One may not intentionally water his lawn on Shabbat (See here.)
A shul congregant told me that he has a sprinkler that waters his lawn based on the weather forecast. It is not set on any type of clock so he is not intentionally setting the system to turn on Shabbat. He says that the system measures the water table in the lawn and combines this with a weather "feed" that it gets from a website. If the lawn is "dry", it will turn on the sprinkler that day. However, if the weather report predicts rain, it will not turn on, as the rain will do its job.
Can someone having this type of system have his lawn watered on Shabbat, or has he violated any melacha?
See this article for an example of this type of system and a brief explanation of how it works.
Please ignore concerns of mar'it ayin for purposes of answering this question.