A glossary of the often complex terms describing the amazing nature of ‘groundwater’

A glossary of the often complex terms describing the amazing nature of ‘groundwater’

May 11, 2023

In a little under two years, hydrologists at the Arizona Department of Water Resources have completed studies of groundwater conditions in two regions at opposite ends of south-central Arizona - the Pinal Active Management Area south and east of Phoenix and the Lower Hassayampa Sub-basin, an area west of the White Tank mountains and northwest of Phoenix.

Both of those groundwater studies - known as groundwater models - have examined the physical processes that govern the movement and storage of groundwater in an aquifer system.

Both of the models employed state-of-the-art “numerical” modeling strategies - a computer-based process that is considered industry-wide to be the best-available scientific tool for regional-scale applications. ADWR has developed numerical models for all five initial AMAs.

Soon, the Department anticipates completing work on its most comprehensive modeling project to date - a numerical model of the entire Phoenix Active Management Area.

Like all scientific endeavors, groundwater modeling has its own language.

Some terms and phrases are self-evident. An “acre-foot?” Why, it’s enough water to cover an acre of land a foot deep. Nothing tough about that, right?

Others, like the sadz-inducing “cone of depression” or “hydraulic conductivity” present the average person with a bit more complexity.

The following set of groundwater-related words, terms and phrases should prove helpful to non-experts who are seeking to better understand the nature of the water beneath our feet.


100-year Assured Water Supply: Water of sufficient quantity and quality is available to sustain a proposed development for 100 years and will be consistent with the management goal and management plan of the active management area.

Illustration of the dimensions of an "Acre-Foot" of water

Acre-foot: The volume of water required to cover 1 acre to a depth of 1 foot and is equal to 43,560 cubic feet or 325,851 gallons. In Arizona, 1 acre-foot is typically enough water to serve 3 single-family homes for a year.

Active Management Area (AMA): In Arizona, an active management area (AMA) is a designated geographical area where groundwater management is required to ensure sustainable use of the groundwater resources.

Analysis of Assured Water Supply: An Analysis of Assured Water Supply is generally used to prove that water will be physically available for master-planned communities but may be used to demonstrate other criteria required for a Certificate of Assured Water Supply.

Diagram Illustrating an Unsaturated Zone, Unconfined and Confined Aquifers, and the Confining Layer

Aquifer: An underground geological formation of sand, soil, gravel, and rock able to store, transmit, and yield water.

Calibration: Systematic adjustment of aquifer parameter values to obtain simulated heads and fluxes that best fit the observed heads and fluxes.

Certificate of Assured Water Supply: A developer of a proposed subdivision must have a 100-year Assured Water Supply to obtain plat approval and offer lots for sale. They can demonstrate that supply one of two ways: 1) By obtaining a commitment of water service from a water provider that has been designated by ADWR as having an AWS, or 2) by obtaining a Certificate of AWS from ADWR. To obtain a Certificate of AWS, the applicant must demonstrate that the water will be physically, continuously, and legally available for 100 years.

Cone of Depression: A depression or lowering of the water table or potentiometric surface near a pumping well. When groundwater is pumped from a well, the water table or potentiometric surface in the surrounding area drops, creating a cone-shaped depression in the water table or potentiometric surface.

Confined Aquifer: An aquifer that exists where the groundwater is bounded between layers of impermeable substances like clay or dense rock. When tapped by a well, water in confined aquifers is forced up, sometimes above the soil surface. This is how a flowing artesian well is formed. Also known as artesian or pressure aquifers.

Diagram Illustrating a Cone of Depression Near a Pumping Well

Confining Layer: Geologic material with little or no permeability or hydraulic conductivity, such as clay or dense rock. Water does not pass through this layer, or the rate of movement is extremely slow.

Depletion (groundwater): The extraction of water from an aquifer at a rate greater than that of natural recharge; when groundwater is used faster than it is replaced.

Depth to Water: A term used in hydrogeology to refer to the distance between the ground surface and the water table. It is the depth at which a well, borehole, or other groundwater monitoring device encounters groundwater.

Discharge: An outflow of water from a stream, pipe, groundwater aquifer, or watershed; the opposite of recharge.

Discharge Area: The area or zone where groundwater emerges from the aquifer. The outflow may be into a stream, lake, spring, wetland, etc.

Drainage Basin: The drainage area of a stream at a specified location, measured in a horizontal plane, which is enclosed by a drainage divide.

Drawdown: A lowering of the groundwater level caused by pumping.

Drought: An extended period of dry weather with little or no precipitation; often affects crop production and availability of water supplies.

Ephemeral Stream: A stream or part of a stream that flows only in direct response to precipitation; it receives little or no water from springs, melting snow, or other sources; its channel is at all times above the water table.

Gauging Station: A site on a stream, canal, lake, or reservoir where systematic observations of hydrologic data are obtained regularly by permanently installed equipment.

Gradient (Hydraulic):  The change of pressure per unit distance from one point to another in an aquifer. An area said to be “downgradient” is at a lower level and water will flow in that direction.

Groundwater: The water that fills the spaces and pores between particles of soil and rocks and is stored in underground aquifers.

Groundwater Basin: The underground area from which groundwater drains. The basins could be separated by geologic or hydrologic boundaries.In Arizona, the Director of the Department of Water Resources established the boundaries of each groundwater basin and sub-basin through a statutory process. The State’s Active Management Areas are each individual groundwater basins.

Groundwater Flow: The movement of groundwater within an aquifer.

Groundwater Mining: Withdrawal (removal) of groundwater over a period of time that exceeds the recharge rate of the supply aquifer.

