Have a look at Penguin’s Lab for a great way to capture an event when you are not present. In this case the event they are trying to capture pictures of a cave flood with a Flood Triggered Automated Camera System. The system consists of a camera that is connected to a moisture sensor so that the a camera can start taking pictures when the sensor gets wet. Pictures will continue to be taken every 15 minutes until the moisture levels go back to normal. Since it is being installed in a remote location it needed to be self sustaining.
The water sensor is an interesting design since it has the ability of killing the power to the entire system when the conditions are dry. This is done by using a Darlington transistor feeding a relay.