River Discharge

River discharge is the volume of water (cubic metres) that flows in a river per second.

It's measured in cubic metres per second/cumecs.

Factors that affect River Discharge

Precipitation: the more the precipitation, the higher the discharge

Hot weather: the higher the temperature, the lower the discharge due to more evaporation

Removal of water from river (abstraction): reduces discharge

Hydrographs show how the volume of water flowing at a certain point in a river changes over a period of time. Storm hydrographs show river discharge around the time of a storm event. They only cover a short time period e.g. hours or days.

Peak Discharge

The highest point on the graph: where river discharge is at its greatest.

Lag time

The delay between peak rainfall and peak discharge. A shorter lag time increases peak discharge because more water reaches the river in a shorter period of time

Rising limb: The part of the graph up to peak discharge

Falling limb: The part of the graph after peak discharge. A shallow falling limb shows water is flowing in from stores long after it's stopped raining.

Drainage Basin Characteristics

• Larger drainage basins can catch more precipitation so have a higher peak discharge
• Smaller basins have shorter lag times as precipitation has less distance to travel
• Steep-sided drainage basins have shorter lag times because water flows more quickly downhill into the river - increase peak discharge.
• Circular basins are more likely to have a flashy hydrograph because all points on the watershed are roughly the same distance from the point of discharge measurement - lots of water reaching measurement at same time thus increasing peak discharge
• Basins with lots of streams have shorter lag times

Antecedent Moisture (amount of water already present in drainage basin)

• Affects lag time
• If the ground is waterlogged (soil cannot absorb more water) then infiltration is reduced and surface runoff is increased
• Surface runoff is fast so lag time is reduced

Rock Type

• Impermeable rocks don't store water or let water flow through them, reducing infiltration and increasing surface runoff - reducing lag time and increasing peak discharge as more water reaches the river in a short period

Soil Type

• Sandy soils allow infiltration - longer lag times, lower peak discharge
• Clay soils have low infiltration rates - increase surface runoff, reduce lag time, higher peak discharge

Vegetation

• Vegetation intercepts precipitation and slows its movement to the river channel, increasing lag time
• The more vegetation in a basin, the more water is lost through evapotranspiration, reducing peak discharge

Precipitation

• Intense storms generate more precipitation so there will be higher peak discharges
• Type of precipitation affects lag time e.g. snow fallen in winter storm melts over time and flows into river in spring giving long lag time

Temperature

• Hot, dry conditions and cold freezing conditions result in hard ground, reducing infiltration and increasing surface runoff - lag time reduced, peak discharge increases
• High temperatures can increase evapotranspiration so less water reaches the channel thus reducing peak discharge

Human Activity

• Urbanisation means areas of land are mostly concrete, increasing surface runoff - reducing lag time, increasing peak discharge
• Drainage systems - water flows down drains into river before it can evaporate or infiltrate.