AIH Professional Hydrologist - Surface Water Examination key terms

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

The land area that drains all precipitation falling on it to a single outlet point along a stream or river. Boundaries are defined by topographic divides and control downstream response.

The ridge or boundary line separating adjacent watersheds, determined from topography or groundwater flow paths. Water on opposite sides of the divide flows to different outlets.

Water falling from the atmosphere to the ground as rain, snow, sleet, or hail. It is the primary input to most surface water and groundwater systems.

A line drawn on a map connecting points of equal precipitation depth over a period. It is used to estimate areal average rainfall by contour interpolation.

A method of estimating areal rainfall by assigning each rain gauge a polygon of influence based on nearest-gauge distance. The area-weighted gauge values are averaged.

The average depth of precipitation over a watershed or region for a specified period. It is computed from point gauges using methods such as Thiessen polygons, isohyets, or inverse-distance weighting.

The process by which liquid water changes to vapor at a free water surface or from soil. It removes water from lakes, streams, and wet surfaces and is driven by energy and vapor gradients.

The release of water vapor to the atmosphere through plant stomata as water moves from soil into roots and leaves. It is a major loss term in vegetated watersheds.

The combined loss of water to the atmosphere from evaporation of water surfaces and soil plus transpiration by plants. It is often the largest component of the water balance in humid regions.

The maximum evapotranspiration that would occur from a well-watered, vegetated surface under given atmospheric conditions. It represents an atmospheric demand, not an actual flux limited by soil moisture.

Precipitation captured by vegetation and other surfaces that evaporates before reaching the ground. It reduces the depth of water available for infiltration and runoff.

Precipitation that reaches the ground after passing through or dripping from a plant canopy. It differs from stemflow and from intercepted water that evaporates.

Precipitation that is intercepted by vegetation and flows down stems and trunks to the ground. It concentrates water at the base of plants and can enhance local infiltration.

The downward entry of water from the ground surface into soil. Its rate controls how much rainfall becomes surface runoff versus soil water recharge.

The downward movement of water through soil and rock layers below the root zone. It is the process that recharges groundwater and feeds baseflow to streams.

Water held within the pore spaces of the unsaturated soil zone. It sustains plant growth and controls how much rainfall infiltrates or runs off.

The water content remaining in soil after excess water has drained by gravity and drainage has become negligible. It represents the upper limit of plant-available water under free drainage.

The soil water content below which plants can no longer extract water and wilt permanently. Water held more tightly than this is unavailable to vegetation.

The amount of water a soil can store and release to plants, calculated as the difference between field capacity and wilting point. It is used in irrigation and water balance planning.

Water that flows over the land surface or through channels toward a downstream outlet. It includes both surface flow and delayed contributions from soil and groundwater.

The portion of rainfall that travels over the ground surface to streams and channels without infiltrating. It is a fast-responding component of streamflow during storms.

The sustained portion of streamflow derived from groundwater discharge and delayed soil drainage. It maintains flow between storms and is critical for dry-season water supply.

A graph showing stream discharge or stage plotted against time at a specific location. Its shape reflects rainfall input and the watershed's storage and routing characteristics.

The direct runoff hydrograph produced by one unit of effective rainfall applied uniformly over a watershed in a specified duration. It is used to predict storm runoff from rainfall.

The portion of precipitation that becomes direct runoff after losses such as interception, infiltration, and depression storage are removed. It drives the runoff hydrograph.

The time required for water to travel from the most hydraulically distant point in a watershed to the outlet. It governs the timing of peak discharge and the critical storm duration.

The maximum rate of flow occurring during a storm or flood event at a given location. It is a key design value for culverts, bridges, and flood control structures.

The declining limb of a hydrograph after rainfall has ended, reflecting drainage of stored water from the watershed. Its shape is used to estimate baseflow and groundwater contributions.

A statistical method that relates the magnitude of peak flows to their probability of exceedance using observed records. It estimates design floods for specified return periods.

The average interval of time between events of a given magnitude, equal to the reciprocal of annual exceedance probability. A 100-year flood has a 1 percent chance of occurring in any year.

A hypothetical rainfall event with specified depth, duration, and temporal distribution used for hydraulic design. It is paired with a runoff method to estimate design flows.

A relationship between water level (stage) and discharge at a stream gauge, developed from measurements. It allows continuous discharge to be estimated from recorded stage.

An instrument or station that measures and records the water level or discharge of a stream. Records from gauges support flood analysis, water supply planning, and model calibration.

A saturated geologic formation that can store and transmit enough water to supply wells or springs. Its productivity depends on porosity, permeability, and thickness.

A groundwater formation bounded above and below by low-permeability layers, so water is under pressure greater than atmospheric. Water levels in wells rise above the top of the aquifer.

An aquifer whose upper boundary is the water table, which is free to rise and fall under atmospheric pressure. It is recharged directly by infiltration from the land surface.

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

An imaginary surface representing the hydraulic head in a confined aquifer, defined by water levels in wells. It indicates the direction and magnitude of groundwater flow.

A well drilled into a confined aquifer in which the water level rises above the top of the aquifer without pumping. Flow occurs at the surface where the potentiometric surface exceeds land elevation.

The fraction of a rock or sediment volume that consists of open pore space. It determines the maximum volume of water a material can hold.

The ability of a porous medium to transmit water under a hydraulic gradient, reflecting the connectivity of pores. It controls how readily water moves through soil and rock.

A coefficient describing the rate at which water flows through a porous medium under a unit hydraulic gradient. It depends on both the medium and the fluid properties.

The change in hydraulic head per unit distance along a flow path. It is the driving force for groundwater flow and is used in Darcy's law calculations.

The principle stating that groundwater discharge is proportional to hydraulic conductivity, cross-sectional area, and hydraulic gradient. It is the foundation of groundwater flow analysis.

The volume of water released from storage per unit surface area of an unconfined aquifer when the water table declines one unit. It is less than porosity because some water is retained by capillary forces.

The volume of water released from or taken into storage per unit surface area of an aquifer per unit change in head. It governs how much water an aquifer yields under pumping.

The addition of water to an aquifer from precipitation, stream leakage, or irrigation return flow. It replenishes groundwater storage and sustains baseflow to streams.

The removal of water from an aquifer through pumping, springs, seepage to streams, or evaporation. It balances recharge in a steady-state groundwater system.

The downward-curving water table or potentiometric surface around a pumping well, formed by groundwater drawdown. Its shape and extent depend on pumping rate and aquifer properties.

The decline in water level in a well or aquifer caused by pumping or other groundwater discharge. It is measured relative to the pre-pumping water level.

The movement of saline water into freshwater aquifers, often caused by excessive pumping that reverses natural seaward flow. It degrades water quality and can render wells unusable.