Pardon my ignorance, what do you mean by modeling the canal and gate with a constant head?
One of the main goals of this project (besides rehabbing the deteriorating dam structure) is to reduce the amount of effort that goes into opening and closing the headgate slide gates. For almost the entire irrigation season, our client is limited to what they can take from the river. The river flows vary greatly hour to hour, day to day, week to week, and so they have to adjust the opening of the gates. This requires a significant amount of time. They are considering a SCADA system to take care of it, but costs may be prohibitive and even then, we would want to try to minimize the amount of operation (= electricity to run the gates). At the very least, I need to be able to show how often they will need to adjust the gates given average river flows and the constant flow they are allowed (every drop of which they very much need).