You can cut a hole in your reservoir near the top to act as a overflow of coarse you would have to remix nutrients after your solution becomes diluted so if you have a lot of rain it could be wasteful. I would provide some sort of cover from the elements maybe a sunshade setup over your system with some wood or maybe have it against your house.

I live in San Diego and don't think you will have a problem, I never have. It doesn't really rain enough days to mess things up that often. Just adjust your ph and EC after it rains if needed.

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I collect Zero PPM Rain water and use it in my Reservoir. Here in Florida we get 55 inches a year and it all comes during our Rainy season. But the hot sun uses so much water each day that the rain is a welcome replenishment of the Reservoir. Fact is I still have to add water.

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I'm using 8 and 10" net pots for my drip system and I'll have 10 or more of them so rain water has the potential to be a real problem. I plan on getting a little stiff plastic and making a shallow cone that I can put around the trunk of each plant that will shed the water off without letting it get to the bucket.

I have the buckets sitting out right now but they are empty at the moment and with all the rain, they are easily filling up a 1/2 gallon or more per rain storm.

i doubt you will see enough rain between now and harvest to make a difference.

i live on a tropical island. during the rainy season, it can rain weeks on end, nice downpours. i found an old vinyl sign, cut it in sections large enough to drape over the trays and add grommets. i then placed these over my grow trays, and made small cut outs (shape of an X) and had my starter plants grow thru these small holes. it seems to be working out well, plus im sure it keeps the rootzone a bit cooler, and the reflective white cranks on the plants by reflecting the sun upwards.

good luck

"i doubt you will see enough rain between now and harvest to make a difference."

This.

I'm growing stuff in rain gutters and I still need to top up regularly, even after heavy rain. There's just not enough surface area to collect much water (most of the rain is blocked by the foliage)