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Packed Tower Aeration — (Water Quality) A process for the removal of organic contaminants from groundwater. The groundwater flows downward inside a tower filled with materials (the packing) over a large surface area. Air is introduced at the bottom of the tower and is forced upward past the falling water. Individual organic contaminants are transferred from the water to the air, according to the gas and water equilibrium concentration values of each contaminant.

Palatable Water — Water, at a desirable temperature, that is free from objectionable tastes, odors, colors, and turbidity.

Paleobotany — The study of lake sediments, pollens, and microfossils to determine ancient climate and vegetation.

Paleontology — The study of fossils of animal and plant life that existed in remote geological times. The study of these remains enables scientists to trace the evolutionary history of extinct as well as contemporary organisms. Paleontologists also play a major role in unraveling the stratigraphic mysteries of the earth's crust and by using detailed information on how fossils are distributed in sedimentary strata, they help prepare accurate geologic maps, which are essential in the search for oil, water, and minerals.

Paleopedology — The study of fossil soils.

Palmer Drought Severity Index (PDSI) — The PDSI was the first comprehensive drought index developed in the United States. It is a meteorological drought index based on a balance between moisture supply and demand and responds to weather conditions that have been abnormally dry or abnormally wet. The index is calculated based on precipitation and temperature data, as well as the local Available Water Content (AWC) of the soil, i.e., soil moisture. From the index’s inputs, all the basic terms of the water balance equation can be determined, including evapotranspiration, soil recharge, runoff, and moisture loss from the surface layer. The index, however, is less well suited for mountainous regions or areas of frequent climatic extremes. Human impacts on the water balance, such as irrigation or drainage, are not considered. The index roughly ranges from extremes of -6 to + 6, with negative values denoting dry spells and positive values indicating wet periods. There are also a few values in the magnitude of -7 or +7. Ideally, the PDSI is designed so that a -4.0 in South Carolina has the same meaning in terms of the moisture departure from a climatological normal as a -4.0 in Idaho. The following shows the more typical PDSI classifications:
4.00 or more – Extremely wet
3.00 to 3.99 – Very wet
2.00 to 2.99 – Moderately wet
1.00 to 1.99 – Slightly wet
0.50 to 0.99 – Incipient wet spell
0.49 to -0.49 – Near normal
-0.50 to -0.99 – Incipient dry spell
-1.00 to -1.99 – Mild drought
-2.00 to -2.99 – Moderate drought
-3.00 to -3.99 – Severe drought
-4.00 or less – Extreme drought

Palustrine — Pertaining to a Marsh or Wetlands; wet or marsh habitats.

Palustrine Wetlands — Used in the wetlands classification system by the U.S. Fish and Wildlife Service (USFWS) to refer to wetlands that are vegetated-dominated by trees, shrubs, herbaceous plants, mosses or lichens. See Wetlands (General), Wetlands (COE and EPA), Wetlands (USFWS), Wetlands (NRCS), Wetlands, Palustrine, and Wetlands, Benefits. [See Appendix D–2 for an explanation of the Wetland and Deepwater Habitat Classification System according to U.S. Fish and Wildlife Service (USFWS) criteria and more detailed information of these systems.

Parshall Flume — A device used to measure the flow of water in an open channel.

Partial Duration Flood Series — A list of all flood peaks that exceed a chosen base stage or discharge, without regard for the number occurring in a year. Also referred to as Basic-Stage Flood Series or Floods Above a Base.

Partial Penetration — A well constructed in such a way that it draws water directly from a fractional part of the total thickness of the aquifer. The fractional part may be located at the top, the bottom, or anywhere else in the aquifer.

Partial-Record Station — A gaging site where discrete measurements of one or more hydrologic parameters are obtained over a period of time without continuous data being recorded or computed. A common example is a creststate gage partial record station at which only peak stages and flows are recorded.

Particle Count — (Water Quality) Results of a microscopic examination of treated water with a special “particle counter” that classifies suspended particles by number and size.

Particulate Matter — (Water Quality) In water pollution, particulate matter describes solid material in either the solid or dissolved states. Insoluble particulate matter includes particulate substances that either settle from water that is allowed to stand or are removed by passing the water through a filter. Sand, clay, and some organic matter constitute insoluble particulate matter. Dissolved substances that will neither settle if water is allowed to stand nor be removed by passage through a filter, but which will be recovered if the water is allowed to evaporate, are called dissolved particulate matter. Salt is an example of this type of particulate matter. In air pollution, particulate matter is used to describe either solid particles or liquid droplets that are carried by a stream of air or other gases.

Particulate Organic Matter (POM) — Material of plant or animal origin that is suspended in water. The amount of this type of material suspended in water can be estimated by first removing the suspended material from the water by filtration, followed by either a direct measurement of the amount of carbon retained on the filter or by estimating the amount of carbon present from the weight lost upon heating the filter in excess of 500EC (932EF). Generally, the greater the amount of particulate matter present, the more severe the water pollution problem.

Particulate Phosphate — That portion of the total amount of phosphate (PO4–3) suspended in water that is attached to particles and will not pass through a filter. The aggregates can be either inorganic or organic. This form of phosphate must be solubilized before it can be used as a plant nutrient.

Particulates — Very small solids suspended in water. They can vary in size, shape, density, and electrical charge and can be gathered together by Coagulation and Flocculation. Also see Particulate Matter.

Particulate Transport — Movement of undissolved particles in subsurface water.

Parts Per Billion (PPB) — The number of “parts” by weight of a substance per billion parts of water. Used to measure extremely small concentrations.

