I have found that one good rain for several hours will fill a 400 gallon tank. A large cistern to store water during the dry months will ensure a year-round supply of water. Case in point, the other day. After the first good rain in March, my tank is overflowing with each subsequent rain until some time in December. Another concern is water quality.
Rainwater is clean and pure.
There are special lead-free paints for tin roofs which are approved for rainwater collection. It is also very easy to implement a filter such as sand or charcoal. There are also many commercial resources available. There are places in the world with the same rainfall pattern as the Osa where the people have been living on rainwater for years. There are also areas with less rainfall where people rely on catchment as the only water source.