Monthly water situation report Wessex Area Summary – November 2020 Wessex received ‘normal’ rainfall in November at 84% LTA (71 mm). There were multiple bands of rain throughout November; the most notable event occurred on the 14 November, when 23% of the month’s rain fell. The last week of November was generally dry. The soil moisture deficit gradually decreased throughout November, ending the month on 7 mm, which is higher than the deficit this time last year, but lower than the LTA. When compared to the start of the month, groundwater levels at the end of November had increased at the majority of reporting sites. Rising groundwater levels in the Chalk supported the groundwater dominated rivers in the south, with the majority of south Wessex reporting sites experiencing ‘above normal’ monthly mean river flows, whilst the surface water dominated rivers in the north had largely ‘normal’ monthly mean flows. Daily mean flows generally peaked around 14-16 November in response to the main rainfall event. The dry end to November caused a recession in flows, with all bar two reporting sites ending the month with ‘normal’ daily mean flows. Total reservoir storage increased, with Wessex Water and Bristol Water ending November with 84% and 83%, respectively. Rainfall Wessex received 71 mm of rainfall in November (84% LTA), which is ‘normal’ for the time of year. All hydrological areas received ‘normal’ rainfall bar the Axe (69% LTA; 61 mm) and West Somerset Streams (71% LTA; 79 mm), which had ‘below normal’ rainfall. The highest rainfall accumulations (for the time of year) were generally in the east and south. There were multiple bands of rain throughout November; the most notable rainfall event occurred in the middle of the month, with 23% of the month’s rain falling on the 14 November. The last week of November was generally dry. The 3 month cumulative rainfall is similarly ‘normal’ for the majority of hydrological catchments. In contrast, 12 month cumulative rainfall is largely ‘above normal’ or higher across Wessex, which reflects the heavy rainfall received at the end of 2019, and in February and October 2020. The cumulative rainfall since the start of the water year (October 2020) has decreased, but remains high at 142% LTA. Rainfall map and graph Soil Moisture Deficit The soil moisture deficit continued to gradually decrease throughout November in response to the rainfall received. At the end of the month, the deficit was 7 mm, which is higher than the deficit this time last year (0.1 mm), but lower than the LTA (18 mm). SMD graph and maps River Flows Monthly mean flows were ‘normal’ or ‘above normal’ at all reporting sites bar Sydling Water (Sydling St Nicholas), which flows were ‘notably high’ at 157% LTA. Groundwater dominated rivers in the south were supported by the ‘normal’ or higher groundwater levels in the Chalk, with the majority experiencing ‘above normal’ monthly mean flows, whilst the surface water dominated rivers in the north had largely ‘normal’ flows. Daily river flows fluctuated throughout the month in response to the bands of rainfall. The main rainfall event led to peak flows generally occurring around 14-16 November, with many of the responsive surface water dominated rivers in the north peaking at ‘exceptionally high’ flows, whilst the majority of the less responsive groundwater dominated rivers in the south, only reached ‘notably high’. The dry end to November led to a recession in flows, with all bar two reporting sites ending the month with ‘normal’ daily mean flows. River flow map and graphs All data are provisional and may be subject to revision. 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 2020 Groundwater Levels When compared to the start of the month, groundwater levels at the end of November had increased at all reporting sites bar the responsive Chantry borehole (monitoring the Old Red Sandstone). On the Chalk, rising groundwater led to Ashton Farm and Woodyates reaching ‘above normal’ levels for the time of year, though levels at Tilshead remained ‘normal’. At the start of the month, groundwater levels at Kingston Russell Road (also monitoring the Chalk) were ‘above normal’, and whilst there was an increase in groundwater throughout November at this reporting site, by the end of the month these levels were ‘normal’ for the time of year. Groundwater map and graphs Reservoir Storage Reservoir levels increased throughout November. At the end of the month, total storage for Wessex Water was 84% and for Bristol Water was 83%, which are both lower than this time last year. Reservoir graphs Flood Warnings and Alerts Flood Warnings Flood Alerts Area Fluvial Coastal Groundwater Fluvial Coastal Groundwater North Wessex 2 7 0 5 20 0 South Wessex 0 5 0 9 20 0 Sites Providing Stream Support River Site Gauge End of Month Status Tetbury Avon Tetbury Brokenborough Off Luckington Brook Luckington Fossway Off Sherston Avon Stanbridge Fossway Off Charlton Stream Charlton Crabb Mill Off Gauze Brooke Hullavington Rodbourne Off Rodbourne Brook Lower Stanton St. Quinton Startley Off Chalfield Brook South Wraxall Great Chalfield (Wessex Water) Off Chalfield Brook Little Chalfield Great Chalfield (Wessex Water) Off Semington Brook Easterton No gauge Off Horscombe Stream Tucking Mill No gauge Off River Wylye Brixton Deverill Brixton Deverill Off River Wylye Kingston Deverill Norton Bavant Off Chitterne Brook Codford Road Codford On Bourne Porton Salisbury Bourne Off River Allen Wyke Down All Hallows Off Gussage Stream Gussage All Saints Bowerswain Off Crichel Stream Long Crichel No gauge Off Pimperne Stream Pimperne No gauge Off South Winterbourne Winterbourne Abbas Winterbourne Steepleton Off Devil’s Brook Dewlish WDX Dewlish Woodsdown Cross Off River Piddle Alton Mill and Morningwell South House & Little Puddle Off 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 – 1; Restrict – 0 Dorset Cease – 1; Restrict – 1 Bristol Avon Cease – 0; Restrict – 0 Somerset Cease – 2; Restrict - 0 Glossary Author: Wessex Hydrology www.gov.uk/environment-agency © Environment Agency 2020 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 2020). Provisional data based on Environment Agency 1km gridded rainfall dataset derived from Environment Agency intensity rain gauges. Return to Summary www.gov.uk/environment-agency © Environment Agency 2020 Above 100% LTA rainfall Below 100% LTA rainfall 1-Month Period for Wessex 200% 150% 100% 50% 0% Aug-19 May-20 Dec-18 Jan-19 Feb-19 Mar-19 Apr-19 May-19 Jun-19 Jul-19 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 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 2020). Provisional data based on Environment Agency 1km gridded rainfall dataset derived from Environment Agency intensity rain gauges. Return to Summary www.gov.uk/environment-agency © Environment Agency 2020 Soil Moisture Deficit Less than historic minimum Greater than historic maximum ..... 1961-1990 long term average ── Latest data ── 2019 0 20 40 60 80 100 Soil moisture deficit (mm) Soil moisture 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, 2020). Return to Summary www.gov.uk/environment-agency © Environment Agency 2020 Figure 2.2: Soil moisture deficits for week ending 29 November 2020. 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, 2020). Crown copyright. All rights reserved. Environment Agency, 100024198, 2020. Return to Summary www.gov.uk/environment-agency © Environment Agency 2020 River Flow Figure 3.1: Monthly mean river flows at indicator sites for November 2020, expressed as a percentage of the respective long term average and classed relative to an analysis of historic November monthly means (Source: Environment Agency). Crown copyright. All rights reserved. Environment Agency, 100024198, 2020. 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 45 November mean flow November mean flow 5.539 cumecs 35 40 6.372 cumecs 130% LTA 127% LTA 35 30 30 25 25 20 20 15 15 10 10 Daily FlowDaily Mean (cumecs) Daily FlowDaily Mean (cumecs) 5 5 0 0 30-Jan-20 30-Mar-20 30-May-20 30-Jul-20