U.S. GEOLOGICAL SURVEY CIR9JLAR 1030 

WATER RESOURCES IN THE 

TWENTY-FIRST CENTURYA STUDY OF THE IMPLICATIONS 

OF CLIMATE UNCERTAINTY 

WATER RESOURCES IN THE 

TWENTY-FIRST CENTURYA STUDY OF THE IMPLICATIONS 

OF CLIMATE UNCERTAINTY 

By Marshall E. Moss and Harry F. Lins 

U.S. GEOLOGICAL SURVEY CffiCULAR 1030 

Cover photograph by Peter L. Kresan, 

University of Arizona 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress Cataloging-in-Publication Data 

Moss, Marshall E. 

Water resources in the twenty-first century. 

(U.S. Geological Survey circular; 1030) 

Bibliography: p. 

Supt. of Docs. no.: I 19.4/2:1030 

1. Climatic changes-United States. 2. Watersupply-United States. I. lins, Harry F. II. Title. 

Ill. Series. 

QC981.8.C5M67 1989 553.7 88-600420 

Free on application to the Books and Open-File Reports Section, 

U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225 
FOREWORD 

The uncertainty associated with global-climate change and attendant implications for water and land resources represents one of the major scientific challenges 

of the 20th century. The geophysical and geochemical complexities inherent to the 

study of climate change are enormous. Disciplines from within the atmospheric, 

hydrospheric, terrestrial, and biological sciences are compelled now, as never before, 

to cooperate in studying the climate system, especially the intricate interactions of the 

processes comprising it. 

For more than a decade, the U.S. Geological Survey has been an active participant in Federal programs designed to enhance scientific understanding of climate 

variability and change as well as the social, economic, and political implications of 

such change. Recent revisions to the U.S. National Climate Program Plan call for a 

significant increase in the role and responsibilities of the Geological Survey. To meet 

the challenge of enhanced participation in the study of climate, the Survey is currently engaged in the development of new programs that build upon and expand our 

previous efforts. 

One of the most fundamental aspects of this expansion involves the development of new insights into the potential water-resources implications of climate 

change. A better understanding of the global-hydrologic cycle is frequently cited as a 

critical climate research need. "Water Resources in the Twenty-First Century-A 

Study of the Implications of Climate Uncertainty'' presents the elements of the 

Geological Survey's program for studying climate and hydrology. It incorporates the 

Survey's strong and traditional approach that couples research, data collection, and 

interpretive studies. We believe that it demonstrates the strength of the U.S. Geological Survey's commitment to the national and international attempts to understand 

the nature and effects of climate change. 

Dallas L. Peck 

Director 

Ill 

CONTENTS 

Foreword III 

Abstract 1 

Introduction 1 

Perspectives on climate, hydrology, and the U.S. Geological Survey 2 

Climate variability, change, and uncertainty 2 

The climate-hydrology continuum 4 

The water-related mission of the U.S. Geological Survey 6 

Elements of a program on the hydrologic implications of climate uncertainty 7 

Research 7 

Process-oriented research 7 

Model-oriented research 12 

Process models 12 

Stochastic models 14 

Data collection 15 

Land-based data 15 

Interface with remote sensing 17 

Water-resources interpretive studies 19 

Strategy for achieving the program goal 21 

References 24 

FIGURES 

1. Graphs showing annual mean concentrations of atmospheric carbon dioxide 

measured at the Mauna Loa Observatory and annual changes in mean 

concentrations 3 

2. Graph showing relation of annual runoff to precipitation and temperature 4 

3. Map showing regions that exhibit a consistent precipitation response signal 

to El Nino-Southern Oscillation episodes 5 

4. Graphs showing population growth in California; land under irrigation; water 

storage capacity of reservoirs in the Sacramento and San Joaquin river basin; 

and historic and projected export of water from the delta by the Federal Central 

Valley and State water projects 10 

5. Maps showing mean sea-level pressure (in millibars minus 1,000) and monthly 

streamflow patterns for winters of 1976-77 and 1982-83 11 

6. Characterization of how the spatial variability of a surface condition such as 

soil moisture, which varies over a scale of meters, is generalized to a single 

value through parameterization 13 

7. Map showing geographic variation of the climate factor for the 100-yearrecurrence-interval flood 14 

8. Map showing locations of U.S. Geological Survey benchmark stations 16 

9. Histogram depicting the percentage of continuously operated stream gages 

in 1987, by drainage area 17 

10. Sketch depicting range of scales addressed by the First International 

Satellite Land Surface Climatology Project field experiment 18 

v 
11. Map showing physiography of the Delaware River basin 20 

12. Diagram showing primary components of the Delaware River basin study 21 

13. Diagrams showing resource balance among the three components of the 

climate-uncertainty program 22 

TABlES 

1. Themes and issues in process-oriented research 7 

2. Components of model-oriented research 12 

3. Research status and priorities 23 

VI 
Water Resources in the Twenty-First Century-A Study 

of the Implications of Climate Uncertainty 

By Marshall E. Moss and Harry F. Lins 

Abstract 

The interactions of the water resources on and within the 

surface of the Earth with the atmosphere that surrounds it are 

exceedingly complex. Increased uncertainty can be attached to the 

availability of water of usable quality in the 21st century, therefore, 

because of potential anthropogenic changes in the global climate 

system. For the U.S. Geological Survey to continue to fulfill its mission with respect to assessing the Nation's water resources, an 

expanded program to study the hydrologic implications of climate 

uncertainty will be required. The goal for this program is to develop 

knowledge and information concerning the potential waterresources implications for the United States of uncertainties in 

climate that may result from both anthropogenic and natural 

changes of the Earth's atmosphere. like most past and current waterresources programs of the Geological Survey, the climateuncertainty program should be composed of three elements: (1) 

research, (2) data collection, and (3) interpretive studies. However, 

unlike most other programs, the climate-uncertainty program 

necessarily will be dominated by its research component during 

its early years. 

Critical new concerns to be addressed by the research component are (1) areal estimates of evapotranspiration, (2) hydrologic 

resolution within atmospheric (climatic) models at the global scale 

and at mesoscales, (3) linkages between hydrology and climatology, 

and (4) methodology for the design of data networks that will help 

to track the impacts of climate change on water resources. Other 

ongoing activities in U.S. Geological Survey research programs will 

be enhanced to make them more compatible with climateuncertainty research needs. 

The existing hydrologic data base of the Geological Survey 

serves as a key element in assessing hydrologic and climatologic 

change. However, this data base has evolved in response to other 

needs for hydrologic information and probably is not as sensitive 

to climate change as is desirable. Therefore, as measurement and 

network-design methodologies are improved to account for climatechange potential, new data-collection activities will be added to the 

existing programs. One particular area of data-collection concern 

pertains to the phenomenon of evapotranspiration. 

Interpretive studies of the hydrologic implications of climate 

uncertainty will be initiated by establishing several studies at the 

river-basin scale in diverse hydroclimatic and demographic settings. 

These studies will serve as tests of the existing methodologies for 

studying the impacts of climate change and also will help to define 

subsequent research priorities. A prototype for these studies was 

initiated in early 1988 in the Delaware River basin. 

INTRODUCTION 

Recently, climate has attained the status that Charles 

Dudley Warner long ago claimed for the weather; that is, 

"everybody" talks about it. But counter to the old saw about 

weather, ''somebody'' is doing something about the climate 

of the Earth-'' somebody'' is changing it. However, it is 

uncertain whether the changes at any point or at any time 

will be beneficial or detrimental. At this time, the only 

certainty pertaining to climate change is that its potential 

impacts on humankind will be pervasive. Describing the trend 

within the scientific community toward more integrated 

interdisciplinary studies of the Earth to cope with the 

uncertainties of change, Bretherton (1987) states: 

This movement gains a sense of urgency from signs 

that human activities are altering our global environment in unprecedented ways, with consequences for 

our children and grandchildren that we are barely able 

to comprehend. Although some of the changes may 

well be beneficial, others may not. Without an 

adequate understanding of what is happening, all are 

likely to be stressful. 

Among the resources most likely to be impacted significantly 

is water. 

Water is one of the physical elements that must be 

studied intensively in order to develop a better understanding of the potential for climatic change. Thus, the hydrologist 

is a necessary component in developing the science of global 

environmental change in addition to serving as a linkage to 

the water-resources decisionmakers, who need the anticipated 

new knowledge to decrease the stress that Bretherton so aptly 

describes. Traditionally, the U.S. Geological Survey (USGS) 

has played a significant role in the development of the 

world's hydrologic knowledge and information. However, 

the water-resources mission of the Geological Survey, 

discussed in more detail subsequently, cannot be effectively 

fulfilled in the 21st century without active participation in 

the national and international programs investigating global 

climate change. This document provides a basis for planning 

the evolution of the Geological Survey water-resources 

Introduction 1 
activities in light of the potential for climate change in a 

decadal timeframe. 

This document is the synthesis of ideas derived from 

discussions with many of the authors' colleagues both from 

within and from outside the U.S. Geological Survey. The 

authors particularly would like to acknowledge the contributions of Mark Ayers, Julio Betancourt, Fred Nichols, Dave 

Peterson, and Tom Winter. 

PERSPECTIVES ON CLIMATE, HYDROLOGY, AND 

THE U.S. GEOLOGICAL SURVEY 

The concept of climate is an evolving one, and today 

there are usages within the scientific community for this term 

that are at variance with each other. Therefore, a working 

definition for the purposes of this document is provided here: 

Climate-the unconditioned, statistical (probabilistic) 

description of the sequence of measures of a suite of 

properties that describe the atmospheric components of 

the environment of a locality or region. 

In other words, climate comprises the likelihoods, 

independent of weather-forecasting skill, of such properties 

as sunshine or percentage of cloud cover, amount and 

intensity of precipitation, or the magnitude and direction of 

wind. Weather is a realization or happening at any particular 

instant from within the realm of possibilities described by 

the appropriate climate descriptions for that instant. 

