Hertfordshire and North London

Monthly water situation report Hertfordshire and North London

Summary – August 2021 The Hertfordshire and North London Area received 84% of its long term average rainfall for August. Soil moisture deficits increased across the Area from last month and ended August marginally below the long term average. Our river flow indicator sites ranged between normal and exceptionally high monthly mean flows. Groundwater levels continued their seasonal decline with the majority of our indicator sites recording normal groundwater levels.

Rainfall The Hertfordshire and North London Area (“the Area”) received 84% of the long term average (LTA) rainfall for August. There was on average only seven rain days (=>1mm), with the largest daily total of 29.2mm recorded at Prestwood Reservoir TBR (Chiltern East Colne) on the 21st. The Chiltern East Colne and Lee Chalk catchments both recorded below normal rainfall while the remaining catchments recorded normal rainfall for August. For the summer period (April-August), the Area received 114% LTA rainfall.

Soil Moisture Deficit/Recharge As a result of the dry weather, there was little effective rainfall with the Area experiencing 51% LTA effective rainfall. The only catchments to see effective rainfall were in the Chiltern East Colne and Lee Chalk areal rainfall units; a pattern that has extended across the summer period, with the Area only receiving 44% LTA effective rainfall during that period (April-August). This has led to soil moisture deficits increasing across the Area during the month and ended August marginally below the LTA at 101mm.

River Flows Our river flow indicator sites ranged between normal and exceptionally high monthly mean flows during August. Most of the Area’s clay and urban river experienced normal flows, with the exception of the River Roding which was notably high. The Chalk rivers in the Lee catchments recorded above normal monthly mean flows apart from the River Lee at Waterhall which recorded normal monthly flows. In the Colne catchments, half our indicator sites on Chalk rivers were notably high, while the River Misbourne was above normal and the River Ver experienced exceptionally high monthly mean flows. One flood alert was issued in Hertfordshire and North London Area for the Tidal River Crane on the 9th August.

Groundwater Levels All of our groundwater indicator sites continued their seasonal decline in groundwater levels during August. The majority of our groundwater sites recorded normal levels, with the exceptions of Wapseys Wood (Mid Chilterns Chalk), Lilley Bottom (Upper Lee Chalk) and Crescent Cottages (Upper Lee Chalk) which ended the month with above normal groundwater levels.

Environmental Impact The table below shows the number of abstraction licence with flow constraints that were in force during August (out of a summer maximum of 32):

Week Commencing 2 August 9 August 16 August 24 August 30 August Number of Constraints 2 2 3 3 3

The source of the River Bulbourne moved downstream to Northchurch Playing Fieldes and the River Chess source moved downstream to Missenden Road above the tennis courts. The dry section on the River Misbourne through the Chalfonts extended downstream from below Chalfont St Giles to just above Gerrards Cross golf course.

Author: Groundwater and Hydrology Contact details: 03708 506 506 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 2021

Rainfall Map

Total rainfall for hydrological areas across the Hertfordshire and North London Area for the current month, classed relative to an analysis of respective historic totals.

Data source: Provisional data based on Environment Agency 1km gridded rainfall dataset derived from Environment Agency intensity rain gauges.

Some features of this map are based on digital spatial data licensed from the UK Centre for Ecology and Hydrology, (© UKCEH) and the Ordnance Survey (© Crown Copyright).

www.gov.uk/environment-agency © Environment Agency 2021

Rainfall Map

Total rainfall (as a percentage) for hydrological areas across the Hertfordshire and North London Area 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). Provisional data based on Environment Agency 1km gridded rainfall dataset derived from Environment Agency intensity rain gauges.

Some features of this map are based on digital spatial data licensed from the UK Centre for Ecology and Hydrology, (© UKCEH) and the Ordnance Survey (© Crown Copyright).

www.gov.uk/environment-agency © Environment Agency 2021

River Flow and Groundwater Status Map

Groundwater site status based on end of month level. Surface water site status based on mean monthly flow.

Some features of this map are based on digital spatial data licensed from the UK Centre for Ecology and Hydrology, (© UKCEH) and the Ordnance Survey (© Crown Copyright). Geological map reproduced with kind permission from

UK Groundwater Forum, BGS © NERC

www.gov.uk/environment-agency © Environment Agency 2021

Colne

Monthly total rainfall (mm) Long term average rainfall (mm)

Chilterns East Colne - Rainfall Chilterns East Colne - Effective Rainfall 200 120

180 100 160

140 80 120

100 60

Rainfall(mm) 40 60

40 Effective Effective Rainfall(mm) 20 20

0 Sep-20 Oct-20 Nov-20 Dec-20 Jan-21 Feb-21 Mar-21 Apr-21 May-21 Jun-21 Jul-21 Aug-21 0 Sep-20 Oct-20 Nov-20 Dec-20 Jan-21 Feb-21 Mar-21 Apr-21 May-21 Jun-21 Jul-21 Aug-21

