Early Runoff More than Theory

This post has been modified based on clarifications by the study’s lead author, which are outlined in her comment, below.

A recent study seems to confirm what many have already surmised: The spring melt from the Sierra snowpack is happening sooner.

To get a handle on the timing of mountain runoff, a team led by Iris Stewart of Santa Clara University pulled together data from 52 stream gauges up and down California. For her study, Stewart says she chose only water courses unaffected by dams and diversions, with at least 20 years of continuous data.

Stewart’s data shows that over the 60 years spanning 1948-2008; 80% of the gauges show the “stream pulse” that accompanies peak runoff, coming consistently sooner in the season–an average of about 10 days sooner, though at least one location had shifted up by more than a month. In fact, combining all of the metrics in the study, Stewart says only one gauge showed a later trend.

The trend seems remarkably consistent. Stewart says that despite a warming trend over the past ten years, she has not seen any acceleration of the trend within that period.

Stewart cautions that there’s more work to do on this and was reluctant to draw broad inferences from the study. Runoff in a particular stream is affected by many factors, including the elevation, slope, aspect (which direction it’s facing), vegetation cover and soil composition. Stewart says further study of these variables will better help identify the most vulnerable streams. But the latest results seem consistent with an earlier study in which Stewart found “earlier runoff on a continental scale.”

Scientists are concerned that as average temperatures rise, California’s mountains will see more rain, less snow–and what snow there is will melt off sooner. Reservoirs can only retain so much runoff at once, so if more of the “frozen reservoir” dissipates earlier in the season, farms and cities stand to be caught short of water before the rains return.

Stewart, an assistant professor at SCU’s Environmental Studies Institute, presented her findings this morning to researchers at the Pacific Climate Workshop (known as PACLIM, the conference does not have a website), a semi-annual gathering of climate scientists doing front-line research around North America. The conference in Pacific Grove is organized by the USGS office in Menlo Park.

Over the course of four days, about 60 researchers will hear findings on the climatic implications for fire, fog, glaciers, the ocean and wildlife, among other topics.

Early Runoff More than Theory 20 April,2009Craig Miller

11 thoughts on “Early Runoff More than Theory”

  1. When can we get a look at the data used in the study. It covers both a cool phase and a warm phase PDO. It will be interesting see if the peak pulse date moved over time, coming earlier during the warm phase and lagging during the cool phase. This is important because we have entered a cool phase PDO. Another issue is timber harvesting in the drainage basin. When was the timber harvested, and did it impact when the pulse came.

  2. I’ve alerted Prof. Stewart to your query. She says she’d like to do some more work on the study before making the raw data widely available–but she may attempt to answer your question when she gets back to campus. The logging issue did come up in the Q&A at the conference and she agreed that this is another variable that should be taken into account.

  3. For Russ and his denialist crowd, the PDO (Pacific Decadal Oscillation), a slight climate variation in the North Pacific region, has become a sort of black box into which they stuff their hopes that anthropogenic climate disruption is just something that those scientists and environmentalists made up.

    It’s a little more plausible that the PDO would have a relationship to something occurring in its local region, but the idea of the PDO as a climate “driver” at any scale is just plain silly.

  4. Thank you for your post and comments. Let me just clarify a couple of points: The reference to “earlier runoff on a continental scale” actually referred to a previous study (Stewart et al., 2005, Journal of Climate) that looked at data from western North America through 2002. The current study focuses on 52 (pared down from the initial 150 based on snowmelt and other data criteria) California gauges. Extending the study period through the water year of 2008, we have not seen an acceleration, but a continuation of streamflow timing trends towards earlier in the year.
    With regards to the high elevation gauges: It is not that high elevations in general appear to be the most vulnerable, but that some of the watersheds that have a relatively high percentage of their area at high elevations are more vulnerable than we would have expected from their elevation and position alone, on par with lower and middle elevation watersheds (some of which show relatively minor or even no response). We are also seeing basin-to-basin differences in timing responses, which cannot be explained by climate or elevation alone. This leads us to hypothesize that watershed characteristics (along with climatic controls) play an important role in identifying the vulnerability of watersheds – which is a main focus of our study. I do agree with the comment by Russell Steele that logging could be an important contributing factor and landcover changes are considered in our work. We will also consider PDO influences on the trend, which were analyzed in detail in the Stewart et al. 2005 paper. At this point the study is in an early phase, I will be happy to share figures and results once we have completed the analysis. At that point we will be also be much better positioned to discuss individual watershed characteristics and responses.

