Annual Water Supply Review


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.

The information provided in this section shows the Water Supply data for the 12 months from April to March. The monthly data for our resource levels can be found here.


Last year’s rainfall was reasonably typical when compared with the 30 year long term average.  However, the months of August 2015 and January 2016 exceeded the long term average by 230% and 14% respectively.

Annual total rainfall was 855mm compared with a long term average of 776mm.



Groundwater Levels

The Company has monitored the groundwater level at Idsworth Well, Rowlands Castle, for many years since the well is unaffected by abstraction and is representative of groundwater conditions in the South Downs chalk.  Around 85% of Portsmouth Water’s abstractions are from underground sources and so groundwater levels are critical to maintaining supplies.

Groundwater in the local aquifer normally fluctuates approximately 9.5m between maximum and minimum annual levels.  Groundwater levels in the Spring of 2015 were approximately 700mm above the long-term average.  During the year groundwater levels were within the “normal” range, with levels only rising to the “high” range for a short period in February due to the heavy rain of January.  Since January, levels have fallen broadly in line with the long term average and were approximately 2.5m more than the long term average in April 2016.

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




Abstraction from the Company’s various sources in 2015/16 was as shown in the table below:


Treated Water Distributed




The annual average distribution input dropped from 168.8 Ml/d in 2014/15 to 166.5 Ml/d in 2015/16.  The volume of water distributed is influenced by many things, including the weather.  The peak week average of 192.9 Ml/d occurred in July with an absolute peak day of 205.0 MI/d due to the warm weather and low rainfall.

Abstraction is drawn from three types of source, the River Itchen Works which treats surface water, boreholes and wells which abstract groundwater from the underground chalk and Farlington Water Treatment Works which treats spring water from Havant and Bedhampton.

Our largest source utilises water from a group of natural springs at Havant and Bedhampton.  Water from the springs is treated at Farlington Water Treatment Works.

The nature of the chalk aquifer of the South Downs ensures that at many sites high quality water is abstracted which requires only minimal treatment.  Some chalk sources are at risk of cryptosporidium oocysts (which can cause severe stomach upsets) being present in the water which require enhanced treatment by membrane filtration.


For the year 2015/16 average leakage is calculated post Maximum Likelihood Estimate (MLE) at 28.1 Ml/d. This is an improvement on last year and is better than the performance commitment of 30.0 Ml/d.


Leakage then remained steady, and significantly below target, throughout the summer and autumn (with the exception of a short summer peak).  By the end of autumn, leakage had dropped as low as 26.0 Ml/d.  An uncharacteristically warm December and subsequent late winter also helped to ensure that the traditional winter peak was not as high as normal. Leakage only began to rise in mid-January and only then reached a high of 31.3 Ml/d in comparison to a winter peak of 33.5 Ml/d in the previous year.

Burst Mains

The number of burst mains experienced in 2015/16 was 219, compared to 294 that occurred in 2014/15. This equates to 66 bursts per 1,000km in the reporting year. The majority of the bursts occurred on 3” – 6” cast iron mains and were generally associated with the swelling and shrinking of clay due to changes in soil moisture and temperature.  The number of bursts typically increases during the winter however with mild conditions, lower numbers were experienced in the winter of 2015/16. A similar level of performance was achieved in 2013/14 and this again was a relatively benign winter. 2014/15 was a higher number of bursts driven in particular by the weather.

We continue to target mains for renewal based on the impact of bursts on customers.  The graph below indicates the general reduction in bursts as a result of our approach to renewing our mains.


Click here to see our 2015 report

Click here to see our 2014 report