Water Situation Report Hertfordshire and North London

Monthly water situation report Hertfordshire and North London

Summary – June 2018 The Hertfordshire and North London Area received only 2% of the long term average rainfall for June, making it the driest June on record since 1910. The soil moisture deficits increased across the Area since last month and ended June larger than the long term average. River baseflows decreased and our flow indicator sites recorded monthly mean flows ranging from exceptionally low to normal. All of our groundwater indicator sites recorded normal levels with the exception of Ballingdon Farm which recorded below normal groundwater levels.

Rainfall It was an exceptionally dry month with the Hertfordshire and North London Area having received only 2% of the long term average (LTA) rainfall for June, making it the driest June on record since 1910. There was an average of 28 dry days across the Area, with Weston, Darnicle Hill and Moreton STW rain gauges having recorded no rainfall for the entire month. There was one localised thunderstorm on the 1st resulting in the largest daily total of 25.6mm at Brent reservoir in the North London catchment. All of our catchments recorded exceptionally low rainfall with all but the Chiltern East Colne having recorded the smallest rainfall totals on record since 1910. For the summer period (April- June) the Area received 80% of the LTA rainfall.

Soil Moisture Deficit/Recharge As a result of the dry weather, the soil moisture deficits greatly increased across the Area since last month and ended June larger than the long term average. There was no effective rainfall recorded across the Area. Despite the lack of effective rain in June, the past couple of months’ heavier rainfall has resulted in the Area having received 140% of the LTA effective rainfall for the summer period.

River Flows In the Chalk catchments, most of our river flow indicator sites recorded below normal monthly mean flows. The exceptions to this were the River Colne, River Mimram and the River Lee at Feildes Weir which recorded normal flows. There was a general decline in base flow across all of our Chalk Rivers. The lack of rainfall had a greater effect on our urban and Clay Rivers with our catchments in the East having recorded notably low flows and our catchments in the West having recorded exceptionally low monthly mean flows. The River Crane was the second lowest June river flow on record (1978) with just 2015 recording a lower monthly mean flow. No flood alerts or warnings were issued for the Area during June.

Groundwater Levels All of our groundwater indicator sites recorded normal levels with the exception of Ballingdon Farm which recorded below normal groundwater levels. As the groundwater recharge period has come to an end, the majority of our indicator sites have shown a fall in groundwater level which is to be expected for this time of year. Therfield Rectory was the only indicator site to record a rise in level as it is common for this site to have a delayed response.

Reservoir Storage The Lee Valley reservoir groups’ levels dropped below average levels while the Thames Valley reservoir groups’ levels fell slightly but remained well above average.

Environmental Impact The abstraction licence flow constraints that were in force in June, out of a summer maximum of 28:

Week Commencing 3 June 11 June 18 June 25 June Number of Constraints 0 1 1 2

The upper Lee catchments have begun their summer drying. The source of the River Rib has moved downstream to Walkern while flows have reduced along the River Ash at Much Hadham. The River Stort has become dry downstream to Clavering and the River Rib has dried up to Buntingford with some low flow at Slate Hall Farm.

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 2018

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: Rainfall calculated using the Environment Agency, South East Soil Moisture Model.

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

www.gov.uk/environment-agency © Environment Agency 2018

Rainfall Map

Total rainfall 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.

Data source: Rainfall calculated using the Environment Agency, South East Soil Moisture Model.

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

www.gov.uk/environment-agency © Environment Agency 2018

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 Centre for Ecology and Hydrology, (© CEH) 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 2018

Colne

Monthly total rainfall (mm) Long term average rainfall (mm) Chilterns - East - Colne (N) - Rainfall Chilterns - East - Colne (N) - Effective Rainfall 120 80

70 100 60 80 50

60 40

30 40

Rainfall(mm) 20

20 Effective Rainfall(mm) 10

0 Jul-17 Aug-17 Sep-17 Oct-17 Nov-17 Dec-17 Jan-18 Feb-18 Mar-18 Apr-18 May-18 Jun-18 0 Jul-17 Aug-17 Sep-17 Oct-17 Nov-17 Dec-17 Jan-18 Feb-18 Mar-18 Apr-18 May-18 Jun-18

RIVER VER AT COLNEY STREET (HANSTEADS) RIVER GADE AT CROXLEY GREEN Ranking derived from data for the period 01/10/1956 to 31/12/2012 Ranking derived from data for the period 01/10/1970 to 31/12/2012 1.4 2.5

1.2 2.0

1.0

1.5 0.8

0.6 1.0

0.4

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

0.0 0.0 30-Aug-17 30-Oct-17 30-Dec-17 28-Feb-18 30-Apr-18 30-Jun-18 30-Aug-17 30-Oct-17 30-Dec-17 28-Feb-18 30-Apr-18 30-Jun-18

RIVER COLNE AT DENHAM COLNE RIVER MISBOURNE AT DENHAM LODGE Ranking derived from data for the period 01/10/1952 to 31/12/2012 Ranking derived from data for the period 01/07/1984 to 31/12/2012 12 1.0

