Variations in Runoff and the Water Cycle Flashcards
What do hydrographs show?
Hydrographs show river discharge over a period of time.
What is river discharge?
The volume of water that flows in a river per second.
What is the volume of water measured in?
Cubic meters per second/ Cumecs
What does high levels of runoff (water flowing over the surface of the land) do to the discharge?
It increases the discharge of a river, increasing its volume.
What is peak discharge?
This is the highest point on the graph, when the river discharge is at its greatest point.
What is lag time?
This 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.
What is rising limb?
This is the part of the graph up to peak discharge. The river discharge increases as rainwater flows into the river.
What is a falling limb?
This is 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 its stopped raining.
What does a flashy hydrograph refer to?
A basin with rapid runoff and not much storage capacity gives a hydrograph with a short lag time and high peak discharge. The graph has steep, roughly symmetrical rising and falling limbs.
How does the size of the drainage basin affect the size of the hydrograph?
A larger drainage basin 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 the drainage basin affect the size of the hydrograph?
A circular basin 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 the steepness of the ground affect the size of the hydrograph?
Water flows more quickly downhil in steep-sided drainage basins, shortening lag time. This means that water has less time to infiltrate the soil, so runoff is higher.
How does rock and soil type affect the size of the hydrograph?
Impermeable rocks and soils don’t store water or let water infiltrate. This increases surface runoff. Peak discharge also increases as more water reaches the river in a shorter period.
How does storms and precipitation affect the water cycle, hydrographs and runoff?
Intense storms generate more precipitation and greater peak discharges than light rain showers.
The larger input of water causes flows, run off, and stores, groundwater, to increase size. Some flows may not be able to occur rapidly enough for the size of the input, increasing runoff.
How does seasonal changes and vegetation affect the water cycle, hydrographs and runoff?
The size of inputs, flows and stores in the water cycle varies with the seasons-e.g. in the UK, summer is normally drier than winter.
During the winter, temperatures may drop below 0 degrees, causing water to freeze. This can reduce the size of flows through drainage basins, while the store of frozen water grows. When temperatures increase again, flows through drainage basins (and outputs) can be much larger as the ice melts.
Most plants show seasonal variation (vegetation usually dies back in winter). Vegetation intercepts precipitaiton and slows its movement to the river channel. Interception is highest when there’s lots of vegetation and deciduous trees have their leaves. The more vegetation there is in a drainage, the more water is lost (through transpiration and evaporation directly from vegetation) before it reaches the river channel, reducing runoff and peak discharge.
How do human activities such as farming practises affect the size of stores in the water cycle?
Ploughing breaks up the surface so that more water can infiltrate, reducing the amount of runoff.
Crops increase infiltration and interception compared to bare ground, reducing runoff. Evapotranspiration also increases, which can increase rainfall.
Livestock, such as cattle, trample and compact the soil, decreasing infiltration and increasing runoff.
Irrigation can increase runoff if some of the water can’t infiltrate. Groundwater or river levels can fall if water is extracted for irrigation.
How do human activities such as land use changes affect the size of stores in the water cycle?
Deforestation reduces the amount of water that is intercepted by vegetation, increasing the amount that reaches the surface. In forested areas, dead plant material on the forest floor helps to hold the water, allowing it to infiltrate the soil rather than run off. When forest cover is removed, the amount of infiltration that can take place decreases.
Construction of new buildings and roads creates an impermeable layer over the land, preventing infiltration. This massively increases runoff, resulting in water passing through the system much more rapidly and making flooding more likely.
How do human activities such as water abstraction affect the size of stores in the water cycle?
More water is abstracted to meet demand in areas where population density if high. This reduces the amount of water in stores such as lakes, rivers, reservoirs and groundwater.
During dry seasons, even more water is abstracted from stores for consumption and irrigation, so stores are depleted further.
