R2 (Coefficient of Determination)  (Statistics) A measure of the fraction of the variance of the Dependent, or Endogenous Variable explained by the Independent, or Exogenous Variable(s).  Where only two variables are involved, i.e., the dependent variable and a single independent variable, the coefficient of determination (R2) is also equal to the square of the Correlation Coefficient.
Race  (1) A strong or swift current of water.  (2) The channel of such a current.  Also, an artificial channel built to transport water and use its energy; a Raceway.
Raceway  A rectangular fish rearing unit that has a continuous flow of freshwater to maintain suitable oxygen, temperature, and cleanliness for intensive production.
Radial Drainage  An arrangement of stream courses in which the streams radiate outward in all directions from a central zone or inward from all directions to a central area.
Radial Flow  The flow of water in an aquifer toward a vertically oriented well.
Radioisotope  Isotopic forms of an element that exhibit radioactivity.  Isotopes are varieties of a chemical element that differ in atomic weight, but are very nearly alike in chemical properties.  The difference arises because the atoms of the isotopic forms of an element differ in the number of neutrons in the nucleus.  For example, ordinary chlorine is a mixture of isotopes having atomic weights of 35 and 37, and the natural mixture has an atomic weight of about 35.543.  Many of the elements similarly exist as mixtures of isotopes, and a great many new isotopes have been produced in the operation of nuclear devices such as the cyclotron.  There are 275 isotopes of the 81 stable elements, in addition to more than 800 radioactive isotopes.
Radionuclides  Radioactive chemicals that are usually naturally occurring and found in drinking water.  Typical radionuclides for which the U.S. Environmental Protection Agency (EPA) has established Maximum Contaminant Levels (MCLs) as part of its enforcement of the Safe Drinking Water Act (SDWA) include radium 226 and 228, gross alpha particle activity, and beta particle activity.
Radius of Influence  The radial distance from the center of a well bore to the point where there is no lowering of the water table or Potentiometric Surface (the edge of its Cone of Depression).
Radon  A radioactive element, chemical symbol Rn, atomic number 86, and atomic weight 222 (Radon-222).  Radon is a colorless, tasteless, odorless, naturally-occurring inert gas derived from the natural breakdown (i.e., radioactive decay) of three radioactive is
Rain  (1) The liquid form of precipitation.  (2) Water falling to earth in drops that have been condensed from moisture in the atmosphere.  Generally larger than 0.02 inches (0.05 cm) in diameter and which fall in still air at velocities usually greater than 10 feet (3.0 meters) per second.
Rain Area  (1) The area indicated on a weather map over which rain fell within a certain period of time.  (2) The area over which rain is falling.  (3) The most rainy portion of a cyclonic storm.
Rainband  (Physics and Meteorology) A dark band in the yellow portion of the solar spectrum near the sodium lines, caused by watery vapor in the atmosphere, and hence sometimes used in weather predictions.
Rain Barrel  A barrel of, or for rain water, particularly a barrel placed so as to catch water dripping from eaves of a house or other buildings.
Rainbow  (Meteorology) A circular bow or arc exhibiting, in concentric bands of light, the several colors of the spectrum, and formed opposite the sun by the refraction and reflection of the sun's rays in drops of rain.  Also, a similar arc may be formed by the m
Raindrop  A drop of rain.
Rainfall  (1) A shower or fall of rain.  (2) The quantity of water that falls as rain only in a specified area and time interval.  Not strictly synonymous with Precipitation.
Rainfall Component  That part of the flow of a channel attributed to rain falling directly on the surface of the channel.
Rainfall Duration  The period of time during which rainfall occurs, exceeds a given intensity, or maintains a given intensity.
Rainfall, Excess  That part of the rain in a given storm which falls at intensities exceeding the Infiltration Capacity of the land.  The volume of rainfall available for direct runoff.  It is equal to the total rainfall minus Interception, Depression Storage, and Absorption.
Rainfall, Excessive  Rainfall in which the rate of fall is greater than certain adopted limits, chosen with regard to the normal precipitation (excluding snow) of a given place or area.  Within the United States, according to the National Weather Service (NWS), it is defined, for states along the southern Atlantic coast and the Gulf coast, as rainfall in which the depth of precipitation is 0.90 inch at the end of 30 minutes and 1.50 inches at the end of one hour, and for the rest of the country as rainfall in which the depth of precipitation at the end of each of the same periods is 0.50 and 0.80 inch, respectively.
Rainfall Frequency  The frequency, usually expressed in years, at which a given rainfall intensity and duration can be expected to be equaled or exceeded.
Rainfall (Precipitation) Intensity Area Curve  A curve which expresses the relation between average rainfall or precipitation depth (or the rate) and the area over which it occurs for a given storm duration.  Each curve generally covers a period of years during which the intensities shown will not, on the average, be exceeded more than once.
Rainfall (Precipitation) Intensity-Duration Curve  A curve showing the relationship between average rainfall or precipitation depth (or the rate) and storm duration in a given area.
Rainfall (Precipitation) Intensity-Duration-frequency Curve  Curves showing the relationship between rainfall or precipitation intensity and duration for different levels of frequency; each curve represents the rainfall intensity-duration which will be equaled or exceeded once in a certain number of years, indicated as the frequency of that curve.
Rainfall (Precipitation) Intensity Frequency  The average time interval between the occurrence of the rainfall or precipitation of a given or greater intensity.
