Water supply discussion needs to focus on challenge of updating infrastructure

Most of us have seen the map, produced by the U.S. Drought Monitor data partnership. It shows the Bay

Area and the entire American Southwest in a chronic state of severe drought.

However, this truly does not depict Marin County. Let me explain why.

First, Mount Tamalpais is a “water factory” with its front row seat to the moist Pacific Ocean's westerly winds. Tam's orographic lift effect raises the Pacific moist air. As the latter rises, it cools and its moisture condenses into rain.

Looking at rainfall data going back to 1951, Marin County (Lake Lagunitas station) averages about 51 inches of rain. If not for Mount Tam, Marin would get about 19 inches of rain on average, which is about what San Francisco gets. Since 1951, Marin, in its driest year (2021), still got over 20 inches of rain — over 1 inch more than San Francisco in an average year.

If not for Mount Tam, the Marin Municipal Water District's existing supply infrastructure could only allow it to cover the needs of a few customers.

San Francisco gets about 4,000 acre-feet per year (AFY) of water supplied by rainfall. In Marin, this would only cover the reservoirs' water evaporation.

MMWD can purchase about 11,000 AFY from Sonoma Water, but that would just meet the mandated water release for Marin's salmon fisheries. None of that water is left for humans.

The former MMWD Board of Directors cited climate change as a primary reason for the 2020-2021 water crisis. But an examination of long-term records does not support this argument.

Marin County's climate is not getting any drier. Marin suffered through a dry period from 1917 to 1950 when the average rainfall was only 40.9 inches vs. 51.8 inches between 1951 and 2021.

If we focus on the 15 driest years since 1917 with rainfall of less than 30 inches, nine out of 34 years during the dry period were among those driest years vs. only six out of the 71 years since 1951.

The proportion of driest years during the dry period at 26.5% is more than three times higher than this same proportion during our more recent period since 1951.

Going back to the 2020-2021 water crisis, it was only surprising that Marin did not incur such a severe crisis earlier. The prior one was nearly half a century ago in 1976-1977. Marin had some drought situations in the 1980s and 1990s, but they were not nearly as severe than the two mentioned ones.

The volatility of Marin's climate and rainfall has not changed much if at all. I'll spare you a discussion on standard-deviation changes over time. The bottom line is that such climate volatility has remained relatively stable since the onset of the historical data.

Is climate change going to turn Marin County into a desert? That is unlikely. Climate change is real, but many areas will actually get more rainfall, not less.

By 2100, the National Oceanic and Atmospheric Administration projects that Marin's average temperature will rise between 2.7 and 5.9 degrees Fahrenheit, depending on greenhouse gas emission scenarios.

Over the same period, it projects that Marin rainfall will actually increase between 4% and 7.5%, with the warmer scenario being associated with the higher rainfall level. Warmer air will bring more moisture from the Pacific that Mount Tam will turn into rain thanks to its orographic lift effect.

We need to stop thinking about our dwindling water supply as a climate change problem. It has to do with a water supply infrastructure that is inadequate to service 192,500 customers when Marin County gets less than 35 inches of rainfall in two consecutive years.