Fresh potable water for drinking and irrigation are going to be one of the 21st century’s great challenges. This sounds counterintuitive, but, hey! That’s why we need to be open to new ideas! check this out:
Sand dam technology may date to Roman times. But it has been perfected in the 21st century by a combination of talent including a British NGO called Excellent Development, a Kenyan farmer named Joshua Mukusya, and Mennonites from North America.
Read about sand dams.