The reason this doesn’t happen is that farmers/farming lobbies have a lot of political power, especially in rural districts and no politician wants to be painted as anti-farming interests.
Unlike cities, farmers go where the water is. I know some HNers think farmers are growing crops in deserts. But that's just laughable. They farm where there is water: rivers, lakes, flood planes, deltas, etc. It is the cities diverting the water from these places.
> They farm where there is water: rivers, lakes, flood planes, deltas, etc. It is the cities diverting the water from these places.
What do you call a place where the lakes, rivers, flood planes, and deltas are dried up? That's right. A desert. If current trends continue that's exactly what you're going to have.