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DROUGHT IN THE SOUTHWEST, 1942-56
EFFECTS OF DROUGHT IN THE RIO GRANDE BASIN
By H. E. THOMAS and Others
ABSTRACT
In headwater areas of the Rio Grande and its principal tributaries, variations in streamflow and in ground water storage and discharge depend upon fluctuations in precipitation, with modifications by geologic factors and by the pattern of water development and use. In downstream areas the surface and ground water resources are replenished not only by local precipitation but also by outflow from the headwaters areas; thus the effects of drought upon those water resources are complex and may be vague and indeterminate.
INTRODUCTION
The Rio Grande basin is one of five areas of the Southwest subdivided for detailed discussion of the effects of drought as distinguished from other water shortages. The Rio Grande (or Ri´o Bravo in Mexico) is an interstate and international stream. It rises in Colorado and flows southward for more than 400 miles across New Mexico, then forms the boundary between Texas and the United States of Mexico for about 1,200 miles to its mouth (fig. 1). The Rio Grande has a total length of more than 1,800 miles and is the second longest river in the United States; its drainage basin encompasses about 182,000 square miles.
Hydrologically the Rio Grande includes an upper river above Fort Quitman, Tex. (about 90 miles downstream from El Paso), which is generated entirely within the United States and almost entirely used up in Colorado and New Mexico, and a lower river that is regenerated chiefly by flow from Mexico.
The upper Rio Grande has a drainage area of about 35,000 square miles, less than a fifth of the water producing area of the Rio Grande basin. According to the report of the Rio Grande Joint Investigation (National Resources Committee, 1938), the total mean annual water production from runoff in the period 1890-1935 was about 3 million acre feet, of which more than half was used consumptively for irrigation in New Mexico and Colorado. The river is generally lowest in average annual flow in the barren reach below Fort Quitman. In the 20 years 1924-43 the average discharge from the upper basin at Fort Quitman was about 200,000 acre feet.
According to records of the International Boundary and Water Commission (1943), the Rio Grande at the gaging station below Presidio, Tex., with drainage area of about 60,000 square miles, had an average annual discharge of 1.4 million acre feet in 1924-43, chiefly from the tributary Ri´o Conchos in Mexico. In the same period the average annual measured runoff of the Rio Grande was 3.5 million acre feet at Laredo, Tex. (drainage basin 133,000 square miles), and 5.1 million at Rio Grande City (drainage basin 174,000 square miles). These figures, as well as those for the upper Rio Grande, are averages based on records for years prior to the beginning of the most recent drought.
The Pecos River, with a drainage basin greater than 38,000 square miles, joins the Rio Grande below the mouth of the Conchos, and is the largest tributary entering the lower Rio Grande from the United States. Like the Rio Grande, the Pecos has headwaters in the Rocky Mountains, its water is used extensively in New Mexico, and the outflow from New Mexico is considerably less than the flow in the river farther up stream. However, the Pecos gains much water as it flows across the Edwards Plateau region in Texas, and as it enters the Rio Grande it has a larger volume than at any other point along its course.
The principal use of water throughout the Rio Grande basin is for irrigation, and the total area irrigated within the basin is estimated (International Boundary and Water Commission, 1950) to have been about 2,700,000 acres in 1950. Of this total about 950,000 acres is in the upper basin above Fort Quitman, "
Water producing area only. The outer rim of the Rio Grande basin encompasses 335,000 square miles, including closed basins between the Pecos and Rio Grande (Thomas and others, 1962b) In the United States, and extensive closed basins bordering the drainage areas of the Rio Conchos, Rio Salado, and Rio San Juan in Mexico.