Estimation of outage allowances

A methodology is described for estimating the allowances or safety margins which can be applied to estimates of water resources to allow for outages ( random or periodic interruptions to supply). The methodology follows recent published UK Government and water-industry guidelines and provides estimat es at two levels: on-average estimates, and risk-related estimates. Example applications are described, and advice is given on ways of improving the s upply/demand balance. The final choice of any outage allowance to be used s hould take account of (a) patterns of demand, and (b) the consequences of f ailure.