In order to quantify effects of such a drought on various aspects of the Colorado River system, several models and simulations were used to forecast the hydrologic, environmental, and economic impacts and damages of a severe sustained drought.
Figure 6. Volumes of water stored in a major Upper Basin reservoir (Lake Powell) and a major Lower Basin reservoir (Lake Mead), under Severe Sustained Drought conditions (top) and under normal conditions (bottom). (Modified from Harding and others,1995)
In order to meet all demands, simulated streamflows were reduced by an average of 6 to 12%. In some months, some stretches of river would be completely dry in order to maintain reservoir storage elsewhere in the system. Water levels in Upper Basin reservoirs (like Powell Dam, Figure 6) would decline to "dead storage" levels [at which point releases of water are mechanically infeasible] during the worst years of the drought. Reservoirs in the Lower Basin (like Hoover Dam, Figure 6) would still have water in active storage during the worst years.
Despite many shortfalls and declines in flow and reservoir
storage, the Colorado River system with its large storage
capacity, proved remarkably resilient to the Severe Sustained
Drought designed by Tarboton (1995). However, the system
is also such that the drought impacts would fall
disproportionally on the Upper Basin users (e.g., Harding and
others, 1995) and on the non-consumptive uses (e.g., Lord and
Figure 7. Backwater along the Colorado River, Grand Canyon (Photo courtesy U.S. Bureau of Reclamation)
The resource categories simulated were: threatened, endangered, or sensitive fish ("listed fish"); nonlisted native fish; wetland and riparian habitats; wildlife refuges; and fish hatcheries. Although water-allocation decisions in Hardy's simulations were partly predicated on some of these environmental resources (especially endangered species), the impacts of the severe sustained drought were substantial and sensitive to allocation decisions.
These environmental resources generally fall into the category of water uses called "non-consumptive uses". Lord and others (1995) concluded from all the studies that non-consumptive uses were, in fact, the most vulnerable uses during drought conditions under existing management rules (the "Law of the River"). Because the current Law of the River and current operating rules are largely designed to sustain withdrawals for consumptive uses (especially in the Upper Basin), Colorado River management substantially increases the severity of environmental losses as well as hydropower losses (another non-consumptive use). In role-playing simulations by Henderson and Lord (1995), a tendency towards slavish adherence to established rights to diversions from the river led to failures to limit environmental impacts and impacts on non-consumptive uses in general. This strategy, which is codified in the Law of the River, effectively minimized drought impacts on the consumptive uses, requiring only the addition of a few water-management improvements within individual states. However, Henderson and Lord (1995) concluded that only reduction in withdrawals for consumptive uses in the Upper Basin would have significantly lessened the overall impacts of the drought.
Figure 8. Economic damages sustained under the Severe Sustained Drought scenario, from consumptive uses (top) and from consumptive and non-consumptive uses (bottom). (Modified from Booker, 1995)
Damages that could be incurred in a severe sustained drought
by consumptive uses and by all users (except environmental
damages--which were not included in Booker's models) are shown
in Figure 8. Damages to consumptive uses were
projected to peak in the final years of the drought at about
$750 million/year. Through the final half of the drought
and during the entire recovery period simulated, the costs
borne by the Southern California Metropolitan Water District
(MWD) were valued at about $250 million/year, mostly because
MWD relies heavily on diversions beyond those guaranteed by
the Law of the River. During the final, deepest years of the
drought (years 17-22), costs to Upper Basin municipal and
energy water uses were even larger. Total damages
(including the non-consumptive uses) reach as high as
$2 billion/year in the final years of the drought. Hydropower
losses and salinity damages eventually could reach levels
comparable with the losses by consumptive uses.
Three main conclusions were indicated by the simulations:
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