In the News

Is There a Drought in the Sierra?
date posted: December 22 2005

Lake Tahoe Report #95
Dec. 3, 2004
By John Cobourn and Heather Segale

The term “drought” seems incongruous when applied to the Sierra. Yet the Truckee, Carson and Walker Rivers, which receive most of their water from Sierra snowstorms, are in the midst of a multiyear drought. Ranchers and urban residents downstream are hoping this winter’s storms will end this drought. Many are encouraged by the stormy start to the winter, which prompted record-breaking early opening dates at some ski resorts. The news of a mild El Nino event currently in progress also has people excited about the possibility of a wet winter. Yet, a recent forecast from National Oceanic and Atmospheric Administration Climate Prediction Center only predicts wetter than normal conditions south of the Tahoe Basin for sometime between January and May.

Drought is defined differently in different areas. Some scientists define a drought year as any year when the average precipitation for a watershed is less than 70 percent of the average annual precipitation. However, water levels in our engineered reservoirs also play a role in defining whether we are in a multiyear drought. This is because downstream users rely on two types of reservoirs, the frozen snowpack and the water held in dams and lakes.

The Tahoe Basin has experienced below-average precipitation each of the last five years. Only one of those years, 2001, was below 70 percent of average, with 57 percent of average precipitation. However, the lake level this fall was below its natural rim, and more than 6 feet below the top of the storage dam in Tahoe City. Other local reservoirs are also low, so the Truckee River Basin is considered to be in a drought status. In fact, many western states – Arizona, New Mexico, Nevada, Utah, Idaho and Montana – are in the midst of a multiyear drought.

It is ironic then, that this October was actually extremely wet locally and in much of California. The Central Sierra Snow Laboratory in Soda Springs at Donner Summit recorded 48 inches of snow in October, surpassing the previous record of 36 inches set in 2000. Record rains also occurred in many parts of southern California in October. Sacramento, with records dating back to the mid-1800s, reported its fifth wettest October ever.

With these impressive statistics and the heavy snows over the Thanksgiving weekend, it is easy to see how some may think we are on the way to an above average snow year. When Gary Barbato, of the National Weather Service in Reno, looked at the top 10 wettest Octobers at Tahoe City, he found that in five of the 10, the winters went on to receive above-average precipitation. Of the others, two were average, two were below average (60 percent to 89 percent), and one was much below average (below 60 percent). Up at the Snow Lab in Soda Springs, the previous snowiest October, in 2000, led to a disappointing 61 percent of average accumulation for that winter.

So, while we can hope, we cannot safely predict that this winter will put an end to the current drought. Even with ongoing research on weather patterns such as El Nino, La Nina and other regional trends, scientists can only wait and see.

One interesting result of multiyear droughts in the past has been a temporary improvement in Lake Tahoe’s water quality. In the famous seven-year drought from 1987 to 1993, Tahoe had six years in a row with below-average snowpack. If one were to look at the UC Davis Secchi Depth data for only the years 1983 to 1992, it would appear that the water clarity trend in the lake was improving. In fact, the long-term declining trend in water transparency resumed when the normal pattern of stormy winters returned. It could turn out again that our current drought will yield a similarly deceptive short-term improvement in clarity, starting in 1997.

The interconnections of ocean currents, sea temperatures, regional long-term weather patterns and their impacts on water quality in Lake Tahoe are complicated indeed. When one factors in research and predictions about global warming, the picture becomes even more complex. Next week we will examine some predictions about climate change.