Variations in runoff and the water cycle Flashcards
what is a flood hydrograph?
Flood hydrographs are graphs that show how a drainage basin responds to a period of rainfall. They are used to plan for flood situations and times of drought.
what is a flashy hydrograph?
Flashy hydrographs have a steep rising limb and a small lag time. This indicates the river discharge increases rapidly over a short period of time, indicating rainwater reaches the river very quickly. This means the river is more likely to flood.
What is river discharge?
The volume of water flowing through a river channel; measured at any given point in cubic metres per second.
What are the stages of a flood hydrograph?
1) Peak discharge- this is the highest point on the graph, when the river discharge is at its greatest.
2) Lag time- this is the delay between peak rainfall and peak discharge. This happens as water takes its tie to flow into the river. A shorter lag time can can increase peak discharge because more water reaches the river in a shorter period of time.
3) Rising limb- This is the part of the graph, before peak discharge. The rising limb gives an indication of how fast water is reaching the channel and represents the level of water rising in the channel.
4) Falling limb- This is the part of the graph, after peak discharge.The falling limb is when discharge decreases and the river’s level falls.
How are runoff and hydrograph shaped by different factors?
> Size of drainage basin- larger drainage basin catches more precipitation, so they have a higher peak discharge than smaller basins.
Smaller basins usually have shorter lag times as there is less distance for water to travel.
> Shape of drainage basin- circular basins are more likely to have flashy hydrographs (steep rising limb and a small lag time) than long, narrow basins. This is because all points on the watershed are roughly the same distance from the point of discharge measurement.
> Ground steepness- 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.
> Rock and soil type- 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.
What physical factors change the water cycle?
Storms and precipitation:
>Intense storms generate more precipitation and significant peak discharges than light rain showers.
>The larger input of water causes flows e.g. runoff and stores, e.g. groundwater to increase in size
>Infiltration may not be able to occur quickly enough, therefore increasing the risk of surface runoff.
Seasonal changes:
>The size of inputs, flows and stores varies within seasons- e.g. in the UK, summer is typically drier than winter.
> During the winter, temperatures may drop below 0 degrees, causing water to freeze. This can reduce the size of flows through the drainage basin, while the store of frozen water grows.
> Vegegation intercepts precipitation 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 basin, the more water is lost (through evaporation and transpiration), before it reaches the water channel, reducing runoff and peak discharge.
What human factors change the water cycle?
Farming practices:
Farming practices can affect infiltration in many ways:
>Ploughing (turning the soil) 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.
>Livestock, such as cattle, trample and compact the soil, decreasing infiltration and increasing runoff.
>Irrigation (artificially watering the land) can increase runoff if some water can’t infiltrate. Groundwater or river levels can fall if water is extracted for irrigation.
Land use change:
> Deforestation reduces the amount of precipitation 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 runoff. Infiltration decreases when removed.
>Construction of new buildings and roads created an impermeable layer over the land, preventing infiltration. This massively increases runoff, resulting in the flow of water increasing, making flooding more likely.
Water abstraction:
>More water is abstracted (taken from stores) to meet demand in areas where population density is high. This reduces the amount of water in stores such as lakes, rivers and groundwater.
>During drier seasons, even more water is abstracted from stores (especially groundwater) for consumption and irrigation, so stores are depleted further.
