Water Supply

C Reed, J Reed & C Green
Publication Date (Web): 10 February 2017
DOI: https://doi.org/10.21139/wej.2017.009

Outage and headroom are concepts that are included in some form in many water utilities’ approach to water resource planning. Outage represents an allowance for a reduction in source or treatment capacity. Headroom represents uncertainty, of either the yield that sources can supply, or the demand that is forecast. These concepts therefore provide a framework for considering risk and uncertainty in the supply-demand balance in a clear and transparent manner. Watercare supplies water to the Auckland region and plans to meet a current peak demand of 500 ML/d. As part of its long term planning, Watercare has carried out assessments of outage and headroom using probabilistic techniques, which enable it to plan to a known level of uncertainty or risk. 

The assessment commenced with a literature review of available international methods. The review found that the UK Water Industry Research (UKWIR) methodologies are the most structured and detailed available methods, complemented by scenario analysis where single factors (such as population) have a disproportionate impact on the analysis outcome. 


At any given time it is unlikely that a water utility will have all of its sources and treatment works available at their specified capacity. Reductions in capacity can be due to planned and unplanned maintenance and changes in raw water quality – such as turbidity or pollution events. Watercare used historic data, combined with operational knowledge, to estimate the duration, magnitude and expected frequency of possible outage events. Monte Carlo modelling enabled these data to be combined to produce a frequency distribution of outage events and a likely outage figure for the peak month to be identified.


Headroom represents an allowance for uncertainty in the supply-demand balance.  The methodology applied by Watercare is a Monte-Carlo based probabilistic method, which enables an understanding of the likelihood of the combined outcomes of each specified principal uncertainty in the supply demand balance that lies outside Watercare’s direct operational control. It does not consider uncertainties within its operational control. The output of the method is a probability distribution, from which Watercare can select a value that reflects the exposure to uncertainty that it can accept and that is also acceptable to customers. The benefit of this methodology is that the outcome can be a single value of a reasonable buffer, or uncertainty allowance, for each part of the planning period. 

Watercare has successfully applied these methodologies to both its metropolitan Auckland supply area and to the small community of Waiuku, therefore quantifying and improving its understanding of the relative impact of key risk factors on these systems. The paper describes the process developed to assess these uncertainties and recommends approaches that could be taken to other municipal supplies in Australia or New Zealand.


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