Parts Per Million (PPM) — The number of “parts” by weight of a substance per million parts of water. This unit is commonly used to represent pollutant concentrations. Large concentrations are expressed in percentages.

Parts Per Thousands (PPT) — An expression of concentration which indicates one unit is contained in a total of a thousands units. It is normally used to specify the salinity of water and commonly indicated by the symbol “‰”.

Pathogens — Microorganisms that can cause disease in humans, animals, plants, or other organisms. Pathogens include bacteria, viruses, and parasites and can be found in sewage, runoff from animal farms, and wild animals. Fish and shellfish contaminated by pathogens, or the pathogens themselves, can cause serious illnesses.

PCBs (Polychlorinated Biphenyls) — A group of synthetic, toxic industrial chemical compounds once used in making paint and electrical transformers which are chemically inert and not biodegradable. PCBs were frequently found in industrial wastes, and subsequently found their way into surface and ground waters. As a result of their persistence, they tend to accumulate in the environment. In terms of streams and rivers, PCBs are drawn to sediment, to which they attach and can remain virtually indefinitely. Although virtually banned in 1979, they continue to appear in the flesh of fish and other animals.

Peclet Number — Relationship between the advective and diffusive components of solute transport; expressed as the ratio of the product of the average interstitial velocity and the characteristic length, divided by the Coefficient of Molecular Diffusion. Small values indicate diffusion dominates; large values indicate advection dominates.

Penstock —
(1) A gate or sluice used in controlling the flow of water.
(2) A tube or trough for carrying water to a water wheel, or a pipe carrying water to an electric turbine.

Percent Saturation — The amount of a substance that is dissolved in a solution compared to the amount that could be dissolved in it.

Percent Sodium — The percent of cationic equivalents in a water which is attributable to sodium.

Perched Ground Water — Ground water in a saturated zone of material underlain by a relatively impervious stratum which acts as a barrier to downward flow and which is separated from the main ground water body by a zone of unsaturated material above the main ground water body.

Perched Streams — Perched streams are either Losing Streams or Insulated Streams that are separated from the underlying ground water by a zone of aeration. Also see Stream.

Perched Water Table — The top of a Zone of Saturation that bottoms on an impermeable horizon above the level of the general water table in the area. Is generally near the surface, and frequently supplies a hillside spring.

Perchloroethylene (PCE) (Tetrachloroethylene) — A solvent often used for degreasing and in dry cleaning which sometimes makes its way into water wells and other ground water supplies. Studies have shown that high concentrations of the chemical can cause liver and kidney damage, including cancer, in animals. In humans, however, not enough information is available to say it is a definite carcinogen. The U.S. Environmental Protection Agency’s (EPA) safe drinking water standard for tetrachloroethylene is 0.005 parts per million (ppm). Also referred to as Tetrachloroethylene and Perclene.

Percolating Waters — 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. May be rainwater slowly infiltrating through the soil or water seeping through the banks or the bed of a stream, but these waters have left the flow of the stream so that they no longer may be characterized as a part of the stream flow. It is presumed that ground waters percolate.

Percolation —
(1) The movement, under hydrostatic pressure, of water through the interstices of a rock or soil. Also, the movement of water within a porous medium such as soil toward the water table without a definite channel.
(2) The entrance of a portion of the streamflow into the channel materials to contribute to ground water replenishment.
(3) Slow seepage of water through a filter.

Percolation, Deep — The amount of water that passes below the root zone of the crop or vegetation.

Percolation Path — The course followed by water moving or percolating through any permeable material or under a dam which rests on a permeable foundation.

Percolation Pond — Refers to a pond (usually man-made) designed to allow treated wastewater effluent to percolate slowly into the ground. The pond acts as a holding facility while gravity allows the water to percolate or seep through the soil or other unconsolidated medium into the local water table (usually the surfacial aquifer).

Percolation Rate — The rate, usually expressed as a velocity, at which water moves through saturated granular material. Also applies to quantity per unit of time of such movement and has been used erroneously to designate Infiltration Rate or Infiltration Capacity.

Percolation Test — (1) A procedure to measure the drainage characteristics of the soil on a lot. Such tests are required in the proper design of septic tank drainfields. (2) A soil test to determine if soil will take sufficient water seepage for use of a septic tank.

Perennial Stream — A stream that flows from source to mouth throughout the year.

Perennial Yield (Ground Water) — The amount of usable water of a ground water reservoir that can be withdrawn and consumed economically each year for an indefinite period of time. It cannot exceed the sum of the Natural Recharge, the Artificial (or Induced) Recharge, and the Incidental Recharge without causing depletion of the groundwater reservoir. Also referred to as Safe Yield.

Perfected Water Right —
(1) A completed or fully executed water right. A water right is said to have been perfected when all terms and conditions associated with it have been fully accomplished, e.g., the diversion has been effected and the water applied to beneficial use.
(2) A water right to which the owner has applied for and obtained a permit, has complied with the conditions of the permit, and has obtained a license or certification of appropriation.
(3) A water right which indicates that the uses anticipated by an applicant, and made under permit, were made for Beneficial Use. Usually it is irrevocable unless voluntarily canceled or forfeited due to several consecutive years of nonuse. Also referred to as a Certified Water Right.

Perfected Water Permit — A permit issued after the permittee has initiated Beneficial Use of water in accordance with the terms and conditions of the conditional water permit. The perfected water permit is the instrument of conveyance of a water right.

Perfection — The process of meeting terms and conditions of a water right permitting process which results in a Perfected Water Right.

