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gases prevent some of the longer wavelength reradiated energy from escaping back into space. By absorbing some of this outgoing radiation, the greenhouse effect keeps the planet warm and comfortable. Recent concern is focused on manmade changes in the composition and concentration of greenhouse gases that may cause overheating of the planet, or global warming. Since the industrial revolution, the concentration of CO2 in the atmosphere has increased by about 25 percent, largely because of the increased output of CO2 caused by burning fossil fuels. To reduce the human contribution of greenhouse gases to the atmosphere, it has been suggested that we should replace high-carbon fuels with low- or zero-carbon forms of energy. In this respect, natural gas may be a significant improvement over coal and oil, although the relative efficiencies and cleanliness of the technologies consuming the fuels are also important. However, cessation of fossil-fuel burning would not immediately affect the atmosphere. Because the residence time of CO2 in the atmosphere is estimated to be several hundred years, reducing the CO2 overload will be a very slow process. Methane is also a greenhouse gas, roughly 25 times more potent than CO2. In contrast to CO2, however, methane has a short residence time in the atmosphere. Like CO2, the amount of atmospheric methane is increasing at an exponential rate. Its sources are poorly quantified, but about one-third is emitted from natural sources such as wetlands, volcanic regions, termites, and decomposition of methane hydrates. Of the remaining two-thirds, more than half comes from agricultural activities (rice paddies and cattle), and the rest comes from petroleum activities, biomass burning, and landfills. The geologic factors governing the generation, migration, and entrapment of energy gases are complex and are ongoing research endeavors of the USGS. In addition, the USGS gathers information on the size and nature of these accumulations, which also contributes to a basic understanding of the Nation's energy gas resources. Methane, the most important energy gas, is formed in many ways in nature and is an intrinsic part of the natural environment. However, only under certain conditions is the gas trapped within the Earth's crust in sufficient quantities to allow economic recovery. Moreover, by gathering geologic data on methane emissions and by studying the geologic aspects of global climate, the USGS provides informa

tion that can be used in developing national policies on environmental issues.

Monitoring the High Plains Aquifer

he High Plains aquifer underlies one of

the major agricultural areas in the United States. About 20 percent of the irrigated land in the United States is in the High Plains, and nearly 30 percent of the ground water used for irrigation in the United States is pumped from the High Plains aquifer. The aquifer underlies about 174,050 square miles in Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming.

Water levels in some parts of the High Plains aquifer declined as much as 100 feet from the start of widespread irrigated agriculture (about 1940) to 1980. These declines and continued irrigation development in the region led in 1988 to a U.S. Geological Survey (USGS) program to monitor annual water-level changes in the High Plains aquifer. As stated in the Omnibus Water Resources Development Act of 1986 (Public Law 99-662), the USGS in cooperation “...with the States of the High Plains region is authorized and directed to monitor the water levels of the Ogallala [High Plains] aquifer, and report annually to Congress.”

Water levels in some parts of the High Plains aquifer declined as much as 100 feet from the start of widespread irrigated agriculture (about 1940) to 1980...and led in 1988 to a U.S. Geological Survey program to monitor annual water-level changes in the High Plains aquifer.

Although this aquifer is known as the Ogallala aquifer, the different geologic units

and ages of the deposits that constitute the aquifer necessitated a more inclusive designation. The High Plains aquifer consists mainly of one or more hydraulically connected geologic units of late Tertiary or Quaternary age; the Ogallala Formation generally is the principal unit. The average saturated thickness was about 190 feet in 1980 but exceeded 1,000 feet in north-central Nebraska. About 66 percent of the estimated drainable water in storage by volume in the High Plains aquifer in 1980 (3,250 million acre feet) was in Nebraska. Water levels in the High Plains aquifer are monitored through an extensive observation-well network. This network comprises many smaller networks of observation wells maintained by Federal, State, and local agencies. Local water and natural-resource conservation districts are responsible for the largest number of the observation wells within these networks. The USGS Water Resource Districts are responsible for compiling waterlevel data from various local networks and maintaining the water-level database in most of the High Plains States. The observation wells are, for the most part, privately owned irrigation wells that are well suited for monitoring water-level change because their large diameters and large pumping capacities make them less prone to plugging. Water-level measurements in the High Plains usually are made during winter and early spring when water levels generally have recovered fully from pumping during the previous irrigation season and represent the highest water levels during the year. A small number of specially designed observation wells are equipped with recording devices for continuous monitoring of water levels in critical areas. The High Plains water-level monitoring network changes from year to year. A small number of wells are lost from the network each year as a result of well failures (collapsing or plugging) and are not replaceable. In come areas, however, additional observation wells are added to better define local waterlevel changes. Between 1980 and 1993, waterlevel changes in the High Plains aquifer were based on measurements from 6,206 wells. Observation wells added to the network after 1980 permitted water-level change from 1992 to 1993 to be based on observations from 8,053 wells. Water-level declines were substantial in several areas of the High Plains before 1980. The estimated average area-weighted waterlevel decline from predevelopment (1940) to

