The idea is to clean and re-use river water. We can also import and purify sea water – freeing the country from dependence on natural rain.
We use solar driven vacuum pumps – as have loads of free sunlight during the day. The UV light passes through cloud, so the extra water will not decrease the power we get.
We only need 1 W, to raise the water by 1m, and the water boils into clean water vapour. We have a chain of vacuum pumps to carry the vapour to above a reservoir. We vent the vapour, and get a fine mist – which fills the reservoir.
The river water will form a thick, dark silt. Which we take off, and dry. It will contain dangerous levels of heavy metals, so we are best dumping it in the deep sea.
We dredge the shallow sea river estuaries, and get new soil – we can wash the salt out of, and import to be new soil – washed down from the highlands.
So we end periods of drought and famine. Bangladesh will become a massive exporter of food TO the world.
I am due to visit Bangladesh in September. I am just back from Turkey, where we can also use this idea – in a lush land. Cheap, safe fresh water.