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Glossary of Terms

The volume of water necessary to cover one acre to a depth of one foot; equal to 43,560 cubic feet or 325,851 gallons.
A case that has been heard and decided by a judge. In the context of an adjudicated groundwater basin, landowners or other parties have turned to the courts to settle disputes over how much groundwater can be extracted by each party to the decision.
Of or pertaining to or composed of alluvium.
A general term for deposits of clay, sand, gravel, or other particulate material that has been deposited by a stream or other body of running water in a streambed.
Appropriative Right
The right to use water that is diverted or extracted by a nonriparian or nonoverlying party for nonriparian or nonoverlying uses. In California, surface water appropriative rights are subject to a statutory permitting process while groundwater appropriation is not.
A geologic formation(s) that is water bearing. A geological formation or structure that stores and/or transmits water, such as to wells and springs. Use of the term is usually restricted to those water-bearing formations capable of yielding water in sufficient quantity to constitute a usable supply for people's uses.
Best Management Practice (BMP)
A technique, process, activity, or structure used or developed to reduce the pollutant content of a storm-water discharge.
Blue Line Stream
A perennial (continuous flow) or intermittent (seasonal flow) creek, stream or watercourse indicated by a solid or broken blue line on a U.S. Geologic Survey 7.5 minute series quadrangle map.
Confined Aquifer
An aquifer that is bounded above and below by formations of distinctly lower permeability than that of the aquifer itself. An aquifer containing confined ground water.
Confined Groundwater
Groundwater under pressure whose upper surface is the bottom of an impermeable bed or a bed of distinctly lower permeability than the material in which the confined water occurs. Confined groundwater moves under the control of the difference in head between the intake and discharge areas of the water body.
Deep Percolation
Percolation of water through the ground and beyond the lower limit of the root zone of plants into groundwater.
Domestic Well
A water well used to supply water for the domestic needs of an individual residence or systems of four or fewer service connections.
Evapotranspiration (ET)
The quantity of water transpired (given off), retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces.
Water that occurs beneath the land surface and fills the pore spaces of the alluvium, soil, or rock formation in which it is situated. It excludes soil moisture, which refers to water held by capillary action in the upper unsaturated zones of soil or rock.
Groundwater Basin
An alluvial aquifer or a stacked series of alluvial aquifers with reasonably well-defined boundaries in a lateral direction and having a definable bottom.
Groundwater Overdraft
Develops when long-term groundwater extraction exceeds aquifer recharge, producing declining trends in aquifer storage. Overdraft is usually evident by declines in surface-water levels and stream flow, reduction or elimination of vegetation, land subsidence, and seawater intrusion.
Groundwater Recharge
The natural or intentional infiltration of surface water into the zone of saturation.
A graphic plot of changes in elevation of water level against time.
Imported Water
Water brought into the county from outside its boundaries (e.g. State water Project).
The flow of water downward from the land surface into and through the upper soil layers.
Integrated Regional Water Management Plan (IRWM)
A Strategic Plan for Sustainable Water Resources to Meet Human and Environmental Needs in San Luis Obispo County.
Land Subsidence
The lowering of the natural land surface due to groundwater (or oil and gas) extraction.
Leaky Confining Layer
A low-permeability layer that can transmit water at sufficient rates to furnish some recharge from an adjacent aquifer to a well.
Low-Impact Development (LID)
An innovative storm water management approach with a basic principle to design the built environment to remain a functioning part of an ecosystem rather than exist apart from it. LID's goal is to mimic a site's predevelopment hydrology by using design techniques that infiltrate, filter, store, evaporate, and detain runoff close to its source.
Any withdrawal of groundwater more than the safe yield.
Overlying Right
Property owners above a common aquifer possess a mutual right to the reasonable and beneficial use of a groundwater resource on land overlying the aquifer from which the water is taken. Overlying rights are correlative (related to each other) and overlying users of a common water source must share the resource on a pro rata basis in times of shortage. A proper overlying use takes precedence over all non-overlying uses.
Perched Groundwater
Groundwater supported by a zone of material of low permeability located above an underlying main body of groundwater.
Percolating Groundwater
Underground waters whose course and boundaries are incapable of determination. Waters, which pass through the ground beneath the earth's surface without a definite channel.
Perennial Yield
The amount of usable water of a groundwater basin that can be withdrawn and consumed economically each year for an indefinite period of time. It cannot exceed the sum of the natural recharge, artificial recharge, and incidental recharge, without causing depletion of the basin.
