On Private Water Supply resilience

While some of the water utilities are starting to prepare for hosepipe bans (or have implemented them), this summer has again brought the resilience of Private Water Supplies (PWS) in the North of Scotland back into focus. High pressure weather systems that were prevalent from the end of February to May brought prolonged dry weather with subsequent impacts on a number of PWS. Irrespective of the cause, these blocking events are quite prevalent, with similar episodes in a number of recent years.

PWS records are generally poor (which in itself is problematic), but the DWQR estimates around 3% of the Scottish population is served by a private water supply. These are a mix of groundwater (boreholes), springs, surface water abstractions from rivers and lochs and the odd bit of rainwater harvesting.

All of these supplies are typically characterised by relatively low usage in absolute terms. For example, using typical demand estimates, a 3 bedroomed house might be expected to have a peak demand of 1200l/day (6 people x 200l per person). While a number of PWS have multiple connected properties, one to two connected dwellings per supply is common.

When new PWS are being proposed for new build houses, there is typically a requirement to assess the suitability of the supply, in both quantitative and qualitative terms. In conveyancing, there is also a need to assess Private Water Supply suitability.

An example of one the many varied and creative approaches to constructing PWS

Undertaking these assessments is subject to a number of challenges including:

• The sources (particularly in the case of surface water abstractions from burns) are extremely small. In the course of being involved in this work over the last few years I've seen source catchments as small as 0.05km² either being used or proposed.

• No formal monitoring (or often monitoring of any kind)

• No statutory level of service or yield target (as is required for public supplies)

• No statutory resilience requirements (such as storage provision)

This has (in this authors opinion) led to a bit of a mish-mash of assessment techniques and outcomes in terms of assessing PWS suitability. Part of the work we do involves reviewing existing quantitative assessments. That work indicates that a range of techniques is currently employed, from Lowflow assessment using software such as Lowflows 2 or scaling from a nearby gauged catchment, to direct measurement of a burn.

Analogue methods can sometimes perform poorly in the typical catchments in which they are applied (which is a function of the poor representation of very small catchments in the underlying datasets rather than any inherent issue with the software or method), and even when they perform reasonably they probably give a poor indication of the likely performance in a decent drought. While the measurement approach provides a bit more useful information by using at-site data, it can often be pot luck as to when the measurement is undertaken. Some assessments using measurement acknowledge the likelihood of the flow on the day of measurement, whereas other seem to blithely ignore it and state yield values as if flows were to remain unchanged from the day they were measured.

Statistical assessments often use metrics like the Q₉₅ (flow exceeded for 95% of the time, on average) as software can output this easily or it can be calculated. But it should be evident that the Q₉₅ is not a reliable guarantee of water availability during periods of drought; in small catchments with very little sub-surface storage, recession curves can be much steeper than in larger, monitored catchments (though not always, the opposite is also true, the point is that the metric itself is unreliable). In Spring 2025 a number of monitored catchments dropped below Q₉₅ (and in some cases Q₉₉) values for considerable periods of time.

Given the relatively low abstraction rates (for example using the demand for a single house noted above, the average abstraction rate over a day would equate to around 0.014l/s) the main risk to a surface PWS is it drying up entirely - even at extremely low flow rates a significant proportion of a single house's demand can be met. Unfortunately, current hydrological methods are not well suited to assess or model the risk of these very small catchments completely drying up and yet we know this is a risk that has been realised on many occasions in recent years.

PWS serving a single house. How to reliably assess the risk of it drying up?

What could be done to improve PWS resilience?

Despite all the above, there remain a number of ways in which both the assessment of supplies and the resilience of those supplies can be improved.

1. Set a target for performance. Most supplies will not be able to meet anything close to the public water supply requirements (such as ensuring supply during a 1 in 40 drought). Setting a target for PWS would have the advantage of (1) helping  develop methods which focus on establishing a reliable assessment of whether supplies meet a stated performance level and (2) helping owners, local authority staff and purchasers of supplies understand their vulnerability to drought.

2. Develop more appropriate assessment techniques which can reliably assess the risk of surface springs and burns drying up.

3. Make better use of monitoring. Sensor cost has dropped in the last few years, with a much wider variety of datalogging and low-cost telemetry (think LoraWAN) or similar).

4. Provide better advice on PWS resilience. By the time a PWS user runs out of water, it is too late and a Local Authority typically has to provide support. If users were made aware of PWS storage levels or burn levels dropping more than usual, it might focus behaviour more on water management. Some users will be very aware of what's going on, but many will not be.

The next comment piece will focus in on assessment techniques, specifically looking at  the concept of 'no-flow' assessment for PWS. We are in the early stages of developing a project to improve the assessment of PWS resilience by making better use of information gathered in the last few dry periods; if you're interested in finding our more or linking up then please get in touch!