I think the first task will be to locate a suitable sensor for the inlet pipe. Domestic showers use them, either to detect when a shower tap is opened (pressure drops) and switch on a pump, or to detect failure of the water supply (pressure drops) and switch off an electric heater. There are other appliances (clothes-washer, dish-washer, .....) which also use them, to sense water level by sensing small pressure changes. You may be able to 'rescue' one from a junk appliance. When you have one you can determine what connections it needs and proceed from there.
If you can locate a suitable sensor then, as pointed out above, timers would be unnecessary. That would simplify the set-up considerably and would make use of a micro (or complex logic) unnecessary, too.