I've criticized the idea of a water-energy-food-land-climate-everything else nexus for some time, but the underlying problem with the concept just hit me: it's an excuse for water managers who do not want to take the blame for their inability to sustainably manage their supplies.
Nexus logic "says" that energy must be managed to reduce the load on water sources. That means, in other words, that energy demand for water is too high.
Fine, but how about reducing the quantity of water demanded from energy users by, e.g., raising its price (administratively) or rationing its quantity (in a market)?* Water managers can do either and get rid of the problem.**
Bottom Line: There is no water-energy nexus. There's only water managers' failure to ration their valuable product.
* "Demand" consists of a "demand curve" that's a function of technology, tastes, substitutes, etc. That demand can be reduced by a change in taste, etc. Quantity demanded -- given that curve -- will fall if the price of water rises (holding tastes, etc. constant).
** With political support, but that would be forthcoming for any manager who says that action is necessary for sustainability.