Water Situation Report Wessex Area

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Monthly water situation report Wessex Area

Summary – August 2021 Wessex received ‘below normal’ rainfall overall in August, at 74% LTA (50 mm); though, this was split between higher rainfall across north Wessex and lower rainfall in the south. Two main periods of rainfall occurred over 1 – 9 August and 19 – 21 August, at 71% and 22% of the month’s total, respectively. The period between these events was largely dry, as was the last week of August. The soil moisture deficit fluctuated in response to the changing wet and dry conditions. The deficit steadily increased at the end of August, ending the month on 76 mm, which is lower than the LTA (81 mm) but higher than the deficit this time last year (72 mm). Monthly mean river flows were largely ‘normal’ or ‘above normal’. Daily mean flows increased in response to the main periods of rainfall, to then recede with the dry end to the month. The majority of reporting sites ended August with ‘normal’ river flows. Groundwater levels decreased at all reporting sites, with end of month levels ranging from ‘notably high’ to ‘normal’. Despite groundwater levels falling, all reporting sites were classed the same as July bar Chantry (monitoring the Old Red Sandstone), which decreased from ‘above normal’ to ‘normal’ . Total reservoir storage decreased to 79% for Wessex Water and to 66% for Bristol Water.

Rainfall Wessex received 50 mm of rainfall in August (74% LTA), which is ‘below normal’ for the time of year. Rainfall totals were highest in north Wessex where catchments largely received ‘normal’ rainfall, and lower towards the south and west, where catchments received ‘below normal’ rainfall. The Axe in north Wessex had the highest rainfall total (76 mm, 96% LTA), which was over double of that received in the Lower Dorset Stour and River Crane (35 mm, 59% LTA) and Poole Harbour and Purbeck (35 mm, 60% LTA) in the south. There were two main periods of rainfall: 1 – 9 August, when 71% of the month’s rain fell, and 19 – 21 August, when 22% of the month’s rain fell. The period between these rainfall events, along with the last week of August, was largely dry.

The 12 month cumulative rainfall totals are ‘above normal’ in the south and west Wessex catchments, whilst the majority of northern catchments are ‘normal’, reflecting the spatial distribution of rainfall received since August 2020. Cumulative rainfall since the start of the water year (October 2020) remains high at 116% LTA.

Rainfall map and graph

Soil Moisture Deficit The soil moisture deficit decreased during the start of the month, following the rainfall and peaking at 52 mm on 10 August. The deficit increased slightly during the drier period that followed, before decreasing once again following the rainfall over 19 – 21 August. The dry end to August led to the deficit increasing steadily, ending the month on 76 mm, which is lower than the LTA (81 mm) but higher than the deficit this time last year (72 mm).

SMD graph and maps

River Flows Monthly mean flows were ‘normal’ or ‘above normal’ at all reporting sites bar Piddle (Baggs Mill) which had ‘notably high’ flows at 124% LTA.

Daily mean flows increased in response to the two main periods of rainfall over 1 – 9 August and 19 – 21 August, with the majority of reporting sites reaching ‘above normal’ or higher flows during these periods. The dry end to August led to a recession in flows, with the majority of reporting sites ending the month with ‘normal’ flows for the time of year.

River flow map and graphs

All data are provisional and may be subject to rev ision. The views expressed in this document are not necessarily those of the Environment Agency. Its officers, servants or agents accept no liability for any loss or damage arising from the interpretation or use of the information, or reliance upon views contained herein. www.gov.uk/environment-agency © Environment Agency 2021

Groundwater Levels Groundwater levels steadily decreased throughout August at all reporting sites bar Chantry (monitoring the Old Red Sandstone), which had a small period of increased groundwater levels as a result of the rainfall received. Groundwater levels ended the month lower than they started at all reporting sites. Month end levels ranged from ‘notably high’ to ‘normal’, with all reporting sites remaining within the same classification bands as July apart from Chantry, which decreased from ‘above normal’ to ‘normal’.

Groundwater map and graphs

Reservoir Storage Total reservoir storage decreased throughout August, with Wessex Water ending the month on 79% and Bristol Water ending the month on 66%, which are both higher than the total storage this time last year.

