Table of Contents

Introduction

Part I

Appendix A

Appendix B

Appendix C

Part II A and B

Part II C and D

Part III

Part IV

References

Summary

Appendix C. Recharge-related Glossary

Arid: Said of a climate characterized by dryness, variously defined as rainfall insufficient for plant life or less than 10 inches or 250 mm of annual rainfall.

Artificial recharge: Deliberate act of adding water to an aquifer by means of a recharge project, also the water so added. Artificial recharge can be accomplished via injection wells, spreading basins, or in-stream projects.

Available-water capacity: The amount of water released from a wet soil between field capacity and the permanent wilting percentage.

Bank storage: Change in storage in an aquifer resulting from a change in stage of an adjacent surface-water body.

Baseflow: Streamflow derived mainly from ground-water seepage into the stream.
Bomb tracer: A tracer resulting from nuclear testing, such as tritium or chlorine-36.
Boundary condition: A mathematical expression of a state of the physical system that constrains the equations of the mathematical model.

By-pass flow: See macropore flow.

Calibration (model application): Process of refining the model representation of the hydrogeologic framework, hydraulic properties, and boundary conditions to achieve a desirable degree of correspondence between the model simulation and observations of the ground-water system.

Capillary flow: The flow that takes place in pores with a diameter less than approximately 0.1 inch or 3 mm in which capillary forces, together with gravity, determine the flow process.

Capillary fringe: Unsaturated zone immediately above the water table containing water in direct contact with the water table.

Conceptual model: An interpretation or working description of the characteristics and dynamics of a physical system.

Confined aquifer: Aquifer that is bounded above and below by formations of significantly lower hydraulic conductivity.

Confining bed: A geological unit of significantly lower hydraulic conductivity than an aquifer stratigraphically adjacent to one or more aquifers.

Consumptive use: Use that makes water unavailable for other uses, usually by permanently removing it from local surface- or ground-water storage as a result of evaporation and/or transpiration. Does not include evaporation losses from bodies of water.

Crop coefficient: Empirically determined coefficient relating potential evapotranspiration to crop evapotranspiration.

Darcy’s equation or Law: A formula stating that the flow rate of water through a porous medium is proportional to the hydraulic gradient. The factor of proportionality is the hydraulic conductivity.

Deep drainage: Drainage of water below the root zone.

Depression-focused recharge: See localized recharge.

Diffuse recharge: Water added to the water table by vertical percolation of precipitation through the unsaturated zone. Also known as direct recharge.

Direct recharge: See diffuse recharge.

Discharge: The volume of water (and suspended sediment if surface water) that passes a given location within a given period of time.

Discharge area: An area in which water is lost naturally from the saturated zone.

Drainage basin: A hydrologic unit consisting of a part of the surface of the earth covered by a drainage system consisting of a surface stream or body of impounded surface water plus all tributaries. The runoff in a drainage basin is distinct from that of adjacent areas. A river basin is similarly defined.

Evaporation: The process of liquid water becoming water vapor, including vaporization from water surfaces, land surfaces, and snow fields, but not from leaf surfaces. Compare with transpiration.

Evapotranspiration: Sum of evaporation and transpiration.

Fallow: The period during which land is left to recover its productivity after cropping, mainly through accumulation of water and nutrients, attrition of pathogens, or a combination of these factors.

Field capacity: The quantity of water held back by soil or rock against the pull of gravity when excess water has drained out of a saturated or near-saturated soil. It is sometimes limited to a certain drainage period (two or three days). Field capacity is thought to be the soil-moisture condition that will promote maximum plant growth, with transpiration occurring at the potential rate (i.e., transpiration is not limited by moisture availability.)

Fingered flow: Unstable flow whereby the percolating water may concentrate at certain points to break into the sublayer in the form of fingerlike or tonguelike protrusions.

Finite-difference method: Numerical technique for solving a system of equations using a rectangular mesh representing the aquifer and solving for the dependent variable in a piece-wise manner.

Finite-element method: Numerical technique for solving a system of equations using an irregular triangular or quadrilateral mesh representing the aquifer and solving for the dependent variable in a continuous manner.

Flow path: The route ground water takes to a distant point.

Flow net: The set of intersecting lines of equal hydraulic-head values and flow lines representing two-dimensional steady flow through a porous medium.

Focused recharge: See localized recharge.

Gaining stream: Stream reach in which the water table adjacent to the stream is higher than the water surface in the stream, causing ground water to seep into the stream, increasing its flow. Also known as effluent stream.

Geographic Information Systems (GIS): Computer-based systems for storing and manipulating geographic (spatial) information.

Ground-water basin: A geologically and hydrologically defined area which contains one or more aquifers which store and transmit water and will yield significant quantities of water to wells.

Ground-water flow system: A set of ground-water flow paths with common recharge and discharge areas. Flow systems are dependent on both the hydrogeologic characteristics of the soil/rock material and landscape position. Areas of steep or undulating relief tend to have dominant local flow systems (discharging in nearby topographic lows such as ponds or streams). Areas of gently sloping or nearly flat relief tend to have dominant regional flow systems (discharging at much greater distances than local systems in major basin topographic lows or oceans).

Hydrogeologic environment: The physical and chemical conditions resulting from the combination of topography, geology, and climate.

Hydrologic budget or water balance: An accounting of the inflow to, outflow from, and storage in a hydrologic unit such as a drainage basin, aquifer, soil zone, lake, or reservoir.

Indirect recharge: Recharge that results from percolation to the water table following runoff and localization in joints, as ponding in low-lying areas and lakes, or through the beds of surface-water courses.

Induced recharge: Recharge to ground water by infiltration, either natural or anthropogenic, from a body of surface water as a result of the lowering of the ground-water level below the surface-water level.

