11 May 2017
Finding could improve water supply forecasting
By Laura Snider
Since the mid-1980s, 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, according to a new study led by the National Center for Atmospheric Research (NCAR).
While this decline was driven in part by the transition from an unusually wet period to an unusually dry period, rising temperatures deepened the trend, the researchers said.
The study paints a detailed picture of how temperature has affected the runoff ratio — the amount of snow and rain that makes it into the river — over time, and the findings could help improve water supply forecasts for the Rio Grande, a water source for an estimated 5 million people.
The study results also suggest runoff ratios in the Upper Rio Grande and other neighboring snow-fed watersheds, such as the Colorado River Basin, could decline further as the climate continues to warm.
“The most important variable for predicting streamflow is how much it has rained or snowed,” said Flavio Lehner, a climate scientist at NCAR in Boulder, Colorado, and lead author of the new study published in Geophysical Research Letters, a journal of the American Geophysical Union. “But when we looked back hundreds of years, we found that temperature has also had an important influence — which is not currently factored into water supply forecasts. We believe that incorporating temperature in future forecasts will increase their accuracy, not only in general but also in the face of climate change.”
Over-predicting water supply
Born in the Rocky Mountains of southern Colorado, the Rio Grande cuts south across New Mexico before