Water And Carbon: The River Exe

Question 1

What is the length of the River Exe?

Answer 1

82.7km

Question 2

Where does the river flow through?

Answer 2

1. Source in the hills of Exmoor
2. Through Tiverton and Exeter
3. Out to sea at Exmouth on the south coast of Devon

Question 3

What are the physical characteristics of the river?

Answer 3

1. Area of the upper catchment is 601 km squared
2. Maximum elevation of 514 m in the north
3. Land is much flatter in the south with lowest elevation of 26 m
4. high drainage density

Question 4

What is the geology of the river?

Answer 4

An estimated 84.4% of catchment is underlain by impermeable rocks, predominantly Devonian sandstones, which accounts for the extensive drainage network

Question 5

What is the land surrounding the river used for?

Answer 5

1. 67% agricultural grassland
2. 15% woodland and arable farming
3. 3% moors and peat bogs on the high ground of Exmoor

Question 6

What is precipitation like along the river area?

Answer 6

Precipitation (1295mm) = evaporation +/- soil water storage (451mm) + runoff (844mm)

Question 7

What is the water balance of the River Exe?

Answer 7

• Rainfall is high, particular over Exmoor. Most rainfall is absorbed by the peaty moorland soils but if soils are saturated or drainage ditches have been dug, water can flow off the hills rapidly
• Runoff accounts for 65% of the water balance, which is quite high compared with other UK rivers. This is because the impermeable nature of most of the bedrock reduces percolation and baseflow whilst drainage ditches on Exmoor reduce amounts of soil water storage.

Question 8

What would a hydrograph of the River Exe look like?

Answer 8

1. River Exe responds relatively slowly to rainfall events
2. After flooding, discharge recedes slowly to reflect the rural nature of the lower catchment (82% of lands use is either woodland or grass)

Question 9

How did the construction of Wimbleball reservoir affect the water cycle and water balance of the River Exe?

Answer 9

1. In 1979 the River Haddeo, an upland tributary of the River Exe, was dammed to create the Wimbleball reservoir which has a surface area of 150 hectares and supplies water to Exeter and parts of East Devon
2. It regulates water flow, ensuring a steady flow regime during the year to prevent the peaks and troughs of water discharge that makes flooding or drought more likely.

Question 10

How did the peatland restoration on Exmoor affect the water cycle and water balance?

Answer 10

1. For decades, drainage ditches have been dug in the peat bogs of Exmoor to make it suitable for farming which has increased the speed of water flow to the Exe, reducing water quality as more silt is carried downstream
2. Peat has also been dug as a fuel, leaving ugly scars in the landscape
3. As the peat surface has dried out, decomposition has occurred which released carbon in the form of carbon dioxide and methane from this important store

Question 11

What is the aim of the Exmoor Mires Project?

Answer 11

Aims to restore 2000 ha of Exmoor to the boggy conditions that would naturally be present by blocking drainage ditches with peat blocks and moorland bales

Question 12

What are the benefits of the Exmoor Mires Project?

Answer 12

1. MORE WATER STORAGE IN UPPER CATCHMENTS - water transfer is slowed, increasing storage capacity and ensuring a steady supply of water throughout the year
2. IMPROVED WATER QUALITY - slower throughflow means that less sediment is carried into the rivers so water is cleaner, less expensive to treat and good for wildlife such as Salmon
3. MORE CARBON STORAGE - peat is essentially carbon and water, making it an important carbon store. Dry peat releases carbon dioxide thorough oxidation so the encouragement of the re-wetting of peat and active peat growth ensure that carbon dioxide is naturally absorbed from the atmosphere and stored

Question 13

What results were drawn from the Exmoor Mires Project?

Answer 13

1. Water tables have started to rise, meaning that more moisture is being retained within the soil
2. Stormflow and flood peaks have reduced whilst baseflow has increased
3. In conclusion, these increases in the water table reflect an increased water storage in the peat mass following restoration and they are consistent with similar changes recorded in other re-wetted peatlands in the UK and Ireland

Question 14

When was the Exmoor Mires Project launched?

Answer 14

The Exmoor Mires project, initially called the MIRE (Moorland Improvement and Restoration on Exmoor) Project, was launched in 1998. In 2010, the project, along with the Dartmoor Mires project, became part of South West Water’s “Upstream Thinking” initiative