I don’t think any of the provided OSPi software has built-in support for using analog sensor to trigger sprinkler events. This blog post has instructions of basic use of the PCF8591T ADC:
http://rayshobby.net/?p=6704
If you have some programming experience, it’s not difficult to modify the existing OSPi software to add ADC support. Because different users may want to use different sensors, this part is largely left to users to customize.