Hydrograph

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A hydrograph is a record of the stage and/or discharge of a river as a function of time.

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.

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 hydrography

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 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 of rainfall 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 (rainfall), 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

• Soil Saturation is dependant on previous rainfall, or otherwise known as Antecedent rainfall.
• The surroundings; Rural or Urban (Could be less impermeable surface, or the surface type could vary)
• Vegetation type (Deforestation and amount of interception)
• Steepness of surrounding land, or 'relief' land
• Drainage density (Number of tributaries)
• Geology (Rock Type; Impermeable=flashier hydrographs. Or Permeable)
• Season dependant; Very dry weather creates a crust on the river bed. Wet winters create increase in dishcarge.
• Soil Type (Clay, sand etc.) Clay would create a flashy hydrograph, but there could be a continium between the two.

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
• Runoff model (reservoir)
• Hydrogeology
• Aquifer test

External links

• The U.S. Geological Survey (USGS) offers real-time streamflow data for thousands of streams in the United States.