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Unformatted text preview: ial evapotranspiration is the loss of water, as a rate or volume, that would occur from a
continuously saturated land surface through combined evaporation from the soil and
transpiration of plants; potential evapotranspiration is a theoretical loss that assumes an
unlimited, constant supply of water.
Potentiometric surface is a hypothetical concept of ground-water hydrology representing the
height or surface to which water in a well rises or maintains if pumping does not occur; for
confined ground water under artesian pressure the potentiometric surface is higher than the
top of the ground-water body, whereas the potentiometric surface of an unconfined groundwater body is similar to the water table.
Precipitation is the discharge (mm) of water in liquid or solid form from the atmosphere onto
the land, water, and ice surfaces of the earth; precipitation includes condensation as dew or
frost, although this component is typically minor in terms of quantity. Q
Quadrat is a small area, possibly a square with 1-m sides, within which samples or observations
of plants, rock fragments, soil conditions, or other land-surface characteristics are taken; most
commonly quadrats are used in ecological studies to document the species occurring in the
area. Often quadrats are positioned along a transect to obtain detailed knowledge or develop a
statistical analysis of vegetation or other characteristics on a landform or plant community. R
Raindrop impact, or rainsplash, are terms expressing the effect that individual raindrops have
on erosion processes; the energy expended, per unit area, by raindrops when they strike a soil
or rock surface can be very high, resulting in the dislodging of soil particles that then are
susceptible to entrainment by water moving downslope as overland flow to rills, gullies, and
Rating curve is a graph relating water discharge to water-surface elevation (gage height, or
stage) at a specified site or cross section of a stream channel.
Reach (of a stream) refers to an uninterrupted part of a stream channel between two points;
generally the two points are where readily recognizable tributary inflows occur, but can also
include features such as meander bends, gorges, or a significant change in geology (which in
turn could be the cause of a gorge or a waterfall).
Recurrence interval of hydrologic events is the average interval of time, generally expressed in
years, within which, for example, the magnitude, or discharge, of a given flood will be equaled
or exceeded. 34 Regime (or regime theory) is the concept that alluvial stream channels are self-forming and
self-adjusting; the term applies only to channels that make at least part of their boundaries
from their transported load, carrying out the process at different places and times in any one
stream channel in a balanced or alternating manner that prevents unlimited growth or removal
of boundaries. Thus, a stream channel is said to be “in regime” when it has achieved an
approximate equilibrium between matter and energy entering a stream reach and matter and
energy leaving the reach.
Regulation of a stream is an imposed alteration of the discharge and discharge fluctuation of
flows; most commonly regulation is caused by the construction of a dam and the control of
water releases to the channel downstream from the dam. Stream regulation by a dam and
reservoir has the effect of storing much of the fluvial sediment that enters the reservoir from
upstream; consequently, the water released from the dam generally has a highly deficient
sediment load and tends to cause downstream bed and bank erosion and related changes in
Rehabilitation, as applied to stream corridors (bottomlands) that have been altered through
human activity, is the establishment or re-establishment of a condition of health and
constructive activity. A goal of rehabilitation, therefore, is to minimize, not eliminate, the
effects of human-induced alterations, thus promoting stable landforms, bioproductivity, and
species diversity. Within industry, reclamation is often used synonymously with rehabilitation
Restoration, as applied to stream corridors (bottomlands) that have been altered through
human activity, is the attempt to recreate the adjusted physical and biological conditions that
were present prior to the alteration; a goal of restoration, therefore, is to minimize and
eliminate the effects of human-induced alterations, thus promoting stable landforms,
bioproductivity, and species diversity.
Reynolds number, as applied to hydrologic processes, is a numerical quantity to describe the
character of flow (typically laminar or turbulent) in a geologic or landscape setting; it is the ratio
of inertial forces to viscous forces, equal to the product of velocity (often mean velocity) and a
linear measure such as diameter or depth divided by the kinematic viscosity of the liquid
Riffle, as applied to alluvial stream channels, is a short, relatively shallow and coarse-bedded
length over which the stream flows at ordinarily higher velocity and greater turbulence than it
does through upstream and downstream pooled reaches where cross-sectional areas of the
channel are greater, bed material is smaller, and velocities and turbulence are less.
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