Monthly water situation report

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Monthly water situation report

Summary – September 2018

An average of 73mm of rainfall fell across Devon and Cornwall in September, which is 80% of the long term average (LTA). The 5 months to September 2018 were the 5

^th

driest on record (data back to 1910). Monthly mean river flows ranged from ‘notably low’ to ‘normal’ across Devon and Cornwall. Groundwater levels at Whitlands, Bussels No7A and Woodleys No1 continued to recede, and SMD dropped to 64mm. Overall, reservoir storage decreased from 62% to 52%.

Rainfall

An average of 73mm of rainfall fell across Devon and Cornwall in September, which is 80% of the LTA for the month and is ‘normal’ for the time of year. However 56mm of the rainfall in Devon and Cornwall fell between the 19^th and 23^rd of the month. Rainfall ranged between 97% LTA (Torridge and Hartland streams), and 67% LTA (Otter Side Axe and Lim). The 6 months to September 2018 were the 11^th driest on record (data back to 1910). However, the dry summer followed a wet spring, with some parts of the area recording the wettest ever March.

Rainfall maps and graph

Soil Moisture Deficit

The soil moisture deficit responded to the rainfall between the 19 and 23 of September, and then decreased to a month end value of 64mm, compared to 103mm last month. The LTA for September is 40mm, therefore the soil in Devon and Cornwall is still drier than average for the time of year.

SMD graph and map

River Flows

Monthly mean river flows ranged from ‘notably low’ to ‘normal’ across Devon and Cornwall. However, due to the flashy nature of many of the catchments in Devon and Cornwall, the month end value is only representative of the rainfall at the end of the month. For example, the flow at East Dart at Bellever went from ‘below normal’, to ‘exceptionally high’ in 5 days as a result of a heavy persistent rainfall event. Total days below Q95 for the 2017-2018 water year (1October to 30 September) averaged 36 days, with the Axe at Whitford below Q95 for 52 days, and the Taw at Umberleigh below Q95 for 49 days (on average we would expect flows to be below Q95 for 18 days a year).

River flow map and graphs

Groundwater Levels

Groundwater levels continued to recede in September. Whitlands, Bussels No7A and Woodleys No1 are all at ‘normal’ levels for the time of year.

Groundwater map and graphs

Reservoir Storage

Overall, total reservoir storage decreased from 62% in August, to 52% in September across Devon and Cornwall. Wimbleball storage dropped to 47%, Colliford dropped to 56% and Roadford dropped to 48% total storage.

Reservoir graphs Glossary

Author: Devon and Cornwall Hydrology Team Contact details: 03708 506506

Devon and Cornwall Area

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Rainfall

Figure 1.1: Total rainfall for hydrological areas across Devon and Cornwall for the current month (up to September), the last three months, the last six months and the last 12 months, classed relative to an analysis of respective historic totals. Final NCIC (National Climate Information Centre) data based on the Met Office 5km gridded rainfall dataset derived from rain gauges (Source: Met Office © Crown Copyright, 2018). Provisional data based on Environment Agency 1km gridded rainfall dataset derived from

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Above 100% LTA rainfall Below 100% LTA rainfall

Figure 1.2: Monthly rainfall totals as a percentage of the 1961 – 1990 long term average. Final NCIC (National Climate Information Centre) data based on the Met Office 5km gridded rainfall dataset derived from rain gauges (Source: Met Office © Crown Copyright, 2018). Provisional data based on Environment Agency 1km gridded rainfall dataset derived from Environment Agency intensity rain gauges. Crown copyright. All rights reserved. Environment Agency, 100026380, 2018.

Soil Moisture Deficit

Less than historic minimum Greater than historic maximum

... 1961-1990 long term average ── Latest data

── 2017

Figure 2.1: Latest soil moisture deficit compared to previous year, maximum, minimum and 1961 – 1990 long term average. Weekly MORECS data for real land use (Source: Met Office © Crown Copyright, 2018).

0%

50%

100%

150%

200%

250%

Oct-16 Nov-16 Dec-16 Jan-17 Feb-17 Mar-17 Apr-17 May-17 Jun-17 Jul-17 Aug-17 Sep-17 Oct-17 Nov-17 Dec-17 Jan-18 Feb-18 Mar-18 Apr-18 May-18 Jun-18 Jul-18 Aug-18 Sep-18

Percentage of LTA

1-Month Period for South West - Devon & Cornwall NCIC

0

20

40

60

80

100

120

140

160

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Soil moisture deficit (mm)

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Month-end Soil Moisture Deficit (mm)

Figure 2.2: Soil moisture deficits for week ending 24^th September 2018. MORECS data for real land use (Source: Met Office © Crown Copyright, 2018). Crown copyright. All rights reserved. Environment Agency, 100026380, 2018.

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River Flow

Figure 3.1: Monthly mean river flows at indicator sites for September 2018, expressed as a percentage of the respective long term average and classed relative to an analysis of historic September monthly means (Source: Environment Agency). Crown copyright. All rights reserved. Environment Agency, 100026380, 2018.

