Monthly water situation report

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Monthly water situation report

Summary – September 2018

September saw a return to the drier conditions, with Wessex receiving only 67% LTA rainfall. Almost all the rainfall occurred between 19 to 22 September, with the majority arriving in one distinct band of rain on 22 September. Daily river flows responded briefly to this rainfall event, then were in recession to the end of the month. The monthly mean flows across the majority of catchments were 'below normal' or 'notably low'. These relatively low monthly mean flows reflected the dry catchment conditions prior to the main rainfall events. Soil moisture deficit increased to the highest deficit observed for this time of year since records began, before the rainfall on 19 to 22 September. Groundwater levels in all the major aquifers were still in recession, and levels remained between ‘below normal' and 'above normal' for the time of year. Total reservoir storage reduced throughout the month.

Rainfall

After a month where Wessex received slightly over 100% LTA rainfall in August, September saw a return to the drier conditions that we observed earlier in the summer, with only 67% LTA (48 mm) rainfall received across Wessex. The total rainfall received between June and September was the lowest received in the area for those four months since 1965. The cumulative rainfall over the past 6 months shows that the majority of the area has received 'notably low' rainfall, with several hydrological areas receiving 'exceptionally low' rainfall.

The south coast of Dorset received less rainfall than the rest of the area, however in general the rainfall was relatively evenly distributed. Over 80% of September's rain fell between 19 to 22 September, with the majority arriving in one distinct band of frontal rain on 22 September.

Rainfall map and graph

Soil Moisture Deficit

The end of August saw a significant reduction in soil moisture deficit following a band of heavy rain, however the deficit subsequently increased immediately after this event, reaching a maximum deficit of 130 mm in mid- September, equalling the maximum deficit observed for this time of year since records began. The rain received from 19 to 22 September had a significant impact on the soil wetness, with the deficit reducing to 102 mm. By the end of September, the soil moisture deficit increased to 113 mm.

SMD graph and maps

River Flows

Daily flows responded to the rainfall event on 22 September, with many sites recording 'above normal' flows;

however the fluvial response was short lived, reflecting the very dry antecedent conditions within the catchments.

As a result of these dry antecedent catchment conditions and lower rainfall, mean monthly flows were lower in September than they were in August.

The groundwater dominated lower Hampshire Avon, Wey, Stour and Sydling St Nicholas catchments recorded 'normal' or ‘above normal’ monthly mean flows, however monthly mean flows across the remainder of the groundwater dominated catchments were 'below normal'. The flashier catchments in the north and west of the area (Bristol Avon, Somerset Frome, Tone, Washford, Brue) ended September with 'below normal' or 'notably low' monthly mean flows.

River flow map and graphs

Wessex Area

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Groundwater Levels

Groundwater levels at all major aquifers across Wessex were still in recession, as is expected throughout early autumn, with levels at the end of September remaining between ‘below normal' (Woodyates - monitoring the Chalk aquifer of south Wessex; and Overcompton - monitoring the Bridport Sands aquifer) and ‘above normal’

(Chantry no.1 – monitoring the carboniferous limestone; and Kingston Russel Road monitoring the Chalk).

Groundwater map and graphs

Reservoir Storage

Reservoir stocks decreased across all of the main reservoirs in Wessex in September, with the total Wessex Water storage well below the 2017 level. The levels have also just dropped below those observed during the summer of 1976, when significant rainfall at the end of September rapidly recharged all the reservoirs. The month ended with 60% and 44% storage across Bristol Water and Wessex Water reservoirs respectively.

Reservoir graphs

Flood Warnings and Alerts

Sites Providing Stream Support

River Site Gauge End of Month Status

River Allen Wyke Down All Hallows On

Chitterne Brook Codford Road Codford On

Gussage Stream Gussage All Saints Bowerswain On

Devil’s Brook Dewlish WDX Dewlish Woodsdown Cross On

River Piddle Briantspuddle Briantspuddle On

South Winterbourne Winterbourne Abbas Winterbourne Steepleton Off

Pimperne Stream Pimperne No gauge Off

Watergates Stream Watergates No gauge On

River Wylye Brixton Deverill Brixton Deverill On

River Wylye Kingston Deverill Norton Bavant On

Rodbourne Brook Lower Stanton St. Quinton Startley On

Tetbury Avon Tetbury Brokenborough On

Sherston Avon Stanbridge Fossway Off

Luckington Brook Luckington Fossway On

Gauze Brooke Hullavington Rodbourne On

Charlton Stream Charlton Crabb Mill On

Chalfield Brook South Wraxall Great Chalfield On

Chalfield Brook Little Chalfield Great Chalfield On

River Bourne Porton No gauge On

Horscombe Stream Tucking Mill No gauge On

Abstraction Licences Subject to Cease or Restrict

Catchment End of Month Status

Hampshire Avon Cease – 2; Restrict – 4 Dorset Frome Cease – 4; Restrict 3 Bristol Avon Cease – 4; Restrict – 0 Somerset Cease – 13; Restrict - 1 Glossary

Author: Wessex Hydrology

Area Flood Warnings Flood Alerts

Fluvial Coastal Groundwater Fluvial Coastal Groundwater

North Wessex 0 0 0 4 14 0

South Wessex 0 0 0 0 5 0

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Rainfall

Figure 1.1: Total rainfall for hydrological areas across Wessex for the current month (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 Environment Agency intensity rain gauges. Crown copyright. All rights reserved. Environment Agency, 100026380, 2018.

