Self-emptying tipping bucket rain gauges are the most popular type of rain gauge"
From another page - The only requirement is that the magnetic switching contact of a rain gauge produces a momentary contact closure when the rain gauge registers its minimum water amount. This type of rain gauge electrical contact is called a “Normally Open” (NO) contact and is found on 90% of rain gauges on the market today.
Concept: Bucket fills up to the point where the weight of the water and design of the 'tipping bucket' tips it over and dumps the water. When it tips, it closes the Normally Open contact for a 'moment'(momentary contact aka push button). Each bucket full and subsequent tip is a certain amount of water that can be translated to rain amount in inches.
I haven't found that inches per bucket tip figure yet but it will depend on the make and model of the bucket. Say it's 0.1 inches. 10 tips would close the contacts 10 times and would be an inch of rain.
The system simply needs a counter which is easy because digital inputs can be a counter already.
Now I just need a tipping bucket rain gauge for the counter to count.
Here's where I'm running into a limitation in the controller software. I can create one counter for each digital input and I can set it to reset at a certain count but that's it. It would be nice to have multiple counters for a single digital input and have the counts reset at different time intervals. That way I could see rain accumulation for this week or this month, year etc.
I'm sure it can be done but it will likely have to be done by writing scripts in the BASIC programming language.
Easy peasy NOT(or simply contact the company and say "Can you give me a script to do this?")
Until I can write or get a script, I can record the monthly amounts in a spreadsheet and reset the counter every year.(once I get a tipping bucket rain gauge)