    1. Professor, thanks for making those clarifications. I’ve edited my original post to take those into account and I apologize for any misinterpretations. We look forward to seeing your evolving work.

  5. Mr Bloom,

    I you want to believe that PDO has no influence on the climate that is OK with me, but the data says other wise. To help readers decide, here is some thoughts by Matt Vooro at ICECAP:

    So what went wrong with the AGW science? It seems that despite IPCC claims carbon dioxide does not seem to be a major climate changer. Ocean currents, ocean surface temperatures and ocean heat sinks are. The most significant being the AMO or Atlantic Multivariate Oscillation and PDO or Pacific Decadal Oscillation.

    1994 2007- VERY WARM- AMO AND PDO BOTH WARM [so called global warming period

    Readers can find the graphics and links to sources here.

  6. Yes, thanks, Dr. Stewart. In case you’re new to the world of climate denialism, a more than sufficient whiff of it can be had by following Russ’ link to the ICECAP “paper.”

    The 2005 paper Dr. Stewart refers to is here. It’s an interesting read, although I don’t think Russ will be too happy with it, especially as Dr. Stewart’s current results confirm that the continuing trend toward earlier run-offs is not being driven by the PDO. They do have a relationship, but it would have been surprising indeed to find no connection between a climate index and a climate trend in the same region.

  7. BTW, Russ, you may want to let Matt Vooro know that the AMO is not the Atlantic Multi*variate* Oscillation but rather is the Atlantic Multi*decadal* Oscillation. Far be it from me to suggest that an inability to keep the terminology straight reflects in any way on Mr. Vooro’s scholarship, though. 🙂

    Also, you might want to double-check his work with regard to the current phase of the AMO.

  8. Dear Mr Bloom

    Let’s not attack the messenger and change the subject, the issue is the data. Lets focus on the data, not a mislabeling of the AMO. Is the PDO and AMO real and do they influence our long term climate trends? For the interest of readers I am providing another link here where they can read and decide for themselves if the PDO and AMO are real. Is the data wrong? What is your alternative explanation? Let’s hear your analysis.

    1. Sounds like it’s time to do a story on the PDO. From what the climatologists tell me, the Pacific Decadal Oscillation is something of a misnomer, in that its cycle is thought to be much longer than 10 years. As to its role in climate patterns, a great deal has yet to be learned about it. It was discovered barely a decade ago (1997) but has been a–pardon me–hot topic in climate circles ever since. I can tell you that at the USGS Pacific Climate Workshop (of which I’ve now attended two), it seems to come up in nearly every presentation. According to Jim Johnstone at UC Berkeley (a presenter at this week’s conference), calling it a climate “driver” is probably premature, though there’s little doubt that it plays a role. In any case, it certainly bears more scrutiny.

  9. Craig,

    To help you get started. General Summary from where PDO was discovered here.
    Fisheries scientist Steven Hare coined the term “Pacific Decadal Oscillation” (PDO) in 1996 while researching connections between Alaska salmon production cycles and Pacific climate. PDO has since been described as a long-lived El Niño-like pattern of Pacific climate variability because the two climate oscillations have similar spatial climate fingerprints, but very different temporal behavior. Two main characteristics distinguish PDO from El Niño/Southern Oscillation (ENSO): first, 20th century PDO “events” persisted for 20-to-30 years, while typical ENSO events persisted for 6 to 18 months; second, the climatic fingerprints of the PDO are most visible in the North Pacific/North American sector, while secondary signatures exist in the tropics – the opposite is true for ENSO. Several independent studies find evidence for just two full PDO cycles in the past century: “cool” PDO regimes prevailed from 1890-1924 and again from 1947-1976, while “warm” PDO regimes dominated from 1925-1946 and from 1977 through (at least) the mid-1990’s (Mantua et al. 1997, Minobe 1997). Minobe (1999) has shown that 20th century PDO fluctuations were most energetic in two general periodicities, one from 15-to-25 years, and the other from 50-to-70 years.

    A couple of more points of interest here and here.

Comments are closed.


Craig Miller

Craig is a former KQED Science editor, specializing in weather, climate, water & energy issues, with a little seismology thrown in just to shake things up. Prior to that, he launched and led the station's award-winning multimedia project, Climate Watch. Craig is also an accomplished writer/producer of television documentaries, with a focus on natural resource issues.

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