Impacts of Climate Change and Land Use  on the Southwestern United States
Coping with Severe Sustained Drought in the Southwest

What kinds of hydrologic, environmental, and economic impacts would result?

Abstracted from Harding and others (1995), Booker (1995), Hardy (1995), Henderson and Lord (1995), Lord and others (1995), and MacDonnell and others (1995)

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.

Hydrologic Impacts

Harding and others (1995) used a computer model of the Colorado River system to simulate the effect of the Severe Sustained Drought on the functioning of Colorado River water-resource systems. Allocations and diversions of Colorado River water were constrained to function within the legal and administrative rules known as the "Law of the River" [MacDonnell and others (1995) provide a very clear exposition of this set of rules]. Within the Law of the River, with inflows of water estimated for the Severe Sustained Drought, and with net water demands [water removed from the river minus water returned] projected to increase at a "medium" rate (averaging 12 million acre-feet per year), the model simulates flows in 107 segments of the river, storage in 14 reservoirs, inflows at 29 points, and 265 sites where water is consumptively used (used and not fully returned because of evaporation or other losses).

Volumes of water stored in Lake Powell and Lake Mead

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)

The drought simulations indicated that, under current management practices, a severe sustained drought would heavily impact the Upper-Basin States of the Colorado River (above Compact Point) but would have limited impact on water use in the Lower Basin. The simulations indicated that supply shortfalls would have to be declared for 8 of 38 years in the Upper Basin; for two years, even the most priviledged (senior) water rights in the Upper Basin would suffer supply shortfalls. In contrast, shortfalls would be declared for two years in the Lower Basin. Some important water users in the Lower Basin, like the Metropolitan Water District of Southern California, rely on water beyond that allocated to them by the Law of the River (either extra water available during wet years or water that is allocated to others but not yet utilized). These diverters of water not specifically covered by the Law of the River would also face supply deficits that were not covered in the model of Harding and others (1995); see Booker (1995) for discussion of the impacts to these uses.

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 others, 1995).

Environmental Impacts

Backwater along the Colorado River

Figure 7. Backwater along the Colorado River, Grand Canyon (Photo courtesy U.S. Bureau of Reclamation)

The impacts of such hydrologic changes on five environmental-resource categories were simulated with human role-players (as decision makers) and computer models of the hydrologic responses, as described by Hardy (1995).

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.

Economic Impacts

Economic damages sustained under the Severe 
Sustained Drought scenario

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)

A severe sustained drought would result in economic costs in all parts of the Colorado River basin. Booker (1995) used an an integrated economic-hydrologic-institutional computer model to evaluate the likely costs. Consumptive uses in the Upper Basin were most vulnerable to large supply shortfalls and large damages because their water rights are junior [take less precedence in the Law of the River] to many downstream uses. Many current uses (especially in California) rely on diversions of "excess" water or water not specifically covered by the Law of the River (such as wet-year flows and water allocated under the Law of the River but not yet used). These excess uses were also quite vulnerable to drought.

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.

Summary of Impacts

Three main conclusions were indicated by the simulations:

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What management alternatives might help?

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