Infiltration: Movement of water from the ground surface into the soil.

Interflow: Subsurface lateral flow that can enter streams quickly enough to contribute to the rising streamflow-hydrograph response to a storm.

Localized recharge: Recharge that results from horizontal surface concentration of water in the absence of well-defined channels, such as sloughs, potholes, and playas. Also called focused or depression-focused recharge.

Losing stream: Stream reach in which the water table adjacent to the stream is lower than the water surface in the stream, causing infiltration from the stream channel, recharging the aquifer and decreasing streamflow. Also known as influent stream.

Macropore flow: The flow that takes place in a wide range of large pores such as cracks in clay soils, rock fractures, fissures in sediments, worm holes, and old root channels. Preferential and by-pass flow are alternative names for macropore flow.

Matric suction: Soil-water potential (energy) resulting from the capillary and absorptive forces due to the soil matrix.

Mountain-front recharge: Recharge that involves complex processes of unsaturated and saturated flow in fractured rocks, as well as infiltration along channels flowing across alluvial fans.

Natural recharge: Naturally occurring water added to an aquifer. Natural recharge generally results from snowmelt and precipitation or storm runoff.

Perched ground water; perching: A superficial body of ground water separated from an underlying main body of ground water by an unsaturated zone due to a sufficiently low hydraulic-conductivity layer that supports this body of perched ground water; the act of causing a body of ground water to form above a low-permeability layer in an unsaturated zone.

Perched water table: Water table of a relatively small ground-water body lying above the general ground-water body.

Percolation: Laminar-gravity flow through unsaturated and saturated earth material.
Permanent wilting percentage: The water content of soil when indicator plants growing in that soil wilt and fail to recover when placed in a humid chamber.

Permanent wilting percentage: The water content of soil when indicator plants growing in that soil wilt and fail to recover when placed in a humid chamber

Phreatophyte: A plant whose roots generally extend downwards to the water table which customarily feeds on the capillary fringe. Phreatophytes are common in riparian habitats. Term literally means water-loving plant.

Piston flow or plug flow: Purely advective flow without dispersion or diffusion of the dissolved components.

Playa: The flat-floored bottom of an undrained desert basin, becoming at times a shallow muddy lake after heavy rainfall; evaporation of the playa lake may leave a deposit of salt or gypsum.

Potential energy: The energy deriving from elevation and/or pressure.

Potential evapotranspiration (PET): The maximum amount of soil evaporation and transpiration from a well-irrigated crop for a given set of environmental conditions.

Potential recharge: Soil-water that percolates below the root zone and has the potential of reaching the aquifer, whereas actual recharge is soil-water that actually reaches the aquifer.

Preferential flow: See macropore flow.

Preferential recharge: Recharge that takes place preferentially through macropores, as opposed to diffuse recharge, which takes place through the entire vadose porous medium.

Recharge area: The area that contributes water to an aquifer. Normally considered to be the natural area of recharge, as contrasted with a constructed recharge basin.

Rejected recharge: Potential recharge that exceeds the rate of flow through an aquifer that is already overfull, and as a result is rejected.

Residence time: The length of time between the input of water as infiltration or recharge and its output as runoff or discharge. Also known as transit time or turnover time.

Residual: In the case of recharge, the remainder of all other hydrologic components in the water-balance equation.

Riparian: Of, or pertaining to, rivers and their banks.

Riparian habitat: Natural home of plants and animals occurring in a thin strip of land bordering a stream or river. Dominant vegetation often consists of phreatophytes.

Semiarid: Said of a type of climate in which there is slightly more precipitation (10 to 20 inches or 250 to 500 mm) than in an arid climate, and in which sparse grasses are the characteristic vegetation.

Sensitivity (analysis): In model application, the degree to which the model result is affected by changes in a selected model input representing hydrogeologic framework, hydraulic properties, and boundary conditions.

Soil-moisture deficit: An estimate of the degree to which soil-moisture content has dropped below field capacity.

Specific discharge: For ground water, the rate of discharge of ground water per unit area measured at right angles to the direction of flow.

Specific yield: The fraction of a saturated bulk volume consisting of water which will drain by gravity when the water table drops; specific yield is less than porosity because some water is too strongly absorbed to the earth material to drain. The ability of an unconfined or water-table aquifer to store water is measured by its specific yield. Specific yield can be several orders of magnitude larger than the storage coefficient, thus producing more water when developed.

Steady-state flow: Characteristic of a flow system where the magnitude and direction of specific discharge are constant in time at any point.

Storativity or storage coefficient: The volume of water released per unit area of aquifer and per unit drop in head. Storage coefficient is a function of the compressive qualities of water and matrix structures of the porous material. A confined aquifer’s ability to store water is measured by its storage coefficient. Storativity is a more general term encompassing both or either storage coefficient and/or specific yield.

Texture (soil): Relative proportions of sand, silt, and clay particles in a mass of soil.

Transmissivity: Flow capacity of an aquifer measured in volume per unit time per unit width. Equal to the product of hydraulic conductivity times the saturated thickness of the aquifer.

Transmission losses: Streamflow losses through seepage in ephemeral streams.

Transpiration: The vaporization of water given off by plants.

Unconfined (or water-table) aquifer: An aquifer in which the water table is at the upper boundary of the ground-water flow system that is at atmospheric pressure.

Unsaturated or vadose zone: The unsaturated (i.e. not completely filled with water) zone lying between the earth’s surface and the top of the ground water.

Water balance: See hydrologic budget.

Water table: The upper boundary of an unconfined aquifer at atmospheric pressure.

Watershed: That surface area which drains to a specified point on a watercourse, usually a confluence of streams or rivers.

Wetting front: The boundary between the wetted region and the dry region of soil during infiltration.