Another reason for optimism about our ability to solve our water messes:

[I]n most regions, the water needed to feed one person decreased even if diets became richer, because of the increase in water use efficiency in food production during the past half-century.

Global Changes and Drivers of the Water Footprint of Food Consumption: A Historical Analysis; Chen Yang and Xuefeng Cui; Water 2014, 6(5), 1435-1452

Thanks to Linus Blomqvist for the pointer.