D3

and about the same acreage is served by major tributaries in the lower basin (Pecos River, 240,000 acres; Ri´o Conchos, 200,000; Ri´o Salado, 125,000; and Ri´o San Juan, 330,000 acres). The remaining 800,000 acres is along the main stem below Fort Quitman, including about 700,000 acres in the lower Rio Grande valley below Rio Grande City.

The headwaters of the Rio Grande are in the rugged San Juan Mountains and Sangre de Cristo Mountains of Southern Colorado. In both ranges there are several peaks that exceed 14,000 feet in altitude, and areas where the mean annual precipitation exceeds 40 inches. In the agricultural valleys along the Rio Grande, however, the average annual precipitation is only 7 to 10 inches, and this is true even in San Luis Valley near the headwaters, where the valley floor ranges from 7,500 to 8,000 feet about sea level. Because of this meager precipitation, irrigation has been prerequisite to successful agriculture along the upper Rio Grande, presumably from the earliest days of habitation. The Spanish explorers in 1540 found that in the many storied towns of the Pueblo Indians of which 17 or 18 are still in existence water was diverted from the river in acequias, or irrigation ditches (Follett, 1898), of which several are still in use. Thus irrigation has been practiced continuously along the upper Rio Grande longer than in any other part of the United States: at least 400 years of recorded history, and probably for several centuries before that.

In comparison to the rest of the Southwest, the upper Rio Grande also has a longer history of water shortages and disputes, and of treaties and decrees and compacts to settle disputes. The town of Albuquerque was founded in 1706, and by 1739 some residents had moved several miles to the south, partly because of shortage of water for the fields at Albuquerque. A water shortage in the early 1890's developed international repercussions because it affected people in Mexico as well as in Texas and southern New Mexico. The shortage was attributed chiefly to the increasing development and use of water for irrigation in San Luis Valley in the preceding decade, but it is to be noted that it coincided with a period of deficiency in precipitation (p. D4). This water shortage was responsible for the "embargo" of 1896 and for the Rio Grande Convention of 1906 between the United States of America and the United States of Mexico. The "embargo" was an order by the Secretary of the Interior which prevented further irrigation development of any magnitude in the Rio Grande basin in Colorado and New Mexico by suspending rights of way across public lands for use of Rio Grande water; the "embargo" was not lifted until 1925. Under the terms of the Treaty of 1906, the United States guaranteed an annual delivery in perpetuity of 60,000 acre feet of water in the Rio Grande at the head of the Mexican Canal near El Paso, Tex.

In 1929 the States of Colorado, New Mexico, and Texas ratified a temporary compact which provided in effect that neither Colorado nor New Mexico would cause or permit the water supply in the Rio Grande to be impaired by new or increased diversions or storage unless and until such depletion was offset by increase of drainage return; this compact was operative until October 1937. In 1938 the same States ratified the Rio Grande Interstate Compact (Witmer, 1956, p. 154-177), which provides for apportionment of the water of the upper Rio Grande basin on the basis of specified indexes of flow at key gaging stations (p. D23-24).

In keeping with the long continued concern over the water of the upper Rio Grande, long records of streamflow are available for many places along the main stem and its principal tributaries; at Embudo, N. Mex., north of Santa Fe, the U.S. Geological Survey initiated its stream gaging program in 1889. However, most of the available records concerning other aspects of hydrology are shorter and less complete, and the data concerning ground water are especially meager.

The report of the comprehensive Rio Grande Joint Investigation (National Resources Committee, 1938), which provided the basic data essential for the negotiation of the Rio Grande Compact of 1938, is of great value in studying the effects of drought upon the basin's water resources. Although that investigation was completed several years before the beginning of the most recent drought, it covered a period (1890- 1935) that began in drought and ended in drought. Because of the intervening wetter years, it was concluded in that report that the median natural streamflow during the period of record was close to the median flow for a much longer period.

In all parts of the basin the natural streamflow has been modified by man to such an extent that there are few places where it can be computed reliably from existing records. Many modifications had been underway long before the beginning of these records, but many others occurred during the period 1890-1935 and have been noted in the report of the Rio Grande Joint Investigation. Such modifications include changes in diversions, reservoir storage, irrigated acreage, and drainage of surface water; changes in groundwater storage; and changes in cover of vegetation that