Looking at tree rings to learn about water supplies
Changes in climate may be one result of the world's continued use of fossil fuels, warn climatologists. These changes would be in addition to those resulting from natural causes such as solar variability, changes in ocean surface temperatures, or volcanic activities, among others. One of the important questions this prospect raises asks what would happen to water supplies.
It is believed that man-induced atmospheric changes [due to accumulation of carbon-dioxide gas released by burning coal] will tend to increase temperature, and the long- range outlook is for general warming. Furthermore, those forthcoming changes in climate will probably not be globally or even hemispherically uniform, but will vary from region to region. For example, climatic changes occurring in the northern Great Plains could be of a different type and different degree of severity than changes occurring in the southern Great Plains.
How the man-induced changes in climate will affect available water supplies is largely unknown, partly because we do not even understand how natural climatic variation affects water supply. Until now, the "normal" water supply in most US river basins for the past 30 to 40 years has been much more than is needed for agricultural, industrial, municipal and other purposes. As needs grow, however, the balance between available water and water requirements will become crucial. Consequently, small climatic variations -- be they natural or man-induced or a combination of the two -- will become very important because of their possible effects on water availability.
Hydrologists at the University of Arizona now are trying to determine what the effects of climatic variations will be on projected water requirements for the year 2000. We are studying in detail how such changes will affect each of the 18 major water regions of the United States.
Part of the news appears to be good: A preliminary study has indicated that, in general, most regions east of the Rocky Mountains would not be drastically influenced by a small climatic variation -- that is, by rises or falls of 2 degrees Celsius in mean annual temperature and 10 percent in total amount of precipitation. It is true that these small changes could create sustained high or low flows in rivers, resulting in local excesses or shortages of water and in flooding and waste disposal problems. And colder temperatures could result in ice on rivers and harbors, causing navigation problems. But most of these problems can be solved by changing operating procedures, by the conjunctive use of ground water, and through engineering and construction.
The bad news has to do with the river basins west of the Mississippi River Basin, which may experience an extended shortfallm in the expected or anticipated supply. Our preliminary study indicates that, in those basins, the increased water evaporation from water surfaces, soil, and plants caused by a rise of 2 degrees Celsius in mean annual temperature accompanied by a 10 percent decrease in total precipitation could result in decreases of 40 to 60 percent in annual surface water supplies! Climate changes of this magnitude have occurred naturally in the past 150 years. Such changes could conceivably be induced in the future by man. The resulting decreases in annual surface water supplies could mean that water supplies will not meet demand in the year 2000.
Why have climatic effects suddenly become of such concern, and why haven't the consequences of climatic variation and its effect on water supply been incorporated into management schemes earlier? The answer is twofold. First, during the past 50 to 60 years, developers of water resources in most basins in the United States have not been required to evaluate the effects of relatively small climatic variations because the "normal" supply (or at least what was thought to be normal) was many times greater than projected requirements.
Second, the present "boom" in most Western states was unanticipated. The increased needs for water to develop energy sources; increased needs by agriculture, expanding metropolitan areas, and relocation; and increased water needs by industry moving from colder climates to the Sunbelt -- these were beyond most water planners' imagination. The result is that, under present climatic conditions, requirements now are roughly equal to supply.
Recent experience allows some insight into what happens when suddenly there is not enough water to go around. Consider the West Coast drought of 1976 and 1977. Although this drought didn't last as long as the great drought of the mid-1930s, and although it covered only a relatively small area, it was severe and it affected large population centers and intensively irrigated agricultural areas. Central and northern California were hit hardest; the 1977 water year (October 1976 to September 1977) was the driest in California since records were first kept 100 years ago; the 1976 water year was the fourth driest. The impact was greatest on municipal water supplies, and extreme conversation and curtailment measures had to be implemented.
Agriculture fared better than expected because of conservation measures and because of a 20 percent increase in the use of ground water. An estimated 7,500 new wells were drilled; additional old wells were reactivated, and others were deepened during 1977. This increased pumpage, largely in the Central Valley, is not a long-term solution, however, as it increases the mining of ground water. (Mining of ground water can be likened to withdrawing more annually from one's saving account than one puts in; the ultimate results is obvious.)
During this same drought, even the comparatively water-rich Northwest Basins experienced shortfalls in supply. Had the drought extended an additional year, "brownouts" would have been inevitable, since most electricity for the region is hydroelectric.
Planning for water excessesm has been done for many years. Flood control structures, flood plain zoning, and land management can and have reduced damages from floods. In many river basins, particularly east of the Rockies, these issues along with water quality control will continue to be the major water management problems. Planning for water deficiency, by contrast, is not very advanced. Our continuing studies evaluating the effects of climatic variation upon water availability and supported by the Division of Atmospheric Sciences, National Science Foundation, will focus on evaluating river basins that may be water deficient in the year 2000 and on establishing possible alterntive water resources.
To do this, we will first determine past variations in supplies by using 400 -year-long records derived from tree-ring data. From these long-term records of the past we can project a reasonably accurate estimate of future natural climatic variations. It is then possible to superimpose estimated man-induced climatic variations and evaluate the combined effect on water availability.
As was illlustrated by the example of the recent California drought, one way to mitigate a water deficiency is to use more ground water. In many areas of the West, ground water is already being used up faster than it is being replenished. Consequently, for each river basin region an inventory must be made of the availability of alternative sources. By using computer models we will be able to develop scenarios of past climatic variations, projected requirements, and water available from ground water and perhaps interbasin transfers; we will then be able to evaluate each basin's resilience to recent past climatic variations and projected future variations. It is our conviction that this sort of analysis will provide water resource managers with the necessary impetus to develop contingency plans for the short-term climatic variations that are sure to come.