Rainfall, Maximization  (1) Moisture Maximization: The process of adjusting precipitation upward to a theoretical value that would have pertained if the moisture content of the air had been at the maximum with other storm conditions remaining unchanged.  (2) Sequential Maximization: Reducing the observed elapsed time between storms to develop a hypothetical severe precipitation sequence.  (3) Spatial Maximization: Reducing the distance between precipitation storms for a hypothetical severe sequence.
Rainfall Rate  The amount of precipitation occurring in a unit of time; generally expressed in inches per hour.
Rainfall (Precipitation), Residual  Rain (or precipitation) that falls at the end of a storm at a rate less than the infiltration capacity.
Rain Forest  A tropical woodland that has an annual rainfall of at least 100 inches (254 centimeters) and often much more, typically restricted to certain lowland areas.
Rain Gauge, also Rain Gage  An instrument for catching and measuring the depth of rainfall.  There are various kinds and sizes of rain gages, most of which catch the rainfall in a collector or cross-sectional area larger than that of the measuring compartment, so that a given depth of water in the latter represents a considerably smaller depth of rainfall catch.
Rain, Heavy  Rain which, at the time of observation, is falling with an intensity in excess of 0.30 inches (0.76 cm) per hour (over 0.03 inches (0.08 cm) in 6 minutes).  Also see Precipitation.
Rain, Light  Rain which, at the time of observation, is falling with an intensity of between a trace and 0.10 inches (0.25 cm) per hour (0.01 inches (0.03 cm) in 6 minutes).  Also see Precipitation.
Rainmaker  One who is supposedly capable of producing rain.
Rain, Moderate  Rain which, at the time of observation, is falling with an intensity of between 0.10 inches (0.25 cm) per hour (0.01 inches (0.03 cm) in 6 minutes) and 0.30 inches (0.76 cm) per hour (0.03 inches (0.08 cm) in 6 minutes).  Also see Precipitation.
Rain, Net  The portion of rainfall which reaches a stream channel or concentration point as direct surface flow.
Rain Sensor  A simple, relatively inexpensive device that measures rainfall and prevents unnecessary irrigation with an automatic controller.
Rain Shadow  A dry region on the lee side of a topographic barrier, usually a mountain range, where the rainfall is noticeably less than on the windward side.
Rainstick  A common term for a type of tubular rattle that mimics the sound of rainfall; a primitive musical instrument in which sound is produced by the movement of particles (sand, stones, etc.) through a hollow tube with an internal matrix that is closed on each end.
Rainwater  Water that has fallen as rain and contains little dissolved mineral matter.
Raised Bogs  See Peatland.
Random Sample  (Statistics) A sample selected in such a manner that all possible samples of the same size have an equal and independent chance of being included.
Random Variable  (Statistics) A variable characterized by random behavior in assuming its different possible values.  Mathematically, it is described by its probability distribution, which specifies the possible values of a random variable together with the probability associated with each value.
Range  (1) Geographic region in which a given plant or animal lives and grows.  (2) Lands that support an understory of periodic cover of herbaceous or shrubby plants suitable for grazing.  The following represent some common range classifications:
(1)  Range, Primary-  Includes areas which are readily accessible, have available water and will be overused before livestock significantly graze other areas;
(2)  Range, Secondary- Areas less preferred by livestock which will ordinarily not be grazed significantly until the primary range has been overused;  
(3)  Range, Suitable- Lands that are or can be made accessible to livestock, that produce forage or have inherent forage producing capabilities, and that can be grazed on a sustained yield basis under given management goals;
(4)  Range, Transitory- Lands temporarily suitable for grazing, but transient over time and/or location, for example, grass may cover an area for a period before being replaced by growth not suitable for forage;  
(5)  Range, Unsuitable- Areas that should not be grazed by livestock because of unstable soils, steep topography, or inherent low potential for forage production.
Range Carrying Capacity  Permitted Animal Unit Month (AUM) production.
Range Condition  The state of the plant community on a range site in relation to the potential natural plant community for that site.  Ratings generally follow categories of poor, fair, good, or excellent.
Range Line  In the generally recognized U.S. Public Land Survey, every 24 miles a station is indicated measuring both east and west from a predetermined principal meridian.  Similar measurements are also taken north and south from a predetermined base line.  In this manner a succession of quadrilaterals are formed, each roughly 24 miles square.  Each of these is subdivided into 16 smaller quadrilaterals roughly six miles square.  In this system of grid lines, north-south lines become Range Lines and east-west lines become Township Lines.
Rankine Scale  A scale of absolute temperature using Fahrenheit degrees, in which the freezing point of water is 491.69? and the boiling point of water is 671.69?.  (Named after William John Macquorn Rankine (1820-1872), Scottish engineer and physicist.)
Rapid Drawdown  Lowering the elevation of water against a bank faster than the bank can drain, leaving a pressure imbalance that may cause the bank to fail.
Rapids  A part of a stream where the current is moving with a greater swiftness than usual and where the water surface is broken by obstructions, but without a sufficient break in slope to form a water fall, as where the water descends over a series of small steps.  It commonly results from a sudden steepening of the stream  gradient, from the presence of a restricted channel, or from the unequal resistance of the successive rocks traversed by the stream.
Rapid Sand Filter  Generally a concrete basin filled with graded gravel and coarse sand. Filtration rates are about 40 times higher than in a slow sand filter, and cleaning is via a backwash operation.
Rapture of the Deep  A state of euphoria and exhilaration that occurs when nitrogen in normal air enters the bloodstream at approximately seven times atmospheric pressure (as in deep-water diving).  Also called Nitrogen Narcosis.
Rare Species  A species of plant or animal which, although not presently threatened with extinction, is in such small numbers throughout its range that it may be endangered if its environment worsens.
