It is not about directly tapping into the rainwater collection, but managing the rainwater run off into ocean.

'Watersheds' are meant for that, and development of that is a serious endevor.

When rain falls on dry ground, some of the water soaks in, or infiltrates the soil. Some water that infiltrates will remain in the shallow soil layer, where it will gradually move downhill, through the soil, and eventually enters the stream by seepage into the stream bank. Some of the water may infiltrate much deeper, recharging groundwater aquifers.

https://www.usgs.gov/special-topics/water-science-school/sci...