actual evapotranspiration

the real rate of evapotranspiration from a land surface.

alternate depths

the two physically allowable water depths for flow in a channel for a given specific energy and a given specific discharge.


a material whose properties (such as intrinsic permeability) depend on the direction of measurement. 


a saturated geological formation that may contain water but does not transmit significant quantities. 


a saturated geological formation that contains and transmits significant quantities of water under normal field conditions. 

aquifer test

an experiment designed to measure the in situ properties of an aquifer, based on the response of the water level in a well to pumping or injecting water. 


a saturated geological formation that neither contains nor transmits significant quantities of water. 


a saturated geological formation that is of relatively low permeability; see aquiclude, aquifuge

artesian aquifer

see confined aquifer

average linear velocity

The average velocity [L T-1] of fluid within the pores of a porous medium, equal to the specific discharge divided by the porosity. 


background low-flow conditions in a stream. 

basin aspect ratio

the ratio of basin length (the direction parallel to flow) to basin depth or thickness (above a low-permeability unit). 

Bernoulli equation

Stating that the sum of the pressure, elevation, and velocity heads in a frictionless fluid is constant along a streamline

body forces

forces that act uniformly on each fluid element; examples include gravitational forces and electromagnetic forces. 

Bowen ratio

B [dimensionless], the ratio of the sensible heatflux (H) to the latent heat flux (El). 

capillary barrier

coarse layers of sediments that impede the movement of water under unsaturated conditions. 

capillary forces

forces that are exerted on soil water due to the strong attraction of water by soil minerals; see capillary-pressure head

capillary fringe

the zone immediately above the water table in which the pores are filled with water but the water is under pressure less than atmospheric. 

capillary-pressure head

y [L], the (negative) pressure head in the unsaturated zone that develops across curved air-water menisci because water is attracted more strongly to soil minerals than it is to other water molecules. 


an area of land, bounded by a divide, in which water flowing across the surface will drain into a stream or river and flow out of the area through a specified point on that stream or river. 

Chézy equation

An equation relating channel mean velocity to channel slope and hydraulic radius via a roughness coefficient C called the Chézy number

Chézy number

C [L1/2 T-1], an empirical roughness coefficient used in the Chézy equation describing open channel flow. 


a physical process occurring in clouds by which falling drops of water repeatedly collide with other drops or droplets, thus forming larger drops. 

cone of depression

the spatial pattern of drawdown in an aquifer as the result of pumping. 

confined aquifer

a permeable formation whose upper boundary is an aquitard; water in a well within a confined aquifer will rise above the top of the aquifer. 

conservation of mass

The law that states that for any particular compartment (usually referred to as a control volume), the time rate of change of mass stored within the compartment is equal to the difference between the inflow rate and the outflow rate. 

continuity equation

an expression of conservation of mass in a flow, stating that the inflow rate minus the outflow rate equals the change in storage. 

convective acceleration

the spatial component of acceleration at a fixed time. 

critical flow

flow that occurs at the minimum value of specific energy allowed for a given. The Froude number equals 1 for critical flow. 

Darcy's law

states that the specific discharge through a porous medium is proportional to the hydraulic gradient


r [M L-3], the mass per unit volume of a substance, defined at a point. 


Q = UA [L3 T-1], the volume flux of water. 

discharge area

a region in which groundwater is moving upward across the water table, thereby discharging into the unsaturated zone above, or to the land surface or a surface-water body such as a lake or stream. 


the boundary of a catchment. Typically topographic highs or ridges, a divide separates an area of land that should drain toward a particular point on a stream or river from surrounding land areas that do not. 


the change in water level in a pumping well or nearby observation well or piezometer. 

effective stress

se [M L-1 T-2], an upward stress (force per unit area) exerted by aquifer solids in the subsurface. 

elevation head

z [L], a component of the total head that may be thought of as the potential energy per unit fluid weight. 


a line of constant hydraulic head; equipotentials and streamlines together constitute a flow net


the physical process involving a phase change from liquid to vapor by which water is returned to the atmosphere. 


et [L T-1], the sum of all processes by which water changes phase (from solid or liquid) to vapor and is returned to the atmosphere. 

