HydrologySJ Marshall, University of Calgary, Calgary, AB, Canada

ã 2013 Elsevier Inc. All rights reserved.

Introduction 1The Global Water Cycle 1Surface Water and Groundwater 1Water Chemistry and Water Pollution 2Aquatic Biology 3Water Resources 3

Introduction

Water is essential to life and is the defining characteristic of Earth, the blue planet. Hydrology is the study of the global water cycle

and the physical, chemical, and biological processes involved in the different reservoirs and fluxes of water within this cycle. This

includes water vapor, liquid water, snow, and ice; indeed, one of the things that makes our planet unique is the fact that water can

be found in all three phases at Earth surface temperatures and pressures. It is the only common substance for which this is true.

In general, hydrologists focus on terrestrial water, while recognizing that the global hydrological cycle includes exchanges of

water between the land surface, ocean, atmosphere, and subsurface. Water in the oceans and atmosphere is mainly studied by

oceanographers and meteorologists, however, and these topics are discussed in the Oceanography and Atmospheric Sciences

sections of the Earth Systems and Environmental Science module. Many hydrologists work at the interface between land surface

water and the atmosphere, studying precipitation and evapotranspiration processes in the field of hydrometeorology. These topics

are discussed in the module on the Global Water Cycle. Other primary subject areas within the Hydrology section include Surface

Water, Groundwater, Aquatic Biology, Water Chemistry, Water Pollution, and Water Resources. This overview introduces each of

these realms of hydrological science.

The Global Water Cycle

The hydrological cycle describes the perpetual flux and exchange of water between different global reservoirs: the oceans,

atmosphere, land surface, soils, groundwater systems, and the solid Earth (Figure 1). Most of the world’s water – approximately

96.3% – is in the world’s oceans, where water molecules have an average residence time of about 3300 years. Glaciers and ice sheets

lock up more than half of the remaining water (Table 1), with 90% of this stored in the Antarctic Ice Sheet. Most of what remains

lies below the surface, in groundwater aquifers, where vast reserves of water are saline or difficult to access.

Freshwater in circulation, on which ecosystems and society so critically depend, therefore makes up only a tiny fraction of

Earth’s total water supply. Surface water constitutes only 0.02% of the global inventory, distributed between rivers, lakes, wetlands,

soils, and the biosphere. The United Nations Environmental Program (UNEP) estimates the global, accessible freshwater supply to

be about 200000 km3. This equates to about 29 million liters of water for each person on the planet. Global water supplies are

bountiful, though not easily accessed or equitably distributed.

Fluxes of water between reservoirs are indicated in Figure 2 and are discussed in the Global Water Cycle section of the ESES

module. There are high rates of turnover in the atmosphere, biosphere, soils, and rivers; the average lifetime of a water molecule in

the atmosphere is 9.2 days, and considerably less than this in the world’s rain belts. Once on the land surface, water can be stored for

extended periods in soils, lakes, groundwater aquifers, vegetation, and seasonal snowpacks. On an annual basis, however, discharge

from the world’s rivers is in near-equilibrium with global precipitation, returning what the ocean gives up through evaporation.

Surface Water and Groundwater

Physical hydrologists study the processes of water movement and storage on and beneath the land, exchanges between different

hydrological reservoirs, and interactions between water and other natural and human systems (e.g., in ecology, agriculture, or civil

waterworks). While surface water makes up a small fraction of the global water reservoir, a large number of hydrologists work in

this area. Subject areas within the SurfaceWater section of the module include consideration of soil water, wetlands, the cryosphere,

rivers, and lakes.

Large reserves of water are stored and routed through subterranean systems, with as much as one third of the world’s population

drawing from groundwater for essential municipal and household use. This includes about one third of the United States and 85%

of India, amongst other countries highly reliant on groundwater supplies. The Groundwater section of the module examines the

Reference Module in Earth Systems and Environmental Sciences http://dx.doi.org/10.1016/B978-0-12-409548-9.05356-2 1

Groundwater

1.75%

Biosphere 0.8%Atmosphere

9.4%

Rivers1.6%

Soil water12.2%

Wetlands8.5%

Surface water0.02%

Global water reserves

Ocean 96.3%

Lakes 67.5%

Surface water

Ice

1.93%

Figure 1 The global water inventory.

Table 1 The global water inventory (km3)

Reservoir Size (km3) World water (%) Freshwater (%)

All Surface

Oceans1 1285400000 96.30 � �Ice Sheets2 25470000 1.91 � �Glaciers2 270000 0.02 � �Permafrost3 22000 0.002 � �Groundwater4 23400000 1.75 � �Fresh 10530000 0.79 98.85Lakes 176400 0.01 � �Fresh 91000 0.007 0.85 74.5Rivers 2120 0.0002 0.02 1.7Soil water 16500 0.001 0.15 13.5Wetlands 11470 0.001 0.11 9.4Biosphere 1120 0.0001 0.01 0.9Atmosphere5 12700 0.001 � �Surface freshwater 122210 0.01 � 100.0Total freshwater 10652210 0.80 100.00 �Global total 1334782310 100.00 � �1Charette and Smith (2010), water only (salts removed, assuming a salinity of 3.5%).2Marshall (2011); glacier density of 900 kg m�3; Antarctic Peninsula classified as glaciers.3Median of Zhang et al. (1999) estimate of 11000–37000 km3 of ice (density 917 kg m�3).4Global estimates vary, making this the most uncertain term in the global water inventory.5Trenberth and Smith (2005).

