Monthly water situation report

(1)

Monthly water situation report

Summary – September 2018

Thames area received 37mm of rainfall during September which is 60% of the Long Term Average (LTA), leaving all rainfall units normal and below normal for the time of year. Over the summer period (April to September), Thames area has received 236mm of rainfall which represents 72% of the LTA for the period. Soil Moisture Deficits were above average for the time of year at the end of the month, leaving soils drier than would be expected as we enter the winter period. Monthly mean river flows were normal and below normal at the majority of our indicator sites. Groundwater levels continued to decline at all indicator boreholes as would be expected at the end of the summer period.

Rainfall

In September Thames area received 37mm of rainfall which is 60% of the Long Term Average (LTA), meaning that eight rainfall units were normal for the time of year, and six were below normal. The month was generally dry, with a wet period between September 18 and 22 which accounted for over 80% of the month’s total rainfall.

The wettest day of the month was September 22 when almost half the month’s total fell in one day, and five of our indicator rain gauges recorded daily totals over 21mm. September sees the end of the summer period which began in April, and over the past six months Thames area has received 236mm of rainfall which is 72% of the LTA for this period.

Soil Moisture Deficit/Recharge

Soil Moisture Deficits decreased slightly in September but still ended the month above average for the time of year, leaving soils much drier than expected at the end of the summer period. As we enter the winter period these dry soils must become fully wetted, before recharge to groundwater resources can begin.

River Flows

Monthly mean river flows were normal or below normal at the majority of our sites in September. Sites in groundwater fed catchments, such as the River Kennet at Theale, were generally below normal reflecting the continued decline of groundwater support to the rivers, and a drier than average month across the area. At Bibury on the River Coln, below normal groundwater levels in the Inferior Oolites combined with a fourth consecutive month of below average rainfall leaving monthly mean flows exceptionally low for the time of year. In contrast sites in flashier, clay dominated catchments such as Sheepbridge on the River Loddon, were normal for the time of year as the wet period during the middle of the month helped boost daily mean flows. Following this quick response to the rainfall, daily mean flows across the area declined again during the final dry week of the month.

Groundwater Levels

As would be expected at the end of the summer period, groundwater levels at all indicator boreholes continued to decline. At the end of September, the majority of sites were normal, including Rockley and Gibbet Cottages in the Chalk of the Berkshire Downs. Fringford and Ampney Crucis in the Great Oolites in the north of Thames area were also normal for the time of year. Three sites were below normal; Jackaments Bottom in the Inferior Oolites, Stonor Park in the Chalk, and Frith Cottage in the Lower Greensand. Marcham, in the Corralian, saw a small decline in levels to the end of September which left it exceptionally low for the time of year.

Environmental Impact

Thames Area

(2)

Rainfall Map (1)

Total rainfall for hydrological areas across Thames for the current month (up to 30 September) 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.

(3)

Rainfall Map (2)

(4)

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. Includes material based on Ordnance Survey 1:50 000 maps with the permission of the controller of Her Majesty’s Stationery Office © Crown copyright. All rights reserved. Environment Agency, 100026380, 2018.

Geological map reproduced with kind permission from UK Groundwater Forum, BGS © NERC

(5)

Rainfall, effective rainfall and river flow (1)

Cotswolds West and River Coln Cotswolds East and River Evenlode

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

Exceptionally high Notably high Above normal Normal

Below normal Notably low Exceptionally low ─── Latest data

0 20 40 60 80 100 120

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

Rainfall (mm)

Cotswolds West - Rainfall

0 20 40 60 80 100 120 140

Rainfall (mm)

Cotswolds East - Rainfall

0 10 20 30 40 50 60 70 80 90 100

Effective Rainfall (mm)

Cotswolds West - Effective Rainfall

0 10 20 30 40 50 60 70 80 90

Cotswolds East - Effective Rainfall

0 1 2 3 4 5

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)

RIVER COLN AT BIBURY Ranking derived from data for the period 01/10/1961 to 31/12/2012

0 2 4 6 8 10 12 14 16 18 20

RIVER EVENLODE AT CASSINGTON Ranking derived from data for the period 01/10/1970 to 31/12/2012

(6)

Rainfall, effective rainfall and river flow (2)

Cherwell catchment and River Cherwell Upper Thames catchment and upper River Thames

0 20 40 60 80 100 120

Rainfall (mm)

Cherwell - Rainfall

0 10 20 30 40 50 60 70 80 90 100

Rainfall (mm)