eexceedance probability

[dimensionless], the relative frequency associated with a random variable attaining a value greater than some specified value. 

field capacity

the relatively constant moisture content that a sandy soil tends to attain following drainage. 


a peak in stream discharge due to inflow of water following a rain or snowmelt event. 

flood routing

determination of river discharge at a point based on knowledge of the discharge at some upstream location (inflow) and the characteristics of the intervening river channel or reservoir. 

flow net

a two-dimensional map of equipotentials (lines of constant hydraulic head) and streamlines in a region of groundwater flow. 


a substance that has no resistance to deformation when subjected to a shearing force. 

free surface

the upper boundary of an open channel flow, between the water and the atmosphere. 

frequency analysis

a statistical technique used by hydrologists for estimating the average rate at which floods, droughts, storms, stores, rainfall events, etc., of a specified magnitude recur. 

friction factor

f [dimensionless], an empirically determined, dimensionless quantity relating head loss to flow properties such as velocity and diameter or depth [U2L/(2gD) for pipe flow]. A friction factor diagram gives values of friction factor f as a function of Reynolds number

friction slope

the slope of the imaginary line that is a distance U2/2g above the water surface in an open channel. 

Froude number

 a dimensionless number that is used to define critical, subcritical, and supercritical flows in channels. 

gage pressure

the fluid pressure relative to atmospheric pressure. The absolute pressure is the sum of the gage pressure and atmospheric pressure. 

gaging station

a facility installed at a selected river site to collect long-term records of flow depth as a function of time. 


water found in the saturated zone of the subsurface. 

groundwater divide

an impermeable vertical or near-vertical boundary separating groundwater flow systems. 

groundwater recession

a decline in groundwater input to a stream through time. 

head loss

hL [L], the loss in head due to friction within the fluid and between the fluid and the side walls of a pipe or channel. 


the condition of spatial variation in physical properties; often applied to the intrinsic permeability of rocks and soil. 


the spatial constancy of physical properties, such as density or intrinsic permeability

Hortonian overland flow

see infiltration-excess overland flow

hydraulic conductivity

K = k(rg/m) [L T-1], the ability of a porous medium to transmit fluid, dependent on both fluid and porous medium properties. 

hydraulic gradient

dh/dl [dimensionless], the change in hydraulic head per unit distance; the driving force in flow through porous media. 

hydraulic head

h = (p/rg) + z [L], the mechanical energy per unit fluid weight, used in the study of flow through porous media. 

hydraulic radius

RH [L], the ratio of the cross-sectional area of flow in a channel to the wetted perimeter. 


a continuous record of streamflow (stage or discharge) as a function of time. 

hydrological cycle

the global-scale, endless recirculatory process linking water in the atmosphere, on the continents, and in the oceans.


the study of the occurrence and movement of water on and beneath the surface of the Earth, the properties of water, and its relationship with the living and material components of the environment. 

hydrostatic equation

an equation describing the relationship between pressure and depth in a fluid at rest. For a homogeneous fluid, the hydrostatic equation is p = rgd


a graph of precipitation versus time. 


the loop-like curve that relates pairs of hydraulic properties of an unsaturated porous medium because volumetric moisture content, capillary-pressure head, and hydraulic conductivity covary along different curves depending on whether the soil is imbibing water or draining. 

incompressible fluid

a fluid for which density is not a function of pressure. 


the movement of rain or melting snow into the soil at the Earth's surface. 

infiltration capacity

the maximum rate at which water can infiltrate into a soil. 

infiltration-excess overland flow

also known as Hortonian overland flow (after its proponent, Robert Horton), a mechanism of runoff generation in which the infiltration capacity of a catchment or portion of a catchment is exceeded by the rainfall intensity, which results in ponding of precipitation at the soil surface and flow across the surface either in sheets or in small rivulets. 


a device to measure infiltration rates with water ponded inside a ring driven into the ground. 


the process by which precipitation (either liquid or solid or both) is temporarily stored either on vegetation surfaces (canopy interception) or on litter surfaces (litter interception); intercepted water either can return to the atmosphere as evaporation or can become stemflow or throughfall

intermediate flow system

a groundwater flow system that is smaller than a regional flow system and that is characterized by flow from a water-table high to a non-adjacent water-table low. 

