The American west has a sprawling network of dams, reservoirs and pipelines that brings a supply of water to its cities and farms. But overexploitation and a two-decade dry spell have put a severe strain on the resources, with reserves dwindling to historic lows in some areas. The situation will only get worse in the coming decades, warn scientists, as surging populations will boost freshwater demand and a hotter, drier climate will bring deeper droughts and more erratic precipitation patterns. …[N]ew water-sourcing approaches are also needed. One such is rainwater harvesting.