Hydrogeologic Mapping

Hydrogeologic mapping is a method of gathering and evaluating geological information to create a three-dimensional depiction of the subsurface material in which groundwater occurs. By mapping the spatial distribution of geologic material with distinctive permeability, the hydrogeologist can understand which geologic units will allow movement of groundwater and which units will restrict groundwater movement.

Hydrologic Cycle

There are about 1,360 million cubic kilometers (326 million cubic miles) of water on Earth. Most of this water is stored in reservoirs such as the oceans, in glaciers and ice caps, underground, and in the atmosphere.


A close look at the world's international basins gives a sense of their significance in terms of area and conflict potential (see map). Their growing number is one indication of this.


The term "hydrosolidarity" describes an increasingly integrated approach to managing water resources that relies heavily on participation and coordination among community stakeholders, water-related management agencies, and local, state, and national governments. A fundamental goal of hydrosolidarity is the cooperative, unified management of shared water resources, whether at the national or the international level.

Ice Ages

Earth has cooled dramatically over the last 50 million years. Ice sheets, flora, and fauna all record those changes.

Ice at Sea

When sea water freezes, ice rarely forms as a continuous, mirror-like sheet. On calm water, the first pattern seen often is that resembling pancakes or lily pads.

Ice Cores and Ancient Climatic Conditions

In polar and alpine regions that are sufficiently cold, snowfall can persist from year to year and consequently can build up in layers. With time, the weight of the snow compresses the deeper snow layers to form solid ice, resulting in a sheet of ice and snow called a glacier.

Infrastructure, Water-Supply

Water-supply infrastructure consists of what is built to pump, divert, transport, store, treat, and deliver safe drinking water. In the United States, this infrastructure consists of vast numbers of groundwater wells, surface-water intakes, dams, reservoirs, storage tanks, drinking-water facilities, pipes, and aqueducts.

Instream Water Issues

Societies rely on fresh-water systems for a variety of purposes: drinking water, food, commerce, recreation, transportation, waste dilution and decomposition, and natural beauty. Rivers and streams are valued for the uses they provide to humans.

Integrated Water Resources Management

Integrated water resources management is the practice of making decisions and taking actions while considering multiple viewpoints of how water should be managed. These decisions and actions relate to situations such as river basin planning, organization of task forces, planning of new capital facilities, controlling reservoir releases, regulating floodplains, and developing new laws and regulations.

International Cooperation

Water, unlike most other natural resources, does not respect political boundaries. The natural flow of water, both on the Earth's surface and underground, routinely crosses these boundaries.

Irrigation Management

In prehistoric times mankind's food came from hunting, fishing, and gathering of wild fruits and berries. Cultivation of seeds was discovered about 5000 B.C.E.

Irrigation Systems, Ancient

Humans are newcomers to Earth, even though their achievements have been enormous. It was only during the Holocene epoch (10,000 years ago) that the development of agriculture occurred, keeping in mind that the Earth and solar system are 4.6 billion years old.

Islands, Capes, and Peninsulas

Islands, capes, and peninsulas are bodies of land that are partially or totally surrounded by water. They vary in size, but inevitably are prominent landmasses.

Isotopes: Applications in Natural Waters

Efforts to understand how natural waters evolve are aided significantly by isotope chemistry. The origin and type(s) of the water, the environment in which the water has resided, the types of reactions that the water has experienced, and the length of time the water has been removed from the hydrologic cycle are just some of the areas where isotopic chemistry has helped hydrologists understand natural waters.

Karst Hydrology

Karst is a term applied to areas where extensive dissolution of rock has led to the development of subterranean channels through which groundwater flows in conduits (enclosed or semi-enclosed channels). These conduits can vary in size from slightly enlarged cracks to tunnels many meters in diameter and many kilometers in length.

Lake Formation

A natural lake is a fairly large body of water occupying an inland basin (low-lying geographic area). Lakes cover only about 1 percent of the continents, and contain less than 0.02 percent of the world's water, but they are important ecosystems and may be sources of water supply in certain regions.

Lake Health, Assessing

The concept of a lake's health is somewhat easy to relate to humans, because they have an idea of what is healthy and well-functioning for humans and what is not. Nevertheless, defining a lake's health in the scientific world is not as clear, and is a subject of debate.

Lake Management Issues

A lake is a reflection of its watershed. Precipitation on the land that does not infiltrate the soil or collect in pools flows downhill, collecting in streams and rivers, which may in turn flow into lakes or reservoirs farther downstream.

Lakes: Biological Processes

The aquatic environment is shaped by complex interactions among a variety of physical, chemical, and biological factors. For example, physical factors such as climate, land topography, bedrock geology, and soil type influence the amount of water flowing in streams and discharging to lakes, as well as the types of materials (chemicals and particulates) found in the water.

Lakes: Chemical Processes

As precipitation flows over the land as runoff, it dissolves or otherwise carries a variety of materials to streams and lakes downgradient. Some of these materials include minerals, such as iron, sulfur, and phosphorus; gases, such as oxygen, carbon dioxide, and nitrogen; microorganisms; organic materials from plants and animals; and pollutants, such as pesticides, metals, and eroded soils.

Lakes: Physical Processes

Most lakes in temperate regions, due to a combination of solar heating and wind-mixing of surface layers, undergo a fairly predictable seasonal pattern, alternating between stratification (layering) and complete mixing.