Rate Structures (Water and Wastewater Treatment)  Rate structures for water and wastewater treatment are generally classified into three primary categories:  declining block, uniform, and inverted.  Details of these primary rate structures are as follows:
(1)  Declining Block Rate- Provides a means of recovering costs from the customer classes under a single rate schedule, recognizing the different water and wastewater demands and costs associated with each customer class.  Under this rate schedule economies of scale are recognized since the price per unit declines as the water customer consumes more water;  
(2)  Uniform Rate- Provides separate rates for each customer class based on the demand, use, and other characteristics of the customer class;
(3)  Inverted-Block Rate- Incorporates a unit charge that increases with increasing water consumption or demands for wastewater treatment; requires a multiple blocking structure with the rate per unit of consumption increasing with each successive level.
Variations and applications of these primary rate structures typically include:
(1)  Unmeasured Usage- Flat rate, irrespective of usage, generally based on (pipe) size of service;
(2)  Constant Block with Service Charge- Constant rate per unit, times the number of units consumed, plus flat rate service charge;  
(3)  Increasing Block with Service Charge- Cost per unit increases with increasing (block) rate of water usage, plus flat rate service charge;
(4)  Decreasing Block with Service Charge- Cost per unit decreases with increasing (block) rate of water usage, plus flat rate service charge;
(5)  Increasing Block with Minimum Allowance- Cost per unit increases with increasing (block) rate of water usage over a minimum charge for minimum amount of use (Lifeline Rate).
Rating Curve  (1) A graphic representation of a Rating Table (see below).  (2) A curve showing the relation between Gage Height and discharge of a stream or conduit at a given Gaging Station.  (3) A curve showing the relation between the discharge of a gage, meter, or other hydraulic structure or instrument and the pertinent hydraulic conditions affecting the discharge, such as pressure, hydrostatic head, and velocity of approach.  If more than one condition affects discharge, a family of curves is needed to represent the rating.
Rating Table  (1) A table showing the relation between two mutually dependent quantities or variables over a given range of magnitude.  (2) A table showing the relation between the Gage Height and the discharge of a stream or conduit at a given Gaging Station.  (3) A table showing the relationship between the stage in a reservoir and its volume.  Also referred to as Discharge Table.
Rational Method (or Formula)  (1) A simple procedure for calculating the direct precipitation peak runoff from a watershed, using the rainfall intensity, the area of the watershed, and the runoff coefficient appropriate for the type of watershed runoff surface.  (2) A technique for estimating peak discharge rates based on average rainfall intensity (i), the drainage area (A), and a coefficient based on watershed characteristics (C).  The discharge in cubic feet per second is derived from the following formula: Q = CiA.  The rational method is commonly applied to areas as large as 5 square miles, but is preferably used for drainage areas under a half square mile.  The 2-, 5-, 10-, 25-, and 50-year flood recurrence discharges can be estimated with this formula.
Rates of Rise and Fall  How rapidly the elevation of the water rises and falls during a flood.
Ravine  (1) A deep, narrow valley or gorge in the earth's surface worn by running water.  (2) A small narrow steep-sided valley that is larger than a gully and smaller than a canyon and that is usually worn by running water.
Raw Sewage  Untreated domestic or commercial wastewater.
Raw Sludge  The material settled out during the primary clarification of sewage.
Raw Water  (1) Water that is direct from the source ground or surface water  without any treatment.  (2) Untreated water, usually that entering the first unit of a water treatment plant.
RCRA  See Resource Conservation and Recovery Act.
Reach (of River)  (1) Most generally, any specified length of a stream, channel, or conveyance.  (2) A length of channel which is uniform in its discharge depth, area, and slope; a relatively homogeneous length of stream having a similar sequence of characteristics.  (3) A length of channel for which a single gage affords a satisfactory measure of the stage and discharge.  (4) The length of a river between two gaging stations.
Readily Water-Soluble Substances  In water pollution, chemicals that are soluble in water at a concentration equal to or greater than one milligram per liter (mg/l).
Reaeration  (1) Absorption of oxygen into water from the atmosphere.  The rate of reaeration is proportional to the oxygen deficit.  (2) Introduction of air into the lower layers of a reservoir.  As the air bubbles form and rise through the water, the oxygen dissolves into the water and replenishes the dissolved oxygen.  The rising bubbles also cause the lower waters to rise to the surface where they take on more oxygen from the atmosphere.
Reaeration (of Streams)  The natural process by which flowing stream water is mixed with the atmosphere, resulting in the addition of Dissolved Oxygen to the water.
Reasonable and Beneficial Use (California)  A requirement in the California State Constitution (Article X, Section 2) that all water resources must be put to beneficial use, preventing waste or unreasonable use or unreasonable methods of use.
Reasonable Pump Lift  A determination of the rate and volume of water to be pumped from an aquifer.  The reasonable pump lift (rate of withdrawal) would include consideration of:
(1)  water quality in the aquifer or the basin, including sea water intrusion, base of fresh water, and lateral or vertical migration of contaminants;
(2)  the ground water management program;
(3)  the thickness of the aquifer;
(4)  the depth of existing wells;
(5)  the capital cost of new wells;
(6)  the net cash flow; and
(7)  the total amount of ground water that can be extracted during one water year by the total number of existing wells.
Reasonable Use  A rule with regard to percolating or riparian water restricting the landowner to a reasonable use of his own rights and property in view of and qualified by the similar rights of others, and the condition that such use not injure others in the enjoyment of their rights.