internal energy

Eu [M L2 T-2], the component of the total energy of a substance that is due to the kinetic and potential energy of the individual molecules. 

intrinsic permeability

k = K(m/rg) [L2], the ability of a porous medium to transmit fluid, independent of the fluid properties. 


line of equal precipitation, in the isohyetal method of estimating areal precipitation. 

isohyetal method

a technique for estimating areal precipitation to a catchment based on representing precipitation structure using lines of equal precipitation known as isohyets. 


a material whose properties (such as intrinsic permeability) do not depend on the direction of measurement. 

laminar flow

a smooth, regular flow in which disturbances are damped out by viscous forces. Laminar flows in pipes and channels occur at Reynolds numbers less than 2000 and 500, respectively. 

land subsidence

the decline of the land surface produced by pumping groundwater wells. 

Laplace equation

an expression of conservation of mass combined with Darcy's law that describes steady two-dimensional groundwater flow in a homogeneous region. Flow nets are graphical solutions to the Laplace equation. 

latent heat

[M L2 T-2], the portion of the internal energy of a substance that cannot be "sensed" (i.e., is not proportional to absolute temperature); latent heat is the internal energy that is released or absorbed during a phase change at constant temperature. 

latent heat flux

El [M T-3], the rate per unit area at which latent heat is transferred to the atmosphere. 

latent heat of vaporization

lv [L2 T-2], the amount of energy per unit mass absorbed during a phase change from liquid to vapor at constant temperature. For evaporation of water at 0°C, lv = 2.5¥106 J kg-1

leaky aquifer

an aquifer that is not perfectly confined, but that has leakage across the surrounding confining layers. 

local acceleration

the change of velocity with time at a fixed location,  [L T-2]. 

local flow system

a groundwater flow system characterized by flow from a water-table high to an adjacent low. 


a relatively large pore, such as a soil pipe, animal burrow or shrinkage crack, in an otherwise fine-grained soil. 

Manning coefficient

n [dimensionless], an empirical channel roughness used in Manning's equation describing open channel flow. 

Manning's equation

 an equation commonly used to calculate channel mean velocity, based on channel geometry and roughness; n is the Manning coefficient


a device used to measure fluid pressure, consisting of a tube filled with fluid and open at one end; the hydrostatic equation is used to relate the pressure at the open end to an unknown pressure at the measurement point. 

mean velocity

U = Q/A [L T-1], the cross-sectionally averaged fluid velocity. 

moisture characteristic

the relationship between moisture content and capillary-pressure head for a porous medium. 

Muskingham method

a numerical method for routing a flood through a river channel. Given an inflow hydrograph, the method predicts the outflow hydrograph for a given stream reach. 

natural basin yield

the average rate of discharge from a basin under natural or undisturbed conditions (i.e., in the absence of anthropogenic groundwater withdrawals or changes in climate or vegetation). 

normal stress

a force per unit area oriented perpendicular to a surface of a fluid or solid object. Pressure is a normal stress. 

numerical method

a method for solving equations by transforming them into one or more algebraic equations that can be solved more easily than the original equation, usually on a computer. 


a device used to measure the flow rate through and the hydraulic conductivity of a porous medium. 


a single tube manometer used to measure pressure head (and thereby hydraulic head) at a point in the subsurface.

piezometric surface

see potentiometric surface

Poiseuille's law

an equation for the velocity of a laminar pipe flow.


f = Vv/Vt [dimensionless], the fraction of the total volume of a porous medium occupied by void space. 

porous medium

a rock, sediment, or soil that contains pores or void spaces. 

potential evapotranspiration

PET, the maximum rate of evapotranspiration from a vegetated catchment under conditions of unlimited moisture supply. 

potentiometric surface

a surface which depicts the distribution of hydraulic heads in a confined aquifer; the water in a well or piezometer penetrating a confined aquifer defines the surface. 


the dominant process by which water vapor in the atmosphere is returned to the Earth's surface either as liquid drops (e.g., rain) or solid particles (e.g., snow) under the influence of gravity. 