Exceptionally high Notably high Above normal Normal

Below normal Notably low Exceptionally low ─── Latest data

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

30-Nov-17 31-Jan-18 31-Mar-18 31-May-18 31-Jul-18 30-Sep-18 30-Nov-18

Daily Mean Flow (cumecs)

Hayle at St Erth

Ranking derived from data for the period 03/01/1968 to 31/12/2012

September mean flow 0.218 cumecs 57% LTA

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Kenwyn at Truro

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Exceptionally high Notably high Above normal Normal Below normal Notably low Exceptionally low ─── Latest data

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Gannel at Gwills

September mean flow 0.111 cumecs

0 5 10 15 20 25 30 35

Camel at Denby

0 5 10 15 20 25 30 35

Fowey at Restormel

0 20 40 60 80 100 120 140 160

Tamar at Gunnislake

0 20 40 60 80 100 120 140

Torridge at Torrington Ranking derived from data for the period 31/07/1960 to 31/12/2012

0 20 40 60 80 100 120 140

Taw at Umberleigh

0 1 2 3 4 5 6 7 8 9 10

East Dart at Bellever

0 10 20 30 40 50 60

Dart at Austins Bridge Ranking derived from data for the period 30/09/1958 to 31/12/2012

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Figure 3.2: Indicator river flow sites for Devon and Cornwall. Daily mean flow compared to an analysis of historic daily mean flows. (Source: Environment Agency).

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0

Teign at Chudleigh Bridge Ranking derived from data for the period 01/01/1958 to 31/12/2012

0 10 20 30 40 50 60 70 80 90 100

Exe at Thorverton

0 5 10 15 20 25

Otter at Dotton

0 5 10 15 20 25 30 35 40

Axe at Whitford

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Groundwater Levels

Figure 4.1: Groundwater levels for indicator sites at the end of September 2018, classed relative to an analysis of respective historic September levels (Source: Environment Agency). Geological map reproduced with permission from UK Groundwater Forum, BGS © NERC. Note: groundwater levels are reported at different times during the month and therefore may not be fully representative of levels at the month end. Crown copyright. All rights reserved. Environment Agency, 100026380, 2018.

Exceptionally high Notably high Above normal Normal

Below normal Notably low Exceptionally low ─── Latest data

86 87 87 88 88 89 89

Dec-15 Apr-16 Aug-16 Dec-16 Apr-17 Aug-17 Dec-17 Apr-18 Aug-18

Level (mAOD)

WHITLANDS

Ranking derived from data for the period Sep-1984 to Dec-2012

23 24 24 25 25 26 26

Level (mAOD)

BUSSELS NO7A

Ranking derived from data for the period Nov-1971 to Dec-2012

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Figure 4.2: Indicator groundwater level sites for major aquifers. End of month groundwater levels compared to analysis of historic end month levels (Source: Environment Agency, 2018).

Reservoir Stocks

── 2018 ── 2017 ── 1995 ... 100% live capacity

Figure 5.1: Reservoir stocks in Devon and Cornwall (Source: South West Water).

30 31 31 32 32 33 33 34 34

Level (mAOD)

WOODLEYS NO1

Ranking derived from data for the period Jan-1966 to Dec-2012

0 20 40 60 80 100

Percentage of live capacity

South West Water - Colliford

0 20 40 60 80 100

South West Water - Roadford

0 20 40 60 80 100

South West Water - Wimbleball

0 20 40 60 80 100

South West Water - Total Reservoir Storage

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Glossary

Term Definition

Aquifer A geological formation able to store and transmit water.

Areal average rainfall The estimated average depth of rainfall over a defined area. Expressed in depth of water (mm).

Artesian The condition where the groundwater level is above ground surface but is prevented from rising to this level by an overlying continuous low permeability layer, such as clay.

Artesian borehole Borehole where the level of groundwater is above the top of the borehole and groundwater flows out of the borehole when unsealed.

Cumecs Cubic metres per second (m³s^-1)

Effective rainfall The rainfall available to percolate into the soil or produce river flow.

Expressed in depth of water (mm).

Flood Alert/Flood Warning Three levels of warnings may be issued by the Environment Agency.

Flood Alerts indicate flooding is possible. Flood Warnings indicate flooding is expected. Severe Flood Warnings indicate severe flooding.

Groundwater The water found in an aquifer.

Long term average (LTA) The arithmetic mean calculated from the historic record, usually based on the period 1961-1990. However, the period used may vary by parameter being reported on (see figure captions for details).

mAOD Metres Above Ordnance Datum (mean sea level at Newlyn Cornwall).

MORECS Met Office Rainfall and Evaporation Calculation System. Met Office service providing real time calculation of evapotranspiration, soil moisture deficit and effective rainfall on a 40 x 40 km grid.

Naturalised flow River flow with the impacts of artificial influences removed. Artificial influences may include abstractions, discharges, transfers, augmentation and impoundments.

NCIC National Climate Information Centre. NCIC area monthly rainfall totals are derived using the Met Office 5 km gridded dataset, which uses rain gauge observations.

Recharge The process of increasing the water stored in the saturated zone of an aquifer. Expressed in depth of water (mm).

Reservoir gross capacity The total capacity of a reservoir.

Reservoir live capacity The capacity of the reservoir that is normally usable for storage to meet established reservoir operating requirements. This excludes any capacity not available for use (e.g. storage held back for emergency services, operating agreements or physical restrictions). May also be referred to as

‘net’ or ‘deployable’ capacity.

Soil moisture deficit (SMD) The difference between the amount of water actually in the soil and the amount of water the soil can hold. Expressed in depth of water (mm).

Rainfall Catchments

Environment Agency, 100026380, 2017.

Monthly water situation report