0.9 10 0.8

0.7 8 0.6

6 0.5

0.4 4 0.3

0.2 Daily Mean Flow (cumecs) Daily Mean Flow (cumecs) 2 0.1

0 0.0 30-Aug-17 30-Oct-17 30-Dec-17 28-Feb-18 30-Apr-18 30-Jun-18 28-Aug-17 28-Oct-17 28-Dec-17 28-Feb-18 30-Apr-18 30-Jun-18

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-2012 Ranking derived from data for the period Sep-1987 to Dec-2012

82 128 81

80 123 79

78 118

Level (mAOD) Level 77 (mAOD) Level 113 76

75 108 Sep-15 Jan-16 May-16 Sep-16 Jan-17 May-17 Sep-17 Jan-18 May-18 Sep-15 Jan-16 May-16 Sep-16 Jan-17 May-17 Sep-17 Jan-18 May-18

BALLINGDON FARM WAPSEYS WOOD OBH Ranking derived from data for the period Jan-1975 to Dec-2012 Ranking derived from data for the period Mar-1988 to Dec-2012

140 57

138 56

136 55 54 134 53 132 52 130 51 Level (mAOD) Level (mAOD) Level 128 50 126 49 124 48 Sep-15 Jan-16 May-16 Sep-16 Jan-17 May-17 Sep-17 Jan-18 May-18 Sep-15 Jan-16 May-16 Sep-16 Jan-17 May-17 Sep-17 Jan-18 May-18

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 90 60

80 50 70

60 40

50 30 40

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

0 Jul-17 Aug-17 Sep-17 Oct-17 Nov-17 Dec-17 Jan-18 Feb-18 Mar-18 Apr-18 May-18 Jun-18 0 Jul-17 Aug-17 Sep-17 Oct-17 Nov-17 Dec-17 Jan-18 Feb-18 Mar-18 Apr-18 May-18 Jun-18

RIVER MIMRAM AT PANSHANGER RIVER ASH (HERTS) AT WARESIDE (MARDOCK) Ranking derived from data for the period 01/12/1952 to 31/12/2012 Ranking derived from data for the period 03/06/1980 to 31/12/2012 1.4 4.0

1.2 3.5

3.0 1.0

2.5 0.8 2.0 0.6 1.5

0.4 1.0 Daily Mean Flow (cumecs) Daily Mean Flow (cumecs) 0.2 0.5

0.0 0.0 30-Aug-17 30-Oct-17 30-Dec-17 28-Feb-18 30-Apr-18 30-Jun-18 30-Aug-17 30-Oct-17 30-Dec-17 28-Feb-18 30-Apr-18 30-Jun-18

RIVER LEE (UPPER) AT HOWE GREEN (WATER HALL) RIVER LEE (MIDDLE) AT FEILDES WEIR Ranking derived from data for the period 01/04/1959 to 31/12/2012 Ranking derived from data for the period 01/10/1883 to 31/12/2012 4.0 35

3.5 30

3.0 25

2.5 20 2.0 15 1.5

10 1.0 Daily Mean Flow (cumecs) Daily Mean Flow (cumecs) 0.5 5

0.0 0 30-Aug-17 30-Oct-17 30-Dec-17 28-Feb-18 30-Apr-18 30-Jun-18 30-Aug-17 30-Oct-17 30-Dec-17 28-Feb-18 30-Apr-18 30-Jun-18

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

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Upper Lee Groundwater

LILLEY BOTTOM OBH CRESCENT COTTAGES Ranking derived from data for the period Jul-1979 to Dec-2012 Ranking derived from data for the period Aug-1968 to Dec-2012

99 92

98 91

97 90

96 89

95 88

Level (mAOD) Level 94 (mAOD) Level 87

93 86

92 85 Sep-15 Jan-16 May-16 Sep-16 Jan-17 May-17 Sep-17 Jan-18 May-18 Sep-15 Jan-16 May-16 Sep-16 Jan-17 May-17 Sep-17 Jan-18 May-18

THERFIELD RECTORY HIXHAM HALL Ranking derived from data for the period Jan-1883 to Dec-2012 Ranking derived from data for the period Jun-1964 to Dec-2012

78 77 91 76 75 86 74 73 81 72

Level (mAOD) Level (mAOD) Level 71 76 70 69 71 68 Sep-15 Jan-16 May-16 Sep-16 Jan-17 May-17 Sep-17 Jan-18 May-18 Sep-15 Jan-16 May-16 Sep-16 Jan-17 May-17 Sep-17 Jan-18 May-18

CAVE GATE Ranking derived from data for the period Jun-1966 to Dec-2012

88 Level (mAOD) Level

84 Sep-15 Jan-16 May-16 Sep-16 Jan-17 May-17 Sep-17 Jan-18 May-18

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 100 60

90 50 80

70 40 60

50 30

40 20 30 Rainfall(mm)