Groundwater Model: A mathematical representation of the physical processes that govern the movement and storage of groundwater in an aquifer system. A groundwater model may be an analytical model or a numerical model. Unlike an analytical model, a numerical model is computer-based and projects how pumping from multiple locations in a region may interact, incorporating multiple variable features of the aquifer system. The numerical model is the best-available scientific tool for regional-scale applications. ADWR has developed numerical models for all five initial AMAs.

Hydraulic Conductivity: A measure of the ability of a porous material, such as soil or rock, to transmit water through its pore spaces. It describes the rate at which water can move through the material under a hydraulic gradient, which is the difference in water pressure between two points divided by the distance between them. Hydraulic conductivity is typically expressed in units of length per time (e.g., meters per day or feet per hour).

Hydraulic Head: A measure of the potential energy of water in a groundwater system, and is defined as the height to which water would rise in a well if it were open to the atmosphere. It represents the total energy per unit weight of water and is made up of two components: elevation head and pressure head.

Impermeable Layer: A layer of material (such as clay) in an aquifer through which water does not flow.

Infiltration: Flow of water from the land surface into the subsurface; also known as recharge.

Infiltration Rate: The quantity of water that enters the soil surface in a specified time interval. Often expressed as the volume of water per unit of soil surface area per unit of time.

Monitoring Well: A non-pumping well, generally of small diameter, that is used to measure the elevation of a water table or water quality.

Mountain-Front Recharge: Natural recharge that occurs at the base of mountains, which then infiltrates into a permeable rock unit.

Municipal Water System: A network of pipes, pumps, and storage and treatment facilities designed to deliver potable water to homes, schools, businesses, and other users in a city or town and to remove and treat waste materials.

Percolation: (1) The movement of water through the openings in rock or soil. (2) The entrance of a portion of the streamflow into the channel materials to contribute to groundwater replenishment.

Permeable Layer: A layer of porous material (rock, soil, unconsolidated sediment); in an aquifer, the layer through which water freely passes as it moves through the ground.

Diagram Illustrating Porosity - The capacity of rock or soil to hold water

Permeability: A measure of how easily water can flow through a porous medium, such as soil or rock. It refers to the ability of a material to allow fluids to pass through it and is typically measured in terms of hydraulic conductivity, which is a function of both the material properties and the fluid properties.

Pore Space: Openings between geologic material found underground. Also referred to as void space or interstices.

Porosity: The ratio of the volume of void or air spaces in a rock or sediment to the total volume of the rock or sediment.

Potable Water: Water of quality suitable for drinking.

Example of Recharge through the Underground Water Storage, Savings, & Replenishment (Recharge) Program

Recharge: Water added to a groundwater aquifer. For example, when rainwater seeps into the ground. Recharge may occur naturally through precipitation or surface water or artificially through injection wells or by spreading water over groundwater reservoirs.

Recharge Rate: The quantity of water per unit of time that replenishes an aquifer.

Recharge Zone or Area: An area where permeable soil or rock allows water to seep into the ground to replenish an aquifer.

Riparian: Pertaining to or situated on the bank of a natural body of flowing water.

Runoff: Water that flows over the land to surface streams, rivers, and lakes.

Safe Yield: Also known as sustainable yield, it is a groundwater management goal of three AMAs in Arizona. “Safe Yield” is an attempt to achieve and then maintain a long-term balance between the annual amount of groundwater withdrawn in an AMA and the annual amount of natural and artificial recharge in the AMA.

Saturated Thickness: Total water-bearing thickness of an aquifer.

SRV: Salt River Valley.

Static Water Level:  The level at which water naturally stands in a well or borehole when no water is being pumped out of or into the well. It is the level at which the water pressure in the well equals the pressure of the surrounding groundwater and is sometimes referred to as the groundwater table or water table.

Stormwater: Runoff that is generated when precipitation from rain and snowmelt flows over land or impervious surfaces and does not percolate into the ground.

Subsidence: A depression of the land surface as a result of groundwater storage loss (pumping).

Transmissivity: A measure of how easily water can move through an aquifer under a hydraulic gradient, which is the difference in water pressure between two points divided by the distance between them. It is a property of the aquifer that describes its ability to transmit water and is expressed in units of length squared per time (e.g., meters squared per day).

Unconfined Aquifer: An aquifer in which the water table is at or near atmospheric pressure and is the upper boundary of the aquifer. Because the aquifer is not under pressure, the water level in a well is the same as the water table outside the well.

Unmet Demand: Unmet demand refers to the amount of water that is needed to meet the water demands of users within an AMA but cannot be met due to limited groundwater availability or other physical or legal constraints.

Water Budget: An accounting of the inflow to, outflow from, and storage changes of water in a hydrologic system.

Water Demand: Water requirements for a particular purpose, such as irrigation, power, municipal supply, plant transpiration, or storage.

Water Table: The top of an unconfined aquifer; indicates the level below which soil and rock are saturated with water. The top of the saturation zone.

Watershed: The land area from which surface runoff drains into a stream, channel, lake, reservoir, or other body of water; also called a drainage basin.

Example of xeriscape using drought tolerant, indigenous plants, shrubs, and ground cover

Well: A man-made opening in the earth through which water may be obtained from beneath the surface.

Wetlands: Areas where water saturation is the dominant factor in determining the nature of soil development and the types of plant and animal communities. Other common names for wetlands are sloughs, ponds, and marshes.

Withdrawal: The removal of water from a surface or groundwater source for use.

Xeriscaping: An environmentally-friendly form of landscaping that uses a variety of indigenous and drought-tolerant plants, shrubs, and ground cover.