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.

The information gathered is reproduced here for information purposes; no guarantee is given as to its accuracy.

Rainfall at Havant

The Rainfall Records at the Company's Head Office at Havant date back to 1886.

April lived up to its reputation with changeable weather experienced throughout the month.  There were only twelve days in the month when no rainfall was recorded and the longest dry spell, of no rain at all, was four days.  The wettest day was the last day of the month, Wednesday 30th, when 28.2mm of rain was recorded at Havant.

Total rainfall for the month of April was 73.8mm compared with the Long Term Average (last 30 years) of 47.62mm.

Groundwater Levels at Idsworth

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

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

By the end of the month, the wet weather during April finally slowed the recession in groundwater levels, but not before they had fallen a further 1.5 metres from the level at the end of March.  By the end of April the groundwater level at Idsworth stood at 21.5m AOD, approximately 1.5 metres below the long-term average.

Despite the fact that groundwater levels are below the long-term average there are no concerns for supplies for the summer of 2008.

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

Water Supplied (Distribution Input)

The Company measures the total amount of water that it puts into supply on a continuous basis. Unsurprisingly supplies peak during the summer periods when demands from both domestic and commercial customers increase usually in response to dry weather.

Daily fluctuations are 'evened out' by storage in treated water service reservoirs but the average daily input over seven days gives an accurate indication of overall water demand.

The changeable weather during April constrained any rise in water demand throughout the month, and as a result the average weekly amount supplied remained at its winter level of between 174 and 176 Ml/d (million litres per day).