Perforation of Wells — Holes in the casing of wells which allow water to flow into the well.

Periodic Station — A site where stage, discharge, sediment, chemical, or other hydrologic measurements are made one or more times during a year, but at a frequency insufficient to develop a daily record.

Periphyton — An assemblage of microorganisms (plants and animals) firmly attached to and growing upon solid surfaces, such as the bottom of a stream, rocks, logs, pilings, and other structures. While primarily consisting of algae, they also include bacteria, fungi, protozoa, rotifers, and other small organisms. Periphyton are useful indicators of water quality.

Permafrost — The part of the earth’s surface that is permanently frozen. Permanently frozen subsoil, occurring throughout the polar regions and locally in perennially frigid areas. Also see Tundra.

Permanent Control — A stream-gaging control which is substantially unchanging and is not appreciably affected by scour, fill, or backwater.

Permanent Hardness — Water hardness that cannot be reduced or removed by heating the water, a reflection of the presence of dissolved calcium, magnesium, iron and other divalent metal ions. These ions will react to form insoluble precipitates.

Permanent Monument — Fixed monuments or reference markers placed away from the dam which allow movements in the horizontal and vertical Control Points on the dam to be observed by using accurate survey procedures.

Permeability —
(1) The capacity of soil, sediment, or porous rock to transmit water; the property of soil or rock that
allows passage of water through it.
(2) For a rock or an earth material, the ability to transmit fluids; the rate at which liquids pass through soil or other materials in a specified direction. It is measured by the rate at which a fluid of standard viscosity can move through a material in a given interval of time under a given Hydraulic Gradient. Permeability for underground water is sometimes expressed numerically as the number of gallons per day that will flow through a cross section of 1 square foot, at 60EF, under a hydraulic gradient of 100 percent. Permeability is equal to velocity of flow divided by hydraulic gradient. The following permeability terms apply:
[1] Very Slow – less than 0.05 inch per hour;
[2] Slow – 0.05 to 0.20 inch per hour;
[3] Moderately Slow – 0.20 to 0.80 inch per hour;
[4] Moderate – 0.80 to 2.50 inches per hour;
[5] Moderately Rapid – 2.50 to 5.0 inches per hour;
[6] Rapid – 5.0 to 10.0 inches per hour; and
[7] Very Rapid – More than 10.0 inches per hour.

Permeability Coefficient — The rate of flow of water through a unit cross-sectional area under a Unit Hydraulic Gradient at the prevailing temperature (Field Permeability Coefficient), or adjusted to 15EC (59EF).

Permeability, Effective — Observed permeability of a porous medium to one fluid phase, under conditions of physical interaction between the phase and other fluid phases present.

Permeability, Intrinsic —
(1) Relative ease with which porous medium can transmit a fluid under a potential gradient, as a property of the medium itself.
(2) Property of a medium expressing the relative ease with which fluids can pass through.

Permeability Soil — The quality of a soil horizon that enables water or air to move through it. The permeability of a soil may be limited by the presence of one nearly impermeable horizon even though the others are permeable.

Permeable — Having pores or openings that permit liquids or gasses to pass through.

Permeable Soils — Soils that water can easily penetrate and spread through.

Permissible Velocity — (Hydraulics) The highest velocity at which water may be carried safely in a channel or other conduit. Also, the highest velocity that can exist through a substantial length of conduit and not cause scouring of the channel. Also referred to as Safe Velocity or Noneroding Velocity.

Permit —
(1) (Water Right) A written document which grants authority to take unused water and put it to Beneficial Use. If all requirements of the permit are satisfied, then the permit for water appropriation can mature into a license or Perfected Water Right.
(2) (Discharge) A legally binding document issued by a state or federal permit agency to the owner or manager of a point source discharge. The permit document contains a schedule of compliance requiring the permit holder to achieve a specified standard or limitation (by constructing treatment facilities or modifying plant processes) by a specified date. Permit documents typically specify monitoring and reporting requirements to be conducted by the applicant as well as the maximum time period over which the permit is valid.

Persistence — The relative ability of a chemical to remain chemically stable following its release into the environment. Persistent chemicals resist biodegradation and thus are of greater concern in the treatment of water and wastes.

Persistent Emergent — Emergent Hydrophytes that normally remain standing at least until the beginning of the next growing season; e.g., cattails (Typha spp.) or bulrushes (Scirpus spp.).

Persistent Pesticides — Pesticides remaining in the environment for more than one growing season or for more than one year after applications.

Peterson Dredge — A device used to collect sediment samples for the identification of bottom-dwelling animals in lakes and streams. The device has the appearance of a closed metal cylinder sectioned in half through the long axis. Weights can be attached to the outside of the cylinder sections to provide a deeper bite into the sediment. The Peterson dredge is particularly useful in the sampling of sediments that have a high content of sand and gravel.

Phagotroph — An organism that obtains nutrients through the ingestion of solid organic matter. This class of organism includes all animals from the simplest, single-celled animal (for example, the protozoa) to the higher life forms. Organisms have some type of device to ingest particles, a digestive system, and a system to discard waste
products.

Phenology — The science of appearances through and attempt to understand nature’s “clocks and calendars.” Today, it has taken on a meaning that links biological events and seasonal weather changes. These events include the hibernation of animals, the sprouting and flowering of plants, changing of leaf color, and the migration of birds.

Phenols — A group of organic compounds that in very low concentrations produce a taste and odor problem in water and which in higher concentrations are toxic to aquatic life. They are byproducts of petroleum refining, tanning, and textile, dye, and resin manufacturing processes.

