Hydrograph
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2008-06-22T09:46:47Z
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Reverted edits by [[Special:Contributions/122.3.196.20|122.3.196.20]] ([[User talk:122.3.196.20|talk]]) to last version by 192.248.40.6
There are two meanings for hydrographs both coming from ''hydro-'' meaning water, and ''-graph'' meaning chart. A hydrograph plots the discharge of a [[river]] as a function of time. This activity can be in response to episodal event such as a [[flood]].
==Terminology==
The discharge is measured at a certain point in a river and is typically time variant.
*'''Rising limb''' - The part of the hydrograph up to the point of peak discharge.
*'''Falling limb''' - The part of the hydrograph after the peak discharge.
*'''Peak discharge''' - The highest point on the hydrograph when there is the greatest amount of water in the river.
*'''Lag time''' - Period of time between peak rainfall and peak discharge.
[[Image:Stream hydrograph.gif|thumb|250px|Stream hydrograph. Increases in stream flow follow rainfall or snowmelt. The gradual decay in flow after the peaks reflects diminishing supply from groundwater.]]
Types of hydrograph can include:
*[[Storm hydrographs]]
*[[Flood hydrographs]]
*[[Annual hydrographs]]
==Surface water hydrograph==
In [[surface water]] hydrology, a hydrograph is a time record of the [[discharge (hydrology)|discharge]] of a [[stream]], [[river]] or [[drainage basin|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 [[drainage basin|watershed's]] response to [[rainfall]] depends on a variety of factors which affect the shape of a hydrograph:
*Watershed [[topography]] and [[geology]] (i.e. [[bedrock]] permeability)
*The area of a basin receiving rainfall
*Land-use (e.g. [[agriculture]], urban development, [[forestry]] operations)
*[[Drainage density]]
*Duration of [[rainfall]] and precipitation intensity and type
*[[Evapotranspiration]] rates
*River geometrics
*The season
*Previous weather
*[[Vegetation]] type and cover
*River conditions (e.g. dams)
*Initial conditions (e.g. the degree of saturation of the soil and aquifers)
*[[Soil]] permeability and thickness
A hydrograph is often compared to a [[hyetograph]] of the watershed.
==Unit Hydrograph==
A '''unit hydrograph''' is used to more easily represent the effect rainfall has on a particular basin. 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.
An '''instantaneous unit hydrograph''' is a further refinement of the concept; for an IUH, the input rainfall is assumed to all take place at a discrete point in time (obviously, this isn't the case for actual rainstorms). Making this assumption can greatly simplify the analysis involved in constructing a unit hydrograph, and it is necessary for the creation of a '''geomorphologic instantaneous unit hydrograph'''.
The creation of a GIUH is possible given nothing more than topologic data for a particular drainage basin. In fact, only the number of streams of a given order, the mean length of streams of a given order, and the mean land area draining directly to streams of a given order are absolutely required (and can be estimated rather than explicitly calculated if necessary). It is therefore possible to calculate a GIUH for a basin without any data about stream height or flow, which may not always be available.
== Factors affecting the hydrograph ==
==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==
*[[Surface water]]
*[[Hydrogeology]]
*[[Aquifer test]]
==External links==
* The U.S. Geological Survey (USGS) offers [http://waterdata.usgs.gov/nwis/rt real-time streamflow data] for thousands of streams in the United States.
[[Category:Hydrology]]
[[Category:Hydraulic engineering]]
[[et:Hüdrograaf]]
[[fr:Hydrographe]]
[[ja:ハイドログラフ]]
[[ru:Гидрограф]]