1.2.3 Variations in Runoff and the water cycle Flashcards
River discharge
The volume of water that flows in a river per second. It is measured in cumecs.
What do high levels of runoff do to river discharge?
High levels of runoff (water flowing on the surface of the land) increase the discharge of a river because more water makes it into the river, increasing its volume.
Hydrographs
Graphs of river discharge over time.
They show how the volume of water flowing at a certain point in the river changes over a period of time.
What do storm hydrographs show?
River discharge around the time of a storm event. They only cover a relatively short period (hours or days).
What does a typical flood hydrograph look like?
[INSERT FLOOD HYDROGRAPH DIAGRAM]
Peak discharge
The highest point on the graph, where river discharge is greatest.
Lag time
The delay between peak rainfall and peak discharge.
This delay happens because it takes time for the rainwater to flow into the river.
A shorter lag time can increase peak discharge because more water reaches the river during a shorter period of time.
Rising limb
The part of the graph up to peak discharge.
The river discharge increases as rainwater flows into the river.
Falling limb
The part of the graph after peak discharge.
Discharge is decreasing because less water is flowing into the river.
A shallow falling limb shows water is flowing in from stores long after it’s stopped raining.
What is a ‘flashy’ hydrograph?
A hydrograph with steep, roughly symmetrical rising and falling limbs.
This drainage basin has rapid runoff and limit storage capacity, giving it a short lag time and high peak discharge.
Physical influences on the shape of storm hydrographs
- Size of drainage basin
- Shape of drainage basin
- Precipitation
- Drainage density
- Saturation
- Rock type – permeability
- Vegetation cover
Human influences on the shape of storm hydrographs
- Urbanisation
- Deforestation
- Agriculture
- Food engineering
How does the size of drainage basin affect a storm hydrograph?
Larger drainage basins catch more precipitation, so they have a higher peak discharge than smaller basins.
Smaller basins generally have shorter lag times because precipitation has less distance to travel, so it reaches the main channel more quickly.
How does the shape of drainage basin affect a storm hydrograph?
Circular basins are more likely to have a flashy hydrograph than long, narrow basins. This is because all points on the watershed are roughly the same distance from the point of discharge measurement.
This means lots of water will reach the measuring point at the same time.
How does ground steepness affect a storm hydrograph?
Water flows more quickly downhill in steep-sided drainage basins shortening lag time.
This also means that water has less time to infiltrate the soil, so runoff is higher.
