Impacts of Climate Change and Land Use  on the Southwestern United States

Impacts of climate change on water resources

Paleohydrology and its Value in Analyzing Floods and Droughts

Robert D. Jarrett
U.S. Geological Survey


This paper is abstracted from Jarrett, R.D., 1991, Paleohydrology and its value in estimating floods and droughts, in Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, D.W., Compilers, National Water Summary 1988-89--Hydrologic Events and Floods and Droughts: U.S. Geological Survey Water-Supply Paper 2375, p. 105-116..

INTRODUCTION

The environmental and economic importance of major floods and droughts emphasizes the need for a better understanding of hydrometeorologic processes and of related climatic and hydrologic fluctuations or variability. In the United States, the average annual flood damage for the 10-year period 1979-88 was $2.4 billion and the average annual number of deaths for the period 1925-88 was 95 (U.S. Army Corps of Engineers, 1989). Droughts lack the dramatic physical results of floods because droughts develop gradually with time and throughout a geographic area, and they affect people and the economy (water supply, groundwater levels, water quality, agriculture, navigation, hydroelectric power, fisheries, and recreation) in different ways. Therefore, identifying the effects and estimating the loss of life and damage from a drought are difficult.

Estimating the magnitude and frequency of large floods and droughts and their effect on people also is difficult. One of the primary missions of the U.S. Geological Survey is to operate a streamflow- gaging-station network to monitor the Nation's water resources and to evaluate streamflow extremes. Estimates of the frequency of floods, droughts, and long-term streamflow variability from short-term (generally much less than 100 years) data records contain much uncertainty. Paleohydrologic techniques offer a way to lengthen a short-term data record and, therefore, to reduce the uncertainty in hydrologic analysis. Paleohydrology, as discussed in this article, is the study of the evidence of the movement of water and sediment in stream channels before the time of continuous (systematic) hydrologic records or direct measurements (Costa, 1987). Paleohydrologic data typically have been used to quantitatively reconstruct hydrologic variability for about the last 10,000 years. Beyond 10,000 years, quantitative paleohydrologic investigations often are hindered by limited evidence of channel changes, which reduces the accuracy of such paleohydrologic estimates. Also, in glaciated areas ice-may have reworked channel deposits and features.

This article briefly reviews paleohydrologic techniques used for analyzing floods, droughts, and hydrologic variability and discusses the application of these techniques in estimating long-term hydrologic records. Emphasis of the review is on those techniques most commonly used for reconstructing stream discharge and for dating floods and droughts. The references cited provide sources for additional information about paleohydrology; excellent reviews of the history of paleohydrology are provided by Costa (1987) and Patton (1987).

DIRECT AND INDIRECT CLIMATIC AND HYDROLOGIC DATA

Direct and indirect climatic and hydrologic data can be used to assess the variability in the hydrologic cycle. Liebscher (1987) provided a summary of the different sources of climatic and hydrologic data available for reconstructing paleohydrologic conditions (fig. 1).

figure 1

Figure 1. Sources of data used to reconstruct climatic or hydrologic conditions. (Modified from Liebscher, 1987). Click to view full-size image.

Direct data can be systematic (measured) or historical. Systematic data, such as streamflow records obtained at gaging stations, have been collected for about the last 100 years as part of scientific investigations. Historical data are recorded episodic observations of streamflow conditions, floods, and droughts that were made before systematic data were collected. In the United States, historical data typically are available for 100 to 200 years (Thomas, 1987). In Egypt and China, historical records are available for several thousands of years (Baker, 1987).

Indirect data, such as pollen, sediment, and tree-ring records, are examples of what is known as proxy data. Each type of proxy data has different problems related to its accuracy. Scientists use proxy data to extend climatic and hydrologic records. For example, tree-ring data have been used to reconstruct past precipitation and temperature for several hundred to thousands of years; deep-sea sediment cores have been used to reconstruct long-term, global temperature fluctuations for thousands to hundreds of thousands of years.

