Reproduced, with permission, from:
Nash, L. L., and P. H. Gleick. 1993. The Colorado River basin and climatic change: The sensitivity of streamflow and water supply to variations in temperature and precipitation. EPA report no. 230-R-93-009. Washington, D.C.: U.S. Environmental Protection Agency, Office of Policy, Planning and Evaluation.
Linda L. Nash
Peter H. Gleick
Pacific Institute for Studies in Development, Environment, and Security
Oakland, California
A Report Prepared for
The United States Environmental Protection AgencyOffice of Policy, Planning, and Evaluation
Climate Change Division
EPA 230-R-93-009
December 1993
THE SENSITIVITY OF STREAMFLOW AND WATER SUPPLY IN THE COLORADO RIVER BASIN TO CLIMATIC CHANGES
Linda L Nash
Peter H. Gleick
June 1993
Pacific Institute for Studies in Development, Environment, and Security1204 Preservation Park Way
Oakland, California 946121
(510) 251-1600
Growing international concern about the greenhouse effect has led to increased interest in the regional implications of changes in temperature and precipitation patterns for a wide range of societal and natural systems, including agriculture, sea level, biodiversity, and water resources. The accumulation of greenhouse gases in the atmosphere due to human activities are likely to have significant, though still poorly understood, impacts on water quality and availability. One method developed over the last several years for determining how regional water resources might be affected by climatic change is to develop scenarios of changes in temperature and precipitation and to use hydrologic simulation models to study the impacts of these scenarios on runoff and water supply. In this paper we present the results of a multi-year study of the sensitivity of the hydrology and water resources systems in the Colorado River Basin to plausible climatic changes.
The Colorado River is one of the most important river systems in the western United States. It is the principal source of water in a semi-arid basin that covers approximately 243,000 square miles, parts of seven states, and reaches into Mexico (Figure ES-1). The study was conducted in two parts: the first part evaluated the effects of changes in temperature and precipitation on runoff using a conceptual hydrologic model developed and operated by the National Weather Service. Among the impacts studied were changes in streamflow into Lake Powell and on three important tributaries of the Upper Colorado River the White River, the East River, and the Animas River. The second phase of the project then evaluated how these hydrologic changes might affect water supply, salinity, and hydroelectricity production throughout the entire Colorado River Basin using the Colorado River Simulation System (CRSS), a reservoir-simulation model developed and operated by the U.S. Bureau of Reclamation.
Two types of climate scenarios were used for these sensitivity studies: hypothetical temperature and precipitation scenarios, and scenarios generated by general circulation models (GCMs) of the climate. The hypothetical scenarios included increases in average temperatures of 2deg. to 4deg.C and increases and decreases in precipitation of 10 and 20 percent. The regional changes in temperature and precipitation from three GCMs were also evaluated. The scenarios chosen reflected both the best understanding and the uncertainty about the expected magnitude of regional climatic changes when the study began.
Our results suggest that certain aspects of the hydrology and water-supply system of the Colorado River Basin are extremely sensitive to climatic changes that could occur over the next several decades. Not only are significant changes in runoff possible, but the ability of the existing water supply system to mitigate the worst effects is limited. For example, the major reservoirs of the Colorado Basin lessen the impacts of reduced flows, but only for a short period of time. Under conditions of long-term flow reductions and current operating rules, these reservoirs are drawn almost completely dry, hydroelectricity production drops dramatically, and salinity in the Colorado River increases to the point where it fails to meet legal standards almost all of the time. The results strongly suggest that the current approaches to water management in the basin will have to be modified to balance the many competing demands and priorities under conditions of altered climate, and that current water allocations may well be threatened.
The principal impacts of changes In temperature and precipitation on runoff in the Colorado Basin are summarized below.
In summary, the hydrologic modeling results suggest that large changes in streamflow may occur in the Colorado River basin as a result of plausible climatic changes. GCM scenarios indicate that runoff in the basin is likely to decrease. The impacts of these potential changes in streamflow would be felt throughout the basin as changes in water deliveries, reservoir storage, and hydroelectricity production.
The changes in runoff determined in the first part of the project were then used to evaluate impacts on several water-supply parameters, including salinity, reservoir levels, deliveries to users, and hydroelectricity generation. Some quite severe effects were seen, assuming no changes in the operating rules of the basin. For example, a 20 percent reduction in natural runoff would cause mean annual reductions in storage of 60 to 70 percent, reductions in power generation of 60 percent, and an increase in salinity of 15 to 20 percent. In contrast, a moderate increase in temperature (2deg.C) and a large increase in precipitation (20 percent) would result in roughly a 20 percent increase in mean annual runoff, a 30 to 60 percent increase in storage, a 40 percent increase in power production, and a 13-15 percent decrease in salinity. The principal impacts on water supply identified with the CRSS model include the following:
The impact of changes in natural runoff on several water-supply parameters is summarized in Table ES-1 and in the sections below.
Water Deliveries to Users
Delivery of water to different users are affected dramatically by different scenarios, depending on streamflow changes and the application of the law of the river. For example, in the base case, deliveries to the Central Arizona Project would ordinarily fall to their minimum level 20 percent of the time and scheduled deliveries are met or exceeded 40 percent of the time. If runoff drops 5 percent, our results suggest that full scheduled deliveries will be met in only 25 percent of the years and that in half of the years, only minimum levels are delivered.
Although the delivery data suggest that Mexico is affected only in extreme cases, the quality of Mexican water decreases significantly. In fact, all lower Basin users would suffer a significant decline in water quality (see Salinity).
