I've seen circuits to use capacitance, but you still need salt water since it must be conductive to form one plate of the capacitor. You arrange a metal cover or top over the water that forms the other plate of the capacitor and drive it with some relatively high frequency alternating supply and measure the current flow with an A.C.microampmeter and calculate the reactance that changes with the height of the water and the closing of the air space.
Oops, you just covered over the bucket!!! That has been done for things like sumps and boat bilges but not for rain gauges. Another scheme I've heard of is to have a smallish calibrated cup that holds maybe the equivelent of half an inch of rain. When this fills its weight causes it to spill out into a bucket, and this tilt/spill clicks some kind of counter. Tom
