Water Scarcity: Impacts on Western Agriculture
514
Water Scarcity: Impacts on Western Agriculture
514Hardcover(First Edition)
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Overview
Although considerable attention has been given to some aspects of declining water supplies for irrigated agriculture in particular areas, this is the first volume to adress in a comprehensive manner the effects of scarce water supplies on agricultural production and the resultant impacts at regional, state, national, and international levels. Over seventy experts, representing all the major physical and social sciences as well as industries examine the issues and conclude that important decisions must be made at all levels of government and private enterprise if the prosperity and quality of life in the region are to be maintained. Specific technical, economic, institutional, and managerial solutions are recommended to forestall an impending water crisis. All segments of society—agriculturalists, urbanites, food processors, land developers, environmentalists, and others—have major stakes in the outcome of any action for future water supplies and distribution in the West.
This title is part of UC Press's Voices Revived program, which commemorates University of California Press's mission to seek out and cultivate the brightest minds and give them voice, reach, and impact. Drawing on a backlist dating to 1893, Voices Revived makes high-quality, peer-reviewed scholarship accessible once again using print-on-demand technology. This title was originally published in 1984.
Product Details
| ISBN-13: | 9780520357433 |
|---|---|
| Publisher: | University of California Press |
| Publication date: | 03/25/2022 |
| Edition description: | First Edition |
| Pages: | 514 |
| Product dimensions: | 6.00(w) x 9.00(h) x 1.30(d) |
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Water Scarcity
Impacts on Western AgricultureUniversity of California Press
Copyright © 1984 Ernest A. Engelbert and Ann Foley ScheuringAll right reserved.
ISBN: 0-520-05313-3
Chapter One
Physical Limitations of Water Resources by John BredehoeftAbstract
In considering the concept of the hydrologic cycle today one must take into account man's influence as an integral part of the functioning of the cycle. Except for the mining of groundwater, the same quantity of water is, on the average, in transit in the hydrologic cycle. Groundwater mining is extensive, especially in Arizona and the High Plains of Texas and New Mexico. Groundwater, however, is a one-time supply; to the extent that we mine it, we are faced with a shortage in the future. Both urban movement to the Southwest and energy development compete with agriculture for the available supply, especially in the areas of critical water supply, southern California and Arizona. Competition is present throughout the semiarid West; anywhere water is fully appropriated, increased urban and industrial supplies must come from agriculture. There seems little doubt that as we approach the limits of available water supply there will be increasing competition for water. In a classic economic sense increased competition implies a shortage.
The water supply of the West is nearly fully utilized. It is difficult to foresee major construction projects which will add significantly to the currently available supply. Several critical areas are now heavily dependent upon mining groundwater, a supply which will be depleted at some point in the future. Urban and energy developments, especially in the Southwest, are competing with agriculture for the available water. This competition will undoubtedly intensify, which poses two major issues for society:
1) How will society, at local, state, and regional levels, cope with the increased competition for water?
2) To what extent can the nation forego irrigated agriculture in the West without significantly decreasing its agricultural output?
It is not the intent of this chapter to address these issues; however, we will attempt to provide an overview of the current availability of water.
The Hydrologic Cycle
Traditionally, when considering the problems of water resources we hydrologists have been prone to think in terms of virgin or natural streamflow. However, it has become increasingly obvious that natural flow is a relict of the distant past. Man has impacted the water resources so dramatically, especially in the arid and semiarid West, that natural flow does not exist except perhaps in the most remote areas.
We must recognize that man's activities are today an integral and inseparable part of the hydrologic cycle. Our current understanding of the hydrologic cycle can be described in a paradigm suggested by Matalas, Landwehr, and Wolman. The three tenets of the active paradigm are:
i) human activity is inseparable from the natural system;
ii) quality is no less a concern than quantity of the water mass as it is distributed and moves through the cycle;
iii) the quantity of the water mass affects and is affected by the quality of the water.
If we accept the active paradigm as best characterizing our concept of the hydrologic cycle, then it is impossible to look at the physical and chemical limitations on water resources without looking at man's activities.
Available Water
Precipitation ultimately is the source of water resources. The average annual precipitation for the United States is depicted in Figure 1.1. That precipitation translates into runoff. West of the 100th meridian much of the land is characterized by less than one inch of runoff. The areas of abundant runoff in the West are easily identified in Figure 1.2. The relative magnitude of the average streamflow of the large rivers in the U.S. is shown in Figure 1.3. The major rivers of interest in the western states are the Columbia, the Colorado, the Sacramento, the Missouri, and their tributaries. Future large-scale surface water diversions must almost certainly come from these river systems.
