Together, hydrology and hydrogeology provide information on how to manage and protect freshwater, humans most essential natural resource. Hydrology and hydrogeology are distinct fields of study that employ different methods and techniques, but they overlap to provide a complete picture of Earth’s freshwater resources.
Hydrology is a branch of engineering that deals with the physical properties of surface freshwater, such as lakes and rivers, and with its chemical interactions with other substances. Hydrogeology is a subfield of geology (study of Earth) that, by definition, specifically addresses groundwater—water moving through tiny openings in rock and soil layers beneath the land surface.
In practice, ground and surface water interact as a single system. Surface water seeps into the ground and groundwa-ter emerges to the surface. Hydrogeologists work to explain the geological effects of surface water in rivers, streams and lakes, and hydrologists lend their technical expertise to the mechanics and chemistry of moving groundwater.
Hydrologists use mathematics and experimental techniques to determine water’s general properties, to make specific observations of freshwater environments, and to design water management systems that contain and direct water.
Britain’s Centre for Hydrology, a government environmental research agency, describes its mission as an effort to answer two questions about the Earth’s freshwater: Why is the natural environment as it is? and What is it likely to be in the future?
Today, humans, not natural processes, manage the flow, distribution, and allocation of almost all of Earth’s surface waters. Dams, levees, and reservoirs (natural or man-made lakes) direct and contain the water of the world’s largest and most heavily used river systems: the Nile, Yangtze, Amazon, Ganges, and Mississippi. U.S. Army Corps of Engineers hydrologists, along with their counterparts at the Bureau of Reclamation (which has jurisdiction over rivers west of the Rockies), control the flow and distribution of all the surface waters of the United States.
They collect numerous measurements of water system conditions such as rainfall totals, lake and reservoir levels, river discharges (the volume of water that flows through a river in a given time), and current speeds, air and water temperatures, and humidity (amount of water vapor in the atmosphere) at specific sites. Then they merge the data into a computerized model that shows how water has moved through the region’s various freshwater reservoirs in the past.
Once a hydrologist has constructed a model of a watershed (land area that contributes water to a stream, lake, or aquifer) or reservoir (body of water), the hydrologist tests it by comparing its calculated predictions with actual results in the natural environment. The model is adjusted to better match the real world. The more times the scientist repeats the process of adding new data to the model and retesting it, the more accurate it becomes.
Computer models help hydrologists predict the ways that natural and man-made changes like droughts (uncommonly dry weather), heavy snows and rains, and new dams across rivers affect water supplies and flows in the future. Many models also include information about the way water physically and chemically interacts with rocks and minerals on its path through the system. These models can be used to track contaminants and predict water quality changes over time.
One example of how hydrologists’ work can affect a regions water supply is in the city of Denver, Colorado when the city had very low reserves of freshwater in its reservoirs. There had been lower than average snowfall in the nearby Rocky Mountains and the city’s water supplies ran low without their usual influx of spring melt water.
Hydrologists at the U.S. Geological Survey in Denver used data and computer models to help the city distribute limited water during the drought period, and to plan for future dry periods. Models of Denver’s surface water flows and seasonal patterns also help water managers protect and regulate the city’s water supply from contamination by such human waste products as agricultural fertilizers and chemicals, industrial wastes, and sewage.
Hydrogeologists are concerned mostly with groundwater and how geologic features affect groundwater storage, flow, and replenishment. Like other geologists, they use observations of rock types and geologic structures on the land surface together with subsurface samples to map folded, faulted (broken), and fractured (cracked) rock layers and bodies beneath the land surface.
Once a hydrogeologist has mapped out the dimensions, physical characteristics, and "plumbing" of a groundwater reservoir, he or she sets out to understand how water moves through the system. Measurements of flows at recharge and discharge points show the rates at which water is entering and leaving the aquifer as well as the time an average water molecule (smallest part of water that has its properties) takes to travel through the system (residence time.) Water levels and pressures in wells indicate flow patterns and rates inside the aquifer.
Groundwater can become polluted. Hydrogeologists also use their understanding of groundwater flow patterns to predict how contaminants might enter an aquifer, move through the system, and reemerge in a distant spring or well. Sometimes an aquifer or soil layer acts as a filter that improves water quality as contaminants move through an aquifer.
Others transport polluted water quickly to a discharge site. Some rock layers even contribute hazardous dissolved chemicals to the ground-water. Hydrogeologists collect water samples and monitor water quality within aquifers. They also conduct laboratory and computer experiments to better understand groundwater’s chemical interactions.
Cities and regions that depend on groundwater require detailed hydrogeologic maps, and a good understanding of how water moves through their aquifer, to effectively manage groundwater resources. In many states and countries, ground-water is common, public property.
Most rules and regulations regarding groundwater consumption and contamination were written in an era when groundwater systems were poorly understood and were considered unending sources of clean freshwater.
Today, many regions with large human populations and fragile natural ecosystems (communities of plants and animals) depend on limited, shared groundwater resources. As such, the actions of some of a groundwater reservoir’s users can negatively affect the water supply and quality for other people, as well as for the aquifer’s plants and animals.