Portsmouth Water has no large raw water storage reservoirs.  It relies almost entirely upon groundwater reserves in the chalk aquifers of the South Downs and abstracts its raw water from wells, boreholes and springs.  It has one river abstraction at Gaters Mill on the River Itchen, a chalk river with a groundwater baseflow.

Those reserves rise and fall with the seasons and so Portsmouth Water pays particular attention to the local hydrogeological data by monitoring rainfall at its Havant Offices, groundwater levels in the chalk at Idsworth on the Hampshire/Sussex border, as well as the total yield from the natural springs at Havant and Bedhampton.

These resources are monitored on a daily basis and reported upon on a monthly basis.  The information here is therefore at times one month behind the most current situation.

The information gathered is reproduced here for information purposes; no guarantee is given as to its accuracy.
<h2>Rainfall at Havant</h2>
The Rainfall Records at the Company’s Head Office at Havant date back to 1886.

The month of December saw below average rainfall with approximately 25% of the long term average rain for the month.  Rain fell on approximately 42% of the days with the maximum daily rainfall approximately 4mm on Wednesday 21 December 2016.

Total rainfall for the month was 22.2mm, lower than the long term average (last 30 years) of 87.5mm.

<a href="https://www.portsmouthwater.co.uk/wp-content/uploads/2017/01/dailyhav-dec16.png"><img class="aligncenter size-full wp-image-5130" src="https://www.portsmouthwater.co.uk/wp-content/uploads/2017/01/dailyhav-dec16.png" alt="dailyhav-dec16" width="1468" height="756" /></a>

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<a href="https://www.portsmouthwater.co.uk/wp-content/uploads/2017/01/ltahav-dec16.png"><img class="aligncenter size-full wp-image-5131" src="https://www.portsmouthwater.co.uk/wp-content/uploads/2017/01/ltahav-dec16.png" alt="ltahav-dec16" width="1468" height="808" /></a>

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<h2>Groundwater Levels at Idsworth</h2>
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The Company has groundwater records from the Idsworth Estate Well, north of Rowlands Castle, which date back to 1931.  Water levels in the Well, which is entirely unaffected by abstraction, give an accurate indication of the groundwater reserves of the South Downs.  In the winter, levels usually rise in response to rainfall, whilst during the summer, levels recede as any summer rainfall either evaporates or is taken up by trees and plants before reaching the aquifer below.  Levels can fluctuate by as much as 20m.

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The Groundwater level is usually recorded in metres AOD (Above Ordnance Datum), Ordnance Datum principally being equivalent to sea level.

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Groundwater levels at the end of the Summer 2016 were close to the long term average.  Autumnal rainfall in September, October and November normally sees the rapid recovery of groundwater levels. Actual Autumnal rainfall has been 55% of the long term average 139mm compared with 252mm.  The low rain fall has continued in December with the four month average dropping to 47% of the long term average.  The low rainfall has resulted in levels being approximately 7.0m below the long term average at the end of December.

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Rainfall normally remains high in January, February and March which is of importance to see groundwater levels recover.

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The graph below shows the change in groundwater level over the last twenty four months compared to the Long Term Average (last 30 years).

<a href="https://www.portsmouthwater.co.uk/wp-content/uploads/2017/01/idsworth-dec16.png"><img class="aligncenter size-full wp-image-5132" src="https://www.portsmouthwater.co.uk/wp-content/uploads/2017/01/idsworth-dec16.png" alt="idsworth-dec16" width="1469" height="939" /></a>