WSP02231

• File Number: 8507
• Description: Rio Grande Project
• State: CO
• Basin: Rio Grande
• Date: 2/1/1981
• Title: Water Res. Reference Base for the 13 ‎(a)‎ Assessment of the Rio Grande Region
• Water Supply Pro - Doc Type: Report/Study

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pervious streambeds. As noted in Table 2, average annual streamf10ws in
these closed basins generally are less than 10,000 acre-feet.

Texas

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Streamf10ws in the Rio Grande immediately below E1 Paso in Texas (WAU
130401) are significantly depleted by surface water diversions primarily for
agricultural irrigation. These losses are apparent in the mean monthly flow
distributions for the Rio Grande at E1 Paso and at Fort Quitman (Figure 3).
The mean annual flow (1938-1978) of the River out of WAU 130401 is approxi-
mately 100,000 acre-feet, whereas about 365,000 acre-feet enter the reach
from New Mexico. As generally has been the case for the entire Rio Grande
during the past two decades, average streamf10ws in this area have decreased
significantly, For example, during the period 1955-1974, the mean annual
flow delivered to Texas was only 261,000 acre-feet, and the mean annual
outflow from WAU 130401 was only 33,000 acre-feet [13]. Under the provisions
of the Rio Grande Compact and the U.S. - Mexican Treaty of 1906, there is no
obligation by upstream users to deliver any water to users or areas
downstream of Fort Quitman, i.e. below WAU 130401.

Substantial tributary inflow to the Rio Grande does occur, however,
downstream of Fort Quitman in WAU 130402. These inflows originate primarily
from sources within Mexico. The Rio Conchos, with a drainage area of 26,000
square miles in Mexico, contributes about 730,000 acre-feet per year [4] to
the Rio Grande. Inflows from other smaller tributaries throughout the length
of the Big Bend reach of the River increase the mean annual flow of the
mainstem to about one million acre-feet near the lower end of WAU 130420 just
upstream of the Pecos River confluence. The seasonal variation of streamflow
in this segment of the River closely tracks the distribution of rainfall on
the watershed, with the highest flows occurring during September. Almost
half of the annual runoff from the Rio Conchos Basin occurs during August,
September and October in response to summer rains associated with the influx
of Pacific moisture from the southwest.

In addition to mainstem flows, Amistad Reservoir also receives a signi-
ficant amount of its total inflow directly from the Devils and Pecos Rivers.

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