In
hydrology,
discharge
is the volume rate of
water flow, including
any suspended solids (i.e. sediment), dissolved chemical species
(i.e. CaCO
_{3}_{(aq)}) and/or biologic material
(i.e. diatoms), which is transported through a given
cross-sectional area. Frequently, other terms synonymous with
discharge are used to describe the volumetric flow rate of water
and are typically discipline dependent. For example, a Fluvial
Hydrologist studying natural river systems may define discharge as
streamflow, whereas an Engineer operating
a reservoir system might define discharge as outflow, which is
contrasted with
inflow.
The
units that are typically
used to express discharge include m³/s (cubic meters per second),
ft³/s (cubic feet per second or cfs) and/or acre-feet per day.
For
example, the average discharge of the Rhine river in
Europe is 2,200 m³/s, 77,704 ft³/s or
~154,000 acre-feet per day.
A commonly applied methodology for measuring, and estimating, the
discharge of a river is based on a simplified form of the
continuity equation. The equation
implies that for any incompressible fluid, such as liquid water,
the discharge (Q) is equal to the product of the stream's
cross-sectional area (A) and its mean velocity (\bar{u}), and is
written as:
- Q=A\,\bar{u}
where
- Q is the discharge ([L^{3}T^{−1}];
m^{3}/s or ft^{3}/s)
- A is the cross-sectional area of the
portion of the channel occupied by the flow ([L^{2}];
m^{2} or ft^{2})
- \bar{u} is the average flow velocity
([LT^{−1}]; m/s or ft/s)
Catchment discharge
The
catchment of a river above a
certain location is determined by the surface area of all land
which drains toward the river from above that point. The river's
discharge at that location depends on the rainfall on the catchment
or
drainage area and the inflow or
outflow of groundwater to or from the area, stream modifications
such as dams and irrigation diversions, as well as evaporation and
evapotranspiration from the area's land and plant surfaces. In
storm hydrology an important
consideration is the stream's
discharge hydrograph, a record of how
the discharge varies over time after a precipitation event. The
stream rises to a peak flow after each precipitation event, then
falls in a slow
recession. Because the
peak flow also corresponds to the maximum water level reached
during the event, it is of interest in flood studies. Analysis of
the relationship between precipitation intensity and duration, and
the response of the stream discharge is mmm by the concept of the
unit hydrograph which represents the
response of stream discharge over time to the application of a
hypothetical "unit" amount and duration of rain, for example 1 cm
over the entire catchment for a period of one hour. This represents
a certain volume of water (depending on the area of the catchment)
which must subsequently flow out of the river. Using this method
either actual historical rainfalls or hypothetical "design storms"
can be modeled mathematically to confirm characteristics of
historical floods, or to predict a stream's reaction to a predited
storm.
The relationship between the discharge in the stream at a given
cross-section and the level of the stream is described by a
rating curve. Average velocities and
the cross-sectional area of the stream are measured for a given
stream level. The velocity and the area give the discharge for that
level. After measurements are made for several different levels, a
rating table or rating curve may be
developed. Once rated, the discharge in the stream may be
determined by measuring the level, and determining the
corresponding discharge from the rating curve. If a continuous
level-recording device is located at a rated cross-section, the
stream's discharge may be continuously determined.
Flows with larger discharges are able to
transport more
sediment downstream.
References
- Buchanan, T.J. and Somers, W.P., 1969, Discharge Measurements
at Gaging Stations: U.S. Geological Survey Techniques of
Water-Resources Investigations, Book 3, Chapter A8, 1p.
- Dunne, T., and Leopold, L.B., 1978, Water in Environmental
Planning: San Francisco, Calif., W.H. Freeman, 257-258 p.
See also
External links