Reasonable Use Theory  A Riparian Owner may make reasonable use of his water for either natural or artificial wants.  However, he may not so use his rights so as to affect  the quantity of quality of water available to a lower riparian owner.
Réaumur, or Reaumur (R)  Relating to, being, or indicated on a thermometer scale that registers the freezing point of water as 0? and the boiling point a 80?.  Named after René Antoine Ferchault de Réaumur.
Recalcitrant  Of a substance that is degraded at an extremely slow rate if at all when released into the environment.  Consequently, this type of material tends to accumulate in water, soil, and biota.
Recarbonation (Recarbonization)  (Water Quality) The process of introducing carbon dioxide as a final stage in the lime-soda ash softening process.  This lowers the pH and converts carbonates to bicarbonates, thereby stabilizing the solution against precipitates of carbonates.
Receiving Waters  (1) Rivers, lakes, oceans, or other water courses or bodies of water that receive waters from another source.  (2) (Water Quality) Bodies of water that receive treated or untreated effluent discharges.
Recessional Moraine  Glacial Till occurring as ridges where the front of a retreating glacier temporarily held a fixed position.  Also see Moraines, Lateral Moraines, and Terminal Moraines.
Recession Hydrograph (Curve)  A Hydrograph which shows the decreasing rate of runoff following a period of rain or snowmelt.  Since Direct Runoff and Base Runoff recede at different rates, separate curves, called direct runoff recession curves, are generally drawn.  Use of the term Depletion Curve in the sense of base runoff recession is not recommended.
Recharge (Hydrologic)  (1) The downward movement of water through soil to groundwater.  (2) The process by which water is added to the Zone of Saturation.  (3) The introduction of surface or ground water to groundwater storage such as an aquifer.  Recharge or replenishment of groundwater supplies consists of three (3) types:
(1)  Natural Recharge which consists of precipitation or other natural surface flows making their way into groundwater supplies;
(2)  Artificial or Induced Recharge which includes actions by man specifically designed to increase supplies in a groundwater reservoirs through various methods such as water spreading (flooding), ditches, and pumping techniques; and
  (3)  Incidental Recharge which consists of actions, such as irrigation and water diversion, which add to groundwater supplies but are intended for other purposes.
Recharge may also refer to the amount of water so added.
Recharge Area (Groundwater)  (1) The land area over which precipitation infiltrates into soil and percolates downward to replenish an aquifer.  (2) The area in which water reaches the Zone of Saturation by surface infiltration.  Infiltration moves downward into the deeper parts of an aquifer in a recharge area.  Also referred to as a Recharge Zone.
Recharge, Artificial  The designed (as opposed to the natural or incidental) replenishment of ground water storage from surface water supplies.  There exist five (5) common techniques to effect artificial recharge of a groundwater basin:
(1)  Water Spreading consisting of the basin method, stream-channel method, ditch method, and flooding method, all of which tend to divert surface water supplies to effect underground infiltration;
(2)  Recharge Pits designed to take advantage of permeable soil or rock formations;
(3)  Recharge Wells which work directly opposite of pumping wells although have limited scope and are better used for deep, confined aquifers;
(4)  Induced Recharge which results from pumping wells near surface supplies thereby inducing higher discharge towards the well; and
(5)  Wastewater Disposal which includes the use of secondary treatment wastewater in combination with spreading techniques, recharge pits, and recharge wells to reintroduce the water to deep aquifers thereby both increasing the available groundwater supply and also further improving the quality of the wastewater.
Also referred to as Induced Recharge.  Also see Natural Recharge, Incidental Recharge, Injection, and Perennial Yield.
Recharge Basin  A surface facility, often a large pond, used to increase the infiltration of surface water into a ground water basin.
Recharge Boundary  An aquifer system boundary that adds water to the aquifer.  Streams and lakes are typical recharge boundaries.
Recharge, Incidental  Ground water recharge (infiltration) that occurs as a result of human activities unrelated to a recharge project, for example, irrigation and water diversion (unlined canals).  Also see Artificial (or Induced) Recharge, Natural Recharge, and Perennial Yield.
Recharge, Induced  See Artificial Recharge, above.  Also see Injection.
Recharge, Natural  The replenishment of groundwater storage from naturally-occurring surface water supplies such as precipitation and stream flows.  Also see Artificial (or Induced) Recharge, Incidental Recharge, and Perennial Yield.
Recharge Rate  The quantity of water per unit of time that replenishes or refills an aquifer.
Recharge Well  Used in conjunction with artificial or induced ground water recharge techniques, the recharge well works directly opposite of pumping wells to induce surface water into the ground water system.  Based on the nature of the soil and rock being recharged, the use of recharge wells typically have limited scope and are better employed for recharging deep, confined aquifers.  Also see Injection.
Recharge Zone  A land area into which water can infiltrate into an Aquifer relatively easily.  The infiltration replenishes the aquifer.  The location is also referred to as a Recharge Area.
Recirculated Water  Water that is used more than one time before it passes back into the natural hydrological system or discharged into a wastewater system.  Also referred to as Recycled Water.
Recirculation  Water reused within a plant unit.  Sometimes, it also means water discharged by one unit and reused by other units in the same plant.
Reclamation  The act or process of reclaiming or converting a resource to another use, as swamp or desert lands to irrigable lands or urban lands.