precipitation intensity

[L T-1], a measure of the rate of precipitation, commonly computed for a specified duration. 


p [M L-1 T-2], the force per unit area acting perpendicular to a surface, or normal stress. 

pressure head

a component of the total head that may be thought of as the "flow work" or the work due to pressure per unit fluid weight [L]. 


one of two components into which a flood hydrograph can be separated (the other is known as baseflow). 

rating curve

a relationship between stage and discharge used to convert continuous measurements of stream depth (stage hydrograph) to a discharge hydrograph. 


a segment of a stream or river channel. 

recharge area

a region in which water is crossing the water table downward, hence recharging the groundwater system. 

regional flow system

a groundwater flow system characterized by flow from a regional water-table high to a regional water-table low. 

residence time

Tr = V/I [T], a measure of the average time a molecule of water spends in a reservoir. The residence time defined for steady-state systems is equal to the reservoir volume divided by the inflow or outflow rate. 

return flow

the process by which groundwater reemerges from the soil at a saturated area and flows downslope as overland flow. 

return period

Treturn [T], a measure of how often (on average) an event (precipitation, flood, etc.) will occur that is greater than some chosen value; the inverse of the exceedance probability.

Reynolds number

R = rUD/m [dimensionless], a dimensionless number representing the ratio of inertial to viscous forces in a flow. Flows with low Reynolds numbers (< 2000 for pipe flow) are laminar; flows with high Reynolds numbers (> 4000 for pipe flow) are turbulent. Flows of different fluids with the same Reynolds number will be similar. 

Richards' equation

an expression of mass conservation in the unsaturated zone, incorporating Darcy's law. Solutions to the Richards' equation provide a history of the pressure distribution in a vertical soil column. 

runoff ratio

the ratio of average annual surface runoff to average annual precipitation for a given land area . 

saturated zone

a region of the subsurface where pores are completely filled with water; the saturated zone is bounded at the top by the water table.

saturation-excess overland flow

a mechanism of runoff generation that is particularly important in vegetated catchments in humid regions in which a shallow water table intersects the ground surface, causing ponding of water at the soil surface and flow across the surface either in sheets or in small rivulets. 

saturation value of moisture content

the volumetric moisture content when all pores of a soil or rock are filled with water. 

saturation vapor pressure

esat [M L-1 T-2], in a system in which both liquid water and water vapor are present, the partial pressure exerted by the water vapor during an equilibrium condition in which the rates of vaporization and condensation are equal. 

sensible heat

[M L2 T-2], that portion of the internal energy of a substance that can be sensed (i.e., is proportional to absolute temperature). 

shallow subsurface stormflow

a mechanism of runoff generation whereby water flows through a shallow, permeable soil horizon, such as when a perched water table forms above a layer of the soil with low permeability; some of the flow may occur along preferred pathways known as macropores

shear stress

t [M L-1 T-2], a tangential force per unit area acting on the surface of a solid or fluid. 

soil horizon

a soil layer defined on the basis of physical and chemical properties and on the history of its formation. 

soil moisture

water that is held in soils and rocks under pressures less than atmospheric; water in the unsaturated zone

specific discharge (open channel flow)

the discharge per unit width of channel in a rectangular open channel, qw = Q/w = Uh [L2 T-1]. 

specific discharge (groundwater flow)

the discharge per unit cross-sectional area of flow through porous media, q = Q/A [L T-1]. 

specific energy

the energy per unit weight of water in a channel with respect to the channel bottom as datu[L]. The total energy H [L] is the sum of the bottom elevation and the specific energy. 

specific energy diagram

a graph of specific energy versus water depth for a given specific discharge. The diagram shows the physically allowable water depths for a given specific discharge and a given specific energy. 

specific heat capacity

cp [L2 Q-1 T-2], a proportionality constant that relates the change in internal energy of a substance to a change in absolute temperature. 

specific yield

Sy [dimensionless], the volume of water produced from an unconfined aquifer per unit aquifer area per unit decline in the water table


the depth of flow in a stream. 

steady flow

a flow that is constant in time at each location in the flow. In steady flow, the local acceleration is zero. 


a physical process by which water is transferred from interception storage to the soil surface by flowing along the stem or trunk of a tree.


tiny pores in the leaves of vascular plants by which gases (including carbon dioxide, oxygen, and water) are exchanged with the atmosphere. 