20 Effective Effective Rainfall(mm) 10 10

0 Jul-17 Aug-17 Sep-17 Oct-17 Nov-17 Dec-17 Jan-18 Feb-18 Mar-18 Apr-18 May-18 Jun-18 0 Jul-17 Aug-17 Sep-17 Oct-17 Nov-17 Dec-17 Jan-18 Feb-18 Mar-18 Apr-18 May-18 Jun-18

RIVER LEE (LOWER) AT LEE FLOOD CHANNEL (LOW HALL) RIVER LEE (LOWER) AT LEA BRIDGE Ranking derived from data for the period 01/01/1980 to 31/12/2012 Ranking derived from data for the period 18/09/1992 to 31/12/2012 35 25

30 20

15 20

15 10

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

0 0 30-Aug-17 30-Oct-17 30-Dec-17 28-Feb-18 30-Apr-18 30-Jun-18 30-Aug-17 30-Oct-17 30-Dec-17 28-Feb-18 30-Apr-18 30-Jun-18

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 120 50

45 100 40

35 80 30

60 25

20 40 15 Rainfall(mm)

10 20 Effective Rainfall(mm) 5

0 Jul-17 Aug-17 Sep-17 Oct-17 Nov-17 Dec-17 Jan-18 Feb-18 Mar-18 Apr-18 May-18 Jun-18 0 Jul-17 Aug-17 Sep-17 Oct-17 Nov-17 Dec-17 Jan-18 Feb-18 Mar-18 Apr-18 May-18 Jun-18

RIVER CRANE AT CRANFORD (CRANFORD PARK) RIVER BRENT AT BRENT (MONKS PARK) Ranking derived from data for the period 03/04/1978 to 31/12/2012 Ranking derived from data for the period 01/12/1978 to 31/12/2012 7 8

6 7

6 5

5 4 4 3 3

2 2 Daily Mean Flow (cumecs) Daily Mean Flow (cumecs) 1 1

0 0 30-Aug-17 30-Oct-17 30-Dec-17 28-Feb-18 30-Apr-18 30-Jun-18 30-Aug-17 30-Oct-17 30-Dec-17 28-Feb-18 30-Apr-18 30-Jun-18

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 Catchment - Rainfall Roding Catchment - Effective Rainfall 120 50

45 100 40

35 80 30

60 25

20 40 15 Rainfall(mm)

10 20 Effective Rainfall(mm) 5

0 Jul-17 Aug-17 Sep-17 Oct-17 Nov-17 Dec-17 Jan-18 Feb-18 Mar-18 Apr-18 May-18 Jun-18 0 Jul-17 Aug-17 Sep-17 Oct-17 Nov-17 Dec-17 Jan-18 Feb-18 Mar-18 Apr-18 May-18 Jun-18

RIVER RODING AT REDBRIDGE RIVER INGREBOURNE AT UPMINSTER (GAYNES PARK) Ranking derived from data for the period 19/01/1950 to 31/12/2012 Ranking derived from data for the period 01/11/1970 to 31/12/2012 16 3.5

14 3.0

12 2.5

10 2.0 8 1.5 6

1.0 4 Daily Mean Flow (cumecs) Daily Mean Flow (cumecs) 2 0.5

0 0.0 30-Aug-17 30-Oct-17 30-Dec-17 28-Feb-18 30-Apr-18 30-Jun-18 30-Aug-17 30-Oct-17 30-Dec-17 28-Feb-18 30-Apr-18 30-Jun-18

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 – June 2018

Summary of rainfall, effective rainfall and soil moisture deficit

Rainfall and Effective Rainfall – June 2018

Rainfall (mm) Effective Rainfall (mm)

Total LTA % of Total LTA % of Area (mm) (mm) LTA (mm) (mm) LTA Chilterns- East - Colne 2 59 3 0 5 0 Lee - Chalk 1 55 1 0 4 0 North London 1 53 2 0 1 0 Lower Lee 0 53 1 0 0 0 Roding Catchment 1 50 2 0 0 0 Hertfordshire and North 1 54 2 0 2 0 London Area Average

Soil Moisture Deficit (SMD) - June 2018

End of Month End of Month Area SMD (mm) SMD LTA (mm) Chilterns- East - Colne 97 77 Lee - Chalk 93 81 North London 100 84 Lower Lee 100 83 Roding Catchment 97 83 Hertfordshire and North 97 82 London Area Average

Rainfall and Effective Rainfall – Summer total for period 1 April 2018 to 30 June 2018

Rainfall (mm) Effective Rainfall (mm)

Total LTA % of Total LTA % of Area (mm) (mm) LTA (mm) (mm) LTA Chilterns- East - Colne 126 170 74 33 30 111 Lee - Chalk 131 154 85 27 23 118 North London 125 155 81 21 12 171 Lower Lee 114 153 75 18 11 161 Roding Catchment 123 143 86 22 10 213 Hertfordshire and North 124 155 80 24 17 140 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