Reservoir graphs

Flood Warnings and Alerts Flood Warnings Flood Alerts Area Fluvial Coastal Groundwater Fluvial Coastal Groundwater North Wessex 0 0 0 0 1 0 South Wessex 0 0 0 0 0 0

Sites Providing Stream Support River Site Gauge End of Month Status Tetbury Avon Tetbury Brokenborough On Luckington Brook Luckington Fossway On Sherston Avon Stanbridge Fossway Off Charlton Stream Charlton Crabb Mill Off Gauze Brooke Hullavington Rodbourne On Rodbourne Brook Lower Stanton St. Quinton Startley On Chalfield Brook South Wraxall Great Chalfield (Wessex Water) On Chalfield Brook Little Chalfield Great Chalfield (Wessex Water) Off Semington Brook Easterton No gauge Off Horscombe Stream Tucking Mill No gauge On River Wylye Brixton Deverill Brixton Deverill On River Wylye Kingston Deverill Norton Bavant On Chitterne Brook Codford Road Codford On Bourne Porton Salisbury Bourne On River Allen Wyke Down All Hallows On Gussage Stream Gussage All Saints Bowerswain On Crichel Stream Long Crichel No gauge Off Pimperne Stream Pimperne No gauge On South Winterbourne Winterbourne Abbas Winterbourne Steepleton Off Devil’s Brook Dewlish WDX Dewlish Woodsdown Cross On River Piddle Alton Mill and Morningwell South House & Little Puddle On River Piddle Briantspuddle Briantspuddle Off Watergates Stream Watergates No gauge On

Abstraction Licences Subject to Cease or Restrict Catchment End of Month Status Hampshire Avon Cease – 4; Restrict – 2 Dorset Cease – 2; Restrict – 3 Bristol Avon Cease – 4; Restrict – 0 Somerset Cease – 3; Restrict - 0 Glossary Author: Wessex Hydrology

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Rainfall

Figure 1.1: Total rainfall (as a percentage) for hydrological areas across Wessex for the current month, the last three months, the last six months, and the last 12 months, classed relative to an analysis of respective historic totals. HadUK data based on the Met Office 1km gridded rainfall dataset derived from rain gauges (Source: Met Office © Crown Copyright 2021). Provisional data based on Environment Agency 1km gridded rainfall dataset derived from Environment Agency intensity rain gauges.

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Above 100% LTA rainfall Below 100% LTA rainfall 1-Month Period for Wessex

200%

150%

100%

50%

May-20 Feb-21 Sep-19 Oct-19 Nov-19 Dec-19 Jan-20 Feb-20 Mar-20 Apr-20 Jun-20 Jul-20 Aug-20 Sep-20 Oct-20 Nov-20 Dec-20 Jan-21 Mar-21 Apr-21 May-21 Jun-21 Jul-21 Aug-21

Figure 1.2: Monthly rainfall totals as a percentage of the 1961 – 1990 long term average. Produced using HadUK data based on the Met Office 1km gridded rainfall dataset derived from rain gauges (Source: Met Office © Crown Copyright 2021). Provisional data based on Environment Agency 1km gridded rainfall dataset derived from Environment Agency intensity rain gauges.

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Soil Moisture Deficit

Less than historic minimum Greater than historic maximum ..... 1961-1990 long term average ── Latest data ── 2020

100

Soil moisture deficit (mm) deficit Soilmoisture 120

140

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

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, 2021).

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Figure 2.2: Soil moisture deficits for week ending 31 August 2021. Top map shows actual soil moisture deficits (mm). Bottom map shows the difference (mm) of the actual from the 1961-90 long term average soil moisture deficits. MORECS data for real land use (Source: Met Office © Crown Copyright, 2021). Crown copyright. All rights reserved. Environment Agency, 100024198, 2021.

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River Flow

Figure 3.1: Monthly mean river flows at indicator sites for August 2021, expressed as a percentage of the respective long term average and classed relative to an analysis of historic August monthly means (Source: Environment Agency). Crown copyright. All rights reserved. Environment Agency, 100024198, 2021. Flows may currently be overestimated at all reporting sites, bar Knapp Mill and Bathford, so the data should be treated with caution.

