Why Does the SFPUC Celebrate a New (Water) Year in October?

For the SFPUC’s Water Resource Engineers, October is the first month of the year –  the Water Year, that is. Every year on October 1, the engineers reset and start tracking water data starting at zero.  The question is, why? 

Let’s start at the beginning. The SFPUC delivers drinking water to 2.7 million customers in four Bay Area counties. Most of the water comes from surface water that collects in reservoirs from rain or melted snow.  The water is transported from these reservoirs to customers through a complicated system of pipelines and tunnels.

The trick is, how does the SFPUC know how much water will collect in these reservoirs each year? How does the SFPUC know when to move water and when to store it? And most importantly, how does the SFPUC ensure that there will be enough water even if there is a drought? 

SFPUC Hydrology and Water Systems Group. Left to Right: Alexis Dufour, Amod Dhakal and  Matt Moses.

Answering those questions is the job of hydrologists with Hetch Hetchy Water and Power and the Hydrology and Water Systems Group in the SFPUC’s Water Enterprise — Alexis Dufour, Matt Moses, Amod Dhakal, Rebecca Pluche, and Amelia Clark. This team uses complicated models based on historic data, observed rain and snowfall information, and other factors to recommend where, when, and how to move water around the system. Their models can predict how much water will be available for our customers in a given year. They can predict how much water will flow into a reservoir during a rain event, and therefore, how high the water level will become in the reservoir. 

 “Anything that involves short-term and long-term planning on the water supply side for the Bay Area portions of the system, our group is involved,” notes Amod Dhakal.

Alameda watershed. San Antonio Reservoir.

In these times of global climate change and more extreme weather events — be they droughts or deluges – the Water Resource Engineers in the Hydrology and Water Systems Group play a crucial role in planning for a reliable water supply future. Matt Moses compares it to insurance. “We all carry a certain amount of insurance in our personal lives. For this it’s a different kind of insurance. What are our preparations for a likely event like a certain length of drought? We find ourselves preparing for those events.”

Which brings it back to the Water Year. Because the San Francisco Bay Area is similar to a Mediterranean climate, the region does not receive much rain for six to eight months of the year. The wet season starts around October and tends to last until May.  So snowfall that falls in 2018 will not turn into water supply in our reservoir until 2019.  Because the wet season spans two calendar years, SFPUC Water Resource Planners track data in Water Years instead, October 1 to September 30. 

Crystal Springs Reservoir in the Peninsula Watershed.

“If we’re receiving 10 inches of precipitation as snow, but we will not see it as water until 6 months when it melts, it would be cumbersome to track it without the Water Year. We might as well keep it all in the same bucket,” explains Alexis Dufour. 

Other water agencies in California and across the West Coast tend to use this Water Year as well, for the same reasons the SFPUC does. Those blessed with wetter climates do not use it as a tracking tool.   

So, why does the SFPUC use the Water Year? Because it makes sense for its local climate.

When the next October first rolls around again, be sure to wish SFPUC crews a Happy New Water Year. 

The HAWS Group works with staff from Hetch Hetchy Water and Power to model how much snowfall in this watershed will end up as water in Hetch Hetchy Reservoir.