D4

is dependent upon ground water. In some places the effects of these modifications by man have reached a sort of equilibrium in subsequent years, perhaps different from natural conditions but nevertheless fairly stable. In other places they tend to reflect or even to enhance the natural fluctuations in supply from precipitation (p. D17).

In accordance with natural divisions, the upper Rio Grande basin comprises three principal areas: the San Luis Valley in Colorado, the Middle Valley in New Mexico, and the Elephant Butte Fort Quitman area in New Mexico, Texas, and Mexico. The Rio Grande enters San Luis Valley near Del Norte, Colo., where streamflow records have been collected at a gaging station since 1889. From these and other streamflow records the total annual production of runoff has been calculated (National Resources Committee, 1938, p. 28-37, 175-183), and is shown in figure 2A. This graph indicates natural fluctuations with least modification by man. The total inflow was less than the long term mean in 10 of the 13 years 1892-1904, the first drought recorded in streamflow records, and was greater than the mean in 17 of the 25 years in the following wet period, 1905-29. The drought of 1930-40 resulted in less than average inflow in 7 of the 11 years. After a short wet period, 1941-45, there were several more years of less than average inflow to San Luis Valley, but these were interspersed with years of more abundant streamflow. In the headwater area the 1892-1904 drought was more severe than any subsequent drought, because it included the least annual runoff and the longest succession of consecutive years of low flow (Gatewood and others, 1962). The Rio Grande headwaters are along the margin of the area of 1942-56 drought (Thomas, 1962), and also in a meteorologic region where annual variations in runoff are not as great as in many parts of the Southwest; as shown by Gatewood and others (1963), the standard deviation for Rio Grande at Del Norte is about 240,000 acre feet. In 62 years of record, the runoff was below the mean for not more than 2 consecutive years, except in 2 periods of 4 consecutive years (1899-1902 and 1953- 56).

The runoff of Rio Grande near Lobatos, Colo., represents the residual flow below San Luis Valley. There was a gradual increase in average annual streamflow depletion (ruled pattern in fig. 2) from about 600,000 acre-feet in the 1890's to more than 800,000 acre-feet in the 1920's, during which time the irrigated area increased from 250,000 to 550,000 acres.

Figure 2B shows the flow of Rio Grande into the Middle Valley as measured at Otowi Bridge near San Ildefonso, N. Mex., the total inflow to the Middle Valley as estimated in the Rio Grande Joint Investigation, the measured residual outflow at San Marcial, and the streamflow depletion within the Middle Valley. Here it was found (National Resources Committee, 1938, p. 37-47) that there had been relatively little change in stream depletion from 1890 to 1935, except that due to variation in water supply; there is an apparent downward trend, in both inflow and outflow in the past 50 years. The hydrographs for the Middle Valley show also the effects of long term climatic fluctuations, including the drought of 1892-1904 (interrupted by a wet 1897) and a wetter quarter of a century beginning in 1905. The drought of the 1930's was not so pronounced as in San Luis Valley, but the drought beginning in 1943 was more so.

Figure 2C shows the inflow to Elephant Butte Reservoir (or flow through the reservoir site prior to 1915) and the diversions and other releases from the reservoir; figure 2D shows the yearend reservoir storage. The inflow reflects major climatic fluctuations even though modified by the developments upstream. The outflow has been small and fairly constant since the construction of Elephant Butte Reservoir except in 1942 when there was spill from the reservoir. In all these graphs there is a general downward trend since 1920, interrupted in the wet years 1941-42.

Figure 2 shows the effects not only of climatic fluctuations but also of human adaptation to them. Once the irrigated acreage had become stabilized, the consumptive use tended to be more nearly constant from year to year than the natural runoff resulting from precipitation. Although the stream diversions were necessarily less in dry years than in wet years, the proportion of the total runoff diverted in a dry year was generally greater. Thus the fluctuations in the annual outflow from San Luis Valley, if expressed in percentage of the long term average, would be more pronounced than the fluctuations in inflow to that valley. There was even more extreme fluctuation in the outflow from Middle Valley, which constitutes the inflow to Elephant Butte Reservoir. By contrast, the small outflow measured at Fort Quitman results from practically complete consumption of water within the upper Rio Grande basin.

A description of the effects of drought upon the upper Rio Grande is difficult because of the problems of segregating the local effects in specific areas, and integrating those effects upon the resource as it moves downstream; and also the problems of discriminating the effects of recurrent drought from the effects of development and control. For this task we have relatively incomplete basic hydrologic data. The pattern