Explaining hydrographs

There are various factors to consider when explaining a hydrograph. Look at the examples below.

Type of drainage basin

Large basins receive more precipitation than smaller basins, therefore they have a larger runoff. Larger size also means a longer lag time as water has a longer distance to travel to reach the main river.

Hydrograph comparisons of large and small drainage basins.

An elongated basin will produce a lower peak flow and a longer lag time than a rounder basin. Basins with a higher drainage density (number of tributary rivers) collect rain water more quickly, therefore the lag time will be shorter.

Type of slope

Flow can be faster down a steep slope, resulting in a steeper rising limb and a shorter lag time.

Hydrograph comparisons of steep and gentle valleys.

Rock/soil type

Permeable rocks mean rapid infiltration and little overland flow, resulting in a less steep rising limb.

Hydrograph comparisons of impermeable and permeable rock types.

Impermeable rocks will lead to an increase in overland flow and a steep rising limb.

Infiltration is generally greater on thick soil, although less porous soils eg clay act as impermeable layers.

Other factors

Urbanisation, with bare concrete and tarmac forming impermeable surfaces, creates a steep rising limb and shortens the time lag.

Hydrograph comparisons of bare and forested landscapes.

In urban areas concrete, tarmac and buildings will channel precipitation to gutter and drainage systems quickly. This water will quickly reach the sewer and river systems, shortening the lag time.

Forest areas will intercept the precipitation, creating a less steep rising limb and lengthening the time lag. Vegetation will intercept precipitation and store and absorb it, therefore preventing the water moving quickly into the river.