A  |   B  |   C  |   D  |   E  |   F  |   G  |   H  |   I  |   J  |   K  |   L  |   M  | N  |   O  |   P  |   Q  |   R  |   S  |   T  |   U  |   V  |   W  |   X  |   Y  |   Z

R

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.

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 isotopes: uranium-238, uranium-235, and thorium-232. These isotopes are typically found in igneous and metamorphic rocks, such as granite and gneiss, and in sedimentary rocks such as organic-rich black shale, phosphatic rock, and coal. Uranium-238 is the most common parent of radon gas because it comprises more than 99 percent of uranium and thorium isotopes found on earth. Radon-222, one of the most common radioactive daughter elements of uranium-238 decay has a relatively short half-life of only 3.8 days. Consequently, radon-222 is relatively unstable and more likely to decay and emit radiation at any particular moment. Daughter isotopes of radon-222 have an even shorter half-life, resulting in a cascade or "burst" of radiation from radon and its daughter products. Radon occurs in groundwater, but not in surface water due to its high volatility. In groundwater, radon will stay in solution until the pressure on the groundwater is decreased. Due to its short half-life and the slow rate of natural groundwater flow, radon in groundwater typically cannot migrate far from its source. Radon is relatively easy to remove from water; several effective options for the treatment of radon in water include storage, adsorption on Granular Activated Carbon (GAC), and aeration. It has been proposed that radon provides about one-half of the radiation to which the average American is exposed. The chemically inert gas enters homes through soil, water, and building materials. The threat is not uniformly distributed across the United States. An important source of personal exposure to radon appears to be drinking water obtained from wells. The threat comes from the inhalation of the gas released from water during showering, bathing, cooking, and other water uses. Ingestion of water does not appear to present a threat.

RAIN—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 moon, or some other source of light, or one formed in spray mist, fog, clouds, etc. Rainbows are circular because the drops, being spherical, are alike in every position. In the case of the ordinary rainbow, or Primary Rainbow, the effective rays are refracted on entering each drop, reflected from its interior surface and refracted again on emerging, then passing to the observer's eye. The red is seen on the outside edge of the bow. Also, there is often seen another larger bow, the Secondary Rainbow, concentric with and near the first. Its formation differs from that of the primary rainbow in that there are two internal reflections, it is much fainter, and it has the red on the inside edge. Faint-colored arcs sometimes seen next to the primary or secondary bow, due to interference, are Supernumerary, or Spurious, Rainbows. White Rainbows are sometimes formed by fog or clouds, but are too minute to give distinctly the concentric bands of color of the ordinary rainbow.

RAINDROP—A drop of rain.

RAINFALL—(1) A shower or fall of rain. (2) The quantity of water that falls as rain 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 per