If my understanding is correct, the model would either have to assume some sort of uniformity in periods of low electrical demand, or it would have to have a model for it. I can help you with the first option, since WEAP can manage that based on the time step scale. To model that, you would two reservoirs, and have a transmission link between them with rules about how much water is transferred and under what conditions. The demand priority of the upper reservoir would have to be higher than the priority of the lower reservoir.
Are you modeling electricity usage? You can try reading through the WEAP-LEAP tutorial (the last chapter of the WEAP tutorial) to get a sense of how that can be done in WEAP.