Phosphorus — A nutrient essential for growth that can play a key role in stimulating aquatic growth in lakes and streams.

Photic —
(1) Penetrated by or receiving light.
(2) Designating or relating to the layer of a body of water that is penetrated by sufficient sunlight for Photosynthesis. Also see Photic Zone.

Photic Zone — The upper water layer down to the depth of effective light penetration where Photosynthesis balances respiration. This level (the Compensation Level) usually occurs at the depth of 1 percent light penetration (i.e., 1 percent of surface light intensity) and forms the lower boundary of the Zone of Net Metabolic Production.

Photoautotroph — An organism which utilizes carbon dioxide (CO2) for cell growth and obtains its energy from the sun.

Photolysis — The breakdown of a material by sunlight. For example, nitrogen dioxide (NO2) is split into nitric oxide (NO) and atomic oxygen (O) by the ultraviolet energy in sunlight. Photolysis is also an important degradation mechanism for contaminants in surface water and in the terrestrial environment.

Photolysis (of Water) — The lysis of water to give oxygen and hydrogen under the influence of light. This process is the source of the free oxygen formed in photosynthesis.

Photosynthesis — The process in green plants and certain other organisms by which carbohydrates are synthesized from carbon dioxide and water using light as an energy source. Most forms of photosynthesis release oxygen as a byproduct. Chlorophyll typically acts as the catalyst in this process.

Phreatic — Of or relating to ground water.

Phreatic Line — The line marking the upper surface of the Zone of Saturation in the soil.

Phreatic Surface — A term equivalent to the Groundwater Surface or the Water Table; the free surface of ground water at atmospheric pressure.

Phreatophyte —
(1) Literally, a water-loving plant, one that thrives in wet sites and/or has the ability to tap deep
saturation zones.
(2) A deep rooted plant that obtains its water from the water table.
(3) A plant that habitually obtains its water supply from the Zone of Saturation, either directly or through the Capillary Fringe.

Phycology — The study of algae.

Physical-Chemical Treatment Processes — A means of wastewater treatment using both physical and chemical processes.

Physical Landscape — Natural land forms and associated natural phenomena of a region.

Phytoplankton — Microscopic floating plants, mainly algae, that live suspended in bodies of water and that drift about because they cannot move by themselves or because they are too small or too weak to swim effectively against a current.

Phytoplankton Die-Off — An abrupt, massive mortality of Phytoplankton resulting from natural or manmade causes.

Phyto-Remediation (Phytoremediation) — The use of plants to cleanse polluted and contaminated soils. Under this process, certain plants (e.g., sunflowers and Indian mustard) are highly effective in removing heavy metals and other toxic wastes through their uptake of water in the soil, a process which may be enhanced by the addition of chemicals to facilitate the absorption process. The plants are then harvested and discarded in a specially designated site of sent to a smelter where the metals can be extracted and sold off. The process, of course, is limited to thedepth of the plant’s roots.

Phytotoxicant — A chemical that can damage or kill pants in aquatic environments.

Phytotoxicity — The ability of chemicals to damage or kill plants in aquatic environments.

Pirate Stream — One of two streams in adjacent valleys that has been able to deepen its valley more rapidly than the other, has extended its valley headward until it has breached the divide between them, and has captured the upper portion of the neighboring stream.

Pitot Tube — An instrument used to measure the velocity of flowing water, with the velocity head of the stream an index of velocity. It consists essentially of an orifice held to a point upstream in the water, connected with a tube in which the rise of water due to velocity head may be observed and measured. It also may be constructed with an upstream and downstream orifice, with two water columns, in which case the difference in height of the water columns in the tubes is the index of velocity.

Place of Use — The specific location, typically documented in a water right permit, where water is applied or used. A water user cannot use water at another location without transferring the right or obtaining a new right.

Place of Use Limitation — In the context of water law, the act of defining a water right so that the owner of the right may not freely change the place of use without consideration of the effect of such change on other water users.

Plankton — Floating or weakly swimming organisms whose migration is controlled by waves and currents. Animals of the group are called zooplankton and the plants are called phytoplankton.

Plankton Bloom — A large quantity of plankton giving water a definite color. Pond water usually appears green because the majority of plankton organisms are greenish, but plankton blooms may also appear black, yellow, red, brown, or blue-green.

Plasmolysis — Shrinkage or contraction of the protoplasm away from the wall of a living plant or bacterial cell, caused by loss of water through Osmosis.

Plate Boundaries — (Geology) According to the theory of Plate Tectonics, the locations where the rigid plates that comprise the crust of the earth meet. The plates move slowly on the molten material beneath in the process called Continental Drift. As the plates meet, the boundaries can be classified as divergent (places where the plates are moving apart, as at the mid-ocean ridges of the Atlantic Ocean), convergent (places where the plates are colliding, as at the Himalayas Mountains), and transform (places where the plates are sliding past each other, as the San Andreas fault in California).

Plate Tectonics — (Geology) The concept that both continents and ocean basis are only the emergent parts of large pieces or plates of the earth’s surface. It is generally agreed that the global surface can be divided into at least twenty discrete plates (seven major and many minor) with each plate moving in a different direction from that of its neighbor. It is this motion that creates the variety of features of the earth as well as leads to instability along the plate edges. The motion of the plates is believed to be caused by tremendous heat and pressure built up beneath the relatively thin veneer of the overlying plates. The motion of the plates is characterized by spreading centers whereby molten rock is forced to the surface to form new crustal rocks, and collision zones where plates meet and the older, heavier plate is forced beneath the newer, lighter plate to be turned into a molten state once again deep beneath the earth’s surface. This subduction process builds up the mountains along the collision line and results in considerable seismic activity. The seven major plates are named for the continents or oceans and include Pacific, Eurasian, African, Australian, North American, South American, and Antarctic.