RIVER VER AT COLNEY STREET (HANSTEADS) RIVER GADE AT CROXLEY GREEN Ranking used data from 01/10/1956 to 31/12/2017 Ranking used data from 01/10/1970 to 31/12/2017 2.5 3.0

2.5 2.0

2.0 1.5

1.5

1.0 1.0

0.5

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

RIVER COLNE AT DENHAM RIVER MISBOURNE AT DENHAM LODGE Ranking used data from 01/10/1952 to 31/12/2017 Ranking used data from 01/07/1984 to 31/12/2017 18 1.6

16 1.4

14 1.2

12 1.0 10 0.8 8 0.6 6

0.4 4 Daily Mean Flow (cumecs) Daily Mean Flow (cumecs) 2 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

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

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Colne Groundwater

AMERSHAM ROAD OBH ASHLEY GREEN STW OBH Ranking derived from data for the period Nov-1991 to Dec-2017 Ranking derived from data for the period Sep-1987 to Dec-2017

82 133

81 128 80

79 123

78 118 Level (mAOD) Level 77 (mAOD) Level 113 76

75 108 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

BALLINGDON FARM WAPSEYS WOOD OBH Ranking derived from data for the period Jan-1975 to Dec-2017 Ranking derived from data for the period Mar-1988 to Dec-2017

142 57 140 56 138 55 136 54 134 53 132 52 130 51 Level (mAOD) Level Level (mAOD) Level 128 50 126 49 124 48 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

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

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Upper Lee

Monthly total rainfall (mm) Long term average rainfall (mm)

Lee Chalk - Rainfall Lee Chalk - Effective Rainfall 180 100

160 90

80 140 70 120 60 100 50 80 40

Rainfall(mm) 60 30 40 20 Effective Effective Rainfall(mm) 20 10

0 Sep-20 Oct-20 Nov-20 Dec-20 Jan-21 Feb-21 Mar-21 Apr-21 May-21 Jun-21 Jul-21 Aug-21 0 Sep-20 Oct-20 Nov-20 Dec-20 Jan-21 Feb-21 Mar-21 Apr-21 May-21 Jun-21 Jul-21 Aug-21

RIVER MIMRAM AT PANSHANGER RIVER ASH AT WARESIDE (MARDOCK) Ranking used data from 01/12/1952 to 31/12/2017 Ranking used data from 03/06/1980 to 31/12/2017 2.0 3.0

1.8 2.5 1.6

1.4 2.0 1.2

1.0 1.5

0.8 1.0 0.6

0.4

Daily Mean Flow (cumecs) Daily Mean Flow (cumecs) 0.5 0.2

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

RIVER LEE AT HOWE GREEN (WATER HALL) RIVER LEE AT FEILDES WEIR (NATURALISED) Ranking used data from 01/04/1959 to 31/12/2017 Ranking used data from 01/10/1883 to 31/12/2017 7.0 50

45 6.0 40

5.0 35

30 4.0 25 3.0 20

2.0 15

10 Daily Mean Flow (cumecs) 1.0 Daily Mean Flow (cumecs) 5

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

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

www.gov.uk/environment-agency © Environment Agency 2021

Upper Lee Groundwater

LILLEY BOTTOM OBH CRESCENT COTTAGES Ranking derived from data for the period Jul-1979 to Dec-2017 Ranking derived from data for the period Aug-1968 to Dec-2017

100 93

99 92

98 91

97 90

96 89

95 88 Level (mAOD) Level Level (mAOD) Level 94 87

93 86

92 85 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

THERFIELD RECTORY HIXHAM HALL Ranking derived from data for the period Jan-1883 to Dec-2017 Ranking derived from data for the period Jun-1964 to Dec-2017

78 77 91 76 75 86 74 73 81 72 Level (mAOD) Level Level (mAOD) Level 71 76 70 69 71 68 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

CAVE GATE Ranking derived from data for the period Jun-1966 to Dec-2017

88 Level (mAOD) Level

84 Nov-18 Mar-19 Jul-19 Nov-19 Mar-20 Jul-20 Nov-20 Mar-21 Jul-21

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

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Lower Lee

Monthly total rainfall (mm) Long term average rainfall (mm)

