Preventing evaporation is reliable, uses no energy, has no...

Preventing evaporation is reliable, uses no energy, has no ongoing costs. All it requires is a scalable cover for the dam:

"In terms of the cost of water, this product will produce savings over a 20 year period, based on an average evaporation rate of 5mm per day (i.e. Brisbane, Queensland location) and a cost of $20 per sqm of product deployed, it will deliver a cost of water at $550 per ML from day one.
As a comparison, the cost of desalination ranges between $1,500 to $3,000 per ML and for recycling the range is between $1,450 to $5,800 per ML. The cost of potable water is rising."