Though most projects will start too late to address the severe drought now plaguing much of the West, they show how to cope with future ones. Together, these projects will treat polluted and even sewage water, capture rainwater, store water in aquifers, and use (or reuse) all of it, often while mimicking or supporting natural processes. The area’s water administrators who, until recently, thought of watersheds as merely rural concerns now recognize that even in Los Angeles, all living things are linked by their common water course and that its proper management is essential to the administrators’ success.

In the last decade the tenets of sustainable watershed management have spread across the country. The city of Los Angeles still imports 89 percent of its water, a proportion that underlines the severity of its water needs, but dozens of other cities (including some Eastern ones) are embracing pieces of Los Angeles’s water sustainability approach.

San Francisco has become a leader in using recycled wastewater for nonpotable purposes like toilet flushing and gardening while reducing its per capita water use to 46 gallons a day, one of the lowest rates in the nation. San Antonio has developed a multifaceted conservation program that has cut the city’s per capita water use by nearly half over the last two decades. Even Philadelphia, which usually has ample water, committed itself in 2011 to a $1.2 billion green infrastructure program that uses storm water capture to prevent environmental degradation. These efforts share the basic tenet that all the water in any watershed — whether tap water, groundwater or toilet water — must be considered part of a constantly circulating hydrological whole.

Los Angeles gets little rain, and what it does get occasionally arrives in the form of harsh, flood-generating storms, like the ones last week. After numerous destructive floods in the first third of the 20th century, the Army Corps of Engineers and the city’s public works department began building a flood-control infrastructure. It was designed to move storm water quickly off city streets and into the Pacific Ocean. All but seven miles of the 51-mile-long Los Angeles River was turned into an ugly concrete conduit that is usually empty.

Flooding stopped, but at a cost. As the region grew, agriculture gave way to urban development, and more and more land was covered with an impermeable layer of pavement and buildings. This meant that even if a storm produced no more rainfall than one a decade earlier, it generated far more runoff. As the water flowed over the city’s hard surfaces, it collected more and more pollutants — animal waste, car oil, toxic chemicals and metals — and deposited them on the beaches and in the sea. Of course, Los Angeles was also importing huge amounts of water, drying out previously pristine areas far to the city’s north. The water-supply infrastructure imported water while the flood-control system exported it, and both processes ravaged the environment.