# Discharge (hydrology)

﻿
Discharge (hydrology)

In hydrology, discharge is the volume rate of water flow, including any suspended solids (i.e. sediment), dissolved chemical species (i.e. CaCO3(aq)) and/or biologic material (i.e. diatoms), which is transported through a given cross-sectional area.[1] 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.[2] For example, the average discharge of the Rhine river in Europe is 2,200 cubic metres per second (78,000 cu ft/s) or 154,000 acre feet (190,000,000 m3) 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 ([L3T−1]; m3/s or ft3/s)
• A is the cross-sectional area of the portion of the channel occupied by the flow ([L2]; m2 or ft2)
• $\bar{u}$ 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

1. ^ 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.
2. ^ Dunne, T., and Leopold, L.B., 1978, Water in Environmental Planning: San Francisco, Calif., W.H. Freeman, 257-258 p.

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• Discharge — in the context to expel or to let go may refer to: A military discharge, issued when a member of the armed forces is released from service Termination of employment, the end of an employee s duration with an employer A patient discharge, the… …   Wikipedia

• Hydrology — Water covers 70% of the Earth s surface. Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability. A… …   Wikipedia

• Infiltration (hydrology) — Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is measured in inches per hour or millimeters… …   Wikipedia

• Groundwater discharge — is the volumetric flow rate of groundwater through an aquifer. Groundwater discharge, Q Total groundwater discharge, as reported through a specified area, is similarly expressed as::Q = frac{dh}{dl}KAwhere: Q is the total groundwater discharge (… …   Wikipedia

• HBV hydrology model — The HBV hydrology model, or Hydrologiska Byråns Vattenbalansmodell, is a computer simulation used to analyze river discharge and water pollution. Developed originally for use in Scandinavia [Bergström, S., 1976. Development and application of a… …   Wikipedia

• Outline of hydrology — The following outline is provided as an overview of and topical guide to hydrology: Hydrology – study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and… …   Wikipedia

• Surface-water hydrology — is a field that encompasses all surface waters of the globe (overland flows, rivers, lakes, wetlands, estuaries, oceans, etc). This a subset of the hydrologic cycle that does not include atmospheric, and ground waters. Surface water hydrology… …   Wikipedia

• GIS and Hydrology — Geographic Information Systems (GIS) has become a particularly useful and important tool in hydrology and to hydrologists in the scientific study and management of water resources. Climate change and greater demands on water resources require a… …   Wikipedia

• Submarine groundwater discharge — (SGD) is the net inflow of fresh water through an aquifer medium to the sea. Depending upon the hydrogeology, nutrient loads, and location, the SGD may vary. It is greatly dependent on the tides. Especially in the estuary regions, SGD is more… …   Wikipedia

• Mainstem (hydrology) — A mainstem is defined as the principal river within a given drainage basin, in the case where a number of tributaries discharge into a larger watercourse. Viewed in terms of the Strahler Stream Order system, the mainstem would be the highest… …   Wikipedia