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The present-day gaging station on the Rio Grande at Embudo, N. Mex. (Photograph from U.S. Geological Survey Professional Paper 778, 1971.)

passage of legislation in 1929 that authorized the USGS cooperative program with State and local agencies was a major factor in the growth of the stream-gaging network as State officials recognized the need for data to effectively manage their water resources. Severe floods and droughts, such as those of the 1930's, recent energy crises, and water-quality legislation all have given impetus to the need for more streamflow data.

From the meager beginning at Embudo, the USGS data-collection networks have grown to more than 50,000 sites where water-quantity, water-quality, or ground-water-level data are obtained, including a network of over 7,000 continuously recording stream-gaging stations and about 3,600 sites where streamflow data are gathered periodically each year.

Hydrologic Function of Wetlands

By Thomas C. Winter

Scientists have long known that wetlands serve many important physical, chemical, and biological functions. In recent years, this awareness has become more widespread, and there is a public demand that wetlands be managed and preserved as essential parts of landscapes and ecosystems. Wetlands have not yet been extensively studied, however, and their functions are not well understood by the public. Because much remains unknown about wetlands, it is difficult to place a value on them. In order to manage wetlands effectively or to defend their preservation, it is critical that their essential function as part of the global ecosystem be understood. That understanding begins with their fundamental hydrology.

Wetlands occur in virtually all physiographic regions. Wetlands can be a dominant part of the landscape, such as the vast tracts of wetland terrane in the arctic and subarctic, or a minor part, such as an oasis in a vast desert. In the temperate, subtropical, and tropical zones, wetlands differ greatly in size and are common features of the landscape. No matter what their size is relative to other parts of the landscape, wetlands are highly visible and significant in most areas because they commonly occur where the focus is on development and agriculture.

The most common man-induced disturbances that affect wetlands are direct filling or drainage and modification of the uplands within the watershed. Wetlands are filled or drained to provide land for development, such as buildings, parking lots, roads, and airports, for agriculture, and for many other uses. Modification to upland watersheds adversely affects down-gradient wetlands because the water, chemical, and biological regimes are changed.

Wetlands occur where a combination of physiographic and hydrologic conditions favor the accumulation and (or) retention of water. Physiographic conditions that tend to enhance the formation

of wetlands include flat to minimal land slope, areas where steep land slopes abut low land slopes, and hummocky topography; hydrologic conditions include soils of low permeability and areas where ground water discharges at the land surface.

Flat and (or) hummocky terrane and soils of low permeability are especially effective in the formation and maintenance of wetlands where surface water or precipitation is the source of water. In such settings, runoff is greatly retarded either by the low gradient or storage in depressions in the topography, and infiltration is also slow because of the low permeability of the soils. Examples of large regions that are characterized by low gradients and shallow depressions include glacial lake plains, such as the Glacial Lake Agassiz plain now occupied by the Red River of the North and the Red Lake peatlands in Minnesota; coastal lowlands along the Atlantic Ocean and Gulf of Mexico; and flood plains of major rivers. An example of a large region that is characterized by numerous depressions in a wide variety of sizes is the glaciated Northeastern and NorthCentral United States. Here, because the landscape is geologically young, an integrated drainage network has not been established, and storage of runoff water in depressions is extensive.

The same landscape features that enhance formation of wetlands from surface-water sources also are important to ground-water-flow systems. Regional slope and local relief of the water table and permeability of the land surface are primary controls on ground-water-flow

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flow

In a generalized landscape that has hummocky topography, ground-waterflow systems are more complex. Here, numerous small, local flow systems form at shallow depths and are underlain by more extensive regional flow systems. Part of the precipitation that falls on any part of a hummocky landscape runs into depressions where much of the water is returned to the atmosphere by evapotranspiration. With respect to local

systems. An additional feature, a break in slope of both land surface and the water table, also commonly results in the formation of wetlands.

Considering wetlands in the framework of ground-water-flow systems is appropriate because, as is the case with most surface water, wetlands are hydrologic features where considerable interaction takes place between ground water and surface water. In a generalized landscape that has uniform, low-gradient land slopes, for example, precipitation falling on an upland will run off slowly because of the low hydraulic gradient. If the soil has low permeability, infiltration is slow, thus enhancing the potential for evapotranspiration because of the long residence time of the water on the land surface. If the upland consists of highly permeable material, infiltration and therefore ground-water recharge is enhanced, the potential for evapotranspiration is less, and little precipitation will run off. Regardless of permeability, regional flow systems predominate: ground water recharges beneath the upland and ground water discharges in the lowland. The lowland is the most favorable location for wetland formation because surface runoff from the upland is coupled with the regional ground-water discharge. Furthermore, the potential for water loss by evapotranspiration is great because of the abundant supply of water from both sources.

The complex and dynamic interactions ... in wetland

ecosystems are a challenge to ... effective

management.

UPLAND Uniform low slope

VALLEY SIDE Uniform steeper slope

ground-water-flow systems throughout the landscape, however, some depressions are areas of recharge, some are areas of discharge, and others serve both functions. In upland areas, for example, some of the recharge from the depressions is to regional flow systems, and, in lowland areas, some of the ground-water discharge to depressions is from regional flow systems. Additional complexities include seasonal flow reversals caused by recharge near wetland edges, bank storage during periods of high water levels in the wetlands, and the presence of phreatophytic plants.

The complex and dynamic interactions between precipitation, surface water, and ground water in wetland ecosystems are a challenge to understanding and then developing effective management of these systems. For example, wetlands that recharge ground water commonly hold water for only part of the year. Because these are easiest to drain, they are commonly drained first. Unfortunately, drainage of wetlands that receive ground-water discharge commonly does not result in a gain of that land for other uses, because not only does drainage not stop the ground-water discharge, it also results in the deposition of salts and (or) in unstable soils. Thus, the wetland is lost and use of the land is lost as well.