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

Bristol Avon at Great Somerford Somerset Frome at Tellisford Ranking used data from 16/12/1963 to 31/12/2017 Ranking used data from 01/09/1961 to 31/12/2017 35 45 August mean flow August mean flow 0.539 cumecs 1.472 cumecs 40 67% LTA 30 106% LTA

35 25 30 20 25

20 15

15 10

10 Daily FlowMean Daily (cumecs) Daily FlowMean Daily (cumecs) 5 5

0 0 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21

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

Bristol Avon at Bathford Bristol Frome at Frenchay Ranking used data from 27/10/1969 to 31/12/2017 Ranking used data from 01/09/1961 to 31/12/2017 140 18 August mean flow August mean flow 5.307 cumecs 0.679 cumecs 16 120 100% LTA 119% LTA 14 100 12

80 10

60 8

6 40

4 Daily FlowMean Daily (cumecs) Daily FlowMean Daily (cumecs) 20 2

0 0 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21

Upper Brue at Lovington Yeo at Pen Mill Ranking used data from 01/10/1964 to 31/12/2017 Ranking used data from 31/10/1963 to 31/12/2017 40 August mean flow August mean flow 30 0.54 cumecs 0.638 cumecs 73% LTA 35 89% LTA

25 30

25 20

20 15 15 10

10 Daily FlowMean Daily (cumecs) Daily Flow(cumecs) Mean Daily 5 5

0 0 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21

Isle at Ashford Mill Tone at Bishops Hull Ranking used data from 01/10/1962 to 31/12/2017 Ranking used data from 01/02/1961 to 31/12/2017 25 August mean flow August mean flow 16 0.655 cumecs 0.939 cumecs 114% LTA 90% LTA 14 20

10 15

8 10 6

Daily Flow(cumecs) Mean Daily Flow(cumecs) Mean Daily 2

0 0 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21

Washford at Beggearn Huish Upper Hampshire Avon at Amesbury Ranking used data from 01/01/1967 to 31/12/2017 Ranking used data from 01/02/1965 to 31/12/2017 6.0 14 August mean flow August mean flow 0.362 cumecs 2.009 cumecs 134% LTA 12 119% LTA 5.0

10 4.0

8 3.0 6

2.0

4 Daily FlowMean Daily (cumecs) Daily FlowMean Daily (cumecs) 1.0 2

0.0 0 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21

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

River Wylye at South Newton Middle Hampshire Avon at East Mills Combined Ranking used data from 01/01/1967 to 31/12/2017 Ranking used data from 01/11/1965 to 31/12/2017 20 August mean flow August mean flow 60 18 1.793 cumecs 7.948 cumecs 99% LTA 101% LTA 16 50 14

12 40

10 30 8

6 20

4 Daily FlowMean Daily (cumecs) Daily FlowMean Daily (cumecs) 10 2

0 0 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21

Lower Hampshire Avon at Knapp Mill Middle Stour at Hammoon Ranking used data from 01/01/1975 to 31/12/2017 Ranking used data from 01/03/1968 to 31/12/2017 100 August mean flow August mean flow 70 90 8.282 cumecs 1.222 cumecs 94% LTA 74% LTA 80 60

70 50 60 40 50

40 30

30 20

20 Daily FlowMean Daily (cumecs) Daily FlowMean Daily (cumecs) 10 10

0 0 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21

Lower Stour at Throop Piddle at Baggs Mill Ranking used data from 01/01/1973 to 31/12/2017 Ranking used data from 01/01/1964 to 31/12/2017

80 August mean flow August mean flow 3.771 cumecs 10 1.354 cumecs 97% LTA 124% LTA 70

8 60

50 6 40

30 4

Daily FlowMean Daily (cumecs) FlowMean Daily (cumecs) 10

0 0 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21

Sydling Water at Sydling St Nicholas Dorset Frome at East Stoke Combined Ranking used data from 01/12/1969 to 31/12/2017 Ranking used data from 01/10/1965 to 31/12/2017 0.8 August mean flow August mean flow 0.118 cumecs 20 3.593 cumecs 0.7 123% LTA 107% LTA

0.6 15 0.5

0.4 10 0.3

0.2

Daily FlowMean Daily (cumecs) FlowMean Daily (cumecs) 0.1

0.0 0 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21

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

Wey at Broadwey Asker at Bridport East Bridge Ranking used data from 01/09/1975 to 31/12/2017 Ranking used data from 01/03/1996 to 31/12/2017 1.4 5.0 August mean flow August mean flow 0.188 cumecs 4.5 0.358 cumecs 1.2 116% LTA 99% LTA 4.0

1.0 3.5

3.0 0.8 2.5 0.6 2.0

0.4 1.5

1.0 Daily FlowMean Daily (cumecs) Daily FlowMean Daily (cumecs) 0.2 0.5

0.0 0.0 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21 31-Oct-20 31-Dec-20 28-Feb-21 30-Apr-21 30-Jun-21 31-Aug-21 31-Oct-21

Figure 3.2: Indicator river flow sites for Wessex. Daily mean flow compared to an analysis of historic daily mean flows. (Source: Environment Agency). Flows may currently be overestimated at all reporting sites, bar Knapp Mill and Bathford, so the data should be treated with caution.

