Water flow or pressure sensor

Hello,
I have an irrigation system done with Sonnoff 4CH PRO R3 and is working very well but, time to time, I’m remaining without water from the main supply.
How can I have an sensor to send me an info/message in order to take an action ? What type of sensor should I use ?
Thanks,
Puiu

floatless relay sensor or liquid level relay (hi/low). you can try those or simply set a timer so water source can replenish.