D7

subirrigation ("subbing") of crops; and

(3) development of pumped wells in the shallow aquifer, especially in years when surface water was insufficient to maintain the "sub."

As of 1951 it is estimated that 1,400 to 1,500 irrigation wells pumped on the order of 500,000 acre feet of water from the shallow aquifer.

In a 291square mile area that includes about half the shallow irrigation wells, it was found (Powell, 1958) that the shallow aquifer had an average saturated thickness of 60 feet, a specific yield of 30 percent, and storage of about 3 million acre feet of water. In a 4 year inflow outflow study by Mutz (in Powell, 1958, p. 120-129) the consumptive use of water within the area ranged from 307,000 acre feet in 1949 to 200,000 acre feet in 1951. The quantities of water diverted from streams and pumped from wells are shown by bar graphs in figure 3B. The effects of these additions to and subtractions from the reservoir are shown in the water levels in three wells within the study area, and similar effects may be presumed in years when no data are available as to diversions or pumpage. Thus, after wet 1941, sub irrigation was moderately effective until the extremely dry year 1951. With record precipitation in 1952 the water table rose again to "subbing" level and was high also in 1953. From 1954 through 1956 the water table failed to rise to the "subbing" level and in early 1957 it was at a record low level. With abundant runoff in 1957 and 1958, the water table was again high enough for "subbing" by June 1958.

Increases in ground water storage such as that recorded in 1952 are responsible for part of the difference between stream inflow to and outflow from San Luis Valley (fig. 2). The average annual depletion of Rio Grande within the valley in the 18 years 1936-53 was 800,000 acre feet, the same as was computed for the years 1927-35, but it ranged from about 500,000 acre feet in the dry years 1940 and 1951 to a million acre feet or more in 1941, 1949, and 1952. If 800,000 acre feet represents the quantity of water that must be taken from the Rio Grande in order to sustain San Luis Valley requirements, then surface water could not fulfill the demand in 1940, 1946, 1951 or 1953, because total surface water inflow to the valley was less than 800,000 acre feet. The difference in recent years has been made up by pumping from wells which, however, has reduced the storage in the ground water reservoir. In subsequent wet years this ground water storage was replenished, and the stream depletion in such years exceeded the consumptive use of water by irrigated crops and miscellaneous non beneficial water uses.

MIDDLE VALLEY AND TRIBUTARIES, NEW MEXICO

The middle section of the upper Rio Grande extends from Lobatos, Colo., near the New Mexico State line, to San Marcial at the head of Elephant Butte Reservoir, a distance of about 270 miles. The northern half of this section is flanked by the southern extensions of the Conejos Range and Sangre de Cristo Mountains, which border San Luis Valley. Throughout most of this reach the Rio Grande flows in canyons or narrow valleys. It is from this part of the drainage area that Rio Grande receives most of its water supply that originates in New Mexico, and most of the water that is used in New Mexico. The principal tributaries are the Rio Chama from the west and several streams rising in the Sangre de Cristos to the east ; winter snows on the higher mountains are an important source of the water in these streams. The annual runoff of the Rio Grande at Otowi Bridge near San Ildefonso, N. Mex., downstream from this producing area, is at least double and in most years is 4 or 5 times as great as the quantity leaving San Luis Valley near Lobatos, Colo.

At a point due west of the city of Santa Fe the Rio Grande emerges from White Rock Canyon and flows southward in a valley that continues all the way to San Marcial, a distance of 150 miles. This is commonly called the Middle Valley; the valley floor is generally 1 to 5 miles wide and is bordered by scarps rising to "mesas" (alluvial fans) that rise hundreds of feet above the valley floor. Most of these lands are within the Middle Rio Grande Conservancy District, which operates El Vado Reservoir (capacity 200,000 acre feet) on the Rio Chama. The mountain ranges bordering the valley are low and receive precipitation chiefly in summer cloudbursts; the tributaries to the Middle Valley therefore are subject to flash floods that produce relatively small total runoff. There is only meager information concerning the tributary inflow and ground water inflow from the sides of the Middle Valley. The report of the Rio Grande Joint Investigation (National Resources Committee, 1938, p. 13) gives the following summary of findings based on available data prior to 1936:

Accurate determination of past stream flow depletion in the Middle Valley is not possible because of the lack of adequate records of tributary inflow and uncertainty with respect to it. An approximation has been derived, based on such data as are available, in order to furnish a reasonable basis for analyses of the effect upon the Elephant Butte Fort Quitman section of present and given future conditions of irrigation development in the San Luis and Middle sections. The mean annual stream flow depletion, 1890-1935, Otowi Bridge to San Marcial, is estimated to have been 586,000 acre feet. The corresponding mean annual tributary inflow derived as a residual