River Exe Flashcards
Describe the location of the River Exe catchment.
Located in Devon in South West England.
The River Exe itself is 82.7km long and travels from Moorland in Exeter and finally ending up at the sea in Exmouth with tributaries in Dulverton.
Describe the catchment of the River Exe.
The area of the catchment is 601km2. The source is on Exmoor, an upland area of moorland and peat bogs rising to 514m above sea level and used for sheep farming and shooting game birds.
The catchment is largely under lain by impermeable sand stone - which leads to a high drainage density.
The upper (and majority of the) catchment is 67% agricultural grassland, woodland and arable farmland.
How long is the River Exe?
87.2km2
What is the area of catchment if the River Exe?
601km2
Where is the source of the River Exe? What is it like?
The source is on Exmoor - an upland area of moorland and peat bogs rising to 514m and used for sheep farming and shooting game birds.
The catchment of the River Exe is underlain by impermeable sandstones. What does this lead to?
A high drainage density.
Describe the relief and the impact on hydrology of the River Exe.
Description: ranges from 514m above sea level in the North to 26m in the South. It’s hilly in the North but much flatter in the South.
Impact on hydrology: the steep catchment area means the rainfall will move faster from the upper course to the lower course.
Describe the area and the impact on hydrology of the River Exe.
Description: 601km2 - a relatively small drainage basin.
Impact on hydrology: water would get the the main channel quicker. Less water is in the river compared to a larger river.
Describe geology and the impact on hydrology in the River Exe.
Description: 84% of the drainage basin is underlain by impermeable rocks - mainly sandstones.
Impact on hydrology: the rainfall cannot infiltrate through the rocks, so more runoff as a result. High levels of saturation below ground.
Describe landuse and the impact on hydrology on the River Exe.
Description: 67% grassland. 15% woodland. 10% arable land. 3% moors/peatbogs.
Impact on hydrology: because 82% is grassland and woodland, water infiltrates and is stored below ground in soil. Only 15% where there is high levels of uptake and usage.
How would you use the water balance equation to calculate runoff?
Precipitation - evaporation (+- soil water storage).
The runoff % of the River Exe is high. Suggests reasons for this.
The impermeable nature of most of the bedrock reduces percolation and baseflow so rainfall travels as runoff from the catchment to the River Exe channel.
Secondly drainage ditches (used to drain water from low lying areas along fields to the river channels) reduce the amount of soil water storage, so the smaller amount of soil becomes saturated more quickly due to a reduction is the soil, so there is more runoff.
Why use a null hypothesis?
Because it is very difficult to prove that a hypothesis is correct, therefore we set a null hypothesis which we can disprove and therefore be confident enough to accept that our hypothesis is valid.
What would you use to establish the relationship between precipitation and runoff in the River Exe?
A statistical technique: Spearman’s ran correlation coefficient.
Using Spearman’s rank correlation coefficient, what does +1 mean?
A perfect positive correlation.
Using Spearman’s rank correlation coefficient, what does 0 mean?
No correlation.
Using Spearman’s rank correlation coefficient, what does -1 mean?
Perfect negative correlation.
How do you calculate the ‘degrees of freedom’ value?
n-2.
How would you use the significance of the Spearman’s rank correlation coefficient and degrees of freedom graph to see if the result from your hypothesis is statistically significant?
If your value is about the 5% confidence line, it means that 5% of the time you can expect your result to occur by chance and 95% of the time your result is statistically significant.
Your value must be above the 5% level to reject your null hypothesis.
How can you reject our null hypothesis?
Your value on the Spearman’s rank correlation coefficient and degrees of freedom graph must be above the 5% level.
There isn’t a strong positive correlation between peak discharge and monthly rainfall in the River Exe. Suggest potential reasons for this.
- not enough data
- rural nature of the catchment
- water is abstracted at some locations along the river and used for agriculture
- reservoir holds water to prevent flashy peaks
- heavy agriculture leads to crops using it and animals drinking it
- drainage ditches which changes the flow of the natural river
Why could the interquartile range be a useful measure than a straightforward range?
Because it disregards the highest and lowest pieces of data so removes any skewing effect of anomalies, as it shows the spread of 50% of the data around the median.
Formula for IQR?
IQR = UQ - LQ
How to calculate lower quartile?
(n+1) / 4th value
How to calculate upper quartile?
3(n+1) / 4th value.
The interquartile could be useful for assessing flood risk. Why?
If the peak flow is greater than the IQR, this could show the risk of flood as it is different than the normal 50%.
1 benefit of using a proportional circle map?
Shows a clear pattern.
2 problems of proportional circles maps?
- hard to precisely measure against a scale so could be inaccurate
- does not show farmland, only towns.
Why were drainage ditches dug in the first place on the River Exe?
Because they dry out pear and make it more suitable for conversion to farmland or grazing more sheep on.
This also means more water enters the channel more quickly, reducing lag time and increasing the flood risk downstream.
What are the problems with drainage ditches on the River Exe?
Dry peat is susceptible to erosion, and the carbon contained within it is dissolves in water and transported downstream. This means carbon is no longer being sequestered in peatlands but is free to exchange from water with the atmosphere and become atmospheric carbon.
Also dissolved organic carbon makes water brown, therefore water companies have to spend more money removing the colour.
What is the potential problem with restoring peat and blocking up drainage ditches?
Landowners have to be convinced of the benefits in order to invest in blocking drainage ditches and potentially lose money from agriculture as their land becomes saturated and boggy once again.
Benefits of blocking drainage ditches up?
Restoration of peat blocks on Exmoor has resulted in a third less water the Moreland during heavy rainfall compared with 3 years ago.
By blocking up drainage ditches, the moorland can now hold more water and release it more slowly, reducing potential flooding elsewhere into major population centres like Exeter.
It’s thought that it also reduces carbon transfers but they haven’t been blocked for long enough for long enough to conclusive evidence of this.
What are the benefits of the Exmoor Mires project?
- More water storage in upper catchments. Water transfers are slowed, increasing storage capacity and ensuring a steady supply of water all year round.
- Improved water quality. Slower transfer of water means that less sediment is carried into the rivers. Water is therefore cleaner, less expensive to treat and good for wildlife eg salmon.
- More carbon storage. Peat is essentially carbon and water, and therefore an important carbon store. Dry peat however releases CO2 through oxidation. By encouraging the re-wetting of peat and active peat growth, CO2 is naturally absorbed from the atmosphere and stored.
- Improved grazing and water supply for animals. Animals benefit from having all year round water supply, as well as improved grazing during drier parts of the year.
What are the aims of the Exmoor Mines project?
It aims to restore 2000ha of Exmoor to boggy conditions that would naturally be present by blocking up drainage ditches with peat bogs and moorland bales.
What happened regarding the Exmoor Mines project as of 2015?
By 2015, 1000ha of peat moorland had been restored and nearly 100km of drainage ditches blocked, raising the water table by 2.65cm.
This has reduced the amount of water that drains from the monitoring area by 2/3rds.
