How does global warming affect flows in the Rio Grande?

New study to help water planners in changing climate

A 2016 Landsat 8 image of the Lower Rio Grande canyons courtesy NASA Earth Observatory.

Staff Report

There are more and more signs that global warming triggered a step-change in many natural systems in the 1980s. A new study, led by scientists with the National Center for Atmospheric Research, tracked a big change in flows in the Rio Grande watershed, a key source of water in New Mexico and Texas.

According to the study, the percentage of precipitation that becomes streamflow in the Upper Rio Grande watershed has fallen more steeply than at any point in at least 445 years.

In another recent study, European researchers showed how major lakes across Central Europe warmed dramatically starting about that same time, and the meltdown of Arctic ice has also accelerated rapidly since then.

In the Rio Grande Basin, the transition was partly caused by a transition from a wet era to a much drier climate regime, but warming temperatures intensified the shift in hydrology. The findings support evidence that snow-fed watersheds across the West could see streamflows decline as temperatures continue to warm.

Just a few months ago, a different set of researchers made similar findings for the Colorado River Basin, concluding that warming temperatures will drive further declines in flows in the decades ahead.

“The most important variable for predicting streamflow is how much it has rained or snowed,” said NCAR scientist Flavio Lehner, lead author of the study. “But when we looked back hundreds of years, we found that temperature has also had an important influence.” Lehner said adding temperatures changes to projections for streamflows will help resource managers plan for the future.

The study, published in the journal Geophysical Research Letters, was funded by the Bureau of Reclamation, Army Corps of Engineers, National Oceanic and Atmospheric Administration (NOAA), and National Science Foundation, which is NCAR’s sponsor.

The Rio Grande’s water is used by  farmers and cities, including Albuquerque, New Mexico, and El Paso, Texas, and water users depend on the annual water supply forecasts to determine who gets how much of the river. The forecast is also used to determine whether additional water needs to be imported from the San Juan River, on the other side of the Continental Divide, or pumped from groundwater.

In recent years, forecasts have tended to over-predict how much water will be available, leading to over-allocation of the river. In an effort to understand this changing dynamic, Lehner and his colleagues investigated how the relationship between precipitation and streamflow, known as the runoff ratio, has evolved over time.

Streamflow data derived from tree-ring climate records outside the Rio Grande watershed helped generate estimates of precipitation going back to 1571. That data was combined with a separate streamflow reconstruction within the basin for the same period. Because these two reconstructions were independent, it allowed the research team to also estimate runoff ratio for each year: the higher the ratio, the greater the share of precipitation that was actually converted into streamflow.

Along with precipitation, the scientists also found that the runoff ratio was reduced when temperatures were warmer. And the influence of temperature strengthened during drier years: When the snowpack was shallow, warm temperatures reduced the runoff ratio more than when the snowpack was deep, further exacerbating drought conditions. The low runoff ratios seen in dry years were two and a half to three times more likely when temperatures were also warmer.

“The effect of temperature on runoff ratio is relatively small compared to precipitation,” Lehner said. “But because its greatest impact is when conditions are dry, a warmer year can make an already bad situation much worse.”

A number of factors may explain the influence of temperature on runoff ratio. When it’s warmer, plants take up more water from the soil and more water can evaporate directly into the air. Additionally, warmer temperatures can lead snow to melt earlier in the season, when the days are shorter and the angle of the sun is lower. This causes the snow to melt more slowly, allowing the meltwater to linger in the soil and giving plants added opportunity to use it.

The extensive reconstruction of historical runoff ratio in the Upper Rio Grande also revealed that the decline in runoff ratio over the last three decades is unprecedented in the historical record. The 1980s were an unusually wet period for the Upper Rio Grande, while the 2000s and 2010s have been unusually dry. Pair that with an increase in temperatures over the same period, and the decline in runoff ratio between 1986 and 2015 was unlike any other stretch of that length in the last 445 years.

Because the existing forecasting models were calibrated on conditions in the late 1980s and 1990s, it’s not surprising that they over-predicted streamflow in the drier period since 2000, Lehner said.

“These statistical models often assume that the climate is stable,” Lehner said. “It’s an assumption that sometimes works, but statistical forecasting techniques will struggle with any strong changes in hydroclimatology from decade to decade, such as the one we have just experienced.”

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