Runoff comes largely from the mountains in the spring as snowmelt. The typical seasonal variation is illustrated by the long-term average monthly runoff for the Clarks Fork of the Yellowstone River near Belfrey, Montana, Figure 1.4. Storage of water, either in surface reservoirs or in aquifers, improves the timing between supply and demand, especially the seasonal demand for agriculture.
Groundwater forms an additional resource. The important aquifers of the western United States are shown in Figure 1.5.
Depletion of Water
Given our picture of surface and groundwater, how much is utilized? Relative water depletion is depicted in Figure 1.6. Depletion is defined as "the total consumptive use plus any water exported from each basin, divided by the total supply". Groundwater mining has been excluded from the long-term supply. This is perhaps the most important single illustration in this paper. Several critical areas show up on the map of depletion:
1) Most of the lower Colorado River basin, southern California, and most of Nevada, where the depletion exceeds 100 percent. The differences are made up from mining groundwater.
2) South-central California, including the San Joaquin and Owens Valleys, where the depletion exceeds 75 percent.
3) The High Plains of Colorado and west Texas, where the depletion exceeds 75 percent.
4) Much of New Mexico, where the depletion exceeds 75 percent.
The depletion map is somewhat misleading, since instream flow requirements are not accounted for, and they are important constraints on water availability.
Groundwater constitutes an important additional source of water. Groundwater withdrawals are shown in Figure 1.7. California and Texas are the two largest users of groundwater, accounting for 37 percent of the total withdrawn nationwide, closely followed by Nebraska, Idaho, Kansas, and Arizona, which together account for an additional 26 percent of the total. These six states account for almost two-thirds of the groundwater withdrawn in the United States.
The relative importance of groundwater as a source of water in the semiarid West is depicted in Figure 1.8. Groundwater constitutes the major source of water, exceeding approximately 50 percent in much of the High Plains, a large portion of Arizona, and parts of California.
Much of the groundwater withdrawn is being mined. The Second National Water Assessment of the U.S. Water Resources Council identified areas of groundwater overdraft-"mining" in my terminology-as shown in Figure 1.9. The principal areas of overdraft identified west of the 100th meridian are (1) the high plains of Texas, New Mexico, Colorado, Oklahoma, and Kansas, and (2) large areas of Arizona. Moderate overdrafts occur over much of the area west of the 100th meridian.
Water Use
How is the water used? Figure 1.10 is a graph of water withdrawals for the period 1950 through 1975 for the entire U.S. The largest withdrawals are for power plant cooling and irrigation. Consumptive use, on the other hand, presents a very different picture. Figure 1.11 shows nationwide water consumption. Irrigation accounts for by far the largest fraction of consumption. In the western states irrigation accounts for more than 90 percent of the consumptive use.
Groundwater use is also interesting; the growth in groundwater withdrawal over the last 25 years has been almost exclusively for irrigation, as is shown in Figure 1.12. In 1977 42 million acres were irrigated, for which the consumption was approximately 82 billion gallons a day (92 million acre-feet per year). Something approaching one third to one half of that water came from groundwater, much of which was mined, as Figure 1.9 indicates.
Eighty-four percent of the fresh water consumed in the coterminous United States is consumed in the 17 western states; most is utilized for agriculture. The acreages irrigated in the 17 western states are given in Table 1.1. California accounts for 23 percent of the total acreage; together, Texas and California account for 42 percent of the total.
Looking at statistics for the nation as a whole may appear to be somewhat misleading. However, since the 17 western states dominate the consumptive use, consuming 84 percent, the statistics for the nation are strongly influenced by the West, where agriculture is the primary consumer of water.
The Lower Colorado Basin
In any overview of the water resources of the semiarid West, the lower Colorado River basin and southern California stand out as the most critical areas for water. Another look at the depletion map, Figure 1.6, indicates that the water supply is more than 100 percent depleted in these areas. This is substantiated by the overdraft of groundwater shown in Figure 1.9.
The Colorado River is the principal long-term source of water for much of this area. Stockton and Jacoby, utilizing tree-ring data, reconstructed Colorado River streamflow back to 1512. Using this record they estimated the mean annual flow at 13.5 million acre-feet. This is approximately 2 million acre-feet less than anticipated when the water rights were divided in the 1922 Colorado River Compact. Unfortunately, the 1922 Compact was based on records of flow during a series of unusually wet years from 1906 to 1920. The availability of water from the Colorado is further complicated by a number of Indian claims upon the river which are as yet unresolved.