Both climate and weather impact the study of the 

Earth's water resources, which is known as hydrology. 

Hydrology, through its linkage with climate, entails the statistical description of water-related environmental conditions; 

but it also, through its relation with weather, involves an 

accounting of the individual realizations in time of such 

conditions. Thus, conceptually, the term ''hydrology'' refers 

to a broader field of study than either climatology or 

meteorology. In this regard, hydrology is more analogous 

to the term ''atmospheric sciences,'' which encompasses both 

climatology and meteorology. With these distinctions drawn, 

it is possible to define a role for the Geological Survey in 

the study of the hydrologic implications of climate 

uncertainty. 

Climate Variability, Change, and Uncertainty 

The classical defmition of climate was a quasi-static 

one; climate was described by the normal or most common 

weather conditions. It was recognized that climate could 

change on a time scale of centuries to millennia, but the 

averages of limited historical data were the best key to what 

might happen weatherwise in the future. Such an approach 

suffered from several shortcomings. First, the averages or 

norms of weather properties do not comprise a sufficient set 

of statistics to describe the atmospheric environment. The 

2 Water Resources and Implications of Climate Uncertainty 

variability and particularly the extremes of weather are often 

more useful for climate applications than are norms. Second, 

examination of longer historical records demonstrated that 

periods of deviation of weather from its norms could last 

significantly longer than could be explained on a statistical 

basis. The Little Ice Age is such an example (Eddy, 1976). 

Third, for many areas of the world, the periods of data 

collection are too short and the numbers of meteorological 

stations are too few to accurately describe the local climate, 

even on a quasi-static basis. Thus, study of any climaterelated phenomenon such as water resources must contend 

with significant uncertainty even before the consideration of 

potential anthropogenic change of the Earth's atmosphere. 

Climate varies periodically on at least three temporal 

scales: (1) daily, (2) annually, and (3) over geological ages. 

Over the daily cycle, changes in insolation and temperature 

are as distinct as night and day; over the annual cycle, clouds, 

precipitation, and winds may vary seasonally as well; and 

at the geologic time scale, ice sheets wax and wane causing 

drastic swings in all climate parameters. The changes 

associated with each of these periods can be attributed 

primarily to the distribution of incident solar radiation across 

the Earth's surface. However, another factor that causes 

variation in the Earth's climate is the makeup of its 

atmosphere, which controls the amount of energy that passes 

from the Earth back into space. Radiative gases in the 

atmosphere, like carbon dioxide, methane, ozone, and water 

itself, are called greenhouse gases. Analogous to the glass 

panes in the gardener's greenhouse, these gases permit the 

passage of the Sun's short-wave energy in to the Earth but 

deter the longer wave energy reradiated by the Earth from 

dissipating into space. Thus, the greenhouse gases temper 

the Earth's climate by retaining energy within the 

atmosphere. 

Since the Industrial Revolution, the use of energy has 

increased immensely. For the last century much of that 

energy has come from the consumption of fossil fuels like 

coal, petroleum, and natural gas. A byproduct of the consumption of fossil fuels is carbon dioxide, which is released 

to the atmosphere. The concentration of carbon dioxide in 

the atmosphere has been measured at the top of Mauna Loa 

in Hawaii since 1958; figure 1A illustrates the significant 

increase in this gas, which frequently has been cited as a 

likely cause of climatic change. Increasing the carbon dioxide 

in the atmosphere is analogous to increasing the thickness 

of the glass panes in a greenhouse-each results in a more 

efficient deterent to the loss of long-wave energy. Retention 

of more energy within the atmosphere eventually will result 

in a global increase in temperature. Perhaps even more 

disturbing than the increase in atmospheric carbon dioxide 

is the sporadic but increasing rate at which it is increasing, 

as is shown in figure 1B. 

The climatic impacts of increasing concentrations of 

carbon dioxide will be compounded by increases in other 
350.----------------------------------------------------, 

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Figure 1. Annual mean concentrations of atmospheric carbon dioxide measured at the Mauna 

Loa Observatory (A) and annual changes in mean concentrations (8). 

greenhouse gases in the atmosphere (Clark, 1982). In 1985, 

a group of eminent scientists convened in Villach, Austria, 

to discuss this problem. They reached a consensus that 

the global-mean temperature would increase by 1.5 to 

4.5° Celsius during the first half of the 21st century (World 

Climate Programme, 1986). However, because of the 

dynamic nature of the Earth's oceans and atmosphere, such 

a change will not occur uniformly over the surface of the 

Earth. Some areas will warm in excess of the global mean, 

others less, and probably some will cool. 

An important aspect of the greenhouse effect is the 

potential for positive or negative feedbacks among the 

phenomena that cause either exacerbation or modulation of 

the initial effect. For example, increased energy within the 

atmosphere will cause added evaporation of water from the 

Earth's surface. If cloud cover is not increased significantly 

Perspectives on Climate, Hydrology, and the U.S. Geological Survey 3 
by the added atmospheric water, the greenhouse effect will 

compound because of the radiative properties of the additional 

water vapor. On the other hand, if cloud cover increases 

significantly as a result of the increased evaporation, the 

clouds will reflect part of the incoming solar radiation and 

thus will modulate the greenhouse effect. 

Concomitant with changes in temperature, other 

climatic factors also can be expected to vary as a result of 

the modified atmosphere. Thus, after several decades in 

which climatic uncertainty seemed to be decreasing as 

knowledge and information increased, it seems that climatic 

uncertainty suddenly has increased dramatically. 

The Climate-Hydrology Continuum 

Unlike climate, the boundaries and definition of 

hydrology have been rather broadly accepted for several 

decades. A typical definition is: 

Hydrology-the science that deals with the waters of the 

Earth, their occurrence, circulation, and distribution, their 

chemical and physical properties, and their reactions with 

their environment, including their relation to living things 

(Federal Council for Science and Technology, 1962). 

This level of acceptance was not always so. Meinzer 

(1942) traces the evolution of the meanings of hydrology 

from the beginning of the 20th century, when the U.S. 

Geological Survey used "hydrology" to denote only the 

study of waters beneath the surface of the Earth. At that time, 

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the study of surface water was referred to as ''hydrography'' 

and "hydrometry." 

Although many hydrologic studies are conducted from 

a perspective that is more akin to weather than climate, there 

is a field of endeavor within hydrology that relates directly 

to climate, and that is the study of the normal behavior of 

aspects of the land phase of the hydrologic cycle. Some of 

the earliest reported hydrologic studies were of this type. 

Meinzer (1942, p. 14-15) attributes the first studies relating 

hydrology to climate to Pierre Perrault and Edme Marriotte, 

who were French contemporaries of the 17th century. Each 

labored independently to demonstrate that rainfall on the 

drainage basin of the Seine River was sufficient to be the 

ultimate source of runoff in the river. Their studies were 

attempts to add evidence to the theory of the hydrologic cycle 

that had been espoused a century and a half earlier by 

Leonardo da Vinci. 

A more recent, but also classic, study is that of 

Langbein and others (1949), which graphically relates the 

normal runoff in rivers of the United States to the climate 

measures of precipitation and temperature (fig. 2). Some of 

the more enduring aspects of the Langbein study were its 

illustration of (1) how runoff, for a given annual precipitation, decreases as temperature increases; (2) how runoff, for 

a given temperature, increases with precipitation; and (3) how 

the numerical difference between precipitation and runoff, 

for a given temperature, increases with precipitation and 

16 18 

Parameter is weighted mean 

annual temperatures, in 

degrees Fahrenheit 

20 22 24 26 28 30 

MEAN ANNUAL RUNOFF, IN INCHES 

Figure 2. Relation of annual runoff to precipitation and temperature. Source: Langbein and others, 1949. 

4 Water Resources and Implications of Climate Uncertainty 
ultimately reaches a constant representing the limiting rate 

of evapotranspiration. Although in more recent years there 

has been an increasing emphasis placed on studying such 

conditions using numerical models, Langbein's fundamental 

points have endured. The study continues to this day as a 

mainstay in the definition of impacts of climate change on 

streamflow (Revelle and Waggoner, 1983). 

A rapidly emerging field of study within hydrology 

focuses on flooding and related sedimentary processes that 

occurred during the period from the mid- to late Holocene 

up to the advent of systematic stream gaging. Such paleoflood 

studies are having significant impacts on flood-probability 

estimation. For example, Webb (1985) determined that the 

frequency of large floods on the Escalante River in southcentral Utah was at a maximum 1,100 to 900 years ago and 

that the largest flood, occurring approximately 950 years ago, 

was seven times larger than the maximum recorded at any 

of the river's gaging stations. These flood-frequency changes, 

in turn, have produced changes in the Escalante's upstream 

alluvial channel. Valley-margin stratigraphy representing 

1,600 years of deposition indicates that since 1,100 years 

ago, when large floods were at a maximum, a marshy flood 

plain gave way to a dry , fire-swept meadow wherein a 24-mwide and 2.5-m-deep arroyo formed . 

Although this arroyo filled with sediments between 500 

and 400 years ago, floods between 1909 and 1940 recut the 

arroyo to up to 100 m wide and 17 m deep. Although land 

EXPLANATION 

use practices have clearly been associated with significant changes in watershed and flood-plain conditions, 

there also has been a subtle climate shift that has increased the amount of summer precipitation and intensity of 

storms. 

Another example, relevant to both floods and droughts, 

is their relation to large-scale atmospheric and oceanic 

phenomena. The occurrence and magnitude of the aperiodic 

El Nino-Southern Oscillation (ENSO) phenomenon, which 

spans the South Pacific Basin, have been shown to relate 

to climatic and hydrologic anomalies from Asia (Shukla 

and Paolino, 1983) to South America (Hastenrath, 1978; 

Ropelewski and Halpert, 1987) and to the western and 

southeastern parts of the United States (Ropelewski and 

Halpert, 1986) (fig. 3). Such associations, although statistically significant, are often weak, and their physical basis 

is poorly understood. Considerably more research is necessary before such associations can be used in warning of 

extreme hydrologic events. 