The capacity of soil, sediment, or porous rock to transmit water.
The ratio of the voids or open spaces in alluvium and rocks to the total volume of the alluvium or rock mass.
Prescriptive Right
Rights obtained through the open and notorious adverse use of another's water rights. By definition, adverse use is not use of a surplus, but the use of non-surplus water to the direct detriment of the original rights holder.
Pumping Level
The level as measured from ground surface at which water stands in a well when pumping is in progress.
Water added to an aquifer or the process of adding water to an aquifer. Ground water recharge occurs either naturally as the net gain from precipitation, or artificially as the result of human influence.
Reclaimed Water
Treated or recycled waste water of a quality suitable for non-potable uses such as landscape irrigation; not intended for human consumption.
Treated Sewage Effluent Water
Treated or recycled waste water of a quality suitable for non-potable uses such as landscape irrigation; not intended for human consumption.
Resource Capacity Study (RCS)
A Resource Capacity Study is prepared by the County at the direction of the Board of Supervisors in order to determine the capacity of the resource being studied, identify thresholds for Level II and III deficiencies, identify alternate measures for avoiding a predicted resource deficiency and evaluate the feasibility and possible funding methods of each measure, provide an estimated timetable for funding and completion of a public works project to correct the resource deficiency and recommend techniques for growth management to be used if needed to extend capacities.
Riparian Right
A right to use surface water, such right derived from the fact that the land in question abuts upon the banks of streams.
Return Flow
1) That part of a diverted flow that is not consumptively used and returned to its original source or another body of water. 2) (Irrigation) Drainage water from irrigated farmlands that re-enters the water system to be used further downstream.
Safe Yield
1) The maximum quantity of water that can be continuously withdrawn from a groundwater basin without adverse effect. 2) The rate at which water can be pumped from wells year after year without decreasing groundwater in storage to the point where pumping lift would become economically unfeasible or where water of poor quality would begin to intrude into the reservoir. The amount of naturally occurring groundwater that can be economically and legally withdrawn from a basin on a sustained basis without producing an undesired result.
Treated or recycled waste water of a quality suitable for non-potable uses such as landscape irrigation; not intended for human consumption.
Recycled Water
Treated or recycled waste water of a quality suitable for non-potable uses such as landscape irrigation; not intended for human consumption.
Specific Yield
As applied to water bearing materials, it is the ratio of the volume of water drained by the force of gravity from a saturated material over a reasonably long period of time, expressed as a percentage of the total volume of the saturated material.
A portion of a basin that can be subdivided for hydrologic study purposes. Hydraulically, a sub-basin is interdependent on the basin as a whole, but is locally independent of pumping depressions and recharge effects.
Supplemental Water
A water source not previously used by an area (e.g. pumping from a deeper, isolated groundwater basin or connecting to the new Nacimiento Water Project).
Surface Supply
Water supply obtained from streams, lakes, and reservoirs.
Surface Water
Water that is on the Earth's surface, such as in a stream, river, lake, or reservoir.
Total Maximum Daily Load (TMDL)
A measure of the amount of contaminants in water. TMDL is used to measure and set targets for water quality.
The product of hydraulic conductivity and aquifer thickness; a measure of a volume of water to move through an aquifer. Transmissivity generally has the units of ft2/day or gallons per day/foot. Transmissivity is a measure of the subsurface's ability to transmit groundwater horizontally through its entire saturated thickness and affects the potential yield of wells.
An essential physiological process in which plant tissues give off water vapor to the atmosphere.
Subsurface flow of groundwater associated with a river or stream that occurs as sub-horizontal flow, roughly parallel to and within the near-surface deposits underlying and directly adjacent to the course of the river and/or its tributaries.
Water Balance
A measure of the amount of water entering and the amount of water leaving a system. Also, see hydrologic budget and hydrologic equation.
Water Conservation
Reducing water use, such as turning off taps, shortening shower times, and cutting back on outdoor irrigation.
Water Efficiency
Replacing older technologies and practices in order to accomplish the same results with less water, for example, by replacing toilets with new low water using models and by installing “smart controllers” in irrigated areas.
Water Quality
Description of the chemical, physical, and biological characteristics of water, usually in regard to its suitability for a particular purpose or use.
Water Wheeling
Occurs when one agency conveys water through another agency's infrastructure. California Water Code requires that wheeling must not harm any other legal user of water.
The total area above a given point on a watercourse that contributes water to its flow; the entire region drained by a waterway or watercourse that drains into a lake, or reservoir.