Apart from more storm water ...
Published by Joseph Cotruvo, President at Joseph Cotruvo & Associates, Water , Environment and Public Health
Apart from more storm water capture and storage and dams the most widely available and sustainable source is recycling sewage. The technologies produce water that is higher quality than 'natural' water. See the 2017 WHO Guideline for direct and indirect water reuse. There is also desalination of sea and brackish water when accessible, but more expensive.