Water Supply News – October 2017

Draft primary wholesale tariffs for 2018/19 published
October 3, 2017

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.

Rainfall at Havant

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

The month of October saw lower than average rainfall with approximately 31% of the long term average rain for the month.  Rain fell on 52% of the days, with the maximum daily rainfall approximately 12mm on Thursday 19 October 2017.

Total rainfall for the month was 30mm, lower than the long term average (last 30 years) of 97mm.

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.  Levels can fluctuate by as much as 20m.

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

Despite the dry winter of 2016/17, ground water remained within the “normal” levels for the spring of 2017.  During the summer, levels fell to a “low” level, but recovered to “normal” levels by the end of August.

It is normal to see ground water levels recover in the autumn as rainfall increases.  September rainfall was above average and October below, with cumulative rainfall for the two months being 78% of the long term average which has seen levels increase, but remaining 1.3m below the long term average at the end of October.

So long as rainfall remains close to average for the year, we do not see the need for drought control measures for 2018, such as temporary use bans, but would encourage all customers to continue to use water wisely.

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