Glossary

        Appropriation--to set apart for or to assign to a particular purpose or use.

Aquifer--saturated underground rock or sediment formation which is sufficiently permeable to transmit water to wells and springs.

 Aquifer Storage -- the ability of the aquifer to store water in interconnected pores and fractures. Aquifer storage is quantified by a values referred to as storativity and specific yield.

Attenuated--to lessen the amount, force or value of : to weaken.

Basalt-- a dark colored igneous (lava) rock. Basalt is differentiated from other igneous rocks by its chemical composition. The basalt of the Eastern Snake River Plain is thought to have intermittently flowed from numerous vents across the Plain over long periods of time.

Boundary conditions-- the physical conditions at the boundaries of a system. Examples are model bottom and no-flow boundaries at the lateral aquifer terminus, fixed flux boundaries representing a fixed inflow or outflow of water across that boundary cell, and fixed head boundaries representing potentiometric head that is held constant by some external force such as a river or lake.  A mathematical represetation of boundary conditions must be specified in a numerical ground water flow model.

Calibration -- in the context of this discussion, calibration refers to the process of adjusting estimates of aquifer characteristics used in numerical ground water models. The calibration usually attempts to minimize differences between simulated and measured characteristics such as aquifer water levels.

Cone of Depression-- the shape of the water table in the area immediately surrounding a pumping well. The water draws down in a radial cone-shape around the pumping well, with the deepest drawdown immediately at the well, tapering off with distance from the pumping well.

Confined Aquifer--an aquifer which is overlain by a confining bed (aquitard) of significantly lower hydraulic conductivity which retards the vertical movement of water.

 Conjunctive Management -- managing surface and ground water as a single system.

 Crop Consumptive Use -- The amount of water used by a plant primarily in the process of transpiration (evaporative) needs. From a basin water resources perspective, this water is considered to be permanently removed from the basin, because little of the transpired water is expected to return to the same basin as precipitation.

Discharge--the release or extraction of water from an aquifer. Typical mechanisms of natural discharge are evapotranspiration by phreatophytes, springs, and drains to surface water bodies. Pumping is a man-caused discharge.

Drawdown--the change in potentiometric head caused by the pumping of groundwater.

Evapotranspiration--loss of water due to the combined effects of evaporation and plant transpiration.

 Gaining Stream--a body of surface water which is gaining water from the inflow of ground water.

 Heterogeneous--a porous medium which has different physical characteristics in different locations.

 Homogeneous--a porous medium which has uniform physical characteristics everywhere.

 Hydraulic Conductivity--the capacity of a porous medium to transmit water through a unit cross-sectional area. Hydraulic conductivity is dependent upon the physical properties of the porous medium and the viscosity of the water and is expressed in units of length/time.

 Hydraulic Connection -- a condition that exists between surface water and ground water when the water table is near or above the bottom of a lake or stream. In this situation, changes in water table change the rate at which a surface water body will gain water from, or lose water to the aquifer.

Hydrograph -- a graph showing changes in flow or stage of a stream, river or lake over time.

Hydrologic Unit Code (HUC) -- a hierarchical classification of drainage basins. An 8 digit HUC indicates region (leftmost 2 digits), sub-region (next two digits), accounting unit (next 2 digits), and cataloging unit (rightmost 2 digits). The Snake River basin is entirely within the Pacific Northwest Region (17).

 Junior Priority Water Right-- within the Prior Appropriation Doctrine, water rights are prioritized according to the date in which they first put water to beneficial use. A water right is junior to another right if the junior right was established at a later date. In periods when insufficient water is available to meet all water rights, junior rights are curtailed, in order of priority date, to provide full supply to more senior rights.

 Losing Stream--a surface stream or lake which is losing water by seepage into the ground.

 Model -- a device which represents an approximation of a field situation. Models can be physical, analytical or numerical.

 A physical model is a physical representation of a larger, more complex system. Examples of physical models are Darcy’s cylinder filled with sand and water and the sand and plexiglass ground-water flow model.

An analytical model is a mathematical representation of a physical system which can be solved using analytical methods. Analytical models are highly dependent upon simplifying assumptions.

A numerical model is a discretized representation of a physical system which is solved iteratively using a computer. Numerical models are less dependent upon simplifying assumptions but are highly data and computation-intensive. Numerical models have the advantage of enabling representation of different areas of the physical system using different physical properties. (Anderson and Woessner, 1992)

 Model Cell--a 3-dimensional volume used in a numerical model to represent a discretized portion of a physical system. Numerical models calculate the flows in and out of each model cell balancing with the flows of the surrounding model cells. The numerical model calculates the potentiometric head or water level at the center of each model cell at the end of each model stress period.