Reproduced from Shiklomanov, I. (1993). World fresh water resources. In: Gleick, P.H. (eds.) Water in crisis: A guide to the world’s fresh water resources. New York: Oxford University

Press, with updates from other sources as indicated.

2 Hydrology

essential processes involved in subsurface water flow, the distribution and health of the world’s groundwater reserves, groundwater

chemistry, and geological considerations of groundwater science, also known as hydrogeology.

Water Chemistry and Water Pollution

While physical hydrologists focus on water quantity and supply, water quality is of fundamental concern for ecological and human

health. Enormous resources are committed to water monitoring, purification, desalination, and wastewater treatment, while access

to clean water and the prevalence of waterborne diseases are among the most serious issues that continue to face the developing

world. Subject areas in the Water Chemistry section of the module include water quality considerations, as well as broader

considerations of river, lake, and groundwater chemistry. This includes basic aspects of water chemistry, nutrient cycling in

lakes, aqueous organic chemistry, and environmental stresses on water chemistry, such as contaminants and acid rain.

Global water cycle

Advection

Advection

Condensation

Condensation Sublimation Snow/ice melt

Precipitation

Precipitation

Transpiration

Evaporation

Evaporation

RunoffRunoff

Infiltration

PercolationPlant uptake

Groundwater discharge

11

50

21

110

39

21

385424

39

495

Figure 2 The global water cycle, with fluxes in 1012 m3 yr�1 after the U.S. University Corporation for Atmospheric Research, https://spark.ucar.edu/longcontent/water-cycle, with updates from Durack et al. (2012). Graphic adapted from NOAA National Weather Service, http://www.srh.noaa.gov/jetstream/index.htm.

Hydrology 3

Aquatic Biology

Aquatic ecosystems support a wide range of organisms, including microorganisms, invertebrates, insects, plants, and fish. Some

hydrologists work in understanding the trophic systems within aquatic ecosystems and their health as a function of environmental

conditions such as water temperature and turbidity. Aquatic biodiversity is a major concern in water conservation and restoration

projects, as well as water resource management. Concern regarding the biological health of wetlands, rivers, and lakes has led to the

idea of ‘ecosystem services’ as a means to quantify or assess the value provided to society by different natural environments,

including aquatic environments. While this lens seems biased to the larger species that are of commercial value (i.e. fish), it is

understood that healthy waters require the full spectrum of organisms as part of an aquatic ecosystem. The section on aquatic

biology provides considerable detail on many of these species.

Water Resources

Water resource management includes consideration of all of the above disciplines of hydrology. Water supplies are allocated and

diverted to a range of agricultural, municipal, industrial, hydroelectrical, and ecological needs. Some of these water uses are

consumptive, removing water from the system (e.g., crop irrigation). Other types of water use return the water to a river, lake, or to

the ground, but the water often requires treatment to restore it to a natural state; sometimes this is not possible (e.g., industrial

tailings ponds).

The balancing act involved in water management includes a broad range of stakeholders and includes water policy and legal

experts. Hydrologists have essential input to these complex and sometimes confrontational deliberations and negotiations. They

also play a central role in applied hydrology – engineering of major waterworks to manage water. Water distribution systems have

been a hallmark of civilization since Babylon, and the modern stamp on this includes major hydroelectric dams and reservoirs,

urban waterworks, and water treatment facilities.

These and other tools help governments to manage water resources in a way that serves societal and ecological needs. However,

water resource management is one of the world’s greatest challenges due to competition for limited resources, regional disparities

in water supply and affluence, mounting global water demand, aquifer depletion, and pollution- and climate-change induced

water stress. Integrated sustainable water resource management is an area requiring innovation, progress, and international

cooperation in the coming decades.

4 Hydrology

Further Reading

Charette MA and Smith WHF (2010) The volume of Earth’s ocean. Oceanography 23(2): 112–114.Durack PJ, Wijffels SE, and Matear RJ (2012) Ocean salinities reveal strong global water cycle intensification during 1950 to 2000. Science 336: 455–458.Margat J (2008) Les eaux souterraines dans le monde. Paris: UNESCO BGRM/Editions.Marshall SJ (2011) The cryosphere. Princeton primers in climate science. N.J.: Princeton University Press, 282 pp.Shiklomanov I (1993) World fresh water resources. In: Gleick PH (ed.) Water in crisis: A guide to the world’s fresh water resources. New York: Oxford University Press.Trenberth KE and Smith L (2005) The mass of the atmosphere: A constraint on global analyses. Journal of Climate 18: 864–875.Yu L (2007) Global variations in oceanic evaporation (1958–2005): The role of the changing wind speed. Journal of Climate 20: 5376–5390.Zhang T, et al. (1999) Statistics and characteristics of permafrost and ground-ice distribution in the northern hemisphere. Polar Geography 23(2): 132–154.