You can probably use one of the sensor ports on OpenSprinkler as ‘program switch’. Have your water level sensor’s output connected to the OpenSprinkler as a sensor, and set it as a program switch to start a program when it’s triggered. I don’t have the full technical details since you didn’t explain what kind of signal your water level sensor outputs. If you are using a ESP8266, you can use one of its GPIO pins as output to produce a digital signal (1 or 0) to indicate the water level reaching target level. Then connect that GPIO pin to either SN1 or SN2 on OpenSprinkler, and GND to GND on OpenSprinkler.