Playa Lake — A temporary lake formed in a Playa. A shallow, intermittent lake in an arid region, occupying a playa in the wet season but drying up in the summer; an ephemeral lake that upon evaporation leaves or forms a playa.

Pleistocene — (Geology) Of, belonging to, or designating the geologic time, rock series, and sedimentary deposits of the earlier of the two epochs of the Quaternary Period. This epoch was characterized by the alternate appearance and recession of northern glaciation and the appearance of the progenitors of human beings. Also commonly referred to as the Ice Age, the Pleistocene covered a period of time from about 2 million years ago to 10,000 years ago and immediately preceded the Holocene Epoch, or the period from 10,000 years ago to the present. The late Pleistocene is generally considered to be the Wisconsinan Age (North America), which extended from about 300,000 years ago to 10,000 years ago and the beginning of the Holocene.

Pleuston —
(1) Plants that float on the surface of bodies of fresh water.
(2) Organisms living in the thin surface layer existing at the air-water interface of a body of fresh water.

Pliocene — (Geology) The epoch immediately preceding the Pleistocene which lasted for about 10 million years’ duration from about 12 million years ago to about 2 millions years ago.

Plug —
(1) The procedure by which a well is sealed after it has been abandoned.
(2) Cement, grout, or other material used to fill and seal a hole drilled for a water well.

Plug Flow — A type of flow that occurs in tanks, basins, or reactors when a slug of water moves through without ever dispersing or mixing with the rest of the water flowing through.

Plume —
(1) (Ecology) A space in air, water, or soil containing pollutants released from a point source.
(2) (Water Pollution) A relatively concentrated mass of emitted chemical contaminants spreading in the environment. In surface water, the effluent added to a receiving stream near a point source. For example, when a heated-water discharge is added to a stream, the heated water does not mix immediately with the stream water. The mass of hot water remains detectable for some distance downstream. In groundwater, the Leachate leaking down-gradient from a site of buried waste material.

Pluvial —
(1) Of having to do with rain; rainy.
(2) To flow, pour, or fill.
(3) (Geology) Formed or caused by the action of rain, as a pluvial deposit.
(4) (Geology) More specifically, the two or more Wisconsin stages, of the late Pleistocene age (epoch), when the western United States waterbasins were filled with lakes. The Early Pluvial period consisted of periods of high humidity so remote as to have left no clear-cut shore features; the Postpluvial period represented a period of desiccation following the last high lake stage.

Pluvial Lake — A lake formed during a pluvial (rainy) period.

Pluvial Period — A period of increased rainfall and decreased evaporation, which prevailed in nonglaciated areas during the time of ice advance elsewhere.

Pluviometer — A Rain Gauge.

Pluvious — Characterized by heavy rainfall; rainy.

Pocosin — An upland swamp of shallow water of the coastal plain of the Southeast United States; a “Dismal”, as used in the southern United States.

Pogonip — A term used in the Western United States denoting a dense winter fog containing frozen particles, formed in the deep valleys of the Sierra Nevada.

Point Bar — A bank on the inside of a meander bend that has built up due to sediment deposition opposite a pool.

Point Discharge — The instantaneous rate of discharge, in contrast to the mean rate for an interval of time.

Point of Compliance (POC) — (Water Quality) For a hazardous waste treatment, storage, or disposal facility, the location, specified by the operations permit of the facility, for downgradient wells. The wells are placed to detect the presence of any contaminants released from the facility into groundwater that will move into the uppermost aquifer in the area.

Point(s) of Diversion — Broadly, the point(s) specified in a water right permit from which water is diverted from a source. Also refers to a river, stream, canal, or reservoir where irrigation water is diverted into an irrigation project.

Point-of-Entry (POE) Treatment Device — (Water Quality) A treatment device applied to the drinking water entering a house or building to reduce the contaminants in the water distributed throughout the house or building.

Point-of-Use (POU) Treatment Device — (Water Quality) An approach to the management of the quality of drinking water that locates a water treatment device at the faucet in an individual household. Such devices are sometimes used in homes supplied by a private well that does not meet drinking water standards.

Point source — A source at a discrete location such as a discharge pipe, drainage ditch, tunnel, well, concentrated livestock operation, or floating craft.

Point Waste Load Allocation — The amount of a particular pollutant a Point Source, e.g., a wastewater treatment facility, can discharge over a specified period of time into a receiving body of water. Allocations are a result of agreed upon water quality standards for a stream.

Polishing — (Water Quality) The removal of low concentrations of dissolved, recalcitrant organic compounds from either water intended for human consumption or wastewater that has been subjected to Primary and Secondary Wastewater Treatment. The passage of water through a charcoal filtering device is a frequently employed polishing technique.

Polyacrylamides (PAMs) — Synthetic polymers with extensive water-retention and water-saving capabilities used to halt erosion and promote dryland farming. The polymers used are long-lasting, gel-forming, water-absorbing materials that can absorb more than 400 times their weight in distilled or pure water. While commercial florists have used PAMs for a number of years, the polymers are now available on a commercial scale to where farmers can incorporate them into the soil like powdered fertilizers and pesticides. In addition to direct application to both soil and irrigation water, the polymers may also be woven into a fabric to cover the surface for weed control and additional moisture conservation.