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Groundwater Levels

Figure 4.1: Groundwater levels for indicator sites at the end of August 2021, classed relative to an analysis of respective historic August levels (Source: Environment Agency). Geological map reproduced with permission from UK Groundwater Forum, BGS © NERC. Crown copyright. All rights reserved. Environment Agency, 100024198, 2021.

Exceptionally high Notably high Above normal Normal Below normal Notably low Exceptionally low ─── Latest data Didmarton 1 Allington No2 Ranking derived from data for the period Nov-1975 to Dec-2017 Ranking derived from data for the period Oct-1974 to Dec-2017

104 81 99 79 94 77

89 75 84 73 79 71

74 Level (mAOD) Level Level (mAOD) Level 69 69 64 67 59 65 Nov-18 Mar-19 Jul-19 Nov-19 Mar-20 Jul-20 Nov-20 Mar-21 Jul-21 Nov-18 Mar-19 Jul-19 Nov-19 Mar-20 Jul-20 Nov-20 Mar-21 Jul-21

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Exceptionally high Notably high Above normal Normal Below normal Notably low Exceptionally low ─── Latest data Chantry No1 Overcompton Ranking derived from data for the period Feb-1974 to Dec-2017 Ranking derived from data for the period Sep-1971 to Dec-2017

156 60

154 59

152 58 150 57

148

Level (mAOD) Level Level (mAOD) Level

146 56

144 55 Nov-18 Mar-19 Jul-19 Nov-19 Mar-20 Jul-20 Nov-20 Mar-21 Jul-21 Nov-18 Mar-19 Jul-19 Nov-19 Mar-20 Jul-20 Nov-20 Mar-21 Jul-21

Tilshead Woodyates Ranking derived from data for the period Jan-1966 to Dec-2017 Ranking derived from data for the period Jan-1942 to Dec-2017

107 98 102 97 93 92 87 88

Level (mAOD) Level Level (mAOD) Level 83 77 72 78 67 Nov-18 Mar-19 Jul-19 Nov-19 Mar-20 Jul-20 Nov-20 Mar-21 Jul-21 Nov-18 Mar-19 Jul-19 Nov-19 Mar-20 Jul-20 Nov-20 Mar-21 Jul-21

Kingston Russell Road Ashton Farm Ranking derived from data for the period Jan-1974 to Dec-2017 Ranking derived from data for the period Feb-1974 to Dec-2017

121 72 119 71 117 70 115 69 113 68 111 67 109

66 Level (mAOD) Level Level (mAOD) Level 107 105 65 103 64 101 63 Nov-18 Mar-19 Jul-19 Nov-19 Mar-20 Jul-20 Nov-20 Mar-21 Jul-21 Nov-18 Mar-19 Jul-19 Nov-19 Mar-20 Jul-20 Nov-20 Mar-21 Jul-21

Figure 4.2: Indicator groundwater level sites for major aquifers. Daily groundwater levels compared to analysis of historic end month levels (Source: Environment Agency, 2021).

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Reservoir Stocks

── 2021 ── 2020 ── 1995 ── 1976 ..... 100% live capacity Wessex Water - Clatworthy Wessex Water - Sutton Bingham

100 100

80 80

60 60

40 40 Percentage of capacity Percentage gross Percentage of capacity Percentage gross 20 20

0 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Wessex Water - Total Reservoir Storage (including Wimbleball) Bristol Water - Blagdon

100 100

80 80

60 60

40 40 Percentage of capacity Percentage gross Percentage of capacity Percentage gross 20 20

0 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Bristol Water - Chew Valley Bristol Water - Total Reservoir Storage

100 100

80 80

60 60

40 40 Percentage of capacity Percentage gross Percentage of capacity Percentage gross 20 20

0 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 5.1: Reservoir stocks in Wessex (Source: Wessex Water and Bristol Water).

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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).

Categories Exceptionally high Value likely to fall within this band 5% of the time Notably high Value likely to fall within this band 8% of the time Above normal Value likely to fall within this band 15% of the time Normal Value likely to fall within this band 44% of the time Below normal Value likely to fall within this band 15% of the time Notably low Value likely to fall within this band 8% of the time

Exceptionally low Value likely to fall within this band 5% of the time Return to Summary

Rainfall Catchments

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