A log of rainfall amounts and storm durations is a great place to start, but you can make your rain log an even better defender by adding details.
Is it only drizzling out there? Take a picture to prove it!
Did You Sample?
Make it incredibly easy to see when you sampled and when you didn't by answering a yes/no question.
Add any written notes that you might need to reference later. Maybe it only discharged to one of your outfalls: your rain log is a great place to note this.
Add Localized Storms
What if the publicly available rain gauges are too far from your site and your site gets dumped on, but the rain gauge says no rain? No problem: you can manually add (or delete) any rain event.