Hydrology and Hydrogeology

Summary

Hydrology is a broad interdisciplinary science that includes the hydrologic cycle and global distribution of water in solid and liquid forms in the atmosphere, oceans, lakes, streams, and subsurface formations. Hydrogeology is a subset of hydrology that focuses on groundwater and related geologic factors governing its distribution and magnitude. Water shortages and increasing pollution in many countries have heightened concern about the availability and quality of water for many people worldwide.

Definition and Basic Principles

Hydrology is the science of water, a unique substance that affects all life on Earth. Although water could exist on Earth without life, life could not exist without water. Water is the most abundant liquid on Earth, covering 71 percent of the planet's surface in its liquid and solid forms. In humans, water constitutes about 92 percent of blood plasma, 80 percent of muscle tissue, 60 percent of red blood cells, and more than half of most other tissues.

Hydrology is a part of many scientific disciplines. The study of water in the atmosphere involves the fields of climatology and meteorology. The study of the hydrosphere includes the fields of physical geography, potamology (rivers), glaciology, cryology (snow, ice, and frozen ground), and limnology (lakes). The study of the lithosphere (the topmost rock layer of the earth) includes the fields of hydrogeology (groundwater location, movement, and magnitude), geomorphology (the science of surface processes and landforms on the earth), and limnology. Given the importance of water to plants and animals, hydrology includes the fields of silviculture (forestry), plant ecology, and hydrobiology (the biology of bodies of freshwater such as lakes).

Other fields related to hydrology by virtue of their strong connection to water resources include watershed management, potable water supply, wastewater management, irrigation, water law, political science (water policy), economics (costs of water projects), drainage, flood control, hydropower, salinity control and treatment, erosion and aspects of sediment control, navigation, lake and inland fisheries, and recreational uses of water.

Background and History

Credit for developing part of the hydrologic cycle, a fundamental component of hydrology, goes to Marcus Vitruvius Pollio, a Roman engineer and writer during the reign of the emperor Augustus, who developed a theory that groundwater is mostly recharged by precipitation infiltrating the ground. This early theory was bolstered by Leonardo da Vinci and Bernard Palissy during the sixteenth century. The seventeenth century was a period of measurement when scientists studied precipitation, evaporation, and stream discharge in the Seine River in France. Hydraulic studies developed during the eighteenth century. In the nineteenth century, the active area of investigation was experimental hydrology, particularly in stream-flow measurement and groundwater. The US government created several important agencies, including the US Army Corps of Engineers in 1802, the US Geological Survey in 1879, and the US Weather Bureau (now the National Weather Service) in 1891.

The nineteenth and twentieth centuries witnessed the increasing use of statistical and theoretical analysis in hydrologic studies. One example of this was the development of the bed-load function in sedimentation research in 1950 by Hans Albert Einstein, the son of Albert Einstein. Research has benefited from the increasing use of computers that can handle larger amounts of data in shorter periods. Several types of sophisticated statistical packages can assist in analyzing increasingly complicated studies in hydrology and hydrogeology.

How It Works

The Hydrologic Cycle. The never-ending circulation of water and water vapor over the Earth is called the hydrologic cycle. This continuous circulation affects all three parts of the global system—the water spreading over Earth's surface (the hydrosphere), the gaseous envelope above the hydrosphere (the atmosphere), and the rock layer beneath the hydrosphere (the lithosphere). The Sun's energy and gravity power this circulation that has no beginning or end.

The oceans cover 71 percent of Earth's surface and account for 86 percent of the moisture in the atmosphere. The evaporated water transported into the atmosphere can travel tens to hundreds of miles before it is returned to the Earth as rain, hail, sleet, snow, or ice. When precipitation gets closer to the Earth's surface, the water may be intercepted and transpired by vegetation, or it can reach the ground surface and eventually flow into streams or simply infiltrate into the ground. Much of the water that reaches plants and the runoff flowing in streams evaporates back into the atmosphere. A portion of the water that infiltrates the ground may penetrate deeper layers in the Earth to become groundwater. This groundwater may return to the streams as the baseflow component of runoff, which will eventually flow into the oceans and evaporate into the atmosphere to complete the hydrologic cycle. Baseflow consists of prior precipitation that accumulated in the watershed and subsurface runoff of current precipitation.

