Annual Water Supply Review 2016/17

Portsmouth Water Retains Industry Leader Position for Service to Customers
August 23, 2017
Water Supply News – August 2017
September 19, 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.


Annual total rainfall was 596mm compared to the 30 year long-term average of 782mm.  October, November and December were 37% of the long term average and impacted on groundwater recharge.  A significantly dry July was experienced with 10mm of rain compared with a long-term average of 50mm, or 20% of the long term average.

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 2016 were approximately 3m above the long-term average.  During the year groundwater levels remained within the “normal” range.  Groundwater levels normally start to recover in October as Autumnal rainfall leads to recharge.  The significantly low rainfall of October, less than average in November and well below average in December saw the three month rainfall of 37% of the 30 year average.  Groundwater levels did not start to recover until February when rainfall approached normal levels.  At the end of March levels were approximately 2m below the 30 year long-term average and recovered to a satisfactory ‘normal’ level.

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 2016/17 was as shown in the table below:

Source Annual Abstraction – Ml/Yr
Source Licence Source Actual 2016/17 Group Licence Group Actual 2016/17
Northbrook 7,487 6,568 7,487 6,568
Lower Upham 640 0
West Street 3,328 3
West Meon 166 27
River Itchen 15,916 7,745
Maindell 2,040 566
Soberton 3,294 2,406 3,294 2,408
Newtown 695 2
Worlds End 8,296 3,854
Lovedean 4,148 1,407
Havant & Bedhampton 35,770 18,211
Walderton 9,955 7,610 23,740 18,043
Woodmancote 1,103 278
Fishbourne 3,741 2,089
Funtington 2,920 1,849
Lavant 9,950 6,217




1,645 10,358 5,763
Westergate 1,134
Slindon 592
Aldingbourne 2,392
Total 116,066 64,595 44,879 32,782

Treated Water Distributed

The annual average distribution followed a broadly similar pattern which responds to day time temperature and importantly, rainfall.

The heavy rainfall of June 120mm, 260% of the long-term average, saw a significant drop in distribution input, whilst the dry June, 10mm, 20% of the long-term average, saw a significant increase.

The average distribution input for 2016/17 rose from 166.5 Ml/d in 2015/16 to 170.0 Ml/d

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 2016/17 average leakage is calculated (post Maximum Likelihood Estimate (MLE)) at 30.37 Ml/d. This is a failure against the target of 29.95 Ml/d. The graph below shows the long term trend in leakage performance.

Three separate rises in leakage during the second half of the financial year have resulted in this outcome.  This leakage pattern was also seen by other water companies within the South East this year. This suggests that the leakage increase was primarily as a result of external environmental factors including soil conditions and temperature.

As part of the Ofwat ODI scheme, rewards and penalties apply at the end of the current period and to average leakage over the five year period. If leakage remained at the average of the two years 2015/16 and 2016/17 for the three years of the current period until 2020 a financial reward would be gained and as a result allowed revenue will be increased by £42,000 over the next price review period (2020-2025). This will mean an increase in customer bills of 3 pence at 2020.

Performance throughout the year 2016/17 is shown in the graph below with further explanation.

Burst Mains

The number of burst mains experienced in 2016/17 was 298, compared to 219 that occurred in 2015/16 and our annual target of 342.  It equates to 88 bursts per 1,000km in the reporting year.

Over 25% of the bursts experienced in the year occurred in January 2017 when there was a number of cold snaps resulting in rapid temperature changes. We noted that last year, that 2015/16 was a relatively benign winter.