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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. Produced using final and provisional NCIC (National Climate Information Centre) data. (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 - Wessex NCIC

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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

20

40

60

80

100

120

140

160

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

Soi l m oi sture d efic it (m m)

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Figure 2.2: Soil moisture deficits for week ending 24 September 2018. 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, 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 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 2 4 6 8 10 12 14 16 18 20 22

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)

Tone at Bishops Hull

Ranking derived from data for the period 01/02/1961 to 31/12/2012

September mean flow 0.584 cumecs 48% LTA

0 5 10 15 20 25

Yeo at Pen Mill

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

0 5 10 15 20 25

Upper Brue at Lovington Ranking derived from data for the period 01/10/1964 to 31/12/2012

0 20 40 60 80 100 120

Bristol Avon at Bathford Ranking derived from data for the period 27/10/1969 to 31/12/2012

0 5 10 15 20 25 30

Daily Mean Flow (cumecs)

Somerset Frome at Tellisford Ranking derived from data for the period 01/09/1961 to 31/12/2012

0 5 10 15 20 25 30 35

Bristol Avon at Great Somerford Ranking derived from data for the period 16/12/1963 to 31/12/2012

0 10 20 30 40 50 60 70

Middle Stour at Hammoon Ranking derived from data for the period 01/03/1968 to 31/12/2012

0 5 10 15 20 25

Dorset Frome at East Stoke Combined Ranking derived from data for the period 01/10/1965 to 31/12/2012

0 1 2 3 4 5 6 7 8 9 10

Piddle at Baggs Mill

0 2 4 6 8 10 12 14 16

Wylye at South Newton Ranking derived from data for the period 01/01/1967 to 31/12/2012

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0 2 4 6 8 10 12

Upper Hampshire Avon at Amesbury Ranking derived from data for the period 01/02/1965 to 31/12/2012

0 10 20 30 40 50 60 70 80

Stour at Throop

0 10 20 30 40 50 60 70 80 90 100

Lower Hampshire Avon at Knapp Mill Ranking derived from data for the period 01/01/1975 to 31/12/2012

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Washford at Beggearn Huish Ranking derived from data for the period 01/01/1967 to 31/12/2012

0 2 4 6 8 10 12 14 16 18 20 22

Bristol Frome at Frenchay Ranking derived from data for the period 01/09/1961 to 31/12/2012

0 2 4 6 8 10 12

Isle at Ashford Mill

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Asker at Bridport East Bridge Ranking derived from data for the period 01/03/1996 to 31/12/2012

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Sydling Water at Sydling St Nicholas Ranking derived from data for the period 01/12/1969 to 31/12/2012

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

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Wey at Broadwey

0 10 20 30 40 50 60

Middle Hampshire Avon at East Mills Combined Ranking derived from data for the period 01/11/1965 to 31/12/2012

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Groundwater Levels

Figure 4.1: Groundwater levels for indicator sites at the end of September, 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.

Exceptionally high Notably high Above normal Normal

Below normal Notably low Exceptionally low ─── Latest data

64 66 68 70 72 74 76 78 80

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

Level (mAOD)

Allington No2

Ranking derived from data for the period Oct-1974 to Dec-2012

63 64 65 66 67 68 69 70 71 72

Level (mAOD)

Ashton Farm

Ranking derived from data for the period Feb-1974 to Dec-2012

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

144 146 148 150 152 154

Level (mAOD)

Chantry No1

Ranking derived from data for the period Feb-1974 to Dec-2012

58 63 68 73 78 83 88 93

Level (mAOD)

Didmarton 1

Ranking derived from data for the period Nov-1975 to Dec-2012

55 56 57 58 59 60

Level (mAOD)

Overcompton

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

78 83 88 93 98

Level (mAOD)

Tilshead

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

67 72 77 82 87 92 97 102

Level (mAOD)

Woodyates

101 103 105 107 109 111 113 115 117 119

Level (mAOD)

Kingston Russell Road

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Reservoir Stocks

── 2018 ── 2017 ── 1995 ── 1976 ... 100% live capacity

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

0 20 40 60 80 100

Percentage of live capacity

Wessex Water - Clatworthy

0 20 40 60 80 100

Wessex Water - Sutton Bingham

0 20 40 60 80 100

Wessex Water - Total Reservoir Storage

0 20 40 60 80 100

Bristol 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).

Monthly water situation report