Hetch Hetchy Haiku and Heteroskedastic Hydrology

First things first. Restore Hetch Hetchy's Haiku Contest ends July 15. There is still time.

We have received entries from new friends, as well as from long-time members. Here is an example sent by an old friend, originally from bonny Scotland: 

Imagine yourself in

Hetch Hetchy, standing waist deep

in grass and flowers.

Entering is easy and fun. Tell your friends. Simply email your haiku to haiku@hetchhetchy.org. More information is below.


Now about that heteroskedastic hydrology. Say it our loud: het - er - o - ske - das - tic. It is a fun word.

I am going to avoid the dictionary definition of "heteroskedastic", but will simply explain what it means in terms of the Tuolumne and other rivers in California.

We all know that we had some decent storms in California this year, much better than we had between 2012 and 2015, but we did not have the "Godzilla El Niño" that had been forecast. The 2016 water year ends September 30, but the Sierra snow has largely melted so we have a pretty good idea what the annual runoff will be.

So, how wet a year is 2016? For the Tuolumne River, the answer is that water year 2016 is wetter than usual, but drier than average. The world "usual", in this context, means the same as the term "median" - something you may have learned in the fifth grade. Half the values fall above the median, half fall below. 

The fact that "usual/median" and "average" are different is what makes hydrology "heteroskedastic". A few really wet years tend to skew the data, increasing the average more so than they increase the actual number of wetter years. As a result, in hydrology, averages tend to be higher than medians. The same is true of the number of runs scored in a baseball game.

Figure 1 below compares Tuolumne River flows in 2016 with median and average values, as well as the three driest years on record and the single wettest year. Figure 1 also shows how the river's flow is distributed between San Francisco and the Turlock and Modesto Irrigation Districts. Due to San Francisco's "junior" water rights, the Districts get almost all the flow in the driest years.


Most importantly for our friends in San Francisco, the City's share of the Tuolumne River in 2016 will be well in excess of its diversions for use in the Bay Area. So San Francisco will be able to substantially increase its levels of stored water in Hetch Hetchy, Cherry, Eleanor and Don Pedro Reservoirs, rather than draw those storage levels down. When Hetch Hetchy Valley is restored, its storage function will need to be replaced - a system improvement that is very doable.

While rivers in northern California tend to have flows higher than both average and median levels in 2016, rivers south of the Tuolumne are not faring so well. See Figure 2.  


Note also, that while some surface reservoir systems have fully recovered in 2016, groundwater levels have not. California has a long-term problem finding enough water to meet the needs of its cities, its farms and its environment. 

San Francisco's needs, however, can be met by capturing river flows downstream of Yosemite. Not a drop of supply need be lost. Restoration is therefore a separate issue from the challenge of balancing water supply and demand in California.

Please pardon the alliteration in this post. But if former Vice President Hubert Horatio Humphrey were alive today, we would surely be seeking his endorsement.

It is time to return Hetch Hetchy to the American people.