Urbanization and Stream Flow. The expansion of cities into open spaces outside the metropolitan area has strongly affected local streams. As impervious cover—in the form of houses, roads, driveways, and large parking areas for shopping malls and office buildings—increases, larger and larger areas of previously water-penetrable surfaces become impervious. Depending on local land-use regulations, impervious cover can easily approach or exceed 80 percent of the total area. The immediate effect of this high impervious percentage is to reduce the amount of water that can infiltrate into the ground. It results in increased overland flow.

Storm drainage systems can also affect stream flow, as runoff is deliberately directed into nearby streams. This rapid exiting of water from the increased impervious area can quickly reduce the lag time between precipitation input and flood runoff. The resultant increase in the stream hydrograph invariably gives rise to peak discharge flows that result in local and regional flooding.

In the light of flooding problems associated with an increase in impervious cover, some counties and states have required new developments, particularly in suburban areas, to give up part of their site for detention basins. These structures are designed to detain, for varying amounts of time, the excess runoff that the new buildings and roads on the site will generate. The basins can reduce flooding and allow sediment to settle out, improving downstream water quality.

Applications and Products

Flow-Duration Curves. One example of the type of analysis commonly employed in hydrogeologic research is to study the variability of stream flow in watersheds with lithologic heterogeneity (differences in their rock formations). The physical attributes of watersheds affect stream-flow variability. Some formations found on coastal plains have large amounts of sand with high infiltration rates and high groundwater yields. Other geologic formations, such as basalt, diabase, and granite, have low infiltration rates and consequently have low groundwater yields. Indeed, the differences in water yield between formations can approach an order of magnitude.

One useful technique that developed in the twentieth century was to use the low-flow and high-flow ends of flow-duration curves to provide helpful information about the hydrogeologic characteristics of any watershed. The flow-duration curve is a cumulative frequency curve showing the time that specified stream discharges were equaled or exceeded. The values are plotted on logarithmic-probability graph paper with discharge in cubic feet per second on the y-axis (ordinate) and the frequency that specified discharges were exceeded in percent on the x-axis (abscissa). The slope of the curve provides a measure of temporal variability—the steeper the curve, the more variable the value plotted. Steeply sloping curves indicate a flashy stream, where the flow is derived mostly from direct runoff, indicating minimal groundwater storage. As a result, that watershed has limited potential for ample groundwater supplies.

Water-Quality Issues. Concerns have long been raised in the scientific community about releasing pharmaceuticals from manufacturing facilities into surface waters. In an early twenty-first-century study, the US Geological Survey (USGS) found that the water released into surface waters by two wastewater treatment plants in New York that received 20 percent of their wastewater from nearby pharmaceutical plants had concentrations of drugs that were ten to one thousand times higher than the water released from twenty-three other plants in the United States that were not treating any waste from pharmaceutical plants. A sampling of the maximum concentrations in the outflows from the two New York plants were 3,800 parts per billion (ppb) for the muscle relaxer metaxalone and 1,700 ppb and 400 ppb, respectively, for oxycodone and methadone, both opioid pain relievers. In stark contrast, the twenty-three plants that did not receive any wastewater effluent from pharmaceutical facilities reported concentrations of less than 1 ppb for these drugs. These early studies continued to be confirmed in the 2010s and 2020s. Studies conducted by the Geological Survey, Environmental Protection Agency, and the National Ground Water Association found that nearly half of all American drinking water contains at least one harmful chemical. In remote areas, the chances of drinking water testing positive were only around 25 percent, but in urban areas, the probability of finding the presence of harmful chemicals was 75 percent.

Treated effluent from wastewater treatment plants is routinely discharged into streams that flow downstream to one or more water treatment plants that distribute potable water to their service areas. A prime example is New Orleans, located close to the mouth of the Mississippi River, the largest drainage basin in North America, and downstream from many wastewater treatment facilities.