Hydroelectricity
Under current operating rules, hydroelectricity production, like reservoir storage, is extremely sensitive to changes in runoff. If flows in the Upper Basin were to decrease by 10 percent, average annual storage decreases by 30 percent and power production drops by 26 percent. A decrease in flows of 20 percent would reduce storage by 63 percent and power production by nearly 50 percent. An increase in flows of 10 percent would increase storage by 28 percent and power generation by 21 percent.
In the lower Basin, a 10 percent decrease in runoff reduces storage by 30 percent and power production by 36 percent. A drop in runoff of 20 percent reduces Lower Basin storage by 50 percent and power production by 65 percent.
Salinity
The most critical concern for the Lower Basin is salinity and salinity is the only water-quality parameter studied. Even in the base-case scenario salinity criteria are consistently exceeded at all points in the Lower Basin for most years. Decreases in runoff of only 5 percent cause salinity criteria to be exceeded in virtually all years. Even if average flows were to increase by 20 percent, salinity criteria are exceeded continuously for long periods.
Under almost no climate-change circumstances can existing water quality criteria be met given projected demands and operating constraints. Our results suggest that at least a 20 percent increase in natural runoff would be necessary to bring the salinity levels in the Lower Basin into compliance with existing criteria, in the absence of other activities to reduce salinity in the river.
Seasonal Timing of Runoff
A variety of recent hydrologic analyses have suggested that changes in the seasonality of runoff may be a major impact of climate change in hydrologic basins dependent on snowfall and snowmelt. One scenario was run to study the effects of shifts in the seasonality of runoff. The results suggest that an increase in temperature of only 2deg.C would shift peak runoff one month earlier, to May, in the Upper Basin. Under current operating conditions, such a shift in timing reduces the overall efficiency with which the system is operated, reducing effective storage and deliveries, and increasing the average annual salinity. We recommend that changes in operations to account for changes in the timing of runoff should be evaluated.
The results of this assessment suggest that violations of the Colorado River Compact are likely to occur under all scenarios of decreased runoff, assuming that no changes in the operating parameters of the system occur. For instance, storage strategies and targets work extremely well in the base case scenarios but are substantially less effective under alternative scenarios. Thus, violations of the Compact would potentially occur even if runoff dropped only 5 percent. The sensitivity of storage to changes in runoff reflect how carefully the current system is operated and how little room there is for forecast error if water supply is to be maximized without resulting in damaging flood-control releases or uncontrolled spills.
As might be expected, the reservoir simulation results presented here suggest that many of the procedures and inputs used in the Bureau of Reclamation model are closely tuned to the historic hydrologic record. While it is likely that many of the severe impacts noted here could be avoided under different operating conditions and rules, we were constrained in the current study from evaluating any alternative operating criteria.
The problem of planning water management in the face of a high degree of climate and hydrological uncertainty cannot be easily resolved; nonetheless, it may be possible to increase flexibility in water management. This flexibility will need to be reflected in technical and operational decisions, as well as in the legal and economic institutions that govern water use in the basin.
The problem of planning is compounded by the fact that we cannot say with certainty whether runoff in the basin will increase or decrease. Most people with an interest in the basin have focused on the prospect of long-term decreases in runoff and the shortages that would result, which is a logical reflection of the region's preoccupation with drought. The fact that average temperatures in the region will almost certainly increase suggests that, if we assume no knowledge about changes in precipitation, we would expect runoff to decrease as a result of increases in evaporation and vegetative water use. This may be reason enough to plan for supply shortages; but increased water storage must be traded off against the need for flood-control space. The greatest risk of climatic change is the potential for streamflow variability to increase substantially, Increasing the frequency of both sustained drought events and high-flow events.
Beyond the scope of this study were several important issues that policymakers and water-supply managers will have to consider. First, the environmental and ecological impacts of changes in water supply have not been addressed here. In general ecosystems are more sensitive to seasonal, monthly, daily, and even hourly changes in streamflow and water quality than to long-term changes. Unlike water supply, the impacts on the environment cannot be adequately assessed using aggregated time periods or large-scale models. Undoubtedly, however, given the predicted rate of climatic change and the potential magnitude of runoff changes examined here, serious ecological problems would occur.
This study has also not taken projected future economic developments nor some future demands into account. Currently the issue of reserved water rights and Native American claims have obscured future demand scenarios in the basin. Because of the large amounts of water involved, these unresolved claims could have dramatic impacts on water allocation throughout the region and thus add to the uncertainty that the basin faces.
Finally, while this study has suggested what the impacts of climate change could be on water supply, it has not addressed the impacts of climate change on water demand. In fact, demands will change both in time and space. Obviously, agricultural water demand will vary as crops and production patterns are altered in response to climatic changes. Ecosystem water requirements will also vary, both in response to increased temperature and as a result of ecological and environmental changes. Urban and industrial usage will change as a result of both changes in climate and changes in population. It is quite possible that changes in demand over the next 50 to 100 years will equal or exceed changes in supply. In all likelihood, the greatest possibilities for adapting to climatic change lie in the area of demand management, particularly in the agricultural and urban sectors, and the potential for conservation and water transfers needs to be assessed from both a quantitative and an institutional perspective. If we are to plan adaptation strategies, future research must address the integrated impacts of climatic change on demand and supply across sectors.
Given the prospect of future climatic changes, it is imperative that we consider how we can increase the residency of our existing water-management systems and minimize the social and environmental impacts of changes in water availability. We need to identify those responses that will provide us with the greatest flexibility in the coming decades and to develop management schemes that recognize both the variability and the dynamic nature of our climate.
Final Report. This work was supported by the U.S. Environmental Protection Agency, Grant #CR816045-01.
