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 *Thames Area Monthly Water Reports*

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David Harvey

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PostSubject: *Thames Area Monthly Water Reports*   Fri May 11, 2012 3:47 pm

WEST THAMES

Summary – April 2012
Rainfall totals for West Thames Area were exceptionally high in all catchment areas in
April 2012. The rainfall was sustained throughout much of the month leading to
exceptionally high daily mean river flows at most key indicator sites. Groundwater levels
in the Upper Thames, Ock and Cotswolds responded with some rise in levels however
ground water levels were generally lower than normal for the time of year.

Rainfall
The overall average rainfall total for West Thames Area was 128 mm, 258 % of the April long term
average (LTA). All areal rainfall units in West Thames area had exceptionally high end of month rainfall
totals for April. The Wey –greensand unit had the highest rainfall total with 161 mm of rainfall, which was
288 % of its April LTA. This was the highest April rainfall total in this unit since records began in 1920.
Similarly, the neighbouring Loddon unit also had its highest April rainfall record with 253 % of its LTA. The
Enborne and The Cut units also recorded their highest April rainfall totals, although their records only
extend as far back as 1961. The Berkshire Downs areal rainfall unit had the lowest April rainfall total with
113 mm, but this was still more than double its April LTA with 222%.

Soil Moisture Deficit (SMD) /Recharge
As a result of the high rainfall totals in April effective rainfall was exceptionally high in all West Thames
Areal rainfall units too. The highest effective rainfall was in the Wey – greensand unit with 540 % of the
April LTA. The Cotswolds West had 508 % of its LTA, and the Upper Thames had 500 % of its LTA. On
average the areas total effective rainfall was 374 % of the April LTA. By the end of the month all areal
rainfall units had soil moisture deficits to 0 mm. This means that at the end of April soils were saturated.
River Flows
With the exceptionally high rainfall in April, daily mean river flows responded rapidly with most key
indicator sites going from exceptionally low to exceptionally high. At the end of April most rivers were still
rising with 11 of the key indicator sites having exceptionally high flows. Owing to the low flows and dry
soils at the start of April the rivers in West Thames Area were able to contain most of the rainfall and
surface water runoff within their channel capacity and floodplains.

As a result of the increase in river flows in all catchments the April month mean flows suggest that most
rivers have responded to the rainfall with 6 key indicator sites now being normal for the time of year. The
Upper Wey month mean for April was notably high and the Lower Wey and Cherwell was above normal.
However the River Coln month mean remained exceptionally low and the River Kennet was notably low. It
is expected the recovery in river flows will be short lived unless we see significantly heavy rainfall in May.

Groundwater Levels
Groundwater levels at some West Area key observation boreholes showed a response to the April rainfall.
In the Oolitic Limestone of the Cotswolds and Upper Thames and in the Corallian of the Ock catchment
groundwater levels increased slightly. Despite this, groundwater levels in most aquifers are lower than
normal for this time of year. Only the Great Oolites as measured at Ampney Crucis in the West Cotswolds
are normal for the time of year and the groundwater levels at Marcham in the Corallian of the Ock are
above normal. At the end of April Stonor Park observation borehole in the West Chilterns remained dry.

FULL REPORT


SOUTH EAST THAMES


Summary – April 2012

April was a very wet month, with significant rainfall in the second half of the month across the
Region. The soil moisture deficits have been reduced, there has been some recharge and
groundwater levels have risen at a number of the key sites. Flows responded to the rainfall.

Rainfall
April was an exceptionally wet month, with the Region receiving 255% of the long term average rainfall.
The month started relatively dry with less than a fifth of the monthly rainfall falling by the 16th. Thereafter,
there was widespread rainfall recorded every day to the end of the month. On average, 60% of the
monthly total fell in the last 8 days of the month. Particularly heavy rainfall fell on the 28 th of April, with
daily totals in excess of 30mm in Kent and South London and North East Areas. The highest daily total
was 40mm, recorded at Motney Hill in the Medway catchment.

The Region had the 2
nd
wettest April on record, after 2000. In the 19 months since October 2010, only 7
months have had above average rainfall. This wet month follows a dry winter from October 2011, where
only 2 months, including April, received above average rainfall. However, despite the heavy rainfall,
some catchments in the west have still had an exceptionally dry 19 months. The Cotswolds have had the
3rd driest 19 month period ending in April and the driest since 1976.

Soil Moisture Deficit, Recharge and Groundwater Levels
The soil moisture deficit has been significantly reduced by this month’s rainfall and as a result, is lower
than would be expected for the end of April. The catchments are now responsive to any further rainfall.
After months of very little recharge, there was about three times the average for April.
Groundwater levels have responded to the rainfall at a number of the key indicator sites. Levels have
risen at Jackaments Bottom (Cotswolds), Ashley Green (Chilterns East), and at Lilley Bottom (Lee Chalk
West). Chilgrove (South Downs) has shown a steep rise in levels and Sweeps Lane (North Downs) has
also responded to the rainfall. However, despite the rises in level, 10 Chalk boreholes are still at
exceptionally low levels for April and 4 sites are notably low. Stonor (Chilterns West) is still dry. Ashley
Green (Chilterns East), Well House Inn (North Downs), Sweeps Lane (North Downs), Little Bucket (North
Downs) and Clanville Lodge (Hampshire Downs) have all recorded the lowest groundwater levels on
record for April.

