7997

• UCREFRP Catalog Number: 7997
• Author: Nash, L. L. and P. H. Gleick.
• Title: The Colorado River Basin and Climatic Change, The Sensitivity of Streamflow and Water Supply to Variations in Temperature and Precipitation.
• USFW Year: 1993.
• USFW - Doc Type: EPA 230-R-93-009,
• Copyright Material: NO

. Reservoir storage and power generation are the variables most sensitive to changes in runoff.
Changes in long-term mean storage in Lake Mead on August 1 are on the order of -70 percent, or-
8,700 thousand acre-feet (tat) for the -20 percent runoff scenario, to +60 percent, or + 7,400 taf for
the + 20 percent runoff scenario.

. Lake Powell falls below minimum power pool 20 percent of the time when runoff drops by 5 percent;
this frequency rises to nearly 60 percent when runoff decreases by 20 percent. The -20 percent
(runoff) scenario causes Lake Mead to go completely dry roughly 25 percent of the time.

. The sensitivity of storage to changes in runoff suggests how carefully the system is currently
managed and that consequently there may be little room for error in forecasting seasonal flows
should the hydrologic regime undergo any significant changes.

. High salinity levels, already a critical concern for the Lower Basin, would be severely exacerbated
by any decreases in runoff.

. While the runoff scenarios modeled in this study may appear extreme, streamflow In the region may
have a much higher variability than is commonly recognized. For instance, the most extreme
scenario modeled in this study, a 20 percent decrease in mean annual runoff, may not even be
Incompatible with the current (non-greenhouse) hydrologic regime. Tree-ring reconstructions
suggest that over the last 500 years, the lowest SO-year mean at Lee Ferry is less than 11 maf, which
corresponds to a 27 percent decrease in natural flow, compared to the 1906-83 instrumental record.

The impact of changes in natural runoff on several water-supply parameters is summarized in Table
ES-1 and in the sections below.

Table ES-1: Sensitivity of water-supply variables to changes in natural flow in the Colorado
River Basin [1J.

Change in Change in Change Change in Change Change
Natural Actual in Power in in
Flow Flow (2J Storage (3J Generation (4J Depletions (5J Salinity (6J
(GAl) (GAl) (%) (GAl) (GAl) (%)
-20 (10-30) (61) (57) (11) 15-20
-10 (7 -15) (30) (31) (6) 6-7
-5 (4-7) (14) (15) (3) 3
5 5-7 14 11 3 (3)
10 11-16 28 21 5 (6-7)
20 30 38 39 8 (13-15)

Notes: (1) Average change compared to the base case over a 78-year simUlation run. Numbers in parentheses represent
DECREASES.
(2) Changes in flow represent the range of changes at five points: Green River, Cisco. Bluff, Lee Ferry, and Imperial
Dam.
(3) Mean storage throughout the basin on August 1.

[41 Mean annual power generation throughout the basin.
[51 Depletions are summarized over the entire basin, although depletions are defined differently in the upper and low
basins. See Hundley (1975) for details.

(6] Changes in salinity repreSent the range of changes at three points: Davis. Parker, and Imperial Dams.

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