Assume Qi to be the normal demand at node i. Qis,j represents the actual supplied demand at node i under calamity event j, the supply level at node i for event j is given as:

This gives the percentage of the demand that a system supplies to node i under calamity event j. The key is to calculate the actual supply demand Qis under the outage that may cause lower than required junction pressure. The less the demand, the greater the impact the calamity is on the system supplied capacity and the more critical the element is to the system.