The problem is that water containing a growing number of pharmaceuticals is entering wastewater treatment plants that are not equipped to remove them. The issue is compounded by the comparable lack of techniques available to water supply treatment facilities. This water-quality issue will most probably increase in importance.

Specific Capacity. Determining specific capacity is a useful procedure to evaluate the magnitude of a well's expected yield. It is obtained by simply dividing the tested well yield in gallons per minute by the drawdown of the well in feet (gpm/ft). Drawdown measures how much the water table is lowered as a well is pumped. The purpose of this test is to ensure that the well can sustain a required minimum yield over the long term.

Pump tests for residential wells should take at least four hours, although some communities require six hours or more. Most states require a minimum pumping rate of 0.5 gpm/ft for residential use. In addition, the original static level of the water table should recover twenty-four hours after the end of the test. For large-scale commercial and industrial users, many states have more stringent standards for pumping, such as a minimum testing period of at least forty-eight hours.

Specific capacity values can vary over several orders of magnitude, from less than 1 gpm/ft to more than 100 gpm/ft, depending on the type of geologic formations present. High permeability and porosity usually result in high specific capacity values, and the converse is expected if the formations are either poorly fractured or have low permeability and porosity.

Water Use. The USGS began estimating water use every five years beginning in 1950. Its reports present a large amount of data on a wide selection of water-use categories. The USGS collects information from all fifty states in the United States, as well as the District of Columbia, Puerto Rico, and the US Virgin Islands.

Some water-use categories have changed over the years, but the overall data collected are very useful. For example, the public supply category pertains to water that is furnished to at least 25 people or serves at least fifteen connections. The distributed water category includes domestic, commercial, and industrial users and contains estimates about system losses, such as leaks and the flushing of pipes. Surface water (streams and lakes) accounts for about two-thirds of the public water supply. The remaining one-third comes from groundwater sources. In New Jersey, the most densely populated state in the nation, around 12 percent of the population uses their own wells for water. This relatively large percentage has remained about the same since the 1980s. This situation presumably reflects large-lot zoning and the high cost of bringing water from distant suppliers to isolated clusters of a few homes.

Water Disputes. Conflicts over the world's water resources have been numerous and lengthy. The list of water conflicts goes back several thousand years, and the growing disparity between well-watered and poorly-watered countries means future conflicts are likely.

The hydrologic imbalance in water supplies is physically based, but social, economic, and political factors play an essential role. Countries with abundant headwater streams can build large dams and use the water for irrigation, resulting in less flow to downstream countries. For example, the headwaters of the Tigris and Euphrates rivers start in Turkey, and any diminishment in flow through Syria and Iraq en route to the Persian Gulf would affect downstream agriculture. Another well-known example is Egypt's almost total dependence on the Nile River, which begins in Ethiopia (Blue Nile) and Lakes Albert and Victoria (White Nile) in east-central Africa. A significant diversion of the waters of the Nile by the upstream states would have a substantial impact on Egypt.

Continued growth in irrigation and population in the semi-arid portion of the southwestern United States has led to severe problems for the Colorado River. The drainage area of the Colorado River is 246,000 square miles (637,137 square kilometers) and flows for 1,450 miles (2,335 kilometers) from its headwaters in the Rocky Mountains in Colorado, Wyoming, and New Mexico through Utah, Arizona, Nevada, and California before emptying into the Gulf of California in Mexico. Although the seven states and Mexico have made various agreements pertaining to water allocation, numerous problems have developed. They will likely worsen in future years, as the initial allocation in the early twentieth century used an average flow based on precipitation values above normal. The allocations would have to be reduced when drier or even more normal precipitation cycles return, resulting in manifold problems for large users in the basin.

Careers and Course Work

Given the interaction that water has with a wide array of disciplines and subdisciplines, anyone entering the field of hydrology can pursue a variety of paths to become proficient in the subject. Although few academic institutions have a hydrology department, many offer courses in hydrology within academic departments such as civil engineering, geology, physical geography, and environmental science.