(United States) Reclamation Act of 1902  An act passed by Congress on June 17, 1902 which created the U.S. Reclamation Service (USRS, renamed the U.S. Bureau of Reclamation, USBR, in 1923) as a separate entity within the U.S. Department of the Interior (USDI), apart from the U.S. Geological Sur
Reclamation District  (Legal) A district created by legislation for the purpose of reclaiming swamp, marshy, or desert lands and making them suitable for cultivation or habitation.  Usually such districts have power to levy assessments, issue bonds and the like.
Reclamation Drought Index (RDI)  The RDI was developed as a tool for defining drought severity and duration, and for predicting the onset and end of periods of drought.  The impetus for the development of the RDI came from the Reclamation States Drought Assistance Act of 1988, which allowed states to seek federal drought relief assistance.  The RDI is calculated at a river basin level and incorporates the supply components of precipitation, snowpack, streamflow and reservoir storage levels.  The RDI differs from the Surface Water Supply Index (SWSI) in the incorporation of a temperature component.  The RDI is adaptable to a particular region and readily accounts for both climate and water supply factors.  RDI classifications include:
4.0 or more - extremely wet
1.5 to 4.0 - moderately wet
0 to 1.5 - normal to mild wetness
0 to -1.5 - normal to mild drought
-1.5 to -4.0 - moderate drought
-4.0 or less - extreme drought
Also see Drought Indexes (Indices).
(United States) Reclamation Service  A bureau of the U.S. Department of the Interior, created under the Reclamation Act of 1902 and organized to reclaim desert lands (chiefly in the West) by irrigation projects for public sale and to develop electric power.  The U.S. Reclamation Service changed its name to the Bureau of Reclamation (BOR) in 1923.
Reclaimed Sewage  Wastewater treatment-plant effluent that has been diverted or intercepted for use before it reaches a natural waterway or aquifer.
Reclaimed Waste Water  Waste water that becomes suitable for a specific beneficial use as a result of treatment or brackish water demineralized for use.  General types of reclaimed waste water include:
(1)Primary Effluent- reclaimed water that only has had sewage solids removed and is typically used only for surface irrigation of tree, fodder, and fiber crops;
(2)Secondary Effluent- reclaimed water that has had sewage solids removed and has been oxidized and disinfected and is used to irrigate golf courses and cemeteries and provide water for pasture and food crops; and
(3)Tertiary Recycled Water- water produced by conventional sewage treatment followed by more advanced procedures including filtration and disinfection, providing it with the broadest range of uses.
Also see Waste Water Reclamation and "Repurified Water."
Reclaimed Water  Refers to water that has received at least Secondary Wastewater Treatment and is reused after flowing out of a wastewater treatment facility.
Reclamation  (1) The process of land treatment that minimizes water degradation, air pollution, damage to aquatic or wildlife habitat, flooding, erosion, and other adverse effects from surface mining operations including adverse surface effects incidental to underground mines, so that mine lands are reclaimed to a usable condition which is readily adaptable for alternate land uses and creates no danger to public health or safety.  The process may extend to affected land surrounding mining lands, and may require backfilling, grading, resoiling, revegetation, soil compaction, stabilization, and other measures.  (2) May also apply to other land uses and land types, for example, the reclaiming of waste, desert, marshy or submerged land for cultivation, preservation, reuse, etc.
Reclamation Act  Refers to the Reclamation Act which was passed by Congress on June 17, 1902 and authorized the U.S. Secretary of the Interior to locate, construct, operate, and maintain facilities for the storage, diversion, and development of waters for the reclamation
Reclamation District  A subdivision of a state created by legislative authority for the purpose of reclaiming  swamp, marshy, or desert lands within its boundaries and rendering them fit for habitation or cultivation, generally with funds raised by local taxation or the issue of bonds, and sometimes with authority to make rules or ordinances for the regulation of the project.
Reclamation Project  A water development project based on the Reclamation Act of 1902 for the irrigation of arid lands, particularly in the western United States, and for other purposes administered by the U.S. Department of the Interior, Bureau of Reclamation (USBR).  Also see Newlands (Irrigation) Project (Nevada).
Reclamation of Wastewater  The process of treating salvaged water from municipal, industrial, or agricultural waste water sources for beneficial uses, whether by means of special facilities or through natural processes.
Reclamation Withdrawal  A withdrawal of public lands in connection with a reclamation project.
Recommended Maximum Contaminant Level (RMCL)  The maximum level of a contaminant in drinking water at which no known or anticipated adverse affect on human health would occur, and that includes an adequate margin of safety.  Recommended levels are nonenforceable health goals.  Also see Maximum Contaminant Level (MCL).
Reconditioning (Water Well)  The deepening, reaming, casing, recasing, perforating, reperforating, jetting, swabbing, installing of liner pipe, packers and seals or any other significant change in the design or construction of a water well.
Reconnaissance  A preliminary inspection or survey of an area, such as a forest, range, watershed, or wildlife area, to gain general information useful for future management.
Reconnaissance Report  An initial planning document to determine whether further investigation is warranted.
Reconstitute  To bring (a liquid in concentrate or powder form) to normal strength by adding water.
Recorder, Steam Flow  A mechanical apparatus which records a continuous record of a water level or other hydrologic factors such as water temperature, flow rates, etc.
Recording Gage  A Gage which provides a continuous recording of the parameter being monitored.  For example, see Stream Gaging.
Recoverable from Bottom Material  The amount of a given constituent that is in solution after a representative sample of bottom material has been digested by a method (usually using an acid or mixture of acids) that results in dissolution of readily soluble substances.  Complete dissolution of all bottom material is not achieved by the digestion treatment and thus the determination represents less than the total amount (that is, less than 95 percent) of the constituent in the sample.  To achieve comparability of analytical data, equivalent digestion procedures would be required of all laboratories performing such analyses because different digestion procedures are likely to produce different analytical results.
