There are some choices of water meter you can find on the market (if you are in the US, you can search on Amazon). These water meters usually have a reed switch inside and sends out clicks (i.e. the switch opens and closes), the frequency of which depends on how fast water is flowing through the meter. We are actually working on getting this supports in the official OpenSprinkler firmware, however, at the moment we are planning to support just one meter so if you need two meters or more you will have to connect some additional wires.