The 20th century has seen a rise in the world's mean sea level of about 1025 cm, with a most likely value of 18 cm (Görnitz, 1995; Warrick et al., 1996; Chapter 4, this book). Rates of sea level rise (SLR) may increase by 2-5 times over present rates within the next 100 years due to projected global warming, posing a threat to low-lying coastal regions (Chapter 8). Nearly half of the observed SLR of 18 cm during the 20th century can be attributed to thermal expansion of the upper ocean and melting of mountain glaciers associated with rising global mean surface air temperature. Greater uncertainty surrounds the contribution to SLR from melting of the Greenland and Antarctic ice sheets, which may be close to zero at present (Table 5.1). Thus, the balance of 20th century SLR beyond that attributable to ocean thermal expansion and melting of small glaciers needs to be accounted for by other processes, including human-induced transformations of the terrestrial hydrologic cycle.

It has long been recognized that anthropogenic activities1 could influence stream runoff, and hence ultimately affect global sea level (Newman and Fairbridge, 1986; Gornitz and Lebedeff, 1987; Chao, 1991; 1994; 1995; Sahagian et al., 1994; Rodenburg, 1994; Gornitz et al., 1997). However, quantitative assessments have been lacking. The Intergovernmental Panel on Climate Change, in reviewing the literature, estimated that these actions contribute between -0.4 to +0.75 mm/yr to sea level (Table 5.1; Warrick et al, 1996). This considerable spread points to the large uncertainty that exists over the magnitude and even the sign of such anthropogenic effects on global sea level. In this chapter, we review the important hydrologic processes and present historical data pertaining to human modification of the land's hydrologic system since the beginning of this century. We also briefly examine improved hydrologic models that consider land cover transformations, and new, promising remote sensing techniques to monitor hydrological impacts.

1 These include groundwater mining, impoundment in reservoirs, vegetation clearance, infiltration beneath reservoirs and irrigated fields, and evaporation from reservoirs and irrigated fields.

Copyright © 2001 by Academic Press.

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Table 5.1

Estimated Contributions to Global Sea Level Rise during the Past 100 Years (cm) (after Warrick et al., 1996)





Thermal expansion




Mountain glaciers




Greenland ice sheet

- 4



Antarctic ice




Impoundment and groundwater mining












Source: Intergovernmental Panel on Climate Change.

Source: Intergovernmental Panel on Climate Change.

5.1.1 An Overview of the Land Hydrologic Cycle

The earth's water resides in four major reservoirs: the ocean, glaciers and ice sheets, terrestrial water, and the atmosphere (Fig. 5.1). The oceans contain 96.5% of the total water reserves, whereas freshwater reserves constitute only 2.5% of the total (Fig. 5.1). Around 69% of freshwater is locked up in glaciers, permafrost, and ice, while another 30% lies in groundwater and soil, leaving less than 1% in lakes and rivers (Jones, 1997; Shiklomanov, 1997). Water is exchanged between these reservoirs through the processes of evaporation, precipitation, and runoff. The net balance between the components of the hydrologic system is expressed as

where P is precipitation, E is evaporation/evapotranspiration, S is change in storage (soils, rocks, lakes), and R is stream runoff. This equation can be

Figure 5.1 The hydrologic cycle. Figures (after Shiklomanov, 1997) are in units of 103 km3.

solved for any time period, but in practice, an annual or flood hydrograph period is selected, such that initial and final conditions are reasonably equivalent.

Around 500,000 km3 of water evaporates annually from the world's oceans, nearly 90% of which precipitates over the ocean and 10% over land (Shikloma-nov, 1993). Ocean-derived moisture together with local land sources accounts for terrestrial precipitation of around 119,000 km3/yr. Of this amount, between 40,700 and 47,000 km3/yr returns to the sea as total runoff (Fig. 5.1; Shikloma-nov, 1993; L'vovich and White, 1990). Over half of the runoff comes from the major southeast Asian rivers draining the Himalayas and the Amazon River in South America. Analysis of historical records of 142 rivers throughout the world with over 50 years of historical data, occupying drainage areas greater than 1000 km2, revealed no consistent trends in streamflow over time (Houghton et al., 1996). On the other hand, large-scale interdecadal variations correlate well with precipitation in certain regions (e.g., Marengo et al., 1994).

A fraction of the water falling over dry soil infiltrates into the ground, at rates depending on the physical properties of the soil and its water content. Water on the surface may evaporate, or, if the soil is saturated, flow downslope as surface runoff into streams and lakes, or slowly percolate toward the water table2 and recharge groundwater. The exact volume of water held in groundwater is uncertain, but exchanges between relatively shallow groundwater stores may occur over tens to hundreds of years, whereas exchanges at deeper levels may take thousands of years or more (Jones, 1997). For example, water in some Saharan aquifers may date back to the wetter, "pluvial" climate following the end of the last glacial period.

Vegetation participates in the hydrological cycle by modifying evaporation and runoff, compared with bare soil. Over vegetated terrain, the plant canopy intercepts a fraction of the falling water, the throughfall reaches the soil and infiltrates into the ground, where plant roots draw water from the soil. Leaves evapotranspire water mainly during the day, driven by solar heating and photosynthesis. Land-use changes, such as clearing of the natural vegetation for agriculture, grazing, or urbanization, in particular impact hydrological interactions among vegetation, soil and atmosphere, such as interception, infiltration, percolation, and évapotranspiration (Table 5.2). Some of these impacts will be discussed in greater detail below.

Within the last 50-100 years, human appropriations of water, although still a relatively small fraction of the total volume of water exchanged, have nonetheless become significant. For example, Postel et al. (1996) estimate that people already utilize 54% of readily accessible river runoff. River runoff is increasingly being managed, by construction of dams for hydroelectric power, flood control, and irrigation, as well as by huge intrabasin transfers of water,

2 The surface separating the saturated zone (i.e., the zone where all pore spaces in the soil, sediment, or rock are filled with water) from the unsaturated zone where pore spaces are occupied by air and some water.

Table 5.2

Hydrological Impacts of Land-Use Change

Land-use change

Hydrological effects




Wetlands drainage

Decreases in interception, évapotranspiration, infiltration capacity, soil porosity; increases in surface runoff, annual flows, flooding; snowmelt hastened; changes in seasonal distribution of runoff.

Decreases in infiltration and percolation, increases in surface runoff, changes in groundwater levels.

Decreases in infiltration, increases in surface runoff, changes in évapotranspiration, changes in seasonal distribution of runoff. Decreases in water retention capacity, évapotranspiration; increases in floods, annual flows.

and channelization of rivers. Other changes in runoff accompany alteration of land use or land cover patterns, such as replacement of forests by grassland or cropland, urbanization, or draining and clearance of wetlands (Table 5.2). Good summaries and case studies of these processes and their consequences can be found in Walling (1987), Newson (1992), Jones (1997), L'vovich and White (1990), Gleick (1993), and Shiklomanov (1993,1997). The major ways in which human transformation of the hydrologic cycle could influence sea level are illustrated schematically in Fig. 5.2 (see color plate).

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