Upper Thames - Rainfall

0 10 20 30 40 50 60 70

Cherwell - Effective Rainfall

0 10 20 30 40 50 60 70

Upper Thames - Effective Rainfall

0 2 4 6 8 10 12 14 16

RIVER CHERWELL AT BANBURY Ranking from data from for the period 01/01/1972 to 31/12/2012

0 10 20 30 40 50 60 70 80

RIVER THAMES AT FARMOOR (NATURALISED) Ranking derived from data for the period 01/10/1992 to 31/12/2012

(7)

Rainfall, effective rainfall and river flow (3)

Thame catchment and River Thame Ock catchment and River Ock

0 10 20 30 40 50 60 70 80 90 100

Rainfall (mm)

Thame - Rainfall

0 20 40 60 80 100 120

Rainfall (mm)

Ock - Rainfall

0 10 20 30 40 50 60

Thame - Effective Rainfall

0 10 20 30 40 50 60 70

Ock - Effective Rainfall

0 5 10 15 20 25

RIVER THAME AT WHEATLEY Ranking derived from data for the period 01/06/1989 to 31/12/2012

0 1 2 3 4 5 6 7 8 9

RIVER OCK AT ABINGDON GS Ranking used data from 01/06/1979 to 31/12/2012

(8)

Rainfall, effective rainfall and river flow (4)

Berkshire Downs and River Kennet Chilterns West and River Wye

0 20 40 60 80 100 120

Rainfall (mm)

Berkshire Downs - Rainfall

0 20 40 60 80 100 120

Rainfall (mm)

Chilterns West - Rainfall

0 10 20 30 40 50 60 70 80

Berkshire Downs - Effective Rainfall

0 10 20 30 40 50 60 70

Chilterns West - Effective Rainfall

0 5 10 15 20 25 30 35

RIVER KENNET AT THEALE Ranking derived from data for the period 01/10/1961 to 31/12/2012

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

RIVER WYE AT BOURNE END (HEDSOR) Ranking derived from data for the period 01/01/1965 to 31/03/2012

(9)

Rainfall, effective rainfall and river flow (5)

Loddon catchment and River Loddon North Downs (Hampshire) and Blackwater

0 10 20 30 40 50 60 70 80 90 100

Rainfall (mm)

Loddon - Rainfall

0 20 40 60 80 100 120

Rainfall (mm)

North Downs Hampshire - Rainfall

0 10 20 30 40 50 60

Loddon - Effective Rainfall

0 10 20 30 40 50 60 70 80 90 100

North Downs Hampshire - Effective Rainfall

0 2 4 6 8 10 12

RIVER LODDON AT SHEEPBRIDGE Ranking derived from data for the period 01/10/1965 to 31/12/2012

0 2 4 6 8 10 12 14 16 18

BLACKWATER AT SWALLOWFIELD Ranking derived from data for the period 01/10/1952 to 31/12/2012

(10)

Rainfall, effective rainfall and river flow (6)

Wey (Greensand) and Lower Wey catchment and upper River Wey lower River Wey

0 20 40 60 80 100 120 140

Rainfall (mm)

Wey Greensand - Rainfall

0 10 20 30 40 50 60 70 80 90 100

Rainfall (mm)

Lower Wey - Rainfall

0 10 20 30 40 50 60 70 80

Wey Greensand - Effective Rainfall

0 10 20 30 40 50 60 70

Lower Wey - Effective Rainfall

0 2 4 6 8 10 12 14 16

RIVER WEY AT TILFORD Ranking derived from data for the period 01/01/1973 to 31/12/2012

0 5 10 15 20 25 30 35 40

RIVER WEY AT WEYBRIDGE Ranking derived from data for the period 01/04/1979 to 31/12/2012

(11)

River Thames

0 10 20 30 40 50 60 70 80

RIVER THAMES AT FARMOOR (NATURALISED) Ranking derived from data for the period 01/10/1992 to 31/12/2012

0 50 100 150 200 250 300

RIVER THAMES AT WINDSOR Ranking derived from data for the period 01/08/1979 to 31/12/2012

0 50 100 150 200 250 300 350

RIVER THAMES AT KINGSTON (NATURALISED) Ranking derived from data for the period 01/01/1883 to 31/12/2012

0 50 100 150 200 250 300 350

RIVER THAMES AT KINGSTON Ranking derived from data for the period 01/01/1951 to 31/12/2012

(12)

Groundwater Levels

99 100 100 101 101 102 102 103 103 104 104

Dec-16 Mar-17 Jun-17 Sep-17 Dec-17 Mar-18 Jun-18 Sep-18

Level (mAOD)