The above examples demonstrate the dependence of 

several hydrologic variables on climate factors, but the 

interrelation is not always so unidirectional; studies of the 

oasis effects of irrigated lands have demonstrated the feedback from living plants, the water surface, and moist soil 

to modify local microclimates (Oke, 1978). However, the 

potential magnitudes of the feedbacks for large-scale 

hydrologic phenomena are just now beginning to be 

c==J Wet --~--------~~~-----------4------------~------------~-- ----~ 

D ory 

60° -- -----------+-------------4-------------4-------------~------------~------------~ 

Figure 3. Regions exhibiting a consistent precipitation response signal to El Nino-Southern Oscillation episodes. Source: 

Ropelewski and Halpert, 1987. 

The Climate-Hydrology Continuum 5 
addressed. For example, Barnett and others (1988) have 

demonstrated the effect that added snow cover and subsequent 

increased soil moisture in Eurasia have on deterring the 

subsequent monsoon in South Eastern Asia. This same study 

shows strong correlations between Eurasian snow cover and 

the atmospheric conditions over all of North America. 

A common characteristic of each of the above examples 

is that significant levels of uncertainty exist with respect to 

the exact forms of the interdependence of climate and 

hydrology and also with respect to the parameters that 

quantify the interdependence at any given time and location. 

Therefore, current uncertainties pertaining to the availability 

and quality of water resources in the 21st century must be 

viewed as a manifestation of the climatic uncertainty 

described in the previous section. 

Because new hydrologic uncertainties exist and the 

current level of scientific understanding does not permit their 

quantification, scenario analysis (Lave and Epple, 1985) has 

become an accepted tool for exploring potential impacts. 

Scenario development in the climate-change arena has 

been based on three separate, but potentially complementary, approaches: ( 1) reconstruction of prehistoric 

climates from geologic evidence (paleoclimatology), 

(2) analysis of aberrant-climate periods in recorded history, 

and (3) computation of future conditions using physically 

based three-dimensional General Circulation Models 

(GCM's). Lamb (1987) has described some of the shortcomings of each of these three. However, any one of today' s 

scenarios points to potential disruptions in certain parts of 

the water-based economies of the world, while providing 

added opportunities for other parts. Within the United States, 

this mix of potential disruptions and opportunities is no more 

evident than it is on the global scale; the potential for 

disruption may be greater in the United States than in many 

other countries, however, because the relatively mature 

development of our water resources makes them more 

vulnerable to perturbations. 

Because of the potential for major impacts on social, 

economic, and environmental conditions in the United States, 

and because of the uncertainties concerning those impacts, 

major scientific studies are being planned and some already 

have begun. These studies are being carried out both at the 

national and global scales. 

The Water-Related Mission of the 

U.S. Geological Survey 

The water-related activities of the U.S. Geological 

Survey are executed by its Water Resources Division. The 

stated mission of Water Resources Division is to provide the 

hydrologic information needed for managing the Nation's 

water resources (Chase and others, 1983). This information 

is developed by conducting hydrologic research, by collecting 

6 Water Resources and Implications of Climate Uncertainty 

basic hydrologic and water-use data, and by performing 

interpretive studies of the Nation's water resources at various 

scales from local to national. These three program components are interfaced with the Nation's information users 

through the release of publications and hydrologic data and 

through data-coordination, cataloging, and retrieval services. 

For almost 100 years, the Geological Survey has been 

providing hydrologic information to the Nation that is 

essential for coping with water-supply uncertainty and for 

protecting the citizenry from the devastations of floods and 

droughts. For a shorter period of time, the Nation also has 

depended on the Geological Survey's information to set and 

implement policies for dealing with the degradation of the 

quality of its water resources. Thus, the Geological Survey 

is well positioned by tradition, as well as by the scientific 

momentum of its current activities, to play a key role in 

providing information to address the recently recognized 

increment of 'hydrologic uncertainty associated with the 

potential for global climate change. In fact, it will be 

impossible for the Geological Survey to continue to meet its 

mission responsibilities in the 21st century if the potential 

impacts of climate change are not taken properly into 

account. 

Although the Geological Survey has not been one of 

the major Federal participants in the climate-change activities 

of the last decade, it has not been isolated from them either. 

In 1976, the Geological Survey convened a workshop to 

stimulate discussion of the effects of climate variation on the 

land and water resources of the Nation (Smith, 1978a,b). 

As a result of the workshop, a Geological Survey climate 

plan was developed (Howard and Smith, 1978) that to date 

has been implemented only to a modest degree. The plan 

is consistent with the responsibilities of the Geological Survey 

as specified in the first U.S. National Climate Program Plan, 

a document called for in the National Climate Program Act 

(Public Law 95-367) and prepared by the National Climate 

Program Office of the National Oceanic and Atmospheric 

Administration in conjunction with other Federal agencies. 

However, as more knowledge and information have 

accumulated and as Federal agency activities have shifted, 

the National Climate Program Plan has been revised 

(National Climate Program Office, 1988), and the responsibilities of the Geological Survey have increased to that of 

a lead agency for the study of the interactions between climate 

and hydrology. 

For the Geological Survey to become a focal point for 

hydrology in the climate-change arena, the significant, but 

diffuse, climate-related activities that currently are being conducted within the Water Resources Division need to be 

brought into perspective; their roles relevant to a common 

goal need to be evaluated, and weaknesses and critical gaps 

need to be identified. By carrying out these steps and 

rectifying weaknesses and gaps as resources can be made 

available, the Geological Survey indeed will have a program 
on the hydrologic implications of climate uncertainty. The 

goal for this program is: 

To develop knowledge and information concerning the 

potential water-resources implications for the United 

States of uncertainties in climate that may result from 

both anthropogenic and natural changes of the Earth's 

atmosphere. 

Knowledge and information, as referred to in this goal, 

encompass fundamental research into the linkages of climate 

and hydrology, collection of basic data, and the development 

of technology that facilitates interpretation of the data in light 

of the research findings. Although the goal is water-resources 

driven, this program cannot be isolated from current and 

future advances in climatology, biology, geochemistry, 

geology, or any of a multitude of interdependent sciences. 

Nor can the national focus of the goal be interpreted as a 

limitation to consider only U.S. science and U.S. data bases. 

The Water Resources Division must be more involved in 

international scientific endeavors, like the International 

Geosphere Biosphere Programme and the World Climate 

Programme, that portend significant progress toward its 

goal. 

ELEMENTS OF A PROGRAM ON THE 

HYDROLOGIC IMPLICATIONS OF CLIMATE 

UNCERTAINTY 

Elements from each of three major categories of the 

traditional activities of the Water Resources Division of the 

Geological Survey-research, data collection, and interpretive studies-are essential in a comprehensive program 

designed to address the hydrologic implications of climate 

uncertainty. Maintenance of the proper balance of activities 

among the three categories at each stage of its evolution will 

be a key to the program's effectiveness. The final chapter 

of this document discusses the program balance; the present 

chapter describes its composition within each of the above 

categories. 

Research 

Like climate, the word research has a diversity of 

meanings even within the scientific community. Therefore, 

for clarity of presentation, the following definition is used 

in this document: 

Research-the development of understanding through the 

conceptualization and testing of new ideas. 

More specifically, hydrologic research is the development of understanding about ( 1) the paths along which water 

moves on and in the land masses of the Earth, (2) the rates 

of movement along various paths, (3) the chemical, biological, and physical character of the constituents transported 

by the water, ( 4) the controls on the rates of movement of 

and interactions among water and its transported constituents 

imposed by the atmosphere, the geosphere, the biosphere, 

and the oceans, and, conversely, (5) the impacts on the 

atmosphere, geosphere, and biosphere caused by the presence 

or absence of water in or on the land. 

The research component of the program is partitioned 

into two types: (1) fundamental or process oriented and 

(2) model oriented. Because one type of research often builds 

on the results of research of the other type, most research 

projects carry out activities in both categories at any given 

time. Thus, the line delimiting these categories of research 

often is difficult to distinguish even within any particular 

project. However, the categories have utility in describing 

the elements of the proposed program. 

Process-Oriented Research 

Process-oriented research, as used herein, refers to the 

development of increased understanding about how the 

processes that control the land phase of the hydrologic cycle 

interact and how they interact with other processes that they 

impact. Because of the intrinsic interconnectivity among 

elements of the Earth's environmental system, the hydrologic 

impacts of climate change ultimately will depend on 

adjustments in the lithosphere and biosphere, as well as in 

the atmosphere. Therefore, the study of any single component 

or pair of components of the Earth's complex system in isolation from others has inherent limitations. Many of the interconnections are poorly understood, and thus fundamental 

research is needed. Such research will provide insights into 

likely changes in advance of their appearance in the environment and subsequently will facilitate development of timely 

guidance for coping with the changes. A list. of the primary 

elements to be addressed by process-oriented research is 

provided in table 1. 

Table 1. Themes and issues in process-oriented research 

Theme 

Problems of scale .................. . 

Extreme conditions ................. . 

Complex environments ............ . 

Special topics ....................... . 

Specific issues 

Infiltration. 

Evapotranspiration. 

Aquifer recharge. 

Surface runoff. 

Floods. 

Droughts. 

Unsaturated zone. 

Lakes. 

Snow and ice. 

Estuaries. 

Biogeochemical cycles. 

Paleohydrology. 

Hydroclimatology. 

Water quality. 

Research 7 
Much is currently known about many of the hydrologic 

processes, particularly as they function in the laboratory or 

on small plots of land. The knowledge developed in the 

laboratory and on small plots inherently is limited to the lower 

end of both the spatial and temporal scales of interest in 

hydrology and frequently is isolated from many of the factors 

that may be relevant in the natural environments in which 

the processes function. 