Polychlorinated biphenyls (PCBs) — A mixture of chlorinated derivatives of biphenyl, marketed under the trade name Aroclor with a number designating the chlorine content (such as Aroclor 1260). PCBs were used in transformers and capacitors for insulating purposes and in gas pipeline systems as a lubricant. Further sale for new use was banned by law in 1979.

Pond, Wastewater Stabilization — (Water Quality) An impoundment area for water, natural or artificial, into which untreated or partially treated wastewater is discharged and in which natural purification and stabilization processes take place under the influence of sunlight, air, and biological activity.

Pondage —
(1) The holding back of water for later release for power development above the dam of a hydroelectric plant to: (a) equalize daily or weekly fluctuations of streamflow, or (b) to permit irregular hourly use of water by the wheels to take care of fluctuations in the load demand.
(2) The water so held back and later released.
(3) The storage capacity available for the use of such water.

Pool —
(1) A location in an active stream channel, usually located on the outside bends of meanders, where the water is deepest and has reduced current velocities.
(2) A deep reach of a stream; a part of the stream with depth greater than the surrounding areas frequented by fish. The reach of a stream between two riffles; a small and relatively deep body of quiet water in a stream or river. Natural streams often consist of a succession of pools and riffles.

Pool-Riffle Ratio — The ratio of pool and riffle areas, or pool and riffle length in a given stream reach.

Pore Pressure — Pressure exerted by fluid in the void space of soil or rock; the interstitial (pore) movement of water that may take place through a dam, its foundation, or its abutments.

Pore Space — That portion of rock or soil not occupied by solid mineral matter and which may be occupied by ground water.

Porosity — Most generally, porosity is the property of containing openings or interstices. In rock or soil, it is the ratio (usually expressed as a percentage) of the volume of openings in the material to the bulk volume of the material. With respect to water, porosity is a measure of the water-bearing capacity of a formation. However, with respect to water extraction and movement, it is not just the total magnitude of porosity that is important, but the size of the voids and the extent to which they are interconnected, as the pores in a formation may be open, or interconnected, or closed and isolated. For example, clay may have a very high porosity with respect to potential water content, but it constitutes a poor medium as an aquifer. More important in this respect are a formation’s Effective Porosity (defined below) and its Specific Retention.

Porosity, Effective — The amount of interconnected pore space in a material available for fluid transmission; expressed as a percentage of the total volume occupied by the interconnecting interstices. Porosity may be primary, formed during deposition or cementation of the material, or secondary, formed after deposition or cementation, such as fractures.

Post-Closure Plan — A document prepared by a hazardous waste treatment, storage, or disposal facility outlining the groundwater monitoring and reporting, waste containment provisions, and security arrangement for the 30–year period following closure.

Potable Water — Water that is drinkable. Specifically, freshwater that generally meets the standards in quality as established in the U.S. Environmental Protection Agency (EPA) Drinking Water Standards for drinking water throughout the United States. Potable water is considered safe for human consumption and is often referred to as Drinking Water. Freshwater that exceeds established chloride and dissolved solids limits is often referred to as slightly saline, brackish, or nonpotable water and is either diluted with fresher water or treated through a desalination process to meet potable-water standards for public supply. Also see Drinking Water Standards and Drinking Water Standards [Nevada]. [Appendix B–3 presents a listing of Nevada’s current drinking water primary and secondary quality standards. Also see Appendix B–1 and B–2 for Safe Drinking Water Act (SDWA) regulated
contaminants and proposed contaminants to be regulated.

Potamon Zone — Stream reach at lower elevations characterized by reduced flow, higher temperature, and lower dissolved oxygen levels. Also see Rhithron Zone.

Potential —
(1) (Hydrology and Hydraulics) Any of several scalar variables, each involving energy as a function of
position or condition; of relevance here is the fluid potential of ground water.
(2) (Water Quality) A water quality issue or problem identified by a river authority as being a potential problem, or a problem without current supporting data.

Potential Evapotranspiration —
(1) The maximum quantity of water capable of being evaporated from the soil and transpired from the vegetation of a specified region in a given time interval under existing climatic conditions, expressed as depth of water.
(2) The water loss that will occur if at not time there is a deficiency of water in the soil for use by vegetation.

Potential Natural Water Loss — The water loss during years when the annual precipitation greatly exceeds the average water loss. It represents the approximate upper limit of water loss from the type and density of vegetation native to a basin, under conditions of actual moisture supply and other basin characteristics, in contrast to Potential Evapotranspiration which represents a hypothetical condition where there is no deficiency of water in the soil for use by the type and density of vegetation that would develop.

Potential Rate of Evaporation — The rate of evaporation under the existing atmospheric conditions from a surface of water that is chemically pure and has the temperature of the atmosphere. Also referred to as Evaporativity.

Potential Yield (or Well Capacity) — The maximum rate at which a well will yield water under a stipulated set of conditions, such as a given drawdown, pump, and motor or engine size. Well capacity may be expressed in terms of gallons per minute, cubic feet per second, or other similar units.

Potentiometric Surface — A surface which represents the static head of ground water in tightly cased wells that tap a water-bearing rock unit (i.e., aquifer). In relation to an aquifer, the potentiometric surface is defined by the levels to which water will rise in tightly cased wells. If the head varies significantly with depth in the aquifer, then there may be more than one potentiometric surface. The Water Table is a particular potentiometric surface for an Unconfined Aquifer.

Potometer — An apparatus for measuring the rate of transpiration in a plant by determining the amount of water absorbed.

