Singapore is essentially a city state so is very limited in how much of that rainfall can be stored long term. If you look on a satellite map you'll see that th

Singapore is essentially a city state so is very limited in how much of that rainfall can be stored long term. If you look on a satellite map you'll see that the undeveloped land is already dominated by water catchment and storage: https://www.google.com/maps/place/Singapore/@1.3931232,103.7270086,39985m/data=!3m1!1e3!4m5!3m4!1s0x31da11238a8b9375:0x887869cf52abf5c4!8m2!3d1.352083!4d103.819836 https://www.google.com/maps/place/Singapore/@1.3931232,103.7... The same is true in many pacific islands, they get plenty of rainfall but it's not always storable or predictable, a lot comes at once from a cyclone/hurricane for instance, but you might get 2 of them a year and then nothing for a decade.