Model Grid--the discretized model domain. The model grid is the physical model area normally overlain by a rectangular grid which defines the model boundaries and model cells.

 MODFLOW -- a numerical ground water flow model code developed by the U.S. Geological Survey. The code has been widely applied to aquifers throughout the United States and the world.

MODRSP -- a modification of the MODFLOW model code to generate response functions.

 Observation Well--a well located some distance from a pumping well which is used to measure changes in water levels during an applied aquifer stress.

Perched Water--a localized zone of water which sits on top of an aquitard. A perched zone is typically unconfined and at a higher elevation than the regional aquifer system. Unsaturated conditions exist below a perched unit.

Playa--the flat floored bottom of an undrained desert basin that becomes at times a shallow lake.

 Prior Appropriation Doctrine –A major doctrine defining water right systems in the western United States. The Prior Appropriation Doctrine operates on a priority system, where a priority is established based on the date water is first put to beneficial use. When water supply is insufficient to meet demands of all water rights, water use of later priority (junior) rights are curtailed to provide a full supply to earlier (senior) rights.

 Potentiometric or Piezometric Head--the elevation to which water rises in a well. Potentiometric head is measured using a sounder or similar depth gauge. The Potentiometric head in a well in an unconfined aquifer is at the elevation of the water table. In a well in a confined aquifer, the potentiometric head is typically at a higher elevation than the top of the aquifer due to the confining pressure.

 Porosity—a measure of the void or pore space within rocks and sediments (the ratio of the volume of void spaces to the total volume).

 Potentiometric Surface--the two-dimensional surface which describes the elevation of the water table. In an unconfined aquifer, the potentiometric surface is at the top of the water level. In a confined aquifer, the potentiometric surface is above the top of the water level because the water is under confining pressure.

Pumping/Injection Well--a well used for removal or injection of water from an aquifer.

 Recharge--mechanisms of inflow to the aquifer. Typical sources of recharge are precipitation, applied irrigation water, underflow from tributary basins and seepage from surface water bodies.

Response Function – an equation or table of numbers that quantitatively describes the response of a system (in this case river gains and losses) at a specific location, to a stress (pumping or aquifer recharge) at another location. The response normally changes with time, consequently, response functions are time dependent. Response functions assume that the response will be proportional to the stress.

Response Ratio – a component of a response function, describing response at a specific location and at a specific time to a stress at another location.

Saturated Thickness--the saturated depth of an aquifer. For a confined aquifer, the saturated thickness at any point in the aquifer is equal to the aquifer thickness. For an unconfined aquifer, the saturated thickness at any point is the distance from the top of the water table to the bottom of the aquifer. As aquifer recharge and discharge conditions vary in an unconfined aquifer, the saturated thickness will change.

Senior Priority Water Right-- see Junior Priority Water Right

Specific Yield--the ratio of the volume of water which will drain from a porous medium by gravity to the volume of the porous medium.

Steady State Model--a numerical model in which model stresses do not vary over time. A steady state model is run until the modeled basin is in equilibrium and no more changes in potentiometric head are calculated.

Stream Depletion -- a decrease in river gains or an increase in river losses resulting from a change in the water table.

Stream Gage -- a station established to measure flow in a river or stream.

Stress Period--an increment of time in a transient simulation during which aquifer recharges and discharges are held constant.

Storativity -- the volume of water an aquifer released from an aquifer per unit surface area of the aquifer and per unit change in head.

Transient Model--a numerical model in which the model stresses (inflows and outflows) and aquifer head vary over time.

Transmissivity--the rate of flow of water through a vertical strip of aquifer which is one unit wide and which extends the full saturated depth of the aquifer. Transmissivity is related to Hydraulic Conductivity by the relationship:

T = Kb

where T = Transmissivity,

K = Hydraulic Conductivity and

b = the saturated thickness of the aquifer.

Transmissivity is expressed in units of length2/time. Because the saturated thickness of an unconfined aquifer changes as aquifer storage changes in response to variation to aquifer recharge and discharge,  transmissivity of the unconfined aquifer will change.

Tributary Valley Underflow-- the rate at which water flows in the subsurface from a tributary basin into a larger basin.

Unconfined Aquifer--an aquifer that is not under pressure.

Water Table--the elevation of the water in an unconfined aquifer.

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