Practicably Irrigable Acreage (PIA) — The standard (as established in Arizona v. California, 373 U.S. 546 [1963], decreed in final form, 376 U.S. 340 [1964], decree amended, 383 U.S. 268 [1966], second supplemental decree entered, 466 U.S. 144 [1984]) for quantifying reserved water rights on an Indian reservation set aside with the intent that its inhabitants pursue agriculture [or, assumed by extension, water-related economic pursuits, e.g., fish hatcheries]. Under this standard Indian tribes are legally entitled to the amount of water needed to irrigate all practicably irrigable acreage within their reservation boundaries. Furthermore, these water rights have a priority date equal to the date at which the reservation was established. Under the Winters Doctrine, which is the foundation of this policy, practicably irrigable acreage must meet two criteria:
(1) the land must be able to reasonably sustain crops; and
(2) the cost of supply water to the crops must not be unreasonable.

Precautionary Drawdown — Release of water from a reservoir or other water impoundment facility initiated by projections that inflows will cause water in the structure to exceed desired levels.

Precipitable Water — The total water vapor contained in an atmospheric column of unit cross-sectional area; expressed in terms of water of the same cross-sectional area.

Precipitant — An agent added to a liquid mixture to encourage the formation of solid materials that will settle from the mixture. For example, alum (aluminum sulfate) is added to sewage to promote the formation of Floc, which facilitates the removal of organic materials from the wastewater.

Precipitate — A solid which forms from a liquid suspension as a result of a chemical reaction. The material (floc) is insoluble in water and will settle out over time.

Precipitation —
(1) The downward movement of water in liquid or solid form from the atmosphere following condensation in the atmosphere due to cooling of the air below the dew point. Includes rain, snow, hail, and sleet.
(2) As used in Hydrology, precipitation is the discharge of water, in liquid or solid state, from the atmosphere, generally onto a land or water surface. It is the common process by which atmospheric water becomes surface or subsurface water. The term “precipitation” is also commonly used to designate the quantity of water that is precipitated. Forms of precipitation include drizzle, rainfall, glaze, sleet, snow, graupel, small hail, and hail. Also, the process of separating mineral constituents from a solution by evaporation (halite, anhydrite) or from magma to form igneous rocks.

Precipitation, Effective — In agriculture, that portion of the rainfall that remains in the soil and contributes to crop growth.

Preference System — State laws or constitutional provisions which establish preferred used for water over other types of uses. Such systems typically rank domestic or municipal use as the highest, then agricultural use second, and industrial and mining uses next.

Prescribed Water Rights —
(1) Water rights to which legal title is acquired by long possession and use without protest of other parties.
(2) Water use rights gained by trespass or unauthorized taking that ripen into a title; on a par with rights to land gained through adverse possession. To perfect the right, the use of water must be adverse, hostile, open and continuous for five continuous years against the recognized water rights holder.

Prescription — A method of acquisition of title or the use of water by immemorial or long-continued enjoyment. The right of prescription may not exist where water rights laws and a strict permitting process is enforced.

Prescriptive Water Rights — Water rights which are acquired by diverting water and putting it to use in accordance with specified procedures, e.g., filing a request with a state agency to use unused water in a stream, river, or lake.

Presedimentation — (Water Quality) A pretreatment process used to remove gravel, sand, and other gritty material from raw water before it enters the main treatment plant. This is usually done without the use of coagulating chemicals.

Preservation — The natural resources policy that stresses the aesthetic aspects of forests, rivers, wetlands, and other areas and tends to favor leaving such areas in an undisturbed state.

Pressure Filter — (Water Quality) A device used to remove fine particulate matter from water. The filter consists of a filter medium, such as sand or anthracite coal, packed in a watertight vessel.

Pressure Filtration — (Water Quality) A process by which liquid is removed from a sludge by using external pressure to force it through a filter.

Presumptive Test — (Water Quality) The first of three steps in the analysis of water or wastewater for the presence of bacteria of fecal origin. Portions of a water sample are inoculated into lactose broth and incubated for 24 hours at 37EC (98.6EF). The presence of acid and gas after that time is a positive test, and the water is presumed to be contaminated.

Price (or Pygmy) Current Meter — A (water) current meter with a series of conical cups fastened to a flat framework through which a pin extends. Flowing water rotates the cups around the pin in a horizontal plane, and acoustical or electrical devices register the number of revolutions, from which the velocity of the water can be computed.

Price Elasticity (of Water) — Defined as the ratio of the percent change in the quantity demanded of water (or any other economic good) and the percent change in price, or nwater = Percent Change in Qwater / Percent Change in Pwater An elastic demand results when the ratio of nwater is greater than unity (>1), implying that a given change in price will result in a greater (percentage) change in the quantity demanded. Under such conditions of “elastic demand” for water, consumers tend to be responsive to changes in the price for water. Conversely, an inelastic demand results when the ratio of nwater is less than unity (<1), implying that a given change in price will result in a smaller (percentage) change in the quantity demanded. Under such conditions of “inelastic demand,” consumers are relatively unresponsive to changes in the price for water. Along any given (downward sloping) demand curve, the elasticity will vary from inelastic, to unity, to elastic as the price rises further.

Primacy —
(1) Term used to denote that individual states have been delegated the authority to implement the requirements, as prescribed by the U.S. Environmental Protection Agency (EPA), of the Safe Drinking Water Act (SDWA) and amendments thereto.
(2) Primary enforcement responsibility for administration and enforcement of the primary drinking water regulations and related requirements applicable to public water systems within a state.

Primary Cost — (FEMA) The cost of providing the basic floodproofing feature – elevation, flood shield, floodwall or levee.

