By Jay Lund and Jeffrey Mount.
Debates over how to manage California’s critically dry water supplies this year have displaced most discussion about water next year.
This year’s drought is bad, but another dry year that begins with even lower groundwater and reservoir levels could be much worse. The state’s reservoir storage is already at near-record lows for this time of year, and accelerated overdraft of groundwater — the state’s most important drought reserves — is likely to limit its availability.
How likely will next water year be dry?
What history tells us about next year
The historical record, imperfect and limited as it is, provides some information on the odds of water conditions improving or worsening.
California’s Department of Water Resources divides all water years (October to September) into five “year-types”: Critically Dry, Dry, Below Normal, Above Normal and Wet. This year the Central Valley is Critically Dry; last year was Dry and the year before that was Below Average.
Table 1 shows the percent of years from the historical record in each category, and the percent of years in each category if the previous year, like this one, was critically dry.
Based on 106 years of record, only 13 percent of years have been Critically Dry. But the odds facing California for next year aren’t as good. In the Sacramento Valley — the state’s largest source of water supply — there’s a 29 percent chance that the 2014-15 water year will also be Critically Dry, and a 64 percent chance that it will be Dry or Critically Dry — not favorable conditions for water management.
In all, there’s a 71 percent chance that next year will be Below Normal or drier and only a 29 percent chance of experiencing an Above Normal or Wet year.
What about El Niño?
The news abounds with hopeful statements about Pacific winds and sea surface temperatures heralding an El Niño. The periodic shift of warm water from the Western to the Eastern Pacific [known as the El Niño Southern Oscillation (ENSO)] is linked to weather extremes over much of the globe.
Meteorologists have long noted that intense El Niño events are commonly associated with high precipitation in Southern California. Though, historically, odds are against improved water conditions next year, an El Niño could end California’s drought.
The relationship between the ENSO index and annual runoff in the Sacramento and San Joaquin river basins since 1950 is plotted in Figure 1. Although ENSO may signal significant weather changes elsewhere in the world, it has little predictive capacity in Northern California where most of the state’s precipitation occurs. (It has better predictive value for Southern California).
Note in Figure 1 that three of the four largest ENSO events are associated with very wet conditions. Two of these – water years 1983 and 1998 – were record-breaking wet years. This seems to offer a glimmer of hope. But the numerous dynamic and statistical modelsthat predict ENSO conditions into the new water year have positive, but disappointingly weak ENSO values. An El Niño may turn out to be closer to La Nada if the projections of these models bear out.
Hope is not a strategy
During a severe drought, water managers and regulators must balance water deliveries in the current year against saving water for unknown conditions in coming years. It is statistically likely the drought will continue into next year. We all hope wet weather returns to California soon. But, given the odds, it makes sense to prepare for another dry year.
Jay Lund is a professor of civil and environmental engineering and director of the Center for Watershed Sciences at UC Davis. Jeffrey Mount, a UC Davis professor emeritus of geology, is a senior fellow at the Public Policy Institute of California.