Paleohydrologic analysis uses many types of proxy data. Evidence of historic and prehistoric floods commonly is preserved in stream channels as distinctive sedimentologic deposits or landforms and also can be preserved as botanical evidence. The interpretation of this evidence provides important supplemental information about the spatial occurrence, magnitude, age, and frequency of floods, droughts, and hydrologic variability.

IMPORTANCE OF PALEOHYDROLOGIC DATA

figure 2

Figure 2. Examples of long-term temperature variability and sea-level changes based on several types of paleohydrological evidence. A, principal trends in average temperature during the last 100,000 years in northwestern Europe. B, Estimated sea-level changes during the past 150,000 years, which primarily reflect storage and release of water from glaciers. (Modified from (A) Knighton, 1984, (B) Moore, 1982). Click to view full-size image.

Until recently, most planning related to water resources rarely has been able to consider long-term hydrologic variability or climatic change; thus, water- resources investigations and planning sometimes are hampered by inadequate and (or) erroneous hydrologic data (Jarrett, 1988). Short records that include large floods or extreme droughts also might cause significant uncertainty in the results of frequency analysis. Because of the small sample of large floods and extreme droughts in the short systematic streamflow record, conventional hydrologic analysis might not always provide the most accurate representation of the frequency of floods and droughts or long-term hydrologic variability. The use of paleohydrologic techniques provides one means of evaluating the hydrologic effects of long-term hydrologic variability and climatic change because it complements existing short- term systematic and historical records, provides information at ungaged locations, and helps decrease the uncertainty in hydrologic estimation. These improved estimates subsequently improve water-resources planning. One distinct advantage of using paleohydrologic data is that these data can be obtained without direct monitoring. Paleohydrologic information can be used in two directions (Baker, 1983)—first, modern hydro- logic data are used to create models of past hydrologic conditions, and second, paleohydrologic data can be used to calibrate and to evaluate modern hydrologic models, which in turn can be used to predict future climatic and hydrologic conditions.

Climatic change involves changes in the solar-energy regime of a given region that affect the hydrologic cycle. The adjustments of the hydrologic cycle to long-term variability and climatic change also are recorded in surficial sediment deposits and landforms. Variations in lake and ocean levels provide an indication of climatic variability, and ocean-bottom sediments have been analyzed to reconstruct temperatures for as much as about 100,000 years ago (Knighton, 1984). Examples of broad averages of long-term temperature variability and sea-level changes based on several types of paleohydrologic evidence are shown in figure 2. Data in these graphs reflect substantial variations in climate and indicate that the present state of the hydrologic cycle is transient, when considered in the context of millennia. Because climate affects water-resources availability, long-term data are needed to assess the effects of climatic change on water resources. Consequently, there is a need to better determine past streamflow to assess current values.

Recent advances have been made in dating techniques, in paleodischarge estimation, and in the determination of the recurrence interval of floods (Baker, 1987; Baker and others, 1988). Many techniques are available in the scientific disciplines of sedimentology, geomorphology, hydraulics, and botany to extend hydrologic records, and the paleohydrologic techniques expand on those techniques to analyze floods, droughts, and hydrologic variability.


PALEOHYDROLOGIC TECHNIQUES FOR DISCHARGE ESTIMATION

Most paleohydrologic techniques used for reconstructing discharge are either for floods or for variations of long-term discharge. Evidence of past high-flow conditions is preserved in sediments and landforms; evidence of past low-flow conditions is not preserved, although recent low-flow conditions generally are preserved in stream channels. Flow characteristics have been estimated from channel geometry and sediments (Wahl, 1984).

ESTIMATING LONG-TERM DISCHARGE

Annual discharge is the arithmetic mean of individual daily mean discharges during a specific water year. The long-term discharge is the arithmetic mean of the annual discharges. Two approaches have been used to estimate the long-term discharge--a geomorphic approach, which uses geomorphic and hydraulic characteristics to estimate the long-term discharge from sediments and landform features preserved in channels, and a botanic approach (dendrochronology), which yields a continuous reconstruction of annual discharge from analysis of tree-ring widths. This continuous record provides estimates of long-term discharge for the assessment of periods that had high (floods) and low (droughts) streamflow.