Primary Productivity — A measure of algal productivity or rate of growth in a body of water, the primary productivity measures the mass of carbon used annually by algae per unit area of lake surface. This measure, also referred to as the Algal Growth Rate, is expressed as an index figure in grams of carbon per square meter per year, and indicates the state of Eutrophication of a body of water. Algal productivity is influenced by the quantities of nutrients that flow into, or fall onto, the lake each year and the number of days of sunshine. Another important factor is the mixing of the lake, which brings up to the surface where algae exist nutrients which have accumulated near the bottom of the lake.

Prime Water — Generally, any water delivered via a canal to a headgate; irrigation water released from storage for deliver to a headgate.

Priming —
(1) The first filling or first seasonal filling of a canal, reservoir, or other structure with water.
(2) Starting the flow, as in a pump or siphon.

Principal Spillway — Allows discharge of water from a reservoir when the water level exceeds the top of the spillway. Principal spillways are used to allow small inflows to be released from the reservoir.

Prior Appropriation Doctrine —
(1) A concept in water law under which a right to a given quantity of water is determined by such a procedure as having the earliest Priority Date.
(2) The system for allocating water to private individuals used in most of the western United States. The doctrine of Prior Appropriation was in common use throughout the arid west as early settlers and miners began to develop the land. The prior appropriation doctrine is based on the concept of “First in Time, First in Right”. The first person to take a quantity of water and put it to Beneficial Use has a higher priority of right than a subsequent user. Under drought conditions, higher priority users are satisfied before junior users receive water. Appropriative rights can be lost through nonuse; they can also be sold or transferred apart from the land.

Priority — The concept that the person first using water has a better right to it than those commencing their use later. An appropriator is usually assigned a “priority date”. However, the date is not significant in and of itself, but only in relation to the dates assigned other water users from the same source of water. Priority is only important when the quantity of available water is insufficient to meet the needs of all those having a right to use water.

Priority Date — The date of establishment of a water right; the officially recognized date associated with a water right. The rights established by application have the application date as the date of priority. Relative to other water rights, the priority date may make a water right senior (predating other rights) or junior (subordinate to other rights).

Priority of Use and Statutory Preferences — Under appropriation water law systems, priority of use refers to the date a water right is acquired, with senior rights prevailing over junior rights. Priority of use must be distinguished from statutory preferences (designations), which refer to statutory statements of preference among types of beneficial use and would come into play, for example, in deciding which of two concurrent water rights should be satisfied first during a shortage of water or which of two competing applications for a water right should be granted.

Probable Maximum Flood (PMF) —
(1) The largest flood for which there is any reasonable expectancy in a particular climatic era.
(2) The most severe flood that may be expected from a combination of the most critical meteorological and hydrological conditions that are reasonably possible in the drainage basin. It is used in designing high-risk flood protection works and siting or structures and facilities that must be subject to almost no risk of flooding. The probable maximum flood is usually much larger than the 100-year flood.

Process Wastewater — Any water that comes into contact with any raw material, product, byproduct, or waste.

Process Water — Any water that comes in contact with a new material or product. The water is often released as wastewater following use.

Profundal Zone — The deep, bottom-water area beyond the depth of effective light penetration. Includes all the lake floor beneath the Hypolimnion.

Project Conveyance Efficiency — The project conveyance efficiency is an expression representing those “losses” (seepage, operation spills, evaporation, etc.) experienced by an irrigation project conveyance system while transporting water from the diversion point to the farm headgate(s). Project Conveyance Efficiency = (Total) Farm Headgate Delivery (in acre-feet per year) divided by (Total) Diversion for Irrigation at Dam (in acre-feet per year) expressed as a percentage.

Project Crop Water Requirement — The project crop water requirement is the annual amount of water required to meet the total project’s crop consumptive use plus leaching requirement, and adjusted for natural precipitation (expressed in acre-feet per year).

Project Dependable Capacity (PDC) — The power capacity that a hydropower system can reliably deliver at any time. Due to the uncertainties inherent in hydrologic forecasting, and hence reservoir system inflows, defining what is meant by a reliable power capacity is crucial is this determination and requires extensive computer modeling of historical inflows to the storage system.

Proof of Beneficial Use — A part of the water right application and permitting process which documents that the water permitted for use has been applied to Beneficial Use. Generally, this process is followed by the issuance of the water right certificate (Perfected Water Right).

Proof of Appropriation — Part of the water right application and permitting process which attests to the fact that water has been withdrawn for the use specified in the original permit request.

Proper Functioning Condition — The functioning condition of riparian/wetlands is a result of interactions among geology, soil, water and vegetation. Riparian/wetland areas are functioning properly when adequate vegetation is present to dissipate stream energy associated with high water flows, thereby reducing erosion and improving water quality; filter sediment and aid floodplain development; improve floodwater retention and groundwater recharge; develop root masses that stabilize streambanks against cutting actions; develop diverse pond and channel characteristics to provide habitat and the water depth, duration and temperature necessary for fish production, waterfowl breeding and other uses; and support greater biodiversity.

Public Scoping — The process of soliciting public comments on the issues to be examined in environmental documents such as an Environmental Impact Statement (EIS) or water planning documents. The process can be carried out by public meetings, soliciting written comments, or both. The identification of issues, alternatives, impacts, mitigation and/or monitoring all may be addressed during the scoping process.

Putrefaction — The biological decomposition of organic matter by bacteria, fungi, and oxidation, resulting in the formation of foul-smelling products, typically of swamps, bogs, and other areas of persistent moisture; a rotting. A biological process most closely associated with Anaerobic conditions.