Many of the questions concerning the hydrologic 

implications of climate uncertainty cannot be addressed 

adequately by improving process understanding at modest 

scales. Therefore, the majority of the elements contained in 

the fundamental research component of the program will be 

oriented toward development of understanding the aggregate 

of processes at work over reasonably large and div~rse areas 

of land-the scale problem (Dooge, 1982). 

An example of a process that is reasonably well 

understood at the small scale is the concept of infiltration, 

which describes the passage of precipitated, ponded, or flowing water from the surface of the Earth into its interior. Much 

is known about infiltration from the laboratory or at small 

field-plot scales (Dooge, 1982). Rates of infiltration depend 

on the nature and water content of the soil or rock that 

receives the infiltrated water, the chemical, physical, and 

biological character of the source water, and in the case of 

precipitation, the rate at which water is supplied to the 

surface. At many larger scales of interest-that is, 

agricultural fields or larger areas-most of the factors that 

control infiltration will vary significantly. Because of the 

complexity of the relations among the controlling factors, 

the knowledge of infiltration derived at small scales is 

difficult to apply at large scales. This quandary is 

compounded by the current inability to collect and interpret 

sufficient quantities of precise and accurate data to describe 

adequately the spatial variability of the controlling factors. 

Thus, gaining more insight on infiltration over medium to 

large areas is an essential. element of the program. 

Another hydrologic process whose understanding is 

impacted by scale is the reverse of infiltration, that is 

evapotranspiration, which takes water out of the Earth and 

the biosphere and returns it to the atmosphere. In most waterbalance studies, evapotranspiration is computed as the 

residual after all other relevant flows and reservoirs have 

been taken into account. There are two primary reasons that 

it is treated so: (1) evapotranspiration is a highly nonlinear 

function of the energy available to transform liquid water 

to water vapor at the surface of the Earth or on a plant 

surface, and (2) the energy balance on the land surface or 

on a plant is highly variable both in space and in time. As 

climate changes, the statistical characteristics of the energy 

balance will change, as will the availability of water at land 

surface. In response to significant shifts in water availability, 

the local plant community also can be expected to change 

(Eagleson and Segaria, 1985). Therefore, evapotranspiration 

8 Water Resources and Implications of Climate Uncertainty 

probably will be very sensitive to climate change. Of the 

major water balance components, evapotranspiration is the 

least documented. Unlike precipitation and runoff, evapotranspiration has not been measured systematically across 

the Nation. Its character at various spatial and temporal scales 

must be determined. 

In many regions of the United States, ground water 

is an abundant resource that has accumulated over long 

periods of time. It is a resource that can be drawn upon at 

rates that vary as functions of the volume of ground water 

in storage and the rates at which it is replenished. The rate 

at which ground water is replenished is known as aquifer 

recharge and, to a first approximation, it is equal to the 

difference between infiltration and evapotranspiration. 

Because these processes are controlled by many interrelated 

factors sensitive to scale, it follows that aquifer recharge must 

be as well. For ground-water development to proceed in a 

wise and measured fashion under climate uncertainty, the 

process of aquifer recharge must be better understood. 

As a first approximation, the difference between 

precipitation and infiltration is surface runoff. Runoff is the 

primary source of water for most floods producing serious 

property damage and loss of life in the United States. As 

a result of its direct linkage to infiltration, the processes 

controlling the generation of surface runoff are poorly 

understood at spatial scales larger than field plots. Thus, even 

if climate change were not a significant issue, research on 

the process of runoff from catchments and basins is an area 

that warrants renewed support. The added uncertainty 

attendant to climate change only serves to augment an existing 

need for research on this phenomenon. 

Another category of the hydrologic continuum requiring 

process-oriented research is extreme hydrologic conditions. 

The extremes of the hydrologic continuum, floods and 

droughts, have been studied largely in statistical contexts. 

Much of water-resource planning is based on the normal or 

average conditions over some standard period of instrumented observations. Such an approach ignores the question of how representative the available data may be of the 

true expected conditions. This question is particularly relevant for decisions that entail planning horizons that extend 

20 to 100 years into the future. 

Historically, water-resource systems have been designed 

on the assumption that future climate conditions, hence 

floods and droughts, would be similar to those observed in 

the systematic record. This assumption, known as stationarity, has long been controversial in hydrology. If significant 

climate change occurs in the coming decades, it will 

invalidate the stationarity assumption and force a change in 

the procedures for estimating the occurrence of hydrologic 

extremes; the 100-year flood of today will not be the same 

as that of the mid-21st century. Importantly, the change will 

not be attributable simply to a longer record of floods, as 

is frequently the case today but, rather, to a shift in the 
character of the causal factors of flooding. In a situation of 

climate change, shifts in the likelihood of flooding or 

droughts would be continual, lasting to the time at which 

the climate change has ceased. Consequently, data-based 

descriptions of floods and droughts will be of limited utility 

unless a better understanding of the causal processes and their 

rates of change relative to ongoing climate change is attained. 

A third class of problems requiring process-oriented 

research involves complex hydrologic environments such as 

the unsaturated zone, lakes, regions of snow and ice, and 

estuaries. Each of these environments plays an important role 

in the hydrologic cycle, and the dynamics associated with 

each often are poorly understood. 

For example, the characterization of the transport of 

water, as vapor or liquid, and other climatically significant 

gases through the unsaturated zone is crucial to understanding 

how the zone functions as a source or sink of such substances. 

To amplify this point, preliminary studies in Nevada have 

recently indicated that the unsaturated zone is a methane 

sink, yet in the past it has been characterized as a methane 

source. On a molecule-per-molecule basis, methane is a much 

more efficient greenhouse gas than carbon dioxide, and 

methane also is increasing in the atmosphere (National 

Research Council, 1986). Thus, the question of whether the 

unsaturated zone is a modulator or an intensifier of climate 

change vis-a-vis methane cycling is an important one that 

is yet to be answered. 

Lakes integrate the hydrologic system of their watershed because they are in direct contact with each of the three 

major water components: atmospheric water, streamwater, 

and ground water. Therefore, lakes reflect changes in the 

supply of water, chemicals, and other materials that enter 

them from their airshed and watershed. In addition, 

depending on the residence time of water and chemicals in 

lakes, internal chemical and biological processes further 

modify the character of lakes. Because of the wide variety 

of lake sizes, geologic settings, and watershed-area to lakearea ratios, lakes can be used to assess climate variability 

and the impact of that variability on their watersheds over 

time scales ranging from days to years (Goldman, 1988). 

Perhaps the greatest role lakes have in the linkage of 

climate and hydrologic processes is their ability to capture 

the historical record of hydrologic processes in their 

sediments. Lake sediments are com only used to study past 

hydrologic, climatic, and land use changes, primarily for the 

last 13,000 years (Dean and others, 1984), and in some cases 

for many tens of thousands of years. 

Snow-covered areas on land, and ice on both land and 

sea, are extremely sensitive barometers of climate variability 

and change. Perhaps more importantly, these areas exert a 

-major influence on the Earth's radiation budget as well as 

on atmospheric and oceanic circulations. Seasonal variations 

in the areal extent of snow are the most significant 

contributors to global albedo changes. Albedo refers to the 

ratio of radiant energy reflected by a surface to that incident 

upon it. The presence or absence of snow, which has the 

highest albedo of any natural feature on the Earth's surface 

(80-90 percent), dramatically affects the absorption of solar 

radiation and, therefore, surface air temperature. Snow also 

provides the water-supply base for parts of the Western 

United States. Answers to questions regarding how climate 

change will affect the spatial distribution and timing of snowfall and consequent water-supply conditions and resourceallocation strategies must be sought. Similarly, numerous 

questions associated with snow and ice dynamics must be 

answered to measure and to model more accurately snow 

and ice volumes and interactions with other hydrologically 

significant variables. Moreover, more detailed information 

on the characteristics and dynamics of glaciers, continental 

ice sheets, and sea ice is essential for deriving better estimates 

of climatic and hydrologic conditions, including changes in 

sea level, from climate models. 

River-estuarine systems, the meeting place of freshwater and saltwater, are among the world's most complex 

hydrodynamic, chemical, and biological environments. They 

are also, as a result, exceedingly sensitive to climatic and 

anthropogenic influences. These systems, which typically 

support large population centers, provide much of the water 

needed for domestic consumption, industry, and agriculture 

(irrigation) (fig. 4). Developing an understanding of how the 

river-estuarine environment varies in association with 

variations in climate and human activity is essential for 

effective resource management. 

One of the more obvious estuarine responses to climate 

variability and change, having major societal and economic 

importance, relates to instream movement of the freshwatersaltwater interface. Saltwater intrusion can seriously degrade 

coastal land and water resources. The projected acceleration 

in the rise of sea level in the 21st century (Revelle, 1983) 

will promote upstream shifts in the position of the interface 

and increase the landward intrusion of saltwater, thus 

endangering municipal freshwater supplies from rivers and 

ground-water reserves. In some regions, reduced streamflows 

also could increase the demand for coastal ground-water 

withdrawals and generate a need to deepen and widen 

shipping channels. 

In addition to the relatively direct impact of climate 

change on coastal water resources, there are a number of 

more complex and subtle interactions among the estuarine 

hydrodynamic, chemical and biological realms. Such 

processes include changes in river-borne sediment transport 

and deposition and changes in estuarine biogeochemistry. 

Preliminary research indicates that realistic modeling of these 

interactions is highly dependent on the development of 

nonsteady -state formulations of estuarine physics and 

associated biogeochemistry with flow. 

The final categories of the hydrologic continuum for 

which expanded process-oriented research is needed include 

Research 9 
30 

A. CALIFORNIA POPULATION " " " 

Cl) 20 

z 

0 

::::::i 

....J 

~ 10 

Projected A" 

o~~~---L __ J_ __ L_ __ L__....J __ ~~ 

u. 

