In news that will likely affect this El Niño year, it turns out that warmer-than-average temperatures in the spring cause more of a reduction in Colorado River flows than was previously known.
That is, a new study led by the University of Arizona Tucson made that finding. This is the first time that a study analyzed the "instrumental historical record" to learn whether there is a corollation between the temperature and that mighty river's water flow. The instrumental historical record is the data from measurements of surface air temperatures and ocean surface temperatures taken over time.
"Forecasts of stream flow are largely based on precipitation," Connie Woodhouse, geographer and dendrochronologist (tree-ring scientist) and study lead author, said. "What we're seeing since the 1980s is that temperature plays a larger role in stream flow and in exacerbating drought."
The upper Colorado River gains most of its force and water from snowpack. But temperatures as snow melts and runs off in March through July can significantly affect how much water drains into the river. In the study, researchers looked at precipitation amounts in the cool season, temperature records, and stream flow amounts from 1906 to 2012.
"In certain years, temperature became a very strong influence. It was a bit of a surprise," said Woodhouse. "If we have a warmer spring, we anticipate that the river flows will be less relative to the amount of snowpack."
The Colorado River is a major water source for much of the West, including seven states and parts of Mexico. Its water is used for agriculture and for cities. In the United States, major cities using Colorado River water include Albuquerque, Denver, Salt Lake City, Los Angeles, Phoenix, Tucson and San Diego.
In their research, the team identified six droughts between 1906 and 2012 in the Upper Colorado River Basin. These were defined as having below-average amount of streamflow with no more than a year of flow that was normal or above average. Those periods were in the years 1931-1940, 1950-1956, 1959-1969, 1972-1977, 1988-1996 and 2000-2012.
The researchers also sourced winter precipitation records and info on temperatures from March to July historically from a database of chronological data for locations all over the U.S., which goes back over 100 years.
Another finding was that the soil moisture level of November, at the start of the winter season, had very little effect on how much water went into the streams. From drought to drought, winter precipitation and the average temperatures in the runoff season varied.
"The 1950s was the driest period, but also the coolest," said Woodhouse. "In contrast, the most recent drought of 2000 to 2012 was the warmest, but only moderately dry."
In the period of the 1980s and since then, average temperatures in the Upper Colorado River Basin during the season of runoff have been increasing.
"If we have a warmer spring, we can anticipate that the flows will be less relative to the amount of snowpack," said Woodhouse. "What we're seeing is not just the future -- it's actually now. That's not something I say lightly."
The findings mirrored existing climate-model predictions.
The study was recently published in Geophysical Research Letters.
