The Penman Equation ...

The Penman Equation approximates the evaporation rate of water from lakes, etc. https://www.sciencedirect.com/science/article/abs/pii/0002157177900073.  We probably can't do much about said evaporation except to cover the lakes, etc., using the floating ping-pong ball coverage approach.

There is, however, another serious water loss problem that we can begin to address, namely the loss of transported and processed/treated water from water mains.  (The 2016 American Water Works Association study revealed losses of 6.8 - 45.5%, with an average loss of 22.6%.)  Water mains are the oldest municipal infrastructures, and leakage from corrosion pits, cracks, joints frequently last for decades until becoming visible as surface water running down streets, potholes, etc.  Especially in view of worsening drought conditions due to climate change, such losses must be addressed by better measurement practices and volume/mass balance methods to detect, locate, and prioritize leakage.