AIH Fundamentals of Hydrology Examination key terms

The continuous movement of water among the atmosphere, land surface, oceans, and subsurface through evaporation, precipitation, runoff, infiltration, and storage. It is the organizing framework for all hydrologic study.

The land area that drains to a common outlet such as a river reach or lake. Its boundary follows topographic divides, so all precipitation inside it flows toward that outlet.

Water reaching the ground from the atmosphere as rain, snow, sleet, or hail. It is the primary input to most hydrologic budgets and is measured by gauges and remote sensing.

The process by which liquid water at a surface or in soil pores changes to vapor and enters the atmosphere. It is driven mainly by solar energy, temperature, humidity gradient, and wind.

The release of water vapor to the atmosphere through plant stomata as plants draw moisture from the soil. It is a major pathway of water loss in vegetated watersheds.

The combined loss of water to the atmosphere from evaporation at soil and water surfaces plus transpiration by plants. It is often the largest outflow term in a watershed water balance.

The maximum evapotranspiration that would occur from a well-watered, fully vegetated surface under given atmospheric conditions. It is an upper bound; actual evapotranspiration falls below it when water is limiting.

The downward entry of water from the ground surface into soil. Its rate depends on soil texture, structure, antecedent moisture, and surface cover, and it controls how much rainfall becomes runoff.

The downward movement of water through soil and rock layers toward the water table. It is the mechanism that recharges groundwater after infiltration has wetted the unsaturated zone.

Water from precipitation that flows over the land surface or through shallow subsurface paths toward streams, lakes, or other water bodies. It is the portion of rainfall not lost to infiltration and evapotranspiration.

Precipitation caught by vegetation, buildings, or other surfaces and held before reaching the ground, where it later evaporates or drips off. It reduces the water available for infiltration and runoff.

Water held within the pore spaces of the unsaturated soil zone. It supports plant growth and governs how much additional rainfall can infiltrate before runoff begins.

The water content remaining in soil after gravity drainage has largely ceased, typically a few days after saturation. It represents water held by capillary forces that plants can readily use.

The soil moisture content below which plants can no longer extract water and wilt permanently. It marks the lower limit of plant-available water in the root zone.

A graph showing streamflow or water level plotted against time at a given location. Its shape reflects rainfall input, watershed characteristics, and storage, and it is used to estimate peak flow and volume.

The direct runoff hydrograph produced by one unit of effective rainfall (such as one inch or one centimeter) falling uniformly over a watershed in a specified duration. It is used to derive hydrographs for other rainfall amounts by scaling and superposition.

The travel time for water to flow from the most hydraulically distant point in a watershed to the outlet. It is used to select storm duration and estimate peak discharge in many design methods.

The maximum rate of streamflow occurring during a storm or flood event, usually expressed as volume per unit time. It is a key design value for culverts, bridges, channels, and floodplain mapping.

The sustained portion of streamflow derived from groundwater discharge and delayed subsurface storage rather than direct storm runoff. It keeps streams flowing between storms and during dry periods.

Water that travels over the ground surface to a stream or other water body, often after rainfall exceeds the infiltration capacity or the soil is saturated. It responds quickly to storms.

A graph or equation relating water level (stage) to discharge at a specific stream cross-section. It is developed from field measurements and lets continuous stage records be converted to flow.

The elevation or height of the water surface in a stream measured relative to a fixed reference datum. Continuous stage records, converted through a rating curve, provide streamflow estimates.

The average interval, in years, between events of a given magnitude or larger, such as the 100-year flood. It expresses probability, not a guaranteed schedule of occurrence.

The low-lying land adjacent to a river that is inundated during high flows. It stores and conveys floodwater and is commonly mapped using return-period flood elevations for land-use regulation.

An instrument that collects and measures the depth of precipitation at a point. Networks of gauges provide the rainfall data used to drive hydrologic models and water-balance calculations.

A geologic formation or group of formations that stores and transmits groundwater in usable quantities. Productive aquifers typically consist of sand, gravel, sandstone, or fractured rock.

A low-permeability layer of clay or unfractured rock that restricts but does not fully stop groundwater flow between adjacent units. It can confine an aquifer beneath it and slow recharge and discharge.

The upper surface of the saturated zone in an unconfined aquifer, where pore water pressure equals atmospheric pressure. Its depth and shape vary with recharge, discharge, and pumping.

An aquifer whose upper boundary is the water table and which is recharged directly by downward percolation from the land surface. Its saturated thickness changes as water levels rise and fall.

An aquifer bounded above and below by low-permeability layers, so its water is under pressure greater than atmospheric. A well tapping it may flow without pumping if the potentiometric surface is above ground.

The imaginary surface representing the level to which water in a confined aquifer would rise in tightly cased wells. It indicates the hydraulic head and the direction of groundwater flow.

A well drilled into a confined aquifer in which water rises above the top of the aquifer, and may flow at the surface, because of internal pressure. It requires no pumping when the potentiometric surface exceeds land elevation.

A measure of a porous medium's ability to transmit water, reflecting both the material and the fluid's properties. Higher values mean water moves more readily; it is central to flow and contaminant-transport calculations.

The intrinsic capacity of a porous material to allow fluids to pass through its connected pore spaces, independent of fluid properties. It is closely related to, but distinguished from, hydraulic conductivity.

The fraction of a material's total volume that consists of open pore space. It indicates how much water a rock or soil can hold, though not how readily that water moves.

The volume of water an unconfined aquifer releases from storage per unit decline in water table, expressed as a fraction of aquifer volume. It is always less than porosity because some water clings to grains.

The total mechanical energy per unit weight of groundwater at a point, combining elevation and pressure components. Differences in head between points drive groundwater flow.

An empirical relationship stating that groundwater discharge through a porous medium is proportional to the hydraulic gradient and the cross-sectional area, with hydraulic conductivity as the constant. It underpins most quantitative groundwater flow analysis.

The addition of water to an aquifer, chiefly by downward percolation from precipitation or surface water. It replenishes storage and sustains water tables and baseflow.

The natural or artificial outflow of groundwater from an aquifer, such as seepage to streams, springs, lakes, wetlands, or wells. It balances recharge over the long term and feeds baseflow.