Designed storage capacity of potable service reservoirs to maintain supply during short term outage events

Hi all I am currently doing some work on developing some operational standards for the design, operation and maintenance of water network assets. A question I am getting challnged on a lot is how much capacity do we build into our new service reservoirs to maintain supply to our customers during short term outage events on incoming water mains or at the local treatment source. It is about managing the risks of maintaining supply against being able to manage the overall quality of the water in the reservoir. We currently are proposing a minimum of 24 hours of available supply made up of either storage and/or alternative supply from rezone etc. if the population serviced by the asset is above a specfic size (44k pop in urban area and 22k pop in rural area) then this increases to 36 hours to allow for our emergency response capabiltiy (alternative water supplies - tankers, static bowsers, bottle water stations etc) and the likely longer duration of any repairs to the treatment plant or feeding trunk main. I am trying to seek views on how members of the Water Network address this issue within their businesses. Do you have a standard methodology you could share with me or I would also welcome your thoughts. I do not think there is a definitive right or wrong answer but would welcome any thoughts / help on this Regards Mick