S [dimensionless], the volume of water produced from a confined aquifer per unit aquifer area per unit decline in the potentiometric surface.


a path defined by the motion of fluid elements in a flow; at any point along a streamline, the flow direction is tangent to the streamline. Conceptually, water may not cross streamlines. In describing groundwater flow, streamlines and equipotentials together constitute a flow net.


a region within a flow net between two streamlines


a force per unit area (SI units: N m-2 or Pa). 

subcritical flow

the (relatively) deep and slow flow corresponding to a given specific energy and a given specific discharge. The Froude number is less than unity for subcritical flow. 


the physical process by which water in the solid phase changes to water vapor and is directly returned to the atmosphere. 

supercritical flow

the (relatively) shallow and rapid flow corresponding to a given specific energy and a given specific discharge. The Froude number exceeds unity for supercritical flow. 

surface forces

forces that act through direct contact on specific surfaces of a fluid or solid body. 


a device for measuring negative capillary-pressure heads in soils. 


a physical process by which water is transferred from interception storage to the soil surface by dripping off the leaves of the canopy. 

time domain reflectometry(TDR)

a method for measuring soil moisture by timing the movement of a high-frequency electromagnetic wave reflected from the open end of two steel rods inserted into the soil. 


a hydrological catchment model based on land-surface topography. 

topographic index

where a is the upslope contributing area per contour length and tanb is the local slope; used by the catchment model TOPMODEL to calculate the water balance for individual blocks within a catchment. 

total head

H [L], the sum of the pressure, elevation, and velocity heads in a frictionless fluid. 

total stress

sT [M L-1 T-2], the weight (a force) of material overlying a plane of unit cross-sectional area in the subsurface. 


T = Kb [L2 T-1], a measure of the ability of an aquifer of thickness b to transmit water. 


the physical process by which water changes phase from liquid to vapor, is released through the stomata of a plant, and returns to the atmosphere. 

turbulent flow

flow with rapid, irregular fluctuations of velocity in space and time. Turbulent flows in pipes and channels occur at large Reynolds numbers

unconfined aquifer

a permeable formation whose upper boundary is the water table

uniform flow

a flow that does not changing from place to place along the flow path; the convective acceleration is zero. 

unit weight

g [M L-2 T-2], the gravitational force per unit volume, rg, acting on a fluid or solid (SI units: N m-3). 

unsaturated zone

the zone in a soil or rock between the Earth's surface and the water table; pores in the unsaturated zone are partly filled with water and partly filled with air. 

vadose zone

see unsaturated zone

vapor pressure

e [M L-1T-2], the actual partial pressure exerted by a vapor within an air mass; related to the concentration of water vapor in air. 

variable contributing area concept

the idea that catchment areas where saturation-excess overland flow develops expand and contract with time over a storm. 

velocity head

a component of the total head that may be thought of as the kinetic energy per unit fluid weight [L]. 

velocity profile

the variation of velocity with distance away from a boundary. 


m [M L-1T-1], a measure of a fluid's ability to resist deformation. Fluids of high viscosity flow more slowly than low viscosity fluids, everything else being equal (SI units: Pa·s). 

volumetric moisture content

q [dimensionless], the volume of water held in a soil or rock per bulk volume of the sample. 

water budget

a calculation of the inflows, outflows, and change in storage for a particular control volume (such as a lake or a catchment) over a particular time period. 

water table

a surface separating the saturated and unsaturated zones of the subsurface, defined as a surface at which the fluid pressure is atmospheric (or zero gage pressure).

water-table aquifer

see unconfined aquifer.


an artificial obstruction such as a step or dam over which all the water in a channel must flow, and that can be used to measure stream discharge. 

well hydrograph

a record of the variation in water level in a well through time. 

Review Questions

Last modified: Oct 15, 1999
VG Model / Samuel Lee / VADOSE.NET