Harmonisation of Reliability Performance Indices for Planning and Operational Evaluation of Water Supply Reservoirs

The planning and operational performance evaluation of water supply reservoirs routinely use the volume-based (Rv) and time-based (Rt) reliability indices but decision making is often complicated by trade-off necessitated by the fact that the two are never the same, with Rv ≥ Rt. This study has resolved the problem by harmonising the two indices. Using data from ten global rivers, simulations of hypothetical reservoirs were carried out to determine capacity for specified demands and Rt values. The corresponding Rv values were then evaluated and the resulting reliability biases (i.e. Rv – Rt) were found. To harmonise the two indices, i.e. to nullify the biases, the concept of water shortage threshold was introduced, which is the minimum quantity of water shortage that can be taken as constituting real failure for the purpose of Rt evaluation; shortage quantities below this will be disregarded. The results showed that the water shortage threshold that nullifies the reliability bias can be as high as 60% of the demand, depending on the runoff variability, the demand and the specified Rt. When averaged over all the situations analysed, the water shortage threshold was found to be 51% of the demand. Although this might appear high, it is argued that it is plausible both within the context of developed economies, where unaccounted-for-water can be much higher than 51%, and of underdeveloped economies where large sections of the population have no access to adequate water supply. In the latter case, a reduction of 50% in water supplied that guarantees uninterrupted supply of the other 50% will be deemed satisfactory and reliable, while for the former, a shortage of 50% that forces a change in behaviour to waste less water will also be deemed satisfactory. The significance and novelty of this study stem from the fact that it has removed the need for the trade-off between the two reliability indices, thus enabling unequivocal characterisation of water supply reservoir performance for effective decision making.