QCI) 

4 B. AREA UNDER IRRIGATION 

Cl)w 

za: 

0 ~ 2 

::::::io 

....JW 

~I 

o~~~---L __ J_ __ L_ __ L__....J--~~ 

Cl) 40~--~------------------------~ 

a: C. RESERVOIR CAPACITY 

w 

1-

w 

~ 

g 20 

~ 

() 

CD 

:::> 

Projected-7 / 

/ 

/ 

/ 

/ 

o o~~--~--~~~~L-~L_~L_--~ 

Cl) 

a: 8 w 

tua: 

~~ 

ow _.J> 

~ ffi 4 

oa.. 

CD 

:::> 

() 

/ 

Projected -7 

" " 

1980 

Figure 4. Population growth in California (A); land under 

irrigation (B); water storage capacity of reservoirs in the 

Sacramento and San Joaquin river basin (0; and historic and 

projected export of water from the delta by the Federal 

Central Valley and State water projects (0). Source: Nichols 

and others, 1986. 

biogeochemical cycles, paleohydrology, hydroclimatology, 

and water quality. A growing body of evidence indicates that 

the rate of cycling of greenhouse gases through the 

hydrosphere, atmosphere, biosphere, and lithosphere has the 

10 Water Resources and Implications of Climate Uncertainty 

potential to perturb the global climate and that climate 

change, conversely, can produce feedbacks to change 

significantly rates of cycling. Carbon dioxide and methane 

are just two of the greenhouse gases that play a significant 

role in the study of climate change. Nitrous oxide, ozone, 

and industrial halocarbons also are related intimately to 

climate and the hydrologic cycle. Because these gases impact 

on and are impacted by the water resources of a region and, 

more broadly, of the globe, the study of biogeochemical 

cycles must be an integral part of the study of the hydrologic 

implications of climate uncertainty. 

Paleohydrology, the study of geologic and biologic 

evidence of past hydrologic events, is another special 

research topic, of increasing fundamental importance with 

respect to climate change. The aim of paleohydrology is to 

describe hydrologic variations that span intervals beyond the 

range of instrumental records, from glacial-interglacial cycles 

to seasonal anomalies, and to identify their causes. The need 

to study past evidence of climatic and hydrologic change is 

derived from the brevity of most instrumental records, which 

are commonly shorter than the planning horizons for applied 

hydrology and water-resources management. Moreover, it 

is important to place the relatively brief systematic record 

in its proper perspective over the longer historic and geologic 

past. The different response times and sensitivities of the 

atmosphere and oceans (from weeks to centuries) suggest 

that high-frequency phenomena (interannual variability) are 

transient responses to a continuously varying climate system. 

Because the full impact of an anthropogenically induced 

warming may take centuries to develop, an understanding 

of long-term climate dynamics may be necessary to model 

the evolution of equilibrium environmental conditions. 

High-resolution proxy data, that is, geologic and other 

surrogate records, are indispensible for improving estimates 

of the annual mean, annual variance, and long-term persistence of hydrologic phenomena. However, proxy records 

suitable for hydrologic analysis are limited both in geographic 

coverage and interpretive methodological development. 

Presently, interannual proxy records are restricted to tree 

rings, sediment varves (both marine and lacustrine), coral 

bands, ice layers, and written records. The continued 

development and expansion of meaningful and usable highresolution and spatially diverse proxy time series are essential 

elements of the research program to supplement inadequacies 

that exist in the recorded data base. 

Hydroclimatology, the study of hydrologic events and 

conditions within their climatologic context, is another special 

topic of process-oriented research. It encompasses large-scale 

interactions between the atmosphere and hydrosphere. 

Hydroclimatology, by focusing on atmospheric forcing, 

provides a mechanism for integrating the physical sources 

of variability in a hydrologic time series with the statistical 

properties of the varying driving force itself. This integration 

has the benefit of enhancing our understanding of hydrologic 
processes and also the quantitative assessment of their 

variability (Hirschboeck, in press; Lins, 1985a,b). 

Recent research indicates that specific regions of the 

globe respond in a consistent manner to large-scale atmospheric and oceanic circulations. These consistent responses, 

or teleconnections, provide a basis for predicting areas of 

water deficits and surpluses attendant to climate change. The 

concept of teleconnections, introduced by Walker (1923) and 

A. WINTER SEA LEVEL PRESSURE PATIERN 

B. WINTER SEA LEVEL PRESSURE PATIERN 

refined by Namias (1981), actually refers to the statistical 

cross-correlation between atmospheric anomaly patterns 

separated by synoptic (1 ,000-2,500 krn) to hemispheric 

distances. Some of the best documented teleconnections, the 

Pacific/North American, the Central North Pacific, and the 

Southern Oscillation patterns, for example, have been clearly 

associated with distinct patterns of streamflow variability in 

western North America, as shown in figure 5. Refinement 

EXPLANATION 

Monthly Streamflow 

0 Below average 

• Above average 

EXPLANATION 

Monthly Streamflow 

13 Below average 

• Above average 

Figure 5. Mean sea-level pressure (in millibars minu s 1,000) and monthly 

streamflow patterns: (A) winter 1976-77 and (8) winter 1982- 83. "Below 

average" refers to the lower quartile flow and "above average" to the upper 

quartile of flow. Source: Peterson and others, 1987. 

Research 11 
of these associations and expanded hydroclimatic analysis 

of regional patterns nationwide, and at monthly time scales, 

offer potential for water-resource planning an~ asses~ment. 

Water-quality aspects of the hydrologic contmuum 

form the final special topic of process-oriented research. In 

recent years, water quality has. received an increas.ing 

emphasis within the hydrologic-research commumty. 

However, little of that emphasis has been directed toward 

the impacts on water quality of climate change. As improved 

understanding of the potential for modified ra~es . of 

streamflow, ground-water recharge, and evapotranspiration 

is developed, this new knowledge will shed light on concomitant changes in the concentrations and residence ti~es 

of various chemical and biological species that compnse 

water quality. 

Model-Oriented Research 

Mathematical models are numerical represen~ations of 

real-world phenomena and the interactions among 

phenomena. In hydrology such models are used to represent 

the essence of existing, fundamental hydrologic knowledge 

and to integrate that knowledge with the hydrologic and 

related data that are available within an area of interest. 

Hydrologic models are used as research tools, as descriptive devices in interpretive water-resources studies, and as 

planning and design instruments in water-resources decisionmaking. Each usage is relevant in coping with climate 

uncertainty. The proper utilization of hydrologic models for 

any purpose requires that the characteristics and limitations 

of the models be known and understood. 

Hydrologic models are necessarily imperfect representations of the phenomena that they are constructed to mimic. 

Inherent errors in the data derived from such models result 

from ( 1) spatial and temporal scale limitations within the 

models' structures, (2) the imperfect state of the hydrologic 

knowledge upon which they are based, (3) the simplifications required in reducing complicated phenomena to relatively simple mathematical expressions, and ( 4) errors present 

in the input data that drive the models. 

As a point of clarification, model-oriented research is 

distinct from model development. Modeling research 

addresses the quantification of errors present in the model 

output and the search to reduce those errors. In contrast, 

model development deals with the initial construction of the 

model and its incorporation into a modeling framework. 

Components of the model-oriented research element of the 

program appear in table 2. 

Table 2. Components of model-oriented research 

Model form Scale/type 

Process......................... General circulation. 

Mesoscale. 

Basin scale. 

Stochastic..................... Disaggregation. 

Risk. 

Data-network analysis and 

design. 

12 Water Resources and Implications of Climate Uncertainty 

Process Models 

To address the full suite of hydrologic implications of 

climate uncertainty, models that simulate hydrologic processes over a wide range of spatial scales are required. At 

one end of the spectrum of spatial scale is a class of models 

known as general circulation models, which are commonly 

referred to as GCM's and occasionally improperly are called 

global climate models. GCM's depict the cir~ulati.on of ~ass 

and energy in the atmosphere in three spatial dimensiOns, 

on a global scale, and at varying levels of detail that include 

components of oceanic and land-surface processes 

(Washington and Parkinson, 1986). In general, attempts are 

made in the model to represent all physical processes believed 

to be important. The variables used to drive GCM's include 

components of the wind field or vorticity, temperature, ~nd 

water-vapor content. Typically, the data are aggregated mto 

cells of several degrees latitude and longitude around the 

globe, and at 6 to 10 levels vertically through the lower 

atmosphere. The stored quantities are then stepped forward 

in time by applying the equations of motion and the first law 

of thermodynamics, so that mass, energy, and momentum 

are conserved. The equations also include terms describing 

surface friction, radiative and latent heating, and other 

physical processes. 

One of the most crucial aspects ofGCM's involves the 

mathematical description of the interactions between the 

Earth's surface and its atmosphere. Typically, the surface 

fluxes of heat, momentum, and moisture are described using 

bulk aerodynamic formulas. Land-surface temperature and 

soil moisture are commonly calculated by equations 

incorporating daily and seasonal heat and moisture storage 

and their vertical fluxes. Significantly, although current 

GCM 's are structurally complex and computationally 

demanding even for the latest generation of supercomputers, 

in many respects their representations of the physical processes and conditions at and immediately below the land 

surface are exceedingly simple and inaccurate. Sensitivity 

studies indicate that climatic simulations are very sensitive to 

changing land-surface processes and properties, particularly 

those of the hydrologic cycle. 

A critical aspect of general circulation modeling 

involves parameterizing the interactions between the surface 

and the atmosphere. Parameterization refers to the expression 

of the statistical effects of various small-scale ( subgrid scale) 

transport processes in terms of the large-scale (gridscale) variables explicitly resolved by the model (Global 

Atmospheric Research Program, 1975). Because of the wide 

range of scales of interacting atmospheric processes in relation to the limited spatial resolution of computational grids 

and observation systems, it is technically and economically 

impractical to observe or explicitly calculate the effec~s 

of small-scale processes in detail. However, these subgnd 

processes are significant, and it is important to rel~t~ their 

statistical effects to measurable or computable conditiOns at 

larger scales. Because the arbitrary specification of smallscale effects does not accommodate full interaction with the 
larger scale resolvable state of the atmosphere, parameterization is required. 