1980 for the High Plains was 9.9 feet, which represents an average decline of about 0.25 foot per year. In some parts of the Central and Southern High Plains, declines were greater than 100 feet. In the Northern High Plains, declines were much smaller and less extensive largely as a result of later irrigation development. The geographic patterns of water-level change in the High Plains since 1980 are similar to the pre-1980 patterns of change. Those areas in which water levels declined substantially between 1980 and 1993 tend to be the same areas in which declines were large before 1980. The magnitude of the declines after 1980 in these areas, however, generally is much smaller than that before 1980. The average water level in the High Plains declined 2.09 feet from 1980 to 1993 in comparison with the average decline of 9.9 feet from predevelopment since 1980 (see table). This difference can be attributed partly to the shorter time period from 1980 to 1993 and partly to a slower annual rate of decline since 1980. The average annual rate of decline decreased from 0.25 foot from predevelopment (1940) to 1980 to about 0.16 foot from 1980 to 1993, 64 percent of the pre-1980 rate. The annual rate of decline from predevelopment to 1980 might have been substantially larger if the total area irrigated in the High Plains was comparable to the area irrigated after 1980. In 1959, the approximate

Average area-weighted water-level changes in the High Plains aquifer from predevelopment to 1980 and from 1980 to 1993

Area-weighted water-level change

(feet)
State
* 1980 to 1993

Colorado........... –4.2 —3.25
Kansas............... –9.9 —7.26
Nebraska........... 0

New Mexico..... –9.8 —3.42
Oklahoma......... —11.3 —.41
South Dakota.... 0 —.90
Texas................. –33.7 —1.96
Wyoming.......... 0 +.63
High Plains....... –9.9 –2.09

"From U.S. Geological Survey Professional Paper 1400-B, published in 1984.

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midpoint of the period from predevelopment to 1980, about 6 million acres were irrigated in the High Plains. Since 1980, about 14 million acres have been irrigated annually in the High Plains. Several factors appear to have contributed to the smaller average water-level decline after 1980, even though the average area irrigated more than doubled: • Precipitation in most parts of the High Plains was above normal during 1980 to 1993. Average annual precipitation for the High Plains was 1.81 inches above normal from 1981 to 1992. • Irrigation development in the High Plains shifted from areas having large potential rates of aquifer depletion to areas where potential rates of depletion were smaller. After 1950, most of the growth in irrigated acreage was in the Northern High Plains, where greater potential recharge and smaller consumptive irrigation requirements resulted in smaller net withdrawals from the aquifer than in the Central and Southern High Plains. More than one-half of the 14 million acres irrigated in the High Plains after 1980 were in the Northern High Plains, largely in Nebraska. • Advances in irrigation technology, which include center-pivot sprinkler irrigation and light-weight gated pipe designed to apply water more evenly and to minimize conveyance and field losses, have greatly

decreased ground-water pumpage require

ments. Irrigation from open ditches, which can result in water losses of as much as 50 percent, is no longer common. Recent sprinkler designs, which include the lowenergy, precise-application method, minimize wind losses from center-pivot irrigation. Surge irrigation permits more uniform field application by the gravity method and lessens the need to exceed crop requirements in certain parts of the field to assure complete coverage. Although much of the excess water applied by various methods in the past was returned to the underlying aquifer through percolation, substantial volumes of water were lost by runoff and evaporation of ponded water.

• Irrigation management practices that mini

mize pumpage costs and conserve water have been widely adopted. Agricultural production has been converted to crops or plant varieties that have smaller consumptive irrigation requirements. Irrigation scheduling, which includes monitoring of soil and water conditions, commonly

minimizes excess application of irrigation water. • Large water-level declines in some areas of the High Plains before 1980 prompted local regulations to control irrigation withdrawals. State and local agencies were granted the authority to monitor and regulate pumpage volumes from existing wells, to regulate well spacing, and to limit new well construction. Irrigation runoff also has been limited in many parts of the High Plains since the late 1970's by regulations that require such practices as reuse pits.