ESTIMATING INSTANTANEOUS PEAK DISCHARGE

Fluvial paleohydrology is concerned with determining the magnitude and frequency of individual paleofloods. For these estimates, geomorphic and hydraulic approaches have been developed. The geomorphic approach combines analysis of the sizes of boulders on streambeds with channel cross-sectional properties (width and depth). The hydraulic approach combines analysis of indirect evidence of the stages corresponding to these floods and channel cross-section properties (fig. 3). Thus, the hydraulic approach uses paleostage indicators determined from sediment deposits in the channel margins or other indicators of maximum paleostage that correspond to flood elevations.

figure 3

Figure 3. An ideal channel for studying slack-water deposits--the Escalante River in Utah. The person to the left is standing on a typical sequence of slack-water deposits that were deposited where the flow velocity decreased in the canyon of the Escalante River. (Source: Robert H. Webb, USGS).

PALEOHYDROLOGIC TECHNIQUES USED TO DATE FLOODS AND DROUGHTS

Many different paleohydrologic techniques are used to date material present on the land surface. Two techniques discussed here are relative- and absolute-dating techniques. Relative dating of stratified flood deposits is based on position of the deposits (Costa, 1978). Absolute-dating techniques most commonly used in paleohydrology are radiocarbon dating and botanic evidence. Radiocarbon dating of organic material is the primary dating tool used in paleohydrology, although tree-ring analysis is used for dating floods and droughts and for reconstructing long- term hydrologic records.

Absolute dating and assignment of a specific age to a sediment sample using radiocarbon dating (Baker, 1987) are derived from laboratory determination of the decay of radioactive carbon-14 to stable carbon-12 in a sample of organic material. Materials used for radiocarbon dating are wood, charcoal, leaves, humus in soils, and other organic material. Samples of material are collected from slack-water deposits, gravel bars, and other depositional features. Recent advances in radiocarbon dating, such as the use of a tandem accelerator mass spectrometer, allow for dating extremely small samples with great precision (Baker, 1987). Samples having an age of 10,000 years generally can be dated with an uncertainty of less than 100 years (Stedinger and Baker, 1987).

Several different types of botanic evidence for determining the age or chronology of floods and droughts occur on flood plains (Sigafoos, 1964; Fritts, 1976; Hupp, 1987). Long-term annual tree-ring chronologies provide information for precisely dating floods and droughts, in some cases to the nearest year. An accurate tree-ring technique that is used to determine dates of floods is the analysis of tree-ring increments or wedge cuttings through scars. Analysis of a scar location in the tree rings can determine the year of flooding. Stewart and LaMarche (1967) provided documentation of the dates of flooding resulting from their investigations of the severe flood of 1964 in northern California. Their investigations in Coffee Creek indicated that many 200- to 400-year-old trees that had survived lesser floods were toppled during the flood of 1964. Costa (1978) indicated that the age of trees growing on flood-deposited sediment or in flood-scoured areas provides a minimum age since the most recent flooding.

RELATION OF PALEOHYDROLOGY TO CLIMATIC VARIABILITY

Commonly, risk from floods and droughts must be evaluated for time scales beyond the length of available systematic records. Examples include floodplain management, hydrologic aspects of dam safety, and siting of nuclear power plants and waste-storage facilities. Assessing climatic or hydrologic variability with systematic hydrologic data (generally collected for much less than 100 years) is difficult, if not impossible. Climatic influences on floods, droughts, and long-term hydrologic variability are discussed in this section.

figure 4

Figure 4. Climatic effect on magnitudes of floods of a given probability, Mississippi River in Minnesota, 1867-1980. (Modified from Knox, 1984). Click to view full-size image.

