Hydrograph
There are two meanings for hydrographs both coming from hydro- meaning water, and -graph meaning chart;
- a record through time of discharge (flow) in a stream, or
- a record through time of water level in an aquifer, measured in a well.
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Surface water hydrograph
In surface water hydrology, a hydrograph is a time record of the discharge of a stream, river or watershed outlet. Rainfall is typically the main input to a watershed and the streamflow is often considered the output of the watershed; a hydrograph is a representation of how a watershed responds to rainfall. They are used in hydrology and water resources planning.
A watershed's response to rainfall depends on a variety of factors:
- watershed topography and geology;
- land-use (eg. agriculture, urban development, forestry operations);
- river conditions (eg. dams);
- initial conditions (eg. the degree of saturation of the soil and aquifers);
A unit hydrograph is used to more eaily represent the effect a particular basin has on rainfall. It is a hypothetical unit response of the watershed to a unit input of rainfall. This allows easy calculation of the response to any arbitrary input, by simply performing a convolution between the rain input and the unit hydrograph output.
A hydrograph is often compared to a hyetograph of the watershed.
Subsurface hydrology hydrograph
In subsurface hydrology (hydrogeology), a hydrograph is a record of the water level (the observed hydraulic head in wells screened across an aquifer).
Typically, a hydrograph is recorded for monitoring of heads in aquifers during non-test conditions (e.g., to observe the seasonal fluctuations in an aquifer). When an aquifer test is being performed, the resulting observations are typically called drawdown, since they are subtracted from pre-test levels and often only the change in water level is dealt with.
See also
External links
- The U.S. Geological Survey (USGS) offers real-time streamflow data for thousands of streams in the United States.