water and carbon cycles: local drainage basin river exe case study

how long is the river exe?

82km

what is the source of the river exe?

hills of Exmoor

what is the river exe’s drainage density?

High drainage density in the network of tributaries (so shorter lag time)

what is the drainage basin’s elevation?

516m at source

Ranges between 500m and 100m in case study area.

what is the drainage basin’s geology?

Impermeable rock (sandstone) underlies 85% of basin → reduced percolation and increased surface runoff.

how do the drainage basin’s characteristics affect the water cycle?

• Low interception of rainfall

• Over 80% farmland - big impact on water cycle

• Drainage ditches increase runoff

• Animal trampling reduces infiltration

what is the drainage basin’s water balance?

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

Rainfall is high (nearly double London’s value), particularly over Exmoor. Around a third of it is absorbed by the peaty moorland soils. However, if saturated or where drainage ditches have been dug, water can flow off the hill rapidly.

Runoff is 65% of the water balance and soil water storage is 35%.

how do drainage ditches impact the river exe?

increase in surface runoff is especially clear.

how does farming impact the river exe?

Farming around the upper reaches of the River Exe. Where livestock are present, trampling of soil takes place and it becomes compacted, increasing surface runoff by up to 50%.

what is the river exe’s river regime?

big seasonal fluctuation - November-March the flow stays above 10 meters3 per second whereas it is below this for the rest of the year

• large number of peaks suggests a flashy river - is a clear distinction between winter and summer weather

what factors of the river exe cause a flashy river regime?

-Only 2.5% of bedrock has high permeability

-Over 80% of basin is agriculture

-Low forest coverage

-Urbanisation has local effects, although urban areas are small

how is a sustainable water supply kept?

Wimbleball Reservoir

Reservoir supplies water to Exeter and parts of East Devon - regulates water flow and ensures a steady flow regime during the year.

Prevents peaks and troughs of water discharge that make flooding or drought more likely → reduce flood risk in Tiverton and Exeter.

Water is actually abstracted from the drainage basin here to supply water to nearby urban areas.

The reservoir acts as a huge surface store containing 120 million litres of water

how is the river exe managed sustainably?

• Exmoor mires partnership:

Restoration of original peatland bogs on Exmoor by blocking drainage ditches. This encourages water to collect and pool, reinstating historic carbon and water stores on the moorlands

what are the impacts of the exmoor mires project on the carbon cycle?

Increased retention of carbon in peat

Reduced carbon emissions from peat loss

what are the impacts of the exmoor mires project on the water cycle?

Increased surface storage

Reduced surface runoff

Reduced flood risk/’flashy’ peaks

how much of the drainage basin has been restored?

154km by 40km → peat is 3000-4000 years old which has a large impact on the carbon cycle

what is some field data?

-Restored peatlands hold a third more water

-Water table has risen by 2.2cm over 10 years

what is the land use around the river exe?

woodland = 13%

arable = 23.5%

grassland = 57%

heath/bog = 1.5%

urban = 4.5%

what is the permeability around the river exe?

moderate permeability = 32%

low permeability = 31.5%

what is the impact of human activity on the river exe?

• moorland drainage: drain water of moorlands to increase food production and area for cattle/sheep

• urbanisation: land use - increasing run off and flood risk

• wimbleball reservoir

• agriculture

how much did the peatland restoration cost?

£2.2m and covered 2000 ha

how significant is human activity in impacting the water cycle?

• urbanisation: increased surface runoff due to impermeable surfaces and reduced groundwater recharge

• agricultural practices: increased runoff and erosion due to artificial drainage systems removing excess water from fields

• reservoirs: wimbleball reservoir - reduces natural flow variability which can affect downstream ecosystems dependant on fluctuations

how significant is human activity in impacting the carbon cycle?

• deforestation and land use change: reduced carbon sequestration from trees and agricultural practices can lead to the degradation of soil quality - release of carbon stored in the soils as co2

• peatland drainage: drainage for agriculture means peatlands dry out which leads to the oxidation of organic material and release of stored carbon as co2

• afforestation and reforestation efforts: aim to enhance carbon sequestration which will increase biomass carbon storage and soil health - allows for better carbon and water retention in the ecosystem

what combined impact does human activity have on the water and carbon cycle?

• soil erosion and carbon release = increased soil erosion from agricultural land use and deforestation impacts water quality and contributes to loss of soil carbon store

are human or physical factors more important in creating flood risk?

physical

• topography: steep gradients in the upper catchment create rapid surface runoff

• soil + geology: 84.4% of the basin is sandstone

• climate: experiences significant rainfall in winter months e.g.i in 2016 river discharge was above 10 cumecs for 5/6 months but was above 10 cumecs for 8 months 2019

human:

• urbanisation + agricultural practices: increased runoff however, peat restoration may mitigate this → restored 1948 hectares

what has the exmoor mires partnership achieved?

• Surveying 3,362 hectares of moorland

• Restoring 1,948 hectares

• Blocking 195,097 meters of ditches with 22,343 ditch blocks installed. ​

what are some of the afforestation efforts?

Exmoor National Park Authority

The Exmoor National Park Authority (ENPA) has initiated a program to rejuvenate and expand woodland areas within the park. This includes recovery work following storm damage and aligns with governmental plans to increase tree cover to 16.5% by 2050. ​Exmoor National Park

Woodland Creation: Yonder Oak Wood

The Woodland Trust is developing Yonder Oak Wood, creating a mosaic of habitats overlooking the Exe Estuary. This project involves planting woods, glades, and individual trees to enhance biodiversity and landscape connectivity. ​

how has farming impacted the river exe?

• Nutrient Runoff: Agricultural activities, especially intensive livestock farming and crop production, have led to increased runoff of nitrates and phosphates into the river. This can result in eutrophication, reducing oxygen levels and harming aquatic life.

• Soil Erosion: Ploughing and overgrazing, particularly on the steeper slopes of Exmoor, have caused soil erosion. Sediment washes into the river, affecting water quality and smothering fish spawning grounds.

• Drainage of Wetlands: To increase arable land, farmers historically drained peat bogs and wetlands, reducing the natural water-holding capacity of the land and increasing flood risk downstream.

what have been some impacts of urbanisation along the river exe?

• Impervious Surfaces: Roads and buildings increase surface runoff, reducing natural infiltration and leading to more frequent and intense flooding.

• Water Pollution: Urban wastewater and surface runoff carry pollutants like oil, heavy metals, and litter into the river.

• River Modification: Parts of the Exe have been canalised or embanked to protect infrastructure. This disrupts natural river processes and can worsen downstream flooding and reduce biodiversity.

what have been some impacts of deforestation on the river exe?

• Reduced Water Retention: Trees help intercept rainfall and promote infiltration. Their removal leads to faster runoff and greater flood peaks.

• Loss of Habitat: Deforestation reduced biodiversity, removed shade (affecting water temperature), and led to more sediment entering the river.

• Peatland Degradation: Tree removal on Exmoor exposed peat soils, leading to drying, oxidation, and the release of stored carbon