To illustrate the problem associated with specifying 

subgrid processes at the grid scale, consider how a typical 

GCM treats soil moisture (fig. 6). At the Earth's surface, 

ATLANTIC 

Figure 6. Characterization of how the spatial variability of 

a surface condition such as soil moisture, which varies over 

a scale of meters, is generalized to a single value through 

parameterization. 

the moisture content of the top 1S em of soil is a highly 

variable quantity, responding to differing vegetation, soils, 

and geomorphological characteristics. Over an area the size 

of a so latitude by so longitude grid square, a map of soilmoisture content would appear as a complex mosaic. Yet, 

the input to the GCM is a single value for the entire grid 

square. Clearly, for most of the globe's land surface, this 

is a poor representation of soil-moisture conditions. 

Linking hydrology and climate through GCM's is very 

difficult. Accounting for the myriad of interacting processes, 

including precipitation, interception, infiltration, runoff, and 

evapotranspiration, with any degree of precision or realism 

is very complicated, and procedures for handling these 

intricacies are largely underdeveloped or totally lacking. 

Significantly, hydrologists, especially those involved 

in surficial-process and ground-water modeling, have been 

addressing many of these same problems and can make major 

contributions toward their solution. Whereas atmo~pheric 

modelers have focused their attention primarily on processes 

over characteristic scales ranging from 10,000 to 

100,000 km2 , hydrologic modelers have dealt with smaller 

scale processes, those occurring over field ( < 1 km2

) or 

catchment (100-1 ,000 km2

) areas. Importantly, hydrologists 

have had to confront many of the same basic issues to 

parameterize field-scale processes in catchment-scale 

models. Although adequate solutions have not been 

found always, those processes significantly affecting 

catchment response have been identified. Thus, the 

hydrologic community has acquired important insights 

necessary in guiding parameterizations from catchment scales 

up to GCM-grid scales. 

An intermediate step of great significance has been 

undertaken by atmospheric modelers to enhance the realism 

of subgrid-scale processes by application of a finer mesh grid. 

This work, generally referred to as mesoscale modeling, is 

a spatially and temporally limited form of general circulation modeling. Their basic physical and mathematical foundations are the same, although more detail is incorporated 

at the mesoscale. Pielke (1984) defines mesoscale as having a characteristic horizontal spatial scale of 10 km2 to 

SO,OOO km2 with a time scale of about 1 to 12 hours. Clearly, 

this scale of analysis is much more consistent with that used 

in hydrology and affords a sound basis for substantial 

interaction between the atmospheric and hydrologic modeling communities. The mesoscale class of models, with the 

inherent ability to nest within the broader GCM's, provides 

a likely pathway for incorporating more realistic hydrologic 

conditions into climate simulations at the global scale. 

Perhaps more important for hydrologists, however, is the 

potential for improving the output of catchment models by 

using the output of mesoscale models as input. 

At the smaller end of the spatial scale are the basin 

or catchment models commonly used by hydrologists. The 

actual information content of such model outputs has seldom 

been investigated, however, and their sensitivities to varying climate conditions are generally unknown. One exception is the small-catchment rainfall-runoff model developed 

by the U.S. Geological Survey (Dawdy and others, 1972). 

Lichty and Liscum (1978) have devised a procedure for 

estimating the information about flood-recurrence intervals 

that the model can extract from weather records. A factor, 

illustrated in figure 7, also has been mapped that defines the 

impact of existing climate conditions on flood frequencies 

in small catchments of the United States east of the Rocky 

Mountains. Studies of this sort will be carried out for other 

models and for characteristics other than floods, and the procedures will be extended to estimate the impacts of climate 

change on the models' abilities to generate hydrologic 

information. 

Research 13 
N.J. 

el. 

100 

,·I I I 

100 200 KILOMETERS 

EXPLANATION 

-1700- Line of equal climatic factor 

Figure 7. Geographic variation of the climate factor for the 100-year-recurrence-interval 

flood. The factor is the ratio of peak-flood discharge that has a probability of exceedance 

of 1 percent in any given year to a mathematical description of the physical characteristics 

of the drainage basin that could yield the flood; the mathematical description was derived 

by regression techniques used on an exhaustive set of drainage characteristics and model 

output. Source: Lichty and Liscum, 1978. 

Stochastic Models 

Because of the inherent imprecision in physically based 

hydrologic models, a special class of models, known as 

stochastic models, has been developed. Stochastic models 

are based on principles of probability and statistics and can 

be used to describe either hydrologic phenomena or the errors 

that derive from physically based hydrologic models. The 

uses of stochastic models are: 

(1) To provide language in which assumptions may 

be stated about observed time series (Parzen, 

1962); 

(2) To provide insight into the most realistic and/or 

mathematically tractable assumptions to be made 

concerning the stochastic processes that are 

adopted as models for time series (Parzen, 1962); 

and 

14 Water Resources and Implications of Climate Uncertainty 

(3) To provide a tool for obtaining approximate solutions to the complex problems that arise in making water-resources decisions. (Moss and Tasker, 

1987). 

However, for stochastic modeling to remain a useful aspect 

of hydrology in the 21st century, its language and 

mathematical constructs must be expanded to describe 

hydrologic processes that are changing in response to climate· 

change. 

A priority area for stochastic-modeling research deals 

with disaggregation models, which develop statistically 

representative data at scales of time and space that are finer 

than those of the data that are available to drive the models. 

The priority results from the mismatch between the relatively 

large grid sizes of the current general circulation models and 

the much smaller scales at which most hydrologic analyses 
must be performed to meet the needs of water-resources decisionmakers. For example, suppose that one wants to know 

the impact of computed values of precipitation on the 

likelihood of flooding in a small catchment contained within 

the 5° grid of a GCM. Because the catchment is much smaller 

than the GCM grid and because the response time of the 

catchment is significantly smaller than the time step of the 

GCM, the time series of precipitation values from the GCM 

must be disaggregated in both time and space to extract the 

desired information. Although disaggregation modeling in 

the time dimension is relatively advanced at the current 

time (Bras and Rodriguez-Iturbe, 1985, p. 147-154), little 

progress has been made in the spatial dimension, and none 

has occurred in the bidimensional domain of space and time. 

Another crucial type of stochastic modeling research 

involves data-network analysis and design. Most, if not all, 

methodologies for the design of hydrologic-data networks 

are based on statistical or stochastic descriptions of the 

hydrologic phenomena to be monitored. Underlying most of 

these methodologies is an assumption of stationarity of the 

stochastic processes. It has been demonstrated in many 

widespread, but isolated, settings that nonstationarities exist 

in hydrologic records (Riggs, 1985). Nevertheless, most 

hydrologic services continue to rely on stationarity assumptions in the allocation of their data-collection resources. The 

potential for anthropogenic climate change has heightened 

awareness of the disparity between underlying assumptions 

and potential realities in the 21st century. 

Moss (1986) presents an approach, dealing with data 

collection in a regional or national framework, that pertains 

to nonstationarity. However, this approach is in an early stage 

of development. 

As mentioned earlier, the large, but unknown, uncertainty contained in all currently available climate-change 

projections necessitates reliance on scenario analysis, which 

is perhaps the most elemental, but still rational, strategy 

for dealing with uncertainty. To progress beyond the use of 

scenario analysis to a more effective strategy, the uncertainties in climate projections must be quantified, and a 

decisionmaking structure for coping with the resulting risks 

must be elucidated. The theory for decisionmaking under 

uncertainty exists (Raiffa, 1970), but the complex stochastic 

nature of the inherent errors contained within the 

hydroclimate models needs to be determined before the 

theory can be brought to bear. 

Data Collection 

The collection of data is a universal requirement of 

scientific endeavor. Until rather recently, hydrology has been 

to a large degree an empirical science; data collection has 

been the basis for most of its successes. In hydrology, as 

with most other sciences, data are used in two general ways: 

(1) the computation of statistics and (2) the calibration and 

testing of models. These two categories of data use are not 

distinct from each other, however. Often models are 

calibrated by means of the statistics of relevant data, while 

the computation of many statistics presumes an underlying 

stochastic model for the data. Although not distinct, the 

differentiation between statistical and modeling uses of data 

is useful in describing the role of data collection in studying 

the hydrologic implications of climate uncertainty. 

Hydrologic-data collection for the computation of 

statistics entails the quantification of spatial and temporal 

hydrologic phenomena both on and beneath the Earth's 

surface. Monitoring of both the quantity and quality of 

the Nation's water resources for compliance with laws, 

compacts, and treaties is a major aspect of this type of data 

usage. Because the resources for data collection are not 

unlimited, most traditional strategies for statistical data 

collection have depended on one of two major approaches: 

synoptic or temporal. Seldom are aspects of both significant 

in a single strategy. Synoptic approaches rely on occasional 

or periodic bursts of data at given times to define the spatial 

distribution of the hydrologic phenomena of interest; a well 

known synoptic data base from outside hydrologic science 

is the decadal U.S. census. Temporal approaches, on the 

other hand, rely on less spatial resolution through the 

establishment of a network of sites at which the phenomena 

of interest are measured continually, or nearly so, through 

time. Because many of the needs for hydrologic information are site specific, the hydrologic data-collection activities 

of the U.S. Geological Survey generally have used temporal 

strategies instead of synoptic ones. 

Land-Based Data 

An example of a temporal network that is relevant in 

addressing questions of climate uncertainties is the U.S. 

Geological Survey Benchmark Network, consisting of 57 

sites at which streamflow and water quality have been 

measured for almost 25 years (Cobb and Biesecker, 1971; 

Lawrence, 1987). The locations of the benchmark sites are 

shown in figure 8. As its name implies, this network was 

established to provide the Nation with records of hydrologic 

conditions at sites that would be affected minimally by human 

activities on their drainage basins. Although the benchmark 

concept did not explicitly anticipate the current concerns 

about climate change, this network of sites is a critical 

component of the data base that will be used to detect and 

track climate influences on the Nation's water resources. 

