runoff

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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.

Handicapping the Snowpack Derby

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On the eve of the season’s fourth Sierra snow survey, David Gorn files a report that poses the question: “Is it time for Californians to redefine the term “drought?” His report airs on The California Report Thursday morning and some additional thoughts appear here:

Anybody who’s lived in California for a while has been trained to watch the snow. When the monthly snow surveys come around, we handicap them like they’re the Triple Crown. We all know what’s at stake: when it doesn’t snow, our reservoirs don’t get enough runoff, and we dive deeper into drought.

The snowpack gives us a sneak preview of the coming summer, when our other water sources dry up. The last big snow measurement of the season happens on Thursday. State officials are hoping for the best but preparing for something less than a miracle.

We get about a third of our water from snowpack runoff. But the biggest number in water circles is not the number of inches of snow. It’s the amount of runoff that snowmelt produces. And that can be deceiving, which may explain the caution that always seems to pervade official post-survey pronouncements.

Case in point: Last year at this time, the snowpack measurement was 100% of normal and state officials were breathing easier. And yet the amount of runoff that snow produced last year was only 58 percent of normal, and that’s frighteningly low.

What accounts for the difference? Department of Water Resources meteorologist Elissa Lynn says that the wind can disperse snow, which happened last year (maybe Nevada had a “windfall”). Also, a hot spring can melt snow before its time, resulting in too much runoff being released too early, leaving too little for the summer months.

A deceptively full Stafford Lake reservoir in northern Marin County. Photo by David Gorn.

A deceptively full Stafford Lake reservoir in northern Marin County. Photo by David Gorn.

A little rainy weather can be deceiving, too. Even though some local reservoirs around the state topped off–or nearly so–with the late-season storms of February and March, some of the people served by those same reservoirs may still face rationing this summer. That’s because many communities draw their water from multiple sources, which may include mountain runoff.

Projections this year are for snowpack runoff to clock in at about 70% of normal. That beats than the 52% and 58% of the previous two years but is still cause for concern.