Geography T1 - Discharge + Factors affecting it

What is discharge?

the amount of water carried by a river at any given time. This is measured in cumecs, that is the cubic metres of water per second moving past a particular point along the river’s course.

Factors affecting discharge in the river, lagtime and time lag between peak discharge and rainfall

1. amount + intensity of rain

2. temperature

3. land use

4. steep slopes (relief)

5. rock type

6. dams and reservoirs

7. water abstraction for human use

8. size of the drainage basin

How does amount and intensity of rain affect river discharge

heavy rainfall will not sink into the ground; instead, it will become overland flow (run-off) and quickly reach the river.

How does temperature affect river discharge

are significant because they affect the form of precipitation. For example, if temperatures are below freezing, precipitation will be in the form of snow. This can take weeks to melt. If the ground is frozen, melting snow on the surface can reach the river quickly.

How does land use affect river discharge

trees and plants will intercept and delay the rain reaching the ground. Bare soil and rock will speed up run-off and reduce the time lag. So too will urban areas covered by tarmac and concrete (built-up areas).
→Arable Farming - Ploughing of soil breaks it up and allows more infiltration. This increases lag times. Conversely, drains carrying water from fields speed water into local rivers and reduce lag times. When fields are harvested and the fields are bare, this reduces interception, resulting in greater over land flow and decreases lag times.
→Pastoral farming – deforestation to make space for animals – can decrease lag time.

How does steep slopes (relief) affect river discharge

will cause rapid surface run-off, so water will reach the river more quickly. Flat land and gentle slopes will lead to water infiltrating into the soil. This will delay it reaching the river.

How does rock type affect river discharge

impermeable rock will not allow rainwater to sink into it, so will speed up run-off. Permeable rock will allow infiltration and percolation of water into the bedrock. This in turn will increase lag time and slow the delivery of the water to the river.

How does dams and reservoirs affect river discharge

regulate and even out discharges. They hold back discharge and so reduce the risk of flooding downstream.

What is water abstraction for human use

the taking of water from rivers, lakes and from below the water table.

How does size of drainage basin affect river discharge

the larger the drainage basin, water will take longer to reach the river, the smaller the drainage basin, water will enter the river quicker.

Shape of drainage basin affecting the shape of the storm hydrograph:

Length of precipitation on shape of storm hydrograph

how do you calculate river discharge?

cross sectional area x river mean (average) velocity

Diagram of a flood hydrograph

how do you calculate drainage density

total length of stream channels/total area of drainage basin