All three reservoirs receive water only from runoff from natural rainfall. The Otay Filtration Plant operates continuously to meet water demands in its service area. In order to operate efficiently, Otay must be near full. As water is removed from Lower Otay Reservoir, its level drops and additional water is usually added from Barrett whose water would be replenished by Morena. Thus, to be consumed, water in Morena must first be drafted to Barrett and then to Lower Otay.
Withdrawals occur until each reservoir reaches a predetermined minimum pool.
After that, no further drafts can occur from that reservoir. Because the water in the reservoirs is replenished only by natural rainfall, the levels of the reservoirs cannot be predicted. As water is consumed, one of the three reservoirs must supply water and its level will drop accordingly. Factors affecting increases and losses in each reservoir include runoff from precipitation, evaporation, leakage through the dam, and the size and shape of the reservoir. Runoff is highly variable and unpredictable.