Lower Lee - Rainfall Lower Lee - Effective Rainfall 180 100

160 90

80 140 70 120 60 100 50 80 40

Rainfall(mm) 60 30 40 20 Effective Effective Rainfall(mm) 20 10

0 Sep-20 Oct-20 Nov-20 Dec-20 Jan-21 Feb-21 Mar-21 Apr-21 May-21 Jun-21 Jul-21 Aug-21 0 Sep-20 Oct-20 Nov-20 Dec-20 Jan-21 Feb-21 Mar-21 Apr-21 May-21 Jun-21 Jul-21 Aug-21

LEE FLOOD CHANNEL AT WALTHAMSTOW (LOW HALL) RIVER LEE AT LEA BRIDGE Ranking used data from 01/01/1980 to 31/12/2017 Ranking used data from 22/07/1992 to 31/12/2017 35 35

30 30

25 25

20 20

15 15

10 10

Daily Mean Flow (cumecs) 5 Daily Mean Flow (cumecs) 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

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

North London

Monthly total rainfall (mm) Long term average rainfall (mm)

North London - Rainfall North London - Effective Rainfall 180 100

160 90

80 140 70 120 60 100 50 80 40

Rainfall(mm) 60 30 40 20 Effective Effective Rainfall(mm) 20 10

0 Sep-20 Oct-20 Nov-20 Dec-20 Jan-21 Feb-21 Mar-21 Apr-21 May-21 Jun-21 Jul-21 Aug-21 0 Sep-20 Oct-20 Nov-20 Dec-20 Jan-21 Feb-21 Mar-21 Apr-21 May-21 Jun-21 Jul-21 Aug-21

RIVER CRANE AT CRANFORD PARK RIVER BRENT AT MONKS PARK Ranking used data from 01/05/1987 to 31/12/2017 Ranking used data from 01/12/1978 to 31/12/2017 7.0 14

6.0 12

5.0 10

4.0 8

3.0 6

2.0 4

Daily Mean Flow (cumecs) 1.0 Daily Mean Flow (cumecs) 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

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

Roding

Monthly total rainfall (mm) Long term average rainfall (mm)

Roding - Rainfall Roding - Effective Rainfall 180 120

160 100 140

120 80

100 60 80

Rainfall(mm) 60 40

Effective Effective Rainfall(mm) 20 20

0 Sep-20 Oct-20 Nov-20 Dec-20 Jan-21 Feb-21 Mar-21 Apr-21 May-21 Jun-21 Jul-21 Aug-21 0 Sep-20 Oct-20 Nov-20 Dec-20 Jan-21 Feb-21 Mar-21 Apr-21 May-21 Jun-21 Jul-21 Aug-21

RIVER RODING AT REDBRIDGE RIVER INGREBOURNE AT UPMINSTER (GAYNES PARK) Ranking used data from 19/01/1950 to 31/12/2017 Ranking used data from 01/11/1970 to 31/12/2017 16 3.0

14 2.5

2.0 10

8 1.5

6 1.0

Daily Mean Flow (cumecs) Daily Mean Flow (cumecs) 0.5 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

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

Reservoir Storage

Thames Water - Lower Thames Reservoirs - Lower Thames Thames Water - Lee Valley Reservoirs - Lower Lee 100 100 90 90 80 80 70 70 60 60 50 50 40 40 30 30 20 20 Percentage of live capacity live of Percentage 10 capacity live of Percentage 10 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

Below minimum Above maximum ─── Average ─── Latest data

Flows in the chalk fed rivers – August 2021

Summary of rainfall, effective rainfall and soil moisture deficit

Rainfall and Effective Rainfall – August 2021

Rainfall (mm) Effective Rainfall (mm)

Total LTA % of Total LTA % of Area (mm) (mm) LTA (mm) (mm) LTA Chilterns- East - Colne 42 59 71 3 5 59 Lee - Chalk 36 55 66 2 5 43 North London 51 55 94 0 0 0 Lower Lee 52 55 95 0 0 0 Roding Catchment 50 53 94 0 0 0 Hertfordshire and North 46 55 84 1 2 51 London Area Average

Soil Moisture Deficit (SMD) - August 2021

End of Month End of Month Area SMD (mm) SMD LTA (mm) Chilterns- East - Colne 96 104 Lee - Chalk 108 107 North London 106 113 Lower Lee 97 111 Roding Catchment 97 108 Hertfordshire and North 101 109 London Area Average

Rainfall and Effective Rainfall – Summer total for period 1 April 2021 to 31 August 2021

Rainfall (mm) Effective Rainfall (mm)

Total LTA % of Total LTA % of Area (mm) (mm) LTA (mm) (mm) LTA Chilterns- East - Colne 311 280 111 26 39 67 Lee - Chalk 268 260 103 20 32 64 North London 298 257 116 0 12 0 Lower Lee 310 257 120 0 11 0 Roding Catchment 298 244 122 0 10 0 Hertfordshire and North 297 260 114 9 21 44 London Area Average

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