The sensor traces have a weak pull-up resistor of 1 MΩ.
The resistor will pull the sensor trace value high until a drop of water shorts the sensor trace to the grounded trace.
Believe it or not this circuit will work with the digital I/O pins of your Arduino or you can use it with the analog pins to detect the amount of water induced contact between the grounded and sensor traces.

Features:

• Grove compatible interface
• Low power consumption
• 2.0cm x 2.0cm Grove module
• High sensitivity

Application Ideas:

• Rainfall detecting
• Liquid leakage
• Tank overflow detector