For this purpose, the principal shortcomings of the 

benchmark network are (1) the short length of records 

currently available, (2) the limited number of sites at which 

data are being collected, and (3) the limited suite of variables 

that are being measured at each site. Obviously, the first 

shortcoming is irreconcilable; one cannot go back in time 

to redress it except rarely through paleohydrologic studies. 

Data Collection 15 
EXPLANATION 

• Hydrologic benchmark station 

Figure 8. Locations of U.S. Geological Survey benchmark stations. 

However, items 2 and 3 will be ameliorated as part of the 

data-collection element of the program. 

Climate data for the benchmark drainage basins are 

sporadic in time and in space; this facet of the network limits 

its utility for studying climate-hydrology interactions. The 

first priority for increased data collection under this program 

will be the installation of permanent climate stations on each 

benchmark drainage basin. Because of the climate variability 

over areas within the range of the benchmark drainage basins 

(approx 5-2,500 km2

), several climate stations may be 

desirable in many of the basins. However, the network-design 

methodology for specifying the ideal number and locations 

of climate stations for each basin currently does not exist. 

Therefore, a single climate station will be installed on each, 

and deployment of other stations will await the methodology 

that is to be developed in the research element of the 

program. 

Even though new sites can be added to the benchmark 

network as funds are made. available, any new site will never 

be as powerful in tracking the impacts of potential climate 

changes as would the original sites. The information 

contained in the charter sites never can be recovered fully 

for the new sites. Nevertheless, if significant climate change 

is not too immediate, time remains for the establishment of 

significant data bases at new locations. Criteria for the new 

sites, both the number of sites and the characteristics of the 

individual sites, also will be developed within the research 

element of the program. 

16 Water Resources and Implications of Climate Uncertainty 

A histogram of the percentage of continuous stream 

gages in the United States by area of their drainage basin, as 

determined from the files of the National Water Data 

Exchange (Williams and Knecht, 1981), appears in figure 

9. The drainage basins of the benchmark network fall on the 

smaller end of the spectrum of gaged watersheds. Because 

of the incongruity between the spatial resolution of general 

circulation models and that of the Benchmark Network, the 

benchmark sites will contribute only a minor amount of 

information for the calibration and testing of general circulation models. Another set of stations at a more appropriate 

areal scale will be needed for this purpose. Currently, the 

Geological Survey is making a detailed search of the existing 

data base to identify stations that either are active now or 

could be reactivated to provide the requisite data for model 

testing and for evaluation of the hydrologic information 

content of the outputs of the GCM's. It is anticipated that 

both reactivation of some discontinued stations and the 

establishment of some new ones will be required to develop 

an adequate data base to achieve these ends. 

One area in which hydrologic data generally are not 

available for either statistical or modeling purposes is that 

of evapotranspiration. The reason for this is that a costeffective methodology does not exist to generate the data 

of the desired temporal and spatial scales. Early in the 

research element of the program, high priority will be given 

to the development of the missing methodology. Once 

the methodology has been demonstrated to be successful, 
50 

45 (45) 

40 

(/) 

z 

u; 35 

< aJ 30 u.. 

0 

w 25 

(.!) 

~ 20 w 

u 

a: 15 w 

a.. 

10 

5 

100 500 1,000 5,000 ~50,000 

DRAINAGE AREA, IN SQUARE MILES 

Figure 9. The percentage of continuously operated stream gages in 1987, by drainage area. 

Number of benchmark stations in each drainage area class appears in parentheses above 

the respective bar. 

it will be deployed as a basic component of the data 

element. 

As has been noted, there is a strong dependency 

between the data and the research elements of the program. 

In a mature stage of scientific development, a balance is 

achieved in the feedback from one element to the other. 

However, because of the major philosophical changes 

required to deal with hydrologic implications of significantly increased climate uncertainty , many research 

components of the program will have to precede the data 

components. Therefore, definition of major data components 

(for example, ground-water levels or water-quality variables) 

will have to evolve as research results are applied in the 

interpretive studies. 

Interface with Remote Sensing 

The movement toward large-scale studies of the Earth, 

with increasingly complex treatments of the interactions 

among the atmosphere-land-ocean-biota system elements, is 

generating a growing demand for a variety of high-quality 

and high-density geophysical measurements. In many areas 

of research, especially modeling, the data emanating from 

existing networks are either inappropriate, geographically 

insufficient, or both. Numerous applications now demand 

near-continuous spatial data of high resolution ( < 1 km). To 

meet these demands, investigators are making more extensive 

use of remotely sensed data. Indeed, the various programs 

of the National Aeronautics and Space Administration 

(NASA), the National Science Foundation, and the National 

Academy of Sciences encompassed under the rubric of Global 

Change, all explicitly incorporate a requirement for a permanent network of remote-sensing systems in orbit to 

complement both existing and new ground-based data 

networks. 

There is a clear role and need for remotely sensed data 

to support studies relating to climate and hydrology. Many 

of the most significant problems in hydrology relate to 

understanding the hydrologic cycle and its interactions with 

the atmosphere, as well as with the biosphere and lithosphere. 

Achieving such an understanding, for areas other than 

homogeneous small-scale basins, requires a combination 

of in-situ and remotely sensed (airborne and spaceborne) 

measurements (National Research Council, 1985). The 

remote measurements are essential for providing an integration, over broad spatial and temporal scales, of the in-situ 

observations. Moreover, water, because of how and where 

it occurs and the transformations it undergoes, is amenable 

to detection in many parts of the electromagnetic spectrum. 

This fact makes it a particularly appropriate quantity for 

sensing by remote means. 

Although U.S. Geological Survey scientists make 

extensive use of remotely sensed data in numerous and 

varied research activities, the agency is not responsible 

for operating and maintaining a data-collection system based 

on remote sensing. It supports such systems, however, by 

Data Collection 17 
providing in-situ observations necessary for both sensor 

calibration and verifying the interpretation of remotely sensed 

data. An example of such support involves ongoing participation by the USGS in NASA's First International Satellite 

Land Surface Climatology Project Field Experiment at the 

Konza Prairie Long Term Ecological Research Site in northeastern Kansas, which began in 1987 (Sellers and Hall, 1987, 

written comrnun.). As part of the experiment, the Geological 

Survey is operating three surface stations designed to measure 

the flux of water vapor and sensible heat to the atmosphere. 

These measurements are important to continuing efforts 

oriented toward regional-scale estimation of microscale 

processes, such as evapotranspiration, from remote-sensing 

platforms (fig. 10). 

SCALE AND SIMULTANEITY 

Satellite 1 0 meters - 8 kilometers 

Airborne flux 15 kilometers 

Airborne radiometry 

1 0 meters - 15 kilometers 

Flux site 

1 0 meters - 1 kilometer 

Canopy, leaf physiology 

1 centimeter - 10 meters 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

Figure 10. Range of scales addressed by the First International Satellite Land Surface 

Climatology Project Field Experiment. Source: Brutsaert and other, 1988. 

18 Water Resources and Implications of Climate Uncertainty 
Another example is the development of an in-situ 

instrument package for measuring the dielectric properties 

of snow and ice. The Snow and Ice Dielectrics System, 

developed collaboratively by the Environmental Research 

Institute of Michigan and the U.S. Geological Survey, is 

specifically designed to facilitate the spatial extrapolation of 

snow water-equivalent estimates derived from airborne 

passive microwave measurements of snow. Activities such 

as these undoubtedly will expand as large-scale integrated 

studies of the Earth focus to a greater degree on increasing 

the detail and versatility of remotely sensed data. 

Water-Resources Interpretive Studies 

The success of any program designed to study the 

hydrologic implications of climate uncertainty must be 

measured in terms of information utility. It is essential that 

the information generated by a program be carefully matched 

to the level and scale at which actual policies and decisions 

affecting the Nation's ability to cope with the uncertainty are 

made. In terms of water resources, an appropriate areal scale 

for information synthesis is the major river basin or regional 

aquifer. Accordingly, the U.S. Geological Survey has 

initiated a pilot study of the Delaware River basin (Ayers 

and Leavesley, 1988) (1) to test its capabilities to develop 

information on the implications of climate uncertainty 

regarding a number of interrelated aspects of the basin's 

hydrology; (2) to determine the utility of such information 

for basinwide water-resources planning in the 21st century; 

(3) to assess methodological performance and needs for 

enhancing the utility of such studies; and (4) to evaluate future 

data requirements for tracking potential impacts of climate 

change. 

Because the Delaware River basin does not encompass 

the full range of hydrologic phenomena susceptible to climate 

change, this study is envisioned as the first of several 

geographically distinct pilot assessments that logically should 

precede a comprehensive attack on the problem of evaluating 

the Nation's water resources in the 21st century. The 

following summary of the Delaware River Basin study 

provides an example of the scientific complexity associated 

with such comprehensive, but meaningful, interpretive 

analyses. 

The Delaware River basin encompasses 12,765 mi2 and 

crosses four distinct physiographic provinces (fig. 11). Its 

runoff processes are quite diverse, and man's influence on 

the movement and storage of water adds considerable 

complexity to the basin system. The Delaware River serves 

as a major source of water for an estimated 20 million 

people both in and outside the basin (Delaware River Basin 

Commission, 1986). 

The availability of water to users is based on many 

complex systems of reservoirs for storage and pipes, tunnels, 

and canals for diversion and delivery. The two largest 

diversions out of the basin are through the New York City 

aquaduct system to New York City and the Delaware and 

Raritan Canal to northeastern New Jersey. The freshwater 

portion of the tidal river below Trenton, the Delaware 

estuary, serves as a source of ground-water recharge for 

aquifer systems supplying water to southern New Jersey as 

well as a water-supply source for the city of Philadelphia 

and many industries by direct diversion. It is critical for these 

supplies that the Delaware Estuary along these reaches 

remain potable, even during periods of prolonged low flow 

(droughts). 

Salinity problems in the estuary have historically been 

associated with New York City diversion rights. The U.S. 

