Some systems have a complex interaction of penalties among different uses.
For example, consider a system having two reservoirs on different rivers which flow together at some point downstream. Suppose that each reservoir has a power plant at the outlet. Suppose also that there are irrigation areas downstream of the plants but upstream of the confluence of the two rivers. This means that there is a zone of water at the top of the reservoir that can be released to meet irrigation needs, and a lower zone that can be released to generate power. In order to impose the system energy demand, a power control channel is required.