Recoverable Ground Water  The amount of water which may be physically and economically withdrawn from the ground water reservoir.
Recovery  The process by which the decline of an endangered or threatened species is arrested or reversed, and threats to its survival are neutralized, so that its long-term survival in nature can be ensured.
Recovery Plan  A document which delineates, justifies, and schedules the research and management actions necessary to support recovery of a species, including those that, if successfully undertaken, are likely to permit reclassification or delisting of the species.
Recreation Resource  Land and water areas and their natural attributes, with or without man-made facilities, that provide opportunities for outdoor recreation.
Recreation Visitor Day (RVD)  A recreation visitor day is use of a site or area for 12 visitor-hours, aggregated as 1 person for 12 hours, 12 persons for 1 hour, or any equivalent combination of continuous or intermittent use.
Recreational Benefit  The value of a recreational activity to the recreationist, usually measured in dollars above the cost of participating in the recreational activity (travel, lodging, entrance fees, etc.).  Used for valuing recreational resources produced through various water-development projects.  A term used synonymously with the consumer surplus associated with the recreational activity.
Recreational Rivers  A classification under the national Wild and Scenic Rivers Act to include those rivers or sections of rivers that are readily accessible by road or railroad that may have some development along their shorelines, and that may have undergone some impoundments or diversions in the past:
(1) Timber Production - Timber harvesting would be allowed under standard restrictions to protect the immediate river environment, water quality, scenic, fish and wildlife, or other values.
(2) Water Supply - Existing low dams, diversion works, riprap and other minor structures are allowed providing the waterway remains generally natural in appearances.  New structures are prohibited.
(3) Hydroelectric Power - No development of hydroelectric power facilities is provided.
(4) Flood Control - Existing flood control works may be maintained.  New structures are prohibited.
(5) Mining - Subject to regulations (i.e., 36 CFR 228) that the Secretaries of Agricultural and Interior may prescribe to protect the values of rivers included in the national system, new mining claims and mineral leases could be allowed and existing operations allowed to continue.  However, mineral activity must be conducted in a way that minimizes surface disturbances, sedimentation and pollution, and visual impairment.
(6) Road Construction - Paralleling roads or railroads could be constructed on one or both riverbanks.  There can be several bridge crossings and numerous river access points.
(7) Agriculture - Lands may be managed for a full range of agricultural uses, to the extent currently practiced.
(8) Recreational Development - Campgrounds and picnic areas may be established close to the river.  However, the recreational classification does not require extensive recreational development..
(9) Structures - Small communities and dispersed or cluster residential development are allowed.  New structures are allowed for both habitation and for intensive recreation use.
(10) Utilities - New transmission lines, gas lines, water lines, etc., are discouraged.  Where no reasonable alternative exists, additional or new facilities should be restricted to existing right-of-way.  Where new rights-of-ways are indicated, the scenic, recreation, and fish and wildlife values must be evaluated in the selection of the site.
(11) Motorized Travel - Motorized travel on land or water may be permitted, prohibited or restricted.  Controls will usually be similar to surrounding lands and waters.
Also see Wild and Scenic Rivers Act, Wild Rivers, and Scenic Rivers.
Rectangular Pattern  An arrangement of stream courses in which tributaries flow into larger streams at angles approaching 90?.
Rectify  (Chemistry) (1) To refine or purify, especially by distillation.  (2) To adjust (the proof of alcoholic beverages) by adding water or other liquids.
Recurrence Interval (Return Period)  (1) A statistical expression of the average time between floods equaling or exceeding a given magnitude.  (2) The average time interval, usually in years, between the occurrence of a flood or other hydrologic event of a given magnitude or larger.  The re
Recycled Water  (1) Water that is used more than one time before it passes back into the natural hydrologic system.  (2) Water that is used more than one time by the same users.  Also referred to as Recirculated Water.
Recycling  The process by which salvaged materials become usable products.
Red Book  (Water Quality) The 1976 publication issued by the U.S. Environmental Protection Agency (EPA), Quality Criteria for Water, that is used as a basis for ambient water quality standards.
Redd  A type of fish spawning area associated with flowing water and clean gravel.  Fishes that utilize this type of spawning area include trout, salmon, some minnows, etc.
Red Data Book  A collection of the available information relative to Endangered and Threatened Species.  Each volume contains colored loose-leaf information sheets arranged by species.  The sheets are updated as the status of a species changes.  Red sheets are used for those species that are endangered; amber for vulnerable; white for rare; green for out of danger; and gray for species that are indicated to be endangered, vulnerable, or rare, but with insufficient information to be properly classified.  The book is maintained by the International Union for Conservation of Nature and Natural Resources.
Red Snow  Snow on which red-pigmented algae has grown, commonly found in Arctic and alpine regions.
Red Tide  A visible red-to-orange coloration of the sea caused by the presence of a bloom or excessive growth of dinoflagellates in marine waters, resulting in a red, brown, green or yellow tint in the water.  The event causes the death of marine biota and the accumulation of toxins in mussels or clams.  Consumption of the toxin-containing shellfish can cause Paralytic Shellfish Poisoning or severe gastric distress in humans.
Reef  A strip or ridge of rocks, sand, or coral that rises to, or near the surface of a body of water.
Reference Wetland  A wetland within a relatively homogeneous biogeographic region that is representative of a specific hydrogeomorphic wetland type.
