1. 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.
2. 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.
The global water inventory
The global water cycle
3. 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 Surface Water 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 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.
4. 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.
5. 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.
6. 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.