A synthesized record of the flow of the Colorado River below all major diversions, in Figure 1.13, portrays the outflow of the river into the Gulf of California. The downward trend of the residual flow, which is caused by an increasing use of water from the Colorado River, is evident. Usage by Mexico as well as by the United States is reflected in the residuals. (Under the terms of a treaty between the United States and Mexico in 1944, supplemented by various "minutes" and negotiations, Mexico is allotted an annual quantity of 1.5 million acre-feet.)
Diversions from the Colorado began considerably before 1900. However, prior to that year, annual net diversions generally were less than 1.0 million acre-feet. The residual flows during 1935-39 were unusually low, largely because of the initial filling of Lake Mead. Low flows from 1960 to 1978 reflect nearly complete use of the river. In 1979 and 1980, major floods in the Lower Colorado River basin downstream from the principal reservoirs resulted in larger outflows.
Clearly all the water in the Colorado is currently utilized. The consumptive use within the basin is compared with entitlements from the river in Figure 1.14. The large consumptive use in Arizona is made up in part by groundwater mining.
The water in the Colorado is also plagued by an increasing load of dissolved salts. This load comes from a number of natural sources and from sources which are the result of man's actions. Approximately one third of the total salt load is the result of irrigation. Another 10 percent or so comes from Flaming Gorge Reservoir and from Lake Mead, where salts are being leached from geologic deposits inundated by the reservoirs. Figure 1.15 attempts to summarize both the concentration of dissolved solids as well as the total salt load.
Water is in short supply in the Lower Colorado River basin. Population statistics indicate a growth in urbanization both in Arizona and southern California. If urban growth is to continue, there will undoubtedly be pressure to shift water away from agricultural use.
Alternatives for Additional Water Supplies
A number of alternatives have been discussed for increasing the water supply. These are categorized for the purpose of discussion into: (1) increased surface storage; (2) increased groundwater development; (3) more efficiency of water utilization; and (4) large-scale interbasin transfers of water.
Increased Surface Storage
Surface storage is the traditional method of providing additional available water. Additional reservoir sites exist in some parts of the western states. Langbein has reviewed historic trends in reservoir development in the U.S. Table 1.2, taken from Langbein, shows the reservoir capacity currently available in a number of the major river basins of the country. Langbein has suggested that a unit capacity of approximately 400 acre-feet of storage per square mile of drainage area represents a potential limit for reservoir development; the Colorado has a potential unit capacity of 400 acre-feet per square mile.
Langbein also plotted the historic trend of reservoir capacity; this plot is shown in Figure 1.16. The growth in capacity for all purposes and for withdrawal has flattened out since 1960. The question is whether this reduction in reservoir construction will continue, or if it is simply an aberration in long-term growth curve.
Our assessment is that surface reservoirs will continue to be increasingly difficult to develop. Recent legislation such as the National Environmental Protection Act (NEPA) makes it easier for environmental groups to voice their interests. Every major new reservoir project seems likely to receive some resistance from opposing groups. Major conflicts will, in many instances, be settled politically. In arid regions such as the lower Colorado River basin, where water is particularly critical, additional reservoirs may evaporate as much or more water as is made available, thereby further concentrating the dissolved salts. Increasing surface storage in the lower Colorado is a losing proposition.
Increased Groundwater Development
Groundwater is already heavily utilized, as has been pointed out, much of its development resulting in mining of water. The increased costs of pumping imposed by increased energy costs have reduced groundwater pumping, especially in areas such as Arizona.
The one area with apparent potential for a major increase in groundwater development is Nebraska. Table 1.3 is a compilation of the water in storage in the Ogallala Aquifer, the result of an ongoing U.S. Geological Survey study of the system. Approximately two thirds of the water in storage is in Nebraska, an enormous reserve of groundwater. Only in Texas and New Mexico has more than 10 percent of the water initially in storage been depleted. The depletion statistics may be somewhat misleading, since it is economically impractical to remove all the water initially in storage; perhaps 50 to 70 percent is a reasonable estimate of what might be removed under favorable economic conditions.
These data indicate that only a small percentage of the water in the Ogallala has been removed. Obviously an enormous quantity of groundwater is still present for development in Nebraska.
More Effective Water Utilization
A number of measures have been suggested to effect better utilization of water available.
Continues...
Excerpted from Water Scarcity Copyright © 1984 by Ernest A. Engelbert and Ann Foley Scheuring. Excerpted by permission.
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