By Lauren Lipuma, Contributing Writer, EOS
The unprecedented drought that has gripped the Southwest United States has severely depleted the Sierra Nevada snowpack, the major source of water for drinking and farming in California. Researchers and water managers thought this past winter’s monster El Niño would bring enough rainfall to help ease the strain on water resources, but whether El Niño rains were enough to replenish the dwindling snowpack remained to be seen.
Here Margulis et al. used daily maps of the Sierra Nevada taken from NASA Landsat satellites and snow survey data collected by California’s Department of Water Resources to determine the snowpack’s current volume and predict how much water is available within it. The team also used the satellite images and historical measurements of the snowpack and of past El Niños to estimate the snowpack’s total volume for each year from 1951 to 2015.
The researchers found that this winter’s strong El Niño did not bring enough rain to replenish the snowpack’s depleted stores. In 2015, the water volume of the snowpack was just 2.9 cubic kilometers (0.7 cubic mile), whereas a typical year is about 18.6 cubic kilometers (4.46 cubic miles), according to the study. Accounting for the 4-year snowpack deficit from the 2012–2015 drought, the researchers conclude it will likely take until 2019 for the snowpack to return to predrought levels, even if there are above-average precipitation years.
The team suggest that their method, which provides unprecedented detail and precision, could be useful in characterizing snowpack water in other mountain ranges, including the Andes and the Himalayas. These areas currently have much less on-site monitoring than in the Sierra Nevada.
The larger goal of the research is to build a detailed, continuous picture of the historical snowpack and diagnose the primary factors that cause it to vary. This information can ultimately improve models for predicting how much water will be available from the snowpack in the future, which will inform water management decisions. (Geophysical Research Letters, doi:10.1002/2016GL068520, 2016)
Analysis of the Sierra Nevada (USA) snowpack using a new spatially distributed snow reanalysis data set, in combination with longer term in situ data, indicates that water year 2015 was a truly extreme (dry) year. The range-wide peak snow volume was characterized by a return period of over 600 years (95% confidence interval between 100 and 4400 years) having a strong elevational gradient with a return period at lower elevations over an order of magnitude larger than those at higher elevations. The 2015 conditions, occurring on top of three previous drought years, led to an accumulated (multiyear) snowpack deficit of ~ −22 km3, the highest over the 65 years analyzed. Early estimates based on 1 April snow course data indicate that the snowpack drought deficit will not be overcome in 2016, despite historically strong El Niño conditions. Results based on a probabilistic Monte Carlo simulation show that recovery from the snowpack drought will likely take about 4 years.