The course path for students is determined by their interests. For example, students interested in hydrogeology would most likely major in geology and possibly minor in geography, so that they could take courses in physical geography, cartography, and geographic information systems (GIS). Students interested in water-quality issues would be drawn to a major in environmental science, with chemistry and biology as suitable minors. Other useful courses include statistics, economics, civil engineering, mathematics, meteorology, and climatology.

Employment opportunities in hydrology include positions in federal, state, and local government, state water project associations (such as the Central Arizona Project), bi-state river basin commissions (such as the Delaware River Basin Commission), industry, professional associations, and nonprofit watershed associations that act as guardians for their local drainage area. Teaching and research positions are also available at colleges and universities.

Social Context and Future Prospects

Providing sufficient clean water for the world's growing population is vital to human survival. The growing importance of an adequate water supply is demonstrated by the increasing number of books, articles, meetings, and commentaries. The Pacific Institute, established in 1987 in Oakland, California, focuses on water issues, including water and human health, controversies over large dams, freshwater conflicts, climate change and water resources, new water laws and institutions, and efficient urban water use. It conducts research, publishes reports, including the biennial series The World's Water, and works with stakeholders to develop solutions.

The prospects for hydrologic research and technological change are good. For example, desalination plants operate in coastal locations in Saudia Arabia, the United Arab Emirates, and Dubai, with minimal alternative water sources. More than two hundred municipal desalination plants operated in the United States, mainly in California, Florida, and Texas. A technological invention in a type of drip irrigation in Israel addresses scarcity and salinity problems in water-short areas. Israel is primarily a desert climate with a scarce water supply. However, they historically produce 20 percent more water than needed. This irrigation method is used in more than half of the irrigated land in Israel. It is also used in California, particularly for high-value crops such as orchards and vineyards.

Unlike most other resources on Earth, water is renewable. Technology and innovative thinking can play essential roles in ensuring adequate clean water supplies for a growing population. For example, the recognition that storm-water runoff, gray water (from dishwashers and washing machines), and reclaimed wastewater could be used for landscape irrigation and some industrial processes resulted in significant water conservation.

In April 2024, the US Environmental Protection Agency (EPA) debuted the first legally enforceable drinking water standards concerning perfluoroalkyl and polyfluoroalkyl substances (PFAS)—National Primary Drinking Water Regulations (NPDWRs). PFAS, sometimes called "forever chemicals," are manmade chemicals that are common in everyday products and are toxic to the human body. This policy requires public water systems to be transparent and vigilant about the amount of PFAS in their customers' drinking water.

Bibliography

Cech, Thomas V. Principles of Water Resources: History, Development, Management, and Policy. 4th ed., John Wiley & Sons, 2018.

Centers for Disease Control and Prevention. "Water Sources." US Department of Health and Human Services, 6 Apr. 2022, www.cdc.gov/healthywater/drinking/public/water‗sources.html. Accessed 10 June 2022.

Fetter, Charles W. Applied Hydrogeology. 5th ed., Prentice Hall, 2022.

Gleick, Peter, editor. The World’s Water. Volume 8: The Biennial Report on Freshwater Resources. Island Press, 2014.

Hiscock, Kevin M., and Victor F. Bense. Hydrogeology: Principles and Practice. 3rd ed., John Wiley & Sons, 2021.

Powell, James L. Dead Pool: Lake Powell, Global Warming, and the Future of Water in the West. 2008. U of California P, 2020.

Spellman, Frank R. The Science of Water: Concepts and Applications. 4th ed., CRC Press, 2022.

Strahler, Alan. Introducing Physical Geography. 6th ed., John Wiley & Sons, 2015.

"U.S. EPA Announces New National Primary Drinking Water Regulations for PFAS." Sidley, 25 Apr. 2024, environmentalenergybrief.sidley.com/2024/04/25/u-s-epa-announces-new-national-primary-drinking-water-regulations-for-pfas. Accessed 20 May 2024.

Water Resources. "Domestic (Private) Supply Wells." US Geological Survey, 1 Mar. 2019, www.usgs.gov/mission-areas/water-resources/science/domestic-private-supply-wells. Accessed 10 June 2022.