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li.'1 ":,1 .."1 ;-'1 -il Ie, :) ~;~ ,,:' These decreases in surface water supplies have been attributed to chronic drought conditions in the Rio Grande Basin, increased infestations of water- consuming phreatophytes, channel deterioration, and changes in upstream water uses [13,14]. As a result of these surface water shortages, there has been extensive use of ground water for irrigation purposes during low flow periods. ;~ ~-' ..: The Water and Power Resources Service (WPRS), formerly the Bureau of Reclamation, and the State of New Mexico [15] have estimated that the total usable average annual surface water supply in the New Mexico portion of the Rio Grande Basin, excluding transmountain imports from the San Juan-Chama Project, is 540,400 acre-feet. Flow in the New Mexico segment of the Pecos River (WAU 130600) is highly variable, not only because of seasonal hydrologic conditions, but also because of tributary and spring inflows, irrigation diversions, transpiration losses to phreatophytes and native vegetation, channel losses to ground water, and surface evaporation. The surface flow of the River is erratic, being influenced by floodflows resulting from heavy rains. The distribution of mean monthly flows out of New Mexico is illustrated in Figure 3 for WAU 130600. As noted in Table 2, the mean annual flow for the Pecos River near the lower end of WAU 130600 is 135,517 acre-feet, based on the 1937-1976 period of record. More recent data indicate that the annual outflow from New Mexico is substantially less; during the period 1956-1965, the recorded flows averaged only 59,000 acre-feet per year. The total usable average annual surface water supply in the Pecos River Basin of New Mexico has been esti- mated to be 205,000 acre-feet [15]. There are a number of closed basins in the Rio Grande Region of New Mexico which accumulate surface runoff, but do not contribute inflows to the Rio Grande mainstem or the Pecos River. These include the Mimbres, Playas and Wamel Basins in southwestern New Mexico (WAU 130302); the Estancia, Jornada del Muerto, Tularosa and Salt Basins in central New Mexico (WAU 130500); and the North Plains and San Agustin Plains areas in the extreme western portion of WAU 130202. No large supplies of surface water are available within these basins. A few streams have perennial flow in their upper reaches, but the flow rapidly disappears into alluvial fans and highly 15 . ,. ,-- r \. \'"i '. ....... " - - , .