Hydrograph
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There are two meanings for hydrographs both coming from hydro- meaning water, and -graph meaning chart. A hydrograph plots the discharge of a river over time. This can be in response to single event such as a flood.
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[edit] Terminology
- 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 - The difference between the peak rainfall and peak discharge.
Types of hydrograph can include:
- Storm hydrographs
- Flood hydrographs
- Annual hydrographs
[edit] 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 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 network
- 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 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.
A hydrograph is often compared to a hyetograph of the watershed.
[edit] Factors affecting the hydrograph
There are multiple factors affecting the discharge and discharge rate of a river, particularly after a high amount of rainfall.
[edit] Relief or gradient of the area
The steeper the slopes, the lower the rate of infiltration and faster the rate of run-off when the soil is saturated (saturated overland flow) or when rainfall intensity (rate per unit of time) is high (infiltration excess over land flow).
[edit] Geology, rock type and soil type
Run-off will occur quickly where impermeable rocks are exposed at the surface or quickly when they underlay soils (limited amount of infiltration). Soils with large amounts of clay do absorb moisture but only very slowly - therefore their permeability is low.
The deeper the soil the more water can be absorbed. Soils which have larger particle sizes (e.g those derived from the weathering of sandstones) have larger infiltration capacities.
[edit] Presence of vegetation
A high amount of vegetation will intercept rainfall, and reduce initial surface flow. Evapotranspiration will occur when water is on leaves and branches, reducing the amount of water on the ground. Evergreen plants are able to transpire throughout the year (assuming temperatures are high enough and moisture is available - not frozen). Deciduous trees have a much larger biomass/leaf area than evergreen conifers and thus transpire more moisture throughout the year (even though they lose their leaves in winter) than evergreen conifers in Britain.
Also, the root systems of trees and other plants will encourage infiltration of the precipitation, rather than generating more surface runoff.
[edit] Land use
[edit] Urbanisation
Impermeable road surfaces, sloping roofs, guttering, and underground sewer and drainage systems help transfer water in an urban area to rivers quickly. The increase of house building in towns and villages as people have opted to move from large settlements (counter- urbanisation) especially on river flood-plains has contributed to the increase responsiveness of river systems (e.g. flooding along the River Severn in October 2000).
[edit] Agriculture
Irrigation and drainage ditches will increase the speed of water transfer (also occurs in fields where farmers plough up and downslope. One solution to this problem is contour ploughing. Ploughing on wet land compresses the subsoil - creating a "plough pan" that can lead to decreased water holding, infiltration and increased run-off/erosion. The afforestation of the areas upriver can increase the interception and infiltration in the area, reducing discharge.
[edit] Water use
Dams and reservoirs slow down the rate of discharge at peak times as water is held back, possibly to protect the low-lying land downstream. Water extracted for industry, irrigation, and domestic use, also reduce discharge.
[edit] Drainage density
This ratio is the length of river course per area of land. The larger the amount of streams and rivers per area the shorter distance water has to flow and the faster the rate of response.
[edit] Precipitation
The rate and intensity of the rainfall will directly affect the amount and rate of overland flow. Snowfall results in less run-off initially, but a sharp rise in temperature may result in a quick thaw and a sharp increase in overland flow, especially if the ground underneath the snow is still frozen, and the melted snow will flow rapidly on ice, into the river, increasing discharge.
[edit] Time of year/season
In the summer, evapotranspiration rates are higher, reducing the amount of surface runoff. As well, photosynthesis in plants will be at a maximum - longer spells of sunlight and higher temperatures will create more opportunities for root systems to absorb water, and leaf systems to transpire water.
[edit] 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.
[edit] See also
[edit] External links
- The U.S. Geological Survey (USGS) offers real-time streamflow data for thousands of streams in the United States.