AMPNEY CRUCIS - GREAT OOLITE Ranking derived from data for the period Dec-1958 to Dec-2012

94 95 95 96 96 97 97 98 98 99 99

Level (mAOD)

FRINGFORD - GREAT OOLITE

Ranking derived from data for the period Sep-1980 to Dec-2012

94 96 98 100 102 104 106

Level (mAOD)

JACKAMENTS BOTTOM - INFERIOR OOLITE Ranking derived from data for the period Jan-1974 to Dec-2012

58.0 58.2 58.4 58.6 58.8 59.0 59.2 59.4 59.6 59.8 60.0

Level (mAOD)

MARCHAM - CORALLIAN

Ranking derived from data for the period Jan-1988 to Dec-2012

105 110 115 120 125

Level (mAOD)

GIBBET COTTAGES - CHALK

Ranking derived from data for the period Jul-1973 to Dec-2012

62 67 72 77 82 87

Level (mAOD)

STONOR PARK- CHALK

Ranking derived from data for the period May-1961 to Dec-2012

128 130 132 134 136 138 140 142 144

Level (mAOD)

ROCKLEY - CHALK

Ranking derived from data for the period Mar-1933 to Dec-2012

85 86 87 88 89 90 91 92 93

Level (mAOD)

TILE BARN FARM - CHALK

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

(13)

Groundwater Levels

94 95 96 97 98 99 100

Level (mAOD)

FRITH COTTAGE - LOWER GREENSAND (HYTHE) Ranking derived from data for the period Jul-1970 to Dec-2012

56 57 57 58 58 59 59

Level (mAOD)

FLASHES - LOWER GREENSAND (FOLKESTONE) Ranking derived from data for the period Apr-1993 to Dec-2012

89 90 91 92 93 94 95 96 97 98 99

Level (mAOD)

MODEL FARM - UPPER GREENSAND Ranking derived from data for the period Feb-1963 to Dec-2012

(14)

End of month flow constraints

Summary of flow constraints

Week ending 02/09/18 09/09/18 16/09/18 23/09/18 30/09/18

Number of flow constraints in force 23 28 29 17 27

(15)

Summary of rainfall, effective rainfall and soil moisture deficit

Rainfall and effective rainfall

Rainfall (mm) Effective Rainfall (mm)

Area 30 day

total

Sept

LTA % LTA 30 day total

Sept

LTA % LTA

Cotswolds West 40 65 61 4 10 40

Cotswolds East 30 56 54 3 7 40

Berkshire Downs 42 63 67 4 7 64

Chilterns West 39 61 64 4 8 51

North Downs Hampshire 40 74 54 4 14 31

Wey - Greensand 37 71 51 4 11 35

Upper Thames 34 58 59 0 1 0

Cherwell 31 53 59 0 1 0

Thame 33 56 59 0 1 0

Loddon 37 60 62 0 1 0

Lower Wey 35 59 60 0 2 0

Ock 33 54 62 0 1 0

Enborne 50 64 78 0 2 0

Cut 33 57 58 0 1 0

Thames Area 37 61 60 2 5 36

Soil moisture deficit

SMD (mm) Area

End Sept End Sept LTA

Cotswolds West 88 49

Cotswolds East 100 61

Berkshire Downs 121 86

Chilterns West 127 85

North Downs Hampshire 116 73

Wey - Greensand 116 83

Upper Thames 129 86

Cherwell 114 77

(16)

Summer rainfall and effective rainfall

Summer period: 01/04/2018 to 30/09/2018

Rainfall (mm) Effective Rainfall (mm)

Area total LTA % LTA total LTA % LTA

Cotswolds West 260 359 72 38 57 67

Cotswolds East 235 325 72 33 43 77

Berkshire Downs 245 338 72 43 43 99

Chilterns West 235 337 70 40 47 85

North Downs Hampshire 269 366 74 55 63 87

Wey - Greensand 281 351 80 55 52 107

Upper Thames 222 319 69 14 15 96

Cherwell 253 326 77 18 17 103

Thame 223 319 70 17 12 146

Loddon 207 318 65 28 16 180

Lower Wey 224 313 72 27 19 143

Ock 212 305 70 15 11 135

Enborne 242 340 71 30 20 147

Cut 204 310 66 26 17 148

Thames Area 236 330 72 31 31 102

This is a first estimate of areal rainfall, effective rainfall (i.e. percolation/runoff) and soil moisture deficit.

There may be significant variation within each area which must be considered when interpreting these data. When additional meteorological data is available estimates are revised, this will affect period totals.

(17)

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).