Factors affecting discharge

Factors affecting discharge

1. Discharge is the volume of water flowing past a point in a river. It is measured in cubic meters per second (cumecs)

2. Rock and soil types

   • Permeable rocks and soils (such as sandy soils) absorb water easily, so surface run off is rare
   • Impermeable rocks and soils (such as clay soils) are more closely packed. Rainwater can't infiltrate, so water reaches the river more quickly
   • Pevious rocks like limestone allow water to pass through joints in the rock and porous rocks like chalk have spaces between the rock particles

3. Rainfall

   • If an area is already saturated (full) of water, further rain will then flow as surface run-off towards the river
   • Heavy continual rain, or melting snow means more water flowing into the river.

4. Relief (shape of the land)

   • Steep slopes mean that rainwater is likely to run straight over the surface before it can infiltrate

5. Land use

   • In urban areas, surfaces like roads are impermeable - water can't soak into the ground. Instead it runs into the drains, gathers speed and joins rain water from other drains eventually spilling into the river.
   • In rural areas, ploughing up and down (instead of across) hillsides creates channels which allow rain water to reach rivers faster - increasing discharge
   • Deforestation means less interception, so rain reaches the ground faster. The ground is likely to become saturated and surface run-off will increase

6. Weather conditions

   • High temperatures increase evaporation rates from water surfaces, and transpiration from plants - reducing discharge
   • Hot dry weather can bake the soil, so that when it rains the water can't soak in. Instead it will run straight off the surface, straight into the river.
   • Long periods of extreme cold weather can lead to frozen ground, so that water can't soak in