Reflectance  The ratio of the upwelling light to incident (downwelling) light.  Reflectance is a strong function of wavelength, and a weak function of incident sunlight and the depth in the water.  The color of natural waters can be objectively specified using their spectral reflectance.  Also see Water Color.
Reforestation  The planting of trees on land from which the forest has been removed.
Regelation  (1) The fusion of two blocks of ice by pressure.  (2) Successive melting under pressure and freezing when pressure is relaxed at the interface of two blocks of ice.
Regime  "Regime theory" is a theory of the formation of channels in material carried by the stream.  Used in this sense, the word "regime" applies only to streams that take at least part of their boundaries from their transported load and part of their transported load from their boundaries, carrying out the process at different places and times in any one stream in a balanced or alternating manner that prevents unlimited growth or removal of boundaries.  A stream, river, or canal of this type is called a "regime stream, river, or canal."  A regime channel is said to be "in regime" when it has achieved average equilibrium; that is, the average values of the quantities that constitute regime do not show a definite trend over a considerable period, generally, approximately a decade.  In unspecialized use, "regime" and "regimen" are synonymous.
Regimen of a Stream  The system or order characteristic of a stream, i.e., its habits with respect to velocity and volume, form and changes in channel, capacity to transport sediment, and amount of material supplied for transportation.  The term is also applied to a stream which has reached an equilibrium between corrosion and deposition or, in other words, to a graded stream.
Regional Response Team (RRT)  An organization under the joint leadership of the U.S. Environmental Protection Agency (EPA) and the U.S. Coast Guard that serves as the organizational unit to provide for planning and preparedness activities related to spills or discharges of oil and hazardous substances and for coordination and technical advice during such spills or discharges in each of the 10 federal regions outlined by the EPA.
Regression  (Statistics) A statistical method for studying and expressing the change in one Variable associated with and dependent upon changes in another related variable or set of variables.
Regression Analysis  (Statistics) Mathematical procedures for attributing the variability of one quantity to changes in one or more other quantities.  Often called "line fitting" or "curve fitting" since it produces an equation that can be used to predict the quantity of interest under many conditions.  The concept is to attempt to fit a mathematical function to a series of data whereby the square of the error terms measuring the differences between the model estimates and actual observations is minimized, hence the term Ordinary Least Squares (OLS) is also used to describe this process.  The standard of regression model is generally termed the Classical Linear Regression (CLR) Model.  The CLR Model has five basic assumptions:
(1)  Linearity- The dependent variable, or the variable to be explained or forecasted, can be calculated as a linear function of a specific set of independent, or explanatory variables;
(2)  Randomness of Disturbance Terms- The expected value of the disturbance term, that is the term showing the differences between the model's estimated values and the actual observed values, is zero;
(3)  Uncorrelated Disturbance Terms- The disturbance terms all have the same variance and are not correlated with each other (see Serial Correlation);
(4)  Data Conformity- The observations on the independent variable can be considered fixed in repeated samples, i.e., it is possible to repeat the sample with the same independent variables;
(5)  Sample Size and Selection- The number of observations is greater than the number of independent variables and that there are no linear relationships, i.e., no significant correlations, between the independent variables (see Multicollinearity).
Regular Program  (FEMA) The phase of the National Flood Insurance Program (NFIP) that makes available increased amounts of flood insurance, with new and substantially improved structures being rated on an actuarial or actual risk basis.
Regulation (of a Stream)  (Hydrology) The artificial manipulation of the flow of a stream.
Regulation Reservoir  A reservoir used in canal and irrigation systems to reduce the mismatch between downstream demands and upstream water supplies in order to maintain a balanced operation.
Regulatory Flood Datum (RFD)  Established plane of reference from which elevation and depth of flooding may be determined for specific locations of the floodplain.  It is the Base Flood plus a Freeboard factor of safety established for  each particular area which tends to compensate for the many unknown and incalculable factors that could contribute to greater flood heights than that computed for a base flood.
Regulatory Floodplain  (1) That portion of the floodplain subject to floodplain regulations (usually the floodplain inundated by the one-percent chance flood).  (2) Flood hazard area within which a community regulates development, including new construction, the repair of substantially damaged buildings, and substantial improvements to existing buildings.  In communities participating in the National Flood Insurance Program (NFIP), the regulatory floodplain must include at least the area inundated by the Base Flood, also referred to as the Special Flood Hazard Area (SFHA).  Also see Floodplain.
Regulatory Floodway  (1) The channel and that portion of the adjacent land area that is required through regulations to pass flood flows without increasing the water surface elevation more than a designated hight.  (2) As adopted into a community's floodplain management ordi
Rehydrate  To cause (something Dehydrated) to take up fluid.
Reimbursable Costs  Those costs associated with a water project that are expected to be recovered, usually from direct beneficiaries, and repaid to the funding entity.  Also see Nonreimbursable Costs.
Rejuvenation  A change in condition of erosion that causes a stream to begin more active erosion and a new cycle.
Relation, Doctrine of  See Doctrine of Relation (Nevada).
Relative Humidity  The ratio of the amount of water vapor in the atmosphere to the amount necessary for saturation at the same temperature.  Relative humidity is expressed in terms of percent and measures the percentage of saturation.
Release  Any spilling, leaking, pumping, pouring, emitting, emptying, discharging, injecting, escaping, leaching, dumping, or disposing into the environment of a hazardous or toxic chemical or extremely hazardous substance.
Relicensing  The process of renewing a license previously issued by the federal government (commonly involving the Federal Energy Regulatory Commission, FERC) to operate a hydroelectric power plant.
