Variations in Runoff and the Water Cycle. Flashcards
What do hydrographs show (basic)?
They show river discharge over a period of time.
What is river discharge?
The volume (in cubic metres), that flows per second. Measured in cubic metres per second (cumecs.)
What increases river discharge (primary factor)?
Primarily, an increase in runoff does this as more water enters the river, increasing the volume.
What are hydrographs?
These are graphs of river discharge over time. They show how the volume of water flowing at a certain point changes over a period of time. Flood/storm hydrographs show river discharge around the time of a storm event. They only cover a relatively short time period (hours or days.)
What are the four main parts labelled on a flood hydrograph?
Peak discharge, lag time, rising limb and falling limb.
What is peak discharge?
The highest point on the graph, where river discharge is highest in the period of time covered by the graph.
What is the lag time?
The delay between peak rainfall and peak discharge. This shows how long it takes for the river to fill after the peak rainfall has been reached. It highlights the delay that happens because it takes time for the rainwater to flow into the river. A shorter lag time can increase peak discharge because more water is reaching the river during a shorter period of time.
What is the rising limb?
This is the part of the graph leading up to peak discharge. As more rainwater fills the river, the rising limb increases until peak discharge.
What is the falling limb?
This is the part of the graph after peak discharge. Discharge is decreasing as less water is flowing into the river. A shallow falling limb shows water is flowing in from stores long after rainfall stopped; emphasises the flows of water prior to rainfall ending.
What characteristics does a flashy hydrograph have?
This will be commonly seen at a basin with a rapid runoff and little storage capacity near a river. This will see a short lag time and high peak discharge, with steep, roughly symmetrical rising and falling limbs.
What factors relating to the basin itself affect the runoff and hydrograph shape?
Size of basin, shape of basin, ground steepness and rock and soil type.
How does the size of a basin affect the runoff and shape?
Larger basins collect more precipitation and have a bigger peak discharge than smaller basins. Smaller basins generally have a shorter lag time due to shorter travel distances of water.
How does the shape of basin affect runoff and shape?
Circular basins are more likely to produce a flashy hydrograph than longer, narrow basins. As all the 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 the ground steepness affect runoff and shape?
Water flows quicker downhill in steep-sided basins, shortening the lag time, it also means water has less time to infiltrate soil, so runoff is higher.
How does the rock and soil type affect runoff and shape?
Impermeable rocks and soils don’t store water or let it infiltrate. This increases surface runoff. Peak discharge also increases as more water reaches the surface in shorter periods.
