Operating rule model of a cascade of four hydroelectric power reservoirs was revised using a newly developed stage-area and stage-storage relationships. Stage-area relation is the basis for the development stage-storage relation. The basic concept that used to develop stage-area was the change in area with change in stage is proportional to stage. Simpson one-third numerical integration method is used to compute the net storage volume between stages. The result was verified and used to model the operation of the cascading system. The aim of developing a new operating rule model was to improve the total annual power generation from the system. Critical period studied using long-term daily reservoir level and inflow data, it showed that the length of critical period is equal to calendar year. Four important seasons were involving in the period; these are refill, deplete upper and lower operating level. Mathematical models were used to rank the refill and the deplete order of the reservoirs. Seasonally operating rule model was developed using the results of refill and depletion rankings. The power generated based on the developed model was compared to the long-term historically generated; and it found that the new rule model boosted the daily power production by 5.3 (12 MW per day), and the plant factor by 2. © 2011 IEEE.