River Flows
Rivers across the Region responded to the rainfall this month. However, the monthly flows of the River
Coln at Bibury remain exceptionally low with the 2 nd lowest April flows on record after 1976, and the
groundwater fed rivers Mimram, Test, Itchen, Kennet and Dour are notably low. Clay catchments
responded quickly to the rainfall with a number of peaks in late April. By contrast to the groundwater fed
rivers, the Lymington River at Brockenhurst has recorded the 2 nd highest April flows; the River Rother
(Western) at Iping Mill the 4th highest April flows and the Arun at Alfoldean the 5 th highest April flows on
record. There was considerable variation in the flow classification for April across the key flow sites in the
Region, with 1 river with exceptionally low flows; 5 with notably low flows; 3 below normal; 8 normal; 3
notably high and 1 with exceptionally high flows.

A number of flood alerts were issued across the Region: 41 were issued by the southern flood forecasting
team, and the northern flood forecasting team issued 17 in North East Area and 27 in West Thames Area.

Reservoir Storage/Water Resource Zone Stocks

Reservoir storage is close to 100% for the Lower Thames, Lower Lee and Farmoor, Arlington and
Powdermill Reservoirs. Storage has risen in the all the other reservoirs, with Bough Beech and Weir
Wood close to or just below the long term average for April; and Ardingly, Bewl and Darwell about 60%
full. Despite the heavy rainfall, the Chalk aquifers have not recovered significantly and the groundwater
sources may remain under pressure this summer. The temporary water use restrictions remained in place [b]



Last edited by David Harvey on Fri May 11, 2012 4:37 pm; edited 1 time in total
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PostSubject: Re: *Thames Area Monthly Water Reports*   Fri May 11, 2012 4:36 pm

Adding

NORTH EAST THAMES

Summary – April 2012
The wettest April on record caused river flows to rise dramatically during the third week of the
month. The heavy rainfall did little to improve North East Thames Area’s groundwater resource,
which remained below normal for the time of year.

Rainfall

April was the wettest month on record for the North London, Roding (1961-Present), the Chiltern-East-Colne and
Lower Lee (1920-Present) areal units. The North East Thames Area (“the Area”) recorded 122mm of rainfall or
249% of the Long Term Average (LTA) rainfall for April. Rainfall was recorded on 23 days during the month, with
the heaviest totals on the 9, 17, 18, 23, 24, 25, 27, 28, 29 and 30 April. The largest daily total of 31.8 mm was
recorded on the 28 April at Lilley Manor in the River Mimram catchment.

Soil Moisture Deficit (SMD)/Recharge
The month ended with SMDs close to or at zero across the Area. SMDs started the month high but were wiped
out with the exceptional rainfall. With the reduction in SMD, effective rainfall was available to enable recharge to
the Chalk aquifers. The Chilterns-East-Colne received 394% of its monthly LTA effective rainfall, while the Lee
Chalk received 92% of its LTA. On Average the Area received 165% of its April LTA Effective Rainfall.

River Flows
Flows in the Area’s rivers remained low until the third week of April when river flows responded to the continued
heavy rainfall. The legacy of low flows is reflected in the monthly mean flows were notably low flows were
recorded on the River Mimram, below normal flows on the Rivers Gade, Ver, Misbourne, Colne and Lee at
Waterhall. The impact of the heavy rainfall was seen in the river flows of the urban and clay rivers. Two of our
indicator sites were at normal flows, three were at above normal flows (Rivers Brent , Crane and Roding), while
the River Ingrebourne recorded notably high flows (197% of the LTA monthly mean flow).
As a result, flood alerts were issued for North East Thames Area on 29 and 30 April for the River Ingrebourne,
Upper, Middle and Lower Roding, Cripsey Brook, River Stort, Lower Lee Tribs, Mimshall Brook, River Crane,
River Lee at Hertford, Upper Colne and Radlett Brook, River Pinn and Woodridings Stream, River Brent from
Hendon to Brentford and the Brent Brooks.

Groundwater Levels
Despite the large amount of effective rainfall, groundwater levels remained below normal or lower across the
Area. Noticeable rises in groundwater levels were recorded at our indicator sites in the Colne catchment, while
only small increases were observed in the Lee Chalk. Our indicator sites at Ballingdon Farm and Amersham
Road rose to below normal levels, while our indicator sites at Ashley Green and Wapseys Wood remained
exceptionally low, demonstrating how little recharge the Area has received over the past two years.

[url=FULL REPORT]http://www.environment-agency.gov.uk/static/documents/Research/NET_Area_Water_Situation_Report_April_2012.pdf[/url]
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