It is essential also to understand the hydrologic function of wetlands from the

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Schematic of the hydrologic function of wetlands; that is, the interaction between atmospheric water, surface water, and ground water, in the context of a generalized landscape. The generalized landscape consists of a relatively flat upland and lowland separated by a steeper slope. Furthermore, the land surfaces can be smooth and uniform (A) or hummocky (B). The landscape is generalized because it is characteristic of many physiographic settings and scales, such as high moraines adjacent to lower moraines or glacial lake plains, uplands adjacent to river valleys, high terraces adjacent to lower terraces, and coastal scarps adjacent to coastal lowlands.

Direction of ground - water flow

UPLAND Hummocky, low regional slope

VALLEY SIDE Hummocky, steeper regional slope

LOWLAND Hummocky, low regional

slope

Water table

Direction of ground - water flow

Hydrologic Effects of Climatic Change in the Delaware River Basin

By Mark A. Ayers, David M. Wolock, Gregory J. McCabe, and Lauren E. Hay

perspective of the chemicals that wetlands can transport. For example, if contaminants are released from a wetland that recharges ground water, the contaminants will move through local groundwater-flow systems and discharge into nearby wetlands. If the wetland also recharges regional flow systems, the contaminants could affect a much larger area, eventually discharging into wetlands in the lowlands.

In managing wetlands, it is especially important to be aware of the seasonal reversal of flow conditions near wetland boundaries. A one-time or short-term study of a wetland can be misleading because flow directions commonly reverse near wetland edges. A condition measured at one time, and upon which a management decision is based, may not persist, thereby confounding that decision. Furthermore, the highly dynamic flow regime near wetland boundaries makes it particularly difficult to determine and map the extent of wetlands, presently one of the most highly visible problems facing the Nation today and one that is receiving considerable attention.

In summary, wetlands are a highly dynamic hydrologic feature throughout the landscape. Wetlands interact extensively with the atmosphere by discharging large volumes of water from both surface- and ground-water sources through evapotranspiration. Because they exist along the edges of most surfacewater bodies, wetlands also have an important role in surface-water hydrology by controlling flow velocities, sediment transport, and shore erosion. Wetlands are also important areas of groundwater discharge and, in some settings, are areas of critical ground-water recharge. All of these hydrologic functions and their interactions must be considered in managing wetlands. Currently, however, the data base upon which management schemes must be derived is extremely small and only covers a few localities. Additional studies that investigate the critical role of wetlands in the local, regional, and national hydrologic scheme are needed, therefore, to better understand and quantify the interactive processes of wetlands.

The greenhouse effect is an essential component of the climatic processes that support life on Earth. Without the presence of certain gases, the Earth's atmosphere would be about 50 Fahrenheit degrees colder than it is now. Since the Industrial Revolution in the mid-1800's, concentrations of atmospheric greenhouse gases, especially carbon dioxide (CO2) and methane, have been increasing steadily. Concentrations of atmospheric CO, are expected to double in the next century, reaching levels that probably have not existed on Earth in more than a million years. The current consensus among scientists is that an increase in the concentration of greenhouse gases in the atmosphere will result in global warming. Estimates indicate that the average global air temperature could increase from 2 to 7 Fahrenheit degrees as a result of a doubling of atmospheric CO2, according to the National Academy of Sciences. Despite the general consensus concerning the inevitability of global warming, much uncertainty associated with climatic change still exists. The definition of the processes involved in global warming and their attendant effects on water resources, therefore, present a major scientific challenge for the next few decades.

In 1988, the USGS began an interdisciplinary study of the sensitivity of water resources to the potential effects of climatic change in the 12,765-square-mile Delaware River basin. In view of the uncertainty of climatic-change projections and because the effects of climatic change on basin hydrology are poorly understood, this study focuses on defining the basic relations of water-resource systems to current climate and the effects of simple assumptions of future climatic change on the sensitivity of these systems.

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amounts of precipitation. This climate model is linked directly to the watershed model that simulates daily streamflow.

The second climate model randomly predicts the daily weather pattern over the basin, such as a high-pressure system, a cold-frontal passage, or a warm-frontal passage. The model then assigns values of climate variables (temperature and precipitation) based on the observed relations between the variables and weather patterns in historical records. The approach also accurately replicates the statistics of historical climate records and provides a means of estimating climatic variability over the entire basin. Initial analyses indicate that the frequency of weather patterns for present climatic conditions simulated by global climate models is similar to the frequency of observed weather patterns for the basin. In order to check future scenarios, hydrologists used the model to simulate conditions in which the amount of CO2 in the atmosphere doubled. Data from this scenario are being analyzed to determine the resulting change in frequencies of weather patterns, which will then be used to define temperature and precipitation changes for the basin.

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Watershed Modeling

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Base from Delaware River Basin Commission, 1986.

40 KILOMETERS

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Monthly streamflow models (with and without reservoir data) and a daily streamflow model (without reservoir data) have been developed to analyze the effects of climatic change on streamflow in the basin. Reservoir operations are being added to the daily streamflow model in order to incorporate management options into the assessment of climatic change in the basin.

Several analyses of streamflow sensitivity to climate variables already have been completed. Analyses of the effects of climatic changes on monthly streamflow that were made by using a monthly water-balance model without reservoir data indicate that winter warming would cause an increase in the proportion of precipitation as rain in the northern part of the basin. This effect would reduce snow accumulation, increase winter runoff, and reduce spring and summer runoff. Estimates of total annual runoff indicate that a 5-percent increase over current precipitation amounts would be

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