Signs of the western drought

As I traveled through northern California into Oregon last month, evidence of the severe drought lingering in the west was apparent. Late summer and early fall are typically dry periods in California with the grasses having long since turned golden. But there were perhaps other signs of the on-going drought: water levels at Lake Shasta were very low revealing a large band of reddish soil between the reservoir and the treeline; pines in northern California and southern Oregon appeared stressed; normally glaciated Mount Shasta, nearly the highest point in California, had just thin ribbons of snow on its southern slopes.

Low water levels at Shasta Lake (a major reservoir in northern California).

Pines in the vicinity of Mt. Shasta in northern California and
into southern Oregon looked perhaps unusually stressed with
significant amounts of dead needles.

In California the drought is now in its fourth year, and concern finally reached the level where mandatory reductions in water usage by residential consumers were put into place. Other parts of the west are also in drought. Nevada is nearly as dry as California. Oregon and Washington have had abnormally low precipitation this year. The maps of drought conditions for the United States, updated weekly by the U.S.Drought Monitor, show that the western U.S. as a whole is quite dry.

Mt. Shasta with low snow coverage.

One useful metric of drought conditions is the PalmerDrought Severity Index. The index incorporates both precipitation and temperature information to indicate drought intensity. Values ≤ -4 indicate “extreme drought”; and values between -2 and -4 indicate “moderate” or “severe” drought. Trees are also excellent sentinels of overall environmental conditions because their growth patterns integrate the various stressors they experience such as temperature, soil water content and nutrient availability. Thus tree ring data provide excellent annual historical records of environmental conditions.

On-line I located the work of a team from NOAA’s Nation ClimaticData Center that used hundreds of tree ring samples to study long-term changes in drought intensity throughout the U.S. I downloaded and plotted Palmer Index data for the last 500 years in the northern California region. Tree ring data from 26-55 samples cover this time period. I am definitely not an expert on the science of drought, but to me the data suggest that normally there is substantial year-to-year variation in drought conditions in this region. Some longer-term patterns can also be discerned from the time series. For example, from about 1915 to 1935 – a period coinciding with the dust bowl in the Midwestern US – northern California appeared to be in an extended drought.

Palmer Drought Severity Index data reconstructed for northern California from tree ring data over the last 500 years. The 
blue lines indicate annual values; the thicker red line is a ten year running mean. The time series ends in 2003, so the
current drought is not shown on the figure. Data source: NOAA NCDC.

Recent drought status in the continental U.S. as measured by the Palmer Index. Map source.

Only time will tell if California experiences a fifth year of drought. With a strong El Nino currently developing in the eastern Pacific Ocean, chances are that greater than average precipitation will fall in California this winter and some relief will come. It has also been a warmer year than ever, so more of that precipitation may come as rain rather than snow.