Physical & human factors affecting runoff

A variety of physical & human factors affect runoff (the flow of water over the Earth’s surface). These are summarised below.

Physical Factors

Time of year

In temperate climates, where seasonal change is evident, runoff levels can vary greatly throughout the year.

In summer, runoff levels can be low due to a reduction in rainfall. Soil saturation levels will be low and therefore any rainfall at this point can easily infiltrate into the ground.

However, intense baking of the soil by the sun can lead to the soil becoming effectively impermeable and summer storms can lead to high levels of runoff as the rain is unable to soak in. This can lead to flash floods.

In winter, precipitation may be in the form of snow and the water may be stored on the ground due to low temperatures. Warmer temperatures in spring may lead to snowmelt and this can lead to the soil reaching field capacity quickly. Further meltwater will therefore run over the surface.

Storm conditions

Intense storms with heavy rainfall can lead to soils quickly becoming saturated. This can happen in two ways:

• Prolonged rainfall – moderate to high volumes of rainfall over a sustained period can eventually saturate the soil leading to runoff.
• Intense rainfall – heavy rain in a short period can bounce and then flow over the Earth’s surface as runoff.

Vegetation cover

Vegetation can intercept precipitation and reduce runoff. Leaves and stems can capture rain and prevent it from reaching the ground. It will eventually reach the ground but the process will have been slowed by the vegetation and therefore water will infiltrate into the ground rather than runoff.

Dense vegetation with proteoid roots (hairy roots with a large surface area) can absorb large volumes of water even in storm conditions, which will prevent runoff.

Soil saturation levels

Where field capacity is reached in the soil, no more infiltration can take place and therefore runoff will occur.

Topography & relief

In “v” shaped valleys on steep slopes, runoff will increase due to gravity.

On flatter surfaces runoff is less likely to happen as water will be able to infiltrate into the soil more easily.

Equally, depressions in the Earth’s surface and an undulating relief can allow water to collect and reduce runoff.

Human factors

Agricultural land use

Initially, agricultural land use can have the same impact as vegetation cover, in that crops can intercept precipitation and reduce runoff. However, intensive agriculture where irrigation may be used, can cause waterlogged soils and therefore lead to runoff.

Heavy use of agricultural machinery can compact the soil and reduce its infiltration capacity, making runoff more likely.

Urban land use

Changing greenfield surfaces to impermeable concrete and tarmac as construction takes place can increase the level of runoff in an area.