Did you ever hear of the water table?
http://pubs.usgs.gov/gip/water_dowsing/pdf/water_dowsing.pdf
“...The natural explanation of “successful” water dowsing is that in many areas water would be hard to miss. The dowser commonly implies that the spot indicated by the rod is the only one where water could be found, but this is not necessarily true. In a region of adequate rainfall and favorable geology, it is difficult not to drill and find water!
Some water exists under the Earth’s surface almost everywhere. This explains why many dowsers appear to be successful.”