The shape of a hydrograph

Soils which have larger particle sizes e. The size, shape and relief of the basin are important controls. Generally, in large watersheds the peak flow occurs, when rainfall gets stop.

This situation is never reached in large basins, but may occur in small watersheds and is frequently used as the criterion for design of storm sewers, airport drainage or small culverts.

Unconsolidated soils allow water to infiltrate and so act as a store in a drainage basin. Basins with lots of streams and rivers a high drainage density will have a short lag time and a fairly steep falling limb because water will drain out of them quickly. The time difference between the maximum effective rainfall intensity and the maximum runoff is called the time lag.

The creation of a GIUH is possible given nothing more than topologic data for a particular drainage basin. Water takes longer to reach the trunk stream in a large, round basin than in does in a small, narrow one.

The rising limb is the steep part of the discharge line that has a positive gradient, indicating that the discharge is increasing. Note that during periods when the groundwater level is higher than the water level in the river, water will flow from the groundwater into the stream channel, and the river is considered effluent, draining the surrounding area.

Hydrographs of some watersheds resulted from a single and relatively short duration rainfall, have two or more peaks. For a given rainfall duration, an increase in intensity will increase the peak discharge and the runoff volume, provided the infiltration rate of the soil is exceeded.

A number of factors known as drainage basin controls influence the way in which a river responds to precipitation and have an effect on the shape of the hydrograph.


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. A number of factors known as drainage basin controls influence the way in which a river responds to precipitation and have an effect on the shape of the hydrograph.

Conversely, a short lag time indicates that the precipitation is entering the river fairly quickly. The second is used to plot a bar graph of the rainfall event which precedes the changes in discharge. The way in which the land is used will also have an influence on the hydrograph — vegetation intercepts precipitation and allows evaporation to take place directly into the atmosphere so reducing the amount of water available for overland flow while the large number of impermeable surfaces in urban areas encourages run off into gutters and drains carrying water quickly to the nearest river.

Because the timing, magnitude, and duration of groundwater return flow differs so greatly from that of direct runoff, separating and understanding the influence of these distinct processes is key to analyzing and simulating the likely hydrologic effects of various land use, water use, weather, and climate conditions and changes.

In part this is because these two concepts are not, themselves, entirely distinct and unrelated.

Discharge & Hydrographs

Roughly Circular shapes are common as are more elongated and narrow shapes. The second is used to plot a bar graph of the rainfall event which precedes the changes in discharge. Once the value of K has been determined; equation 6. Meaning of Runoff Hydrograph: In a simple hydrograph, the extent of rising limb is comparatively shorter than the falling The shape of a hydrograph, as a result the area below this limb is less to that of the falling limb.

The shape of rising limb is dependent on the storm and watershed characteristics, both. If a larger amount of rainfall occurs in the upper reaches of a basin, the hydrograph exhibits a lower and broader peak. One is used to plot a line graph showing the discharge of a river in cumecs cubic metres per second at a given point over a period of time.

The afforestation of the areas upriver can increase the interception and infiltration in the area, reducing discharge. The time distribution of runoff the shape of the hydrograph is influenced by climatic, topographic and geological factors.

This is much faster than groundflow, interflow and throughflow so the lag time is reduced. The type of storm is important in that thunderstorms produce peak flows on small basins, whereas large cyclonic or frontal-type storms are generally a determinant in larger basins.

Human Activity Humans will normally cover soil in impermeable materials like tarmac or concrete which will increase surface run off and reduce the amount of water being stored, increasing the peak discharge and reducing the lag time. Different river catchments produce different shapes of hydrograph.

All hydrographs have three characteristics regions viz. First the area closest to the station contributes to the surface runoff, followed by the areas further upstream.The shape of a hydrograph is altered by a few different things.

One factor is the shape of the drainage basin. Drainage basins come in a wide assortment of shapes. The shape of the hydrograph varies according to a number of controlling factors in the drainage basin A number of factors (known as drainage basin controls) influence the way in which a river responds to precipitation and have an effect on the shape of the hydrograph.

A hydrograph shows variations in a river’s discharge over ashort period of time, usually during a rainstorm.• River discharge is the amount of water passing a given pointin the river at a particular time.•.

A hydrograph is a graph showing the rate of flow versus time past a specific point in a river, channel, or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second (cms or cfs).

•Affects the shape of hydrograph affecting time of concentration •Broad shaped –peak flow occur soon because of less time of concentration, narrow hydrograph with high peak •Fan shaped –peak flow occur at longer time interval because of longer time of concentration, broad base.

Physical factors affecting storm hydrographs. There is a range of physical factors that affect the shape of a storm hydrograph. These include: 1.


Large drainage basins catch more precipitation so have a higher peak discharge compared to smaller basins. Smaller basins generally have shorter lag times because precipitation does not have as far to travel.

The shape of a hydrograph
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