J.T. Dugan has coordinated the High Plains Water-Level Monitoring Program since 1990.

D.A. Cox served as database manager and geographic information system specialist for the High Plains Water-Level Monitoring Program during 1993–94.

Watershed Modeling Systems

ompetition for water resources in the

Western United States has become very intense, involving conflicts among agricultural, industrial, and municipal water uses and requirements for hydroelectric peaking power, recreation, and preservation of habitat and endangered species. The job of managing water resources has, as a result, become exceedingly complex. To deal successfully with these complexities, a new generation of highly flexible water-management models and supporting data is needed, both for shortterm (hours to months) operational simulations and long-term (years to decades) planning studies.

As part of a broad effort to apply its scientific research and technical development capabilities to the improvement of resource management, the U.S. Geological Survey (USGS) is providing the U.S. Bureau of Reclamation with improved data handling, reporting, and analytical tools, decision support models, and information to carry out its water-management mission. The purpose of the collaborative program between these two Department of the Interior bureaus is to develop, test, and implement water-resources models and fully integrated data-management systems designed to help water managers and

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The Reclamation Act of 1902 (43 U.S.C. 371 et seq.) authorized the Secretary of the Interior to administer a reclamation program that would provide the arid and semiarid lands of the 17 contiguous Western States (Arizona, California, Colorado, Idaho, Kansas, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, Oregon, South Dakota, Texas, Utah, Washington, and Wyoming) a secure, year-round water supply for irrigation. To perform the mission, the Reclamation Service was Created within the USGS. In 1907, the Reclamation Service was separated from the USGS and in 1923 was renamed the Bureau of Reclamation.

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water users improve the utilization of water and the management of the water-related infrastructure in the Reclamation Act States. The objective of this program are to: • Improve environmental quality (including protection or enhancement of wetlands), improve water quality (including reduction of salinity), preserve in-stream or riparian habitat, and protect and enhance endangered species. • Increase the benefits associated with various water uses, such as agriculture, hydropower, municipal and industrial water supply, and recreation. Of particular interest are increases that can be achieved by using the existing water-resources infrastructure of dams, pipelines, canals, and hydroelectric generating capacity. • Increase the available supply of water by more efficient water management and use or reduction of nonbeneficial system losses (including evaporation or canal seepage). The achievement of such improvements requires an ability to predict the outcomes of a wide range of water-management actions under a wide range of hydrologic conditions. The predictions needed are of two kinds: long-term simulations of the operation of complete water-resource systems under a modified management strategy and shortterm simulations that predict the effects of some specific management action, such as a reservoir release or the delivery of some quantity of water to a point of demand. The techniques used to make either kind of prediction are usually referred to as “modeling.” The two situations described above require planning models and forecasting models, respectively. The computer systems, basic understanding, and data-management requirements for these two types of models are much the same, and the development,

testing, and calibration process for both can be done jointly by the USGS and the Bureau of Reclamation. Recent developments have brought water-resources modeling capabilities to a point where the two agencies are poised to enter an implementation phase. These breakthroughs include improvements in the scientific understanding of processes related to climate and hydrologic interactions, production of runoff, and the interaction of surface and ground waters; improvements in the quality and timeliness of data, including use of satellite data relay, new weather radar (to estimate precipitation intensities), and new land- and satellite-based techniques for measuring snowpack; and improvements in the capability of computer hardware and software. Computer models make it possible to test many management scenarios and provide graphical results that are easy to evaluate. The tools involved are relational and spatial database-management systems integrated with improved models of the water resources of western river basins, as characterized by the USGS Modular Modeling System. The models will be used to test and improve long-term operating policies responsive to the multitude of competing demands for water and other project benefits and to produce state-of-the-art operational analyses (at hourly, daily, weekly, seasonal, and interannual time scales) necessary to manage these water-resources systems efficiently. The benefits of the program are the increased savings and revenues associated with improved operations and the savings in capital expenditures that would result from more efficient water use. Examples of the types of questions for which the models would provide answers include: • How much water would be “saved” by implementing conservation measures such as canal lining? Will these savings affect wetland habitat, ground- or surface-water supplies, or water quality downgradient from the canal? How could the “saved” water be beneficially used, and what are the management options? • How would a new operating policy for a reservoir (for example, a lower normal pool elevation or increased instream flows) affect hydropower revenues, reliability of delivery of water to downstream users, total system losses by evaporation and seepage, reservoir water quality, and water temperature (in the reservoir and downstream)?

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