The River Exe - Carbon and Water Cycles case study

The river Exe flows from its source in the hills of Exmoor, through Tiverton and Exeter, to the sea at Exmouth.

It has an extensive network of tributaries and a high drainage density.

• The area of the upper catchment is around 600km².
• Its maximum elevation of just over 500m is in the North.
• The land is much flatter in the South - the lowest elevation is 25m.

Geology:

• An estimated 85% of the catchment is underlain by impermeable rocks, predominantly sandstone, which explains the extensive drainage networks.

Land Use:

• Most of the land is agricultural grassland, with some woodland and arable farmland.
• On the high ground of Exmoor, there are moors and peat bogs.

Rainfall is high, particularly over Exmoor and much of it is absorbed by the peaty moorland soils. However...

If saturated or where drainage ditches have been dug, water can flow off the hills rapidly.

Runoff accounts for some 65% of the water balance, which is relatively high compared to other UK rivers.

There are 2 main reasons for this:

• The impermeable nature of the bedrock reduces percolation and baseflow.
• Drainage ditches on Exmoor reduce the amount of soil water storage.

The river responds relatively slowly to rainfall events due to the rural nature of the lower catchment.

The construction of the Wimbleball Reservoir.

In 1979 the River Haddeo, an upland tributary of the river Exe, was dammed to create the Wimbleball reservoir. It provides water to Exeter and parts of East Devon. It regulates water flow and this prevents the peaks and troughs of water discharge that make flooding or drought more likely.

Peatland Restoration on Exmoor:

For decades, drainage ditches have been dug in the peat bogs of Exmoor to make it suitable for farming. This has increased the speed of water flow to the Exe, which reduces water quality as more silt is carried downstream. Peat has also been dug as a fuel, leaving behind scars to the landscape. As the peat surface has dried out, decomposition has occurred, releasing carbon from this important carbon store into the atmosphere as CO2 and CH4.

Works to restore the peat bogs by blocking the drainage ditches with peat blocks or moorland bales. This increases water content and returns the ground to the saturated, boggy conditions. These saturated conditions help to retain carbon stored in the peat.

Benefits:

• Improved water quality.
• Better carbon storage.
• Increased biodiversity.
• By 2015 over 1000 hectares of moorland had been restored.
• A great deal of scientific research has been conducted including a detailed study on the impact of peat restoration on water tables.

Results:

From results collected so far, there is an indication that water tables have started to rise., meaning that more moisture is being retained within the soil and flood peaks have reduced.

They Used:

• Dipwell Transects
• Water table measuring stations.
• An electric contact dip metre records the depth of the water table.