Hydrology

History

Hydrology has been investigated and applied for millennia. Ancient Egyptians used basin irrigation. Mesopotamian towns were protected by high earthen walls.

Greeks and Romans built aqueducts; the Chinese built irrigation and flood control works. Ancient Sinhalese constructed complex irrigation works in Sri Lanka, inventing the valve pit (bisokotuwas) for large reservoirs, anicuts, and canals still functioning. In the first century BC, Marcus Vitruvius described a philosophical theory of the hydrologic cycle where mountain precipitation infiltrated to form lowland streams and springs.

Leonardo da Vinci and Bernard Palissy independently reached an accurate representation of the cycle. Quantification of hydrologic variables began in the 17th century. The 18th century brought the Bernoulli piezometer and Bernoulli's equation by Daniel Bernoulli, and the Pitot tube by Henri Pitot. The 19th century saw Darcy's law, the Dupuit-Thiem well formula, and Hagen-Poiseuille's capillary flow equation.

In the 20th century, rational analyses replaced empiricism, and government agencies began hydrological research. Key contributions included Leroy Sherman's unit hydrograph, Robert E. Horton's infiltration theory, and C.V. Theis' aquifer test/equation. Since the 1950s, hydrology has gained a more theoretical basis, aided by advances in physical understanding and computers, especially geographic information systems (GIS).

Branches

Hydrology has many specialized branches. Chemical hydrology looks at water's chemical makeup. Ecohydrology explores how living things interact with the water cycle. Hydrogeology deals with where groundwater is and how it moves.

Hydrogeochemistry studies how water dissolves minerals from the ground and how that changes the water's chemistry. Hydroinformatics applies computer technology to water resource problems. Hydrometeorology examines how water and energy move between the land, water surfaces, and the lower sky.

Isotope hydrology uses water's isotopic fingerprints. Surface hydrology covers water processes on or near the ground. Drainage basin management involves storing water in reservoirs and controlling floods. Water quality concerns the chemistry of rivers and lakes, including both pollution and natural substances.

Applications

Hydrology is applied to calculating rainfall, evapotranspiration, surface runoff, and precipitation. It determines the water balance of a region and agricultural water balance. Applications include designing riparian-zone restoration projects, mitigating and predicting flood, landslide, and drought risk, and real-time flood forecasting and warning. It supports designing irrigation schemes, managing agricultural productivity, and providing drinking water.

Hydrologists design dams for water supply or hydroelectric power, bridges, sewers, and urban drainage systems. The field analyzes impacts of antecedent moisture on sanitary sewer systems, predicts geomorphologic changes like erosion or sedimentation, and assesses contaminant transport risk to establish environmental policy guidelines. It estimates water resource potential of river basins and supports water resources management and engineering.

Themes

The central theme of hydrology is that water circulates through Earth via different pathways and rates. Water evaporates from the ocean, forms clouds, drifts over land, and produces rain. Rainwater flows into lakes, rivers, or aquifers, then either evaporates or returns to the ocean, completing a cycle while changing state. Research concerns water movement between states, within a state, or quantifying amounts in a region.

Some hydrology develops direct measurement methods; others model processes for scientific knowledge or practical prediction. Groundwater, often pumped for drinking, is studied in hydrogeology, which quantifies groundwater flow and solute transport. Describing the saturated zone involves characterizing aquifers by flow direction, groundwater pressure, and depth, measured with a piezometer. Aquifers are described by hydraulic conductivity, storativity, and transmissivity, with geophysical methods for characterization.

The vadose zone also presents problems. Infiltration is water entering soil; some is absorbed, the rest percolates to the water table. Infiltration capacity depends on factors including saturated layer resistance, hydraulic head, capillary action, compaction, surface cover, and temperature. Soil moisture is measured by capacitance probe, time domain reflectometer, tensiometer, solute sampling, or geophysical methods.