Supreme Court (1931; 1954) required the city system to 

maintain certain instream flows at Montague, N. J., to 

provide both an adequate water supply and salinity controls 

in the estuary, an issue of considerable concern to 

downstream interests. Further, ''Good Faith'' agreements 

have included instream flow requirements at Trenton, N. J. 

During the drought of record in 1961-65, and twice since 

1980, emergency water-use restrictions have been placed on 

basin water users to meet basic water-supply demands and 

instream-flow requirements. 

Such experiences illustrate the susceptibility of the 

basin's water-supply to climate variability and emphasize how longer term climate uncertainty poses potentially 

serious problems for the Delaware River basin. Moreover, 

they provide insight on how poorly understood the potential 

effects are of climate change on the water supplies of the 

basin, including the ability to maintain instream-flow 

requirements. 

The Delaware River basin study is designed to 

investigate the hydrologic response of the basin (fig. 12), 

under the existing management infrastructure, to possible 

climatic conditions in the mid to late 21st century. Specific 

objectives include defining the spatial and temporal variability 

of current and projected climate conditions on four important 

aspects of water availability within the basin. These are 

(1) the streamflows and storage associated with the New 

York City and other basin reservoir systems, (2) the ability 

to maintain instream flow requirements, (3) the upstream 

movement of saline water in the Delaware estuary associated 

with changes in sea level and freshwater inflows, and (4) 

the resulting potential intrusion of saline water into aquifers 

adjacent to the estuary. 

Although the Delaware River basin is a unique 

hydrologic system within the United States, the interpretive 

assessment of this basin is intended to serve as a prototype 

for similar studies across the Nation. Clearly, other regions 

will have a different mix of physical system, water-supply, 

legal, and economic problems attendant with a climate 

change, but the general approach to evaluating those problems should be applicable. That approach, involving the 

coupling of advances in process research and data collection with existing hydrologic capabilities, is a robust yet 

flexible design. It should provide a sound framework for 

analysis regardless of the hydrologic region or type of climate 

change. 

Water-Resources Interpretive Studies 19 
EXPLANATION 

_,._ Basin boundary 

- Physiographic 

boundary 

25 KILOMETERS 

Figure 11. Physiography of the Delaware River basin. 

20 Water Resources and Implications of Climate Uncertainty 

42' 
(CLIMATE MODELINGJ 

1 ~ EXISTING CONDITIONS FUTURE SCENARIOS 

~ CLIMATE / SEA ' LEVEL __ _..,. 

r----------__.• ( WATERSHE{ MODELING) 

EVAPOTRANSPIRATION - SOIL MOISTURE - SNOWMELT 

~ + RECHARGE RUNOFF~---_. RESERVOIRS 

GROUND-WATER MODELING ~~--------SALINITY DYNAMICS 

SALTWATER INTRUSION 

+ GROUND-WATER AVAILABILITY SURFACE-WATER AVAILABILITY 

"'-----'-/ 

Figure 12. Primary components of the Delaware River basin study. 

STRATEGY FOR ACHIEVING THE 

PROGRAM GOAL 

The primary facet of a strategy for dealing with the 

hydrologic implications of cliinate uncertainty is the evolution of the resource balance among the three components of 

the program: (1) data collection, (2) research, and (3) interpretive studies. The strategy must be an evolutionary one 

because of the high degree of near-term uncertainty in the 

climate variables that, along with gravity, drive the 

hydrologic cycle. Information that reduces this uncertainty 

must be monitored continually by the program administrators, 

and, as the new information dictates, the program balance 

should be modified to address more effectively the program 

goal. In the early phases of the program, the necessity of 

feedback of information to develop the short-term plan 

dominates the planning process to the extent that long-term 

plans can be viewed only in a statistical or probabilistic 

context. 

The planning process is based on the hypothesis that 

a mature program will consist of a modest balance between 

research and data collection that will continue to generate 

new knowledge and information but that will have as its 

major focus the synthesis and interpretation of the existing 

knowledge and information across broad ranges of both 

hydroclimatic settings and water-resources problems and 

opportunities. This hypothesis dictates that a mature program 

tends to evolve toward the lower-right quarter of the diagram 

given in figure 13A. As is shown in figure 13B, the waterresources activities of the U.S. Geological Survey historically 

have gravitated from a data-collection focus to a balance in 

the 1980's that has in excess of 50 percent of its funds 

expended for interpretive studies. This traditional evolution 

can be attributed to the prevailing assumption that natural 

hydrologic processes are in statistical equilibrium at the time 

scales of a century or so-stationarity. Under such an 

assumption, it is efficient to expend relatively large sums 

collecting and analyzing data that will serve as the basis for 

projections of future water resources. As the data bases 

expand, the marginal utility of the next increment of data 

decreases (Moss, 1970), while the hydrologists' abilities to 

Strategy for Achieving the Program Goal 21 
A. 

100 

0 

B. 

100 

collection 

dominates 

0 

dominates 

Interpretive 

studies 

dominate 

~----------------------~~----------------------~~0 

50 

INTERPRETIVE STUDIES 

1981 

100 

~------------------------~----------------------~~0 

0 50 

INTERPRETIVE STUDIES 

100 

Figure 13. Resource balance among the three components of the climateuncertainty program: (A) areas of single-component dominance and (8) program evolutions. 

22 Water Resources and Implications of Climate Uncertainty 
perform interpretive studies using the historical data bases 

increase. Thus, under the assumption of stationarity, there 

is a natural tendency to evolve from data collection to 

interpretation. During this evolution, research has been 

viewed primarily as a modest investment to address future 

contingencies. 

The potential for climate change provides an impetus 

to the proposed program to start from a different sector than 

have the traditional water-resources programs. As was 

described previously, the lack of sufficient understanding of 

the major interactions of the hydrosphere with the atmosphere 

contributes significantly to the level of uncertainty regarding 

the future characteristics of both. Development of this 

understanding is requisite for useful projections of future 

hydrologic conditions as well as for understanding the 

apparent changes that may be detected as a result of existing 

data-collection activities. Thus, research will be the dominant component of the climate uncertainty program during 

its early stages, as shown in figure 13B. As research 

contributes additional understanding, both interpretive studies 

and data collection will have a strong basis for increased 

levels of effort. 

As shown in table 3, the research needs for this 

program are partitioned into three categories. The first 

category, which is called critical, consists of endeavors that 

currently are underrepresented to a debilitating degree if the 

program goal is to be addressed in a timely manner. It is 

noticable that, with one exception, each of these endeavors 

deals with links between the atmospheric and hydrologic 

sciences. Because each will be carried out at the disciplinary 

frontier of hydrology, each increased activity should be 

planned to take advantage of and interact with complementary activities of other Federal agencies and interdisciplinary, 

international programs. 

The exception to the hydrology-atmosphere interface 

in this research category is network design. This endeavor 

is critical because the data-collection component cannot be 

fully effective until better knowledge and methodologies are 

available to guide investments in data. 

The second category of research endeavors includes 

facets of existing activities that only may need some redirection and (or) added support to assume their proper role in 

the program. With the exception of perhaps paleohydrology 

and biogeochemical cycles, entries in this category are 

traditional strengths of U.S. Geological Survey hydrologic 

research. 

The final category contains lakes and estuaries, which 

have been significant components of U.S. Geological Survey 

research in the past but that will require added emphasis in 

the decade of the 1990's and beyond. Studies currently 

are being conducted within the Survey to define more 

specifically the research needs and opportunities in each 

of these areas. 

Table 3. Research status and priorities 

Status 

Critical concerns ............. . 

Ongoing concerns ............ . 

Evolving concerns ............ . 

Research 

Evapotranspiration. 

Droughts. 

Modeling (general circulation 

and mesoscale). 

o Parameterization. 

o Spatial disaggregation. 

Hydroclimatology. 

Network design. 

Snow and ice. 

Infiltration. 

Runoff. 

Unsaturated zone. 

Floods. 

Recharge. 

Paleohydrology. 

Biogeochemical cycles. 

Water quality. 

Lakes. 

Estuaries. 

As was discussed above, the data-collection component 

of the program initially will not be of the same magnitude 

as the research component. However, there are two datacollection activities that merit particular concern. The first 

of these is augmentation of the benchmark network to provide 

baseline information for tracking hydrologic change. Modest 

initial expansion likely would be followed by additional 

increases as knowledge, methodology, and technology are 

developed. 

The second critical endeavor for data collection is a 

network for estimating areal evapotranspiration. Although 

such a network is an immediate need, methodology and 

technology limit our abilities to deploy the requisite costeffective measurement systems. Interagency collaboration, 

such as is currently being carried out in the Konza Prairie 

of Kansas (Brutsaert and others, 1988), will be required both 

to redress the technological shortcomings as well as to 

implement the ultimate solution. The Geological Survey has 

a key role to play in both aspects. 

The program component with the greatest potential for 

growth is interpretive studies. As information and understanding from the other two components of the program 

accumulate, the demand for interpretive studies can begin 

to be met. Indications are that the demand for this type of 

product already exists as a result of the publicity that climate 

change has received recently. However, current ability to 

provide substantive interpretations of the implications of 

climate change is extremely limited. Thus, a cautious 

approach to performing interpretive studies is in order. 

Studies like that in the Delaware River basin, described 

Strategy for Achieving the Program Goal 23 
earlier, will be repeated in other hydroclimatic and 

demographic settings to test the available methodologies and 

data bases prior to a major expansion of this component. 

These early studies will treat the study areas more as largescale open-air laboratories than as definitive hydrologic 

assessments. Hydrologic findings pertaining to climate 

uncertainty will be placed in the context of the existing levels 

of understanding of the critical issues. As the increments of 

knowledge and information accumulate, interpretations can 

be revisited, and the caveats on such studies can be reduced. 

Eventually, generally accepted procedures can be used, and 

interpretive studies of the hydrologic implications of climate 

uncertainty will become a major component of the waterresources activities of the U.S. Geological Survey. 

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