difference remaining after all other factors had been measured or accounted for. The chloride-increase method was based on the determination of the increase in the concentration of chloride in the ground water as it moved from the outer edge of the vegetation-covered bottom-land area to the river, and computation of the use of water by the vegetation was based on the fact that the quantity of water withdrawn by transpiration was proportional to this increase in chloride content. The slope-seepage method was based on the fact that the rate of use of water by vegetation in a reach of the bottom-land area represented the difference between the rates of ground-water inflow to the bottom-land area and to the river; the rate of ground-water inflow to the river was determined from the sets of seepage 1 2 USE OF WATER BY BOTTOM-LAND VEGETATION measurements, and the rate of ground-water inflow to the bottom-land area was determined on the basis of the hydraulic gradient of the water table at the time of each set of seepage measurements, the transmissibility of the aquifer, and the length of the reach. Although the methods differed greatly, the figure for use of ground water computed by each method was within 20 percent of the mean determined by averaging the results of all six methods. As a part of the investigation, the quality of the waters of lower Safford Valley was studied in detail. The quality-of-water studies included more than 5,000 analyses of surface and ground waters. These analyses showed that surface waters of the area contain 250 to about 6,000 parts per million of dissolved solids and that ground waters contain 200 to more than 10,000 parts per million. The waters of low dissolved-solids concentration contain mostly sodium or calcium and bicarbonate. Highly mineralized waters contain mostly sodium and chloride. Based on the results obtained by the six methods, the total use of water by vegetation during the 12-month period ending September 30, 1944, was 28,000 acre-feet in a total of 9,303 acres in the 46-mile reach of Gila River from Thatcher to Calva. As precipitation and runoff were subnormal in most of the period of the investigation, it is possible that the total use of water in other years may exceed 28,000 acre-feet. Of the total water used, 23,000 acre-feet was derived from the ground-water reservoir, and the remainder was derived from precipitation on the area. Of the 23,000 acre-feet, more than 75 percent was used by saltcedar. PART 1. GENERAL DESCRIPTION

Use of water by bottom-land vegetation in lower Safford Valley, Arizona