Relict  A landform that has survived decay or disintegration (such as an erosion remnant) or that has been left behind after the disappearance of the greater part of its substance (such as a remnant island).
Reliction  An increase of the land by the permanent withdrawal or retrocession of the sea or a river.  The process of gradual exposure of land by permanent recession of a body of water.  The alteration of a boundary line due to the gradual removal of land by a stream serving as the boundary.  Also, the land so left uncovered.  Also see Dereliction.
Reliction, Doctrine of (Nevada)  In a Nevada Supreme Court ruling (State Engineer v. Cowles Bros., 86 Nev. 872, 1964) it was held that the lands so exposed by Reliction should belong to the adjoining land owners.  This held true even for those lands exposed by the recession of a navigable body of water, whose bed is owned by the State of Nevada (e.g., Winnemucca Lake).
Relief Ratio  The average slope of a drainage basin; the ratio of maximum relief to basin length.
Relief Wells (of a Dam)  A vertical well or borehole, usually downstream of impervious cores, grout curtains, or cutoffs, designed to collect and direct seepage through or under a dam to reduce uplift pressure under or within the dam.  A line of such wells forms a relief well curtain.  Also referred to as Drainage Wells.
Remedial Investigation (RI)  (Environmental) An in-depth study designed to gather data needed to determine the nature and extent of contamination at a Superfund site, establish site cleanup criteria, identify preliminary alternatives for remedial action, and support technical and cost analyses of alternatives.  The remedial investigation is usually done with the Feasibility Study (FS).  Together they are usually referred to as the "RI/FS".
Remedial Response  (Environmental) Long-term action that stops or substantially reduces a release or threat of a release of hazardous substances that is serious but not an immediate threat to public health.
Remediation  (Environmental) Cleanup or other methods used to remove or contain a toxic spill or hazardous materials from a Superfund site.
Remnant  A remaining part of some larger landform or of a land surface that has been dissected or partially buried.
Remote Sensing  The measurement or acquisition of information of some property of an object or phenomenon by a recording device that is not in physical or intimate contact with the object or phenomenon under study.  Also, the utilization at a distance (as from aircraft, spacecraft, satellites, or ships) of any device and its attendant display for gathering information pertinent to the environment, such as measurements of force fields, electromagnetic radiation, infrared sensing, land use, water bodies, etc.  Such systems typically employ devices such as cameras, lasers, radio frequency receivers, radar systems, infrared detectors, sonar seismographs, gravimeters, magnetometers, and scintillation counters.
Renewable Energy  A source of energy that is replaced by natural phenomena, such as firewood or the water held by a dam and used for hydroelectric purposes.  Conversely, fossil fuels are a nonrenewable source of energy.
Renewable (Natural) Resource  Natural resources that continuously can be replenished in the course of natural events within the limits of human time.  Also see Natural Resource and Nonrenewable Natural Resource.
Renewable Water Supply  The rate of supply of water (volume per unit of time) potentially or theoretically available for use in a region on an essentially permanent basis.
Replenishment  The act of replenishing an aquifer, usually through artificial recharge, to offset excess groundwater pumping.
Replenishment District  An authority, serving a specific area of water users, which has the ability to combine resources to replenish over drafted aquifers with renewable water supplies.
Replicate Samples  A group of samples collected in a manner such that the samples are thought to be essentially identical in composition.
Representativeness  (Statistics) How well a given sample represents the total population from which it was taken.
Representative Sample  A portion of material or water that is as nearly identical in content and consistency as possible to that in the larger body of material or water being sampled.
Repurified Water  Denotes reclaimed or recycled wastewater that is treated far beyond the most stringent standards current in force and then remixed with fresh water to augment existing water supplies.  Typically, such waters would undergo extensive Tertiary Wastewater Treatment and then be stored in a reservoir (say, for up to one year), subsequently blended with fresh water supplies and then undergo further disinfection through conventional drinking water treatment before being distributed to municipal water users.
Re-Regulating Reservoir  A reservoir for reducing diurnal (daily) fluctuations resulting from the operation of an upstream reservoir for power production.
Reservation  A withdrawal usually of a more or less permanent nature; also, any federal lands of the U.S. government which have been dedicated to a specified public purpose such as a national forest, wildlife refuge, or wilderness area.
Reservation Doctrine, Reserved Rights Doctrine, and Winters Doctrine (or Winters Rights)  The legal rule which states that when the United States reserves public lands for a particular purpose it also reserves sufficient water to accomplish that purpose.  Those who initiate water rights after the date of the reservation are subject to the reserved right.  The doctrine was first announced by the United States Supreme Court in the case of Winters v. United States, 207 U.S. 564 (1908), involving a dispute between an Indian reservation and a rancher.  For many years it was thought that the doctrine only applied to Indian reservations, but in recent years it has been extended to other types of federal reservations, such as national parks and forests.  Also see Winters Rights (Decision) and Practicably Irrigable Acreage (PIA).
Reservation or Withdrawal of Water  The withholding of water from appropriation.  A reservation or withdrawal of water in order to preserve instream values would have the same affect as a Minimum Flow Appropriation for that purpose:  The only essential difference is the procedure used.
Reserve Capacity  Extra treatment capacity built into solid waste and wastewater treatment plants and interceptor sewers to accommodate flow increases due to future population growth.
Reserved Water Rights (Federal)  (1) A category of federal water rights, created by federal law and recognized by judicial decision.  These rights are created when the federal government withdraws land from the public domain to establish a federal reservation such as a national park, fo
Reserves  Natural resources that can be exploited in an economically feasible manner employing current technology.  Compare to Resources.