Living off the grid - homesteading and self-sufficiency

I'm in northern Spain and the last time it rained properly was November 2022 (we had about 120mm in a day, which did a fair bit of damage). Since then, it's been 8mm at most - which is hardly anything.

Tomorrow night we're predicted to get about 50 mm, which would be amazing and fill all our water tanks. We only have rain as a water source and the local rivers are getting more and more dry. If we do get that much rain that'll mean cancelling some plans (can't get to the tarmac road when it's that wet) but boy do we need it :')

Side question - in my area, we measure rainwater in millimiters per centimeter and liters per meter (10 mm = 10 liters per square meter). How do you measure rain in your country?