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December 10, 2014

Rains to provide short-term relief for California drought, says Stanford researcher

Stanford researcher Daniel Swain said the upcoming rainstorms this week – among the largest in recent years – will provide a short-term respite to California's drought, by far the state's most intense drought in the historical record. The rain will be good for ecosystems, salmon runs and reservoirs.

By Paige Miller

People take cover from the rain in San Francisco. Heavy storms are on the way, spelling good news for what has been a historic drought. (Photo: AP Photo/Eric Risberg)

This week's powerful rainstorm will not end California's historic drought, but will provide much-needed relief, a Stanford researcher says.

Daniel Swain, a Stanford doctoral student in the Department of Environmental Earth System Science and author of the California Weather Blog, has some answers. The Stanford News Service recently interviewed him about California's record-breaking drought and the impact of the upcoming rains this week. A fellow in the Rising Environmental Leaders Program in the Stanford Woods Institute for the Environment, Swain has conducted research on the extreme 2013-14 California drought in the context of historical variability.


How much rain are we going to get this week?

It will mostly likely be the biggest storm we have seen in several years, or maybe even in five to 10 years. This is partially because we have had almost no rain in the past few years. The impacts from this storm will be impressive, and will include heavy precipitation and possibly significant flooding in some regions. Also, the drought has killed or weakened many thousands of trees around California, and some of those are going to come down with these strong winds.


How severe is the drought we are in now? Where does it rank in the history of recorded droughts in California?

During the past three years of drought, most of California has accumulated a precipitation deficit equivalent to one to two years of rain. Those are very large numbers, and it's hard to make that back quickly. Even if you do get a particularly wet winter, you'd need to have record rainfall to break the drought, which would bring its own problems, such as flooding.

This is by far California's most intense drought in the historical record when you look at metrics that aggregate temperature and precipitation. In addition to being extremely dry these past few years, 2014 has been California's record-warmest year by a very wide margin. Temperature matters in this calculation because warm conditions dramatically increase rates of evaporation, which simultaneously decreases water availability while increasing demand. This is true for natural ecosystems, agriculture and other human activities.


How much more precipitation do we need to get out of the drought?

This will be the most substantial dose of relief California has received since the start of the drought. It certainly doesn't end the drought, but it's a nice start and will provide some short-term relief. Fire season is essentially over, and landscapes in California are greening – that didn't happen last year until February. Streams are already flowing – or certainly will be after this Thursday. The rain will be good for ecosystems, salmon runs and reservoirs, which were actually dropping until early December. Thanks to the rain, they will be rising uniformly in the short term. But California still has bigger water issues that this storm may not even touch. Over the course of the current drought, we have really overutilized California's groundwater aquifers, and we would need many storms of this magnitude to replenish them.


A federal report released Dec. 8 concluded that natural conditions, not human-caused emissions of greenhouse gases, are the driving force behind California's three-year dry spell. How do these findings correlate with your recent research in the lab of Stanford Associate Professor Noah Diffenbaugh?

To the extent that the study's findings overlap with our research, the results are consistent. We found a strong influence of global warming on the occurrence of a large region of high atmospheric pressure over the Pacific Ocean, which is linked to unusually dry conditions in California. The National Oceanic and Atmospheric Administration study highlights the role of recent Pacific Ocean conditions, combined with natural variations in the atmosphere, in increasing the risk of drought in California. A remaining question is what caused these unusual ocean conditions, which is something we didn't address in our work. It's almost never going to be one or the other – natural conditions or climate change alone – that result in a drought, or any kind of extreme weather event. Sometimes one can have a greater effect over the other. We have always had big swings – from very dry to very wet – but the real question is how climate change is affecting these swings now and in the future.


Why start a blog about the weather? Have you been surprised by the interest in this topic?

I've been really surprised. When I started doing it in 2006, I had a niche audience – probably just a handful of people and me. This week, with the big incoming storm, there are literally hundreds of thousands of people reading it. It was initially a hobby, but it has become a useful tool in initiating broader conversations about environmental issues in California and beyond. It's refreshing to see how interested people seem to be in both weather and climate issues. These conversations have resulted in engagement with an impressively broad audience – from random people with a passing interest in tomorrow's forecast to federal scientists or academics whom I have met in a professional context later on. Before it was essentially just for me, but now it's hopefully serving a broader purpose.



Daniel Swain, School of Earth Sciences: (415) 686-1349,

Clifton B. Parker, Stanford News Service: (650) 725-0224,


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