Climatic change can be interpreted from lake and ocean sediments and mass balance of glaciers (Winter and Wright, 1977; Meier, 1986; Solomon and others, 1987). During numerous periods in the last 10,000 years, the climate has varied from the present climate, mean annual temperature has varied about ±4 degrees Fahrenheit from present values, and mean annual precipitation might have varied by as much as ±20 percent of modern values (Solomon and others, 1987). Because climate is the principal factor that affects floods, droughts, and hydrologic variability, anticipated climatic changes need to be considered. Paleohydrologic data provide a means to assess past climatic variability in water-resources planning.

Knox (1984) suggested that climate has affected flood magnitude in the Mississippi River in Minnesota from 1867 through 1980. The annual flood series was partitioned into four time (climatic) intervals. The intervals were chosen because independent studies of climate indicated that the boundaries of these intervals represent approximate dates of changes in characteristics of large-scale atmospheric circulation patterns that control the extent of air masses and the position of storm tracks. Flood-frequency analysis indicated that the magnitude of floods of a given probability varied substantially between the four climatically defined intervals (fig. 4). The first and last intervals were cooler, more moist, and prone to flooding. The greatest influence is on the magnitude of larger floods. Because flood-plain-management policies are based on estimated magnitudes of large floods, generally the 100-year flood, potential effects of climatic variability might need to be considered.

The Colorado River, which is a major source of surface water for much of the southwestern United States, traverses some of the most arid land in the country. The water in the Colorado River is over allocated and is regulated legally by the Colorado River Compact, which allocates the water to the many users in the United States and Mexico. For the 35 years from 1896 through 1930, the period on which the compact is based, the average discharge of the Colorado River was about 23,500 ft3/s (17 million acre-feet per year). However, from 1931 through 1965, the average discharge was only about 13 million acre-feet per year. A long-term (450 years) streamflow record reconstructed from tree rings (fig. 5) indicated the average discharge to be about 18,600 ft3/s (13.5 million acre-feet per year), much less than the 24,200 ft3/s (17.5 million acre-feet per year) on which the compact is based (Stockton and Boggess, 1983).

figure 5

Figure 5. Estimated Colorado River discharge for 450 years (before 1960) at Lees Ferry, Ariz., based on reconstruction from tree-rings. (Modified from Stockton and Boggess, 1983). Click to view full-size image.

The 450-year streamflow record reconstructed from tree rings indicated that the period on which the compact is based contained the longest series of high-flow years during the entire 450-year reconstructed streamflow records. Also, droughts from 1564 through 1600 and from 1868 through 1892 were of longer duration and greater magnitude than for any period of the gaged record. In the Upper Colorado River Basin, climatic changes related to the possible "greenhouse effect" (slightly warmer temperature and less precipitation) may decrease streamflow by 35 percent (Stockton and Boggess, 1982). These streamflow data reconstructed from the analysis of tree rings indicate that climatic variability might reduce the future availability of water from the Colorado River to support continued development of the Southwest.

CONCLUSIONS

Systematic hydrologic records, generally much less than 100 years long, rarely include infrequent and extraordinarily large floods and droughts, nor do these records reflect long-term hydrologic variability. Paleohydrology complements existing data, extends our hydrologic knowledge, and allows the reconstruction of long-term hydrologic records. The results of incorporating paleohydrologic data into conventional or new techniques are encouraging. The techniques and examples summarized here indicate how the use of paleohydrologic information and techniques can decrease the uncertainty in water-resources planning. They also indicate the value of paleohydrology in the evaluation of floods, droughts, and climatic and hydrologic variability. Paleohydrologic information also provides a means to assess the effects of potential climatic change on hydrology.

Although many paleohydrologic techniques are available, research can improve the understanding of physical processes of floods and droughts, paleohydrologic techniques, the understanding of climatic and hydrologic variability, and statistical procedures to better use historical and paleohydrologic data. The results of this research may reduce the uncertainty of hydrologic modeling, which in turn will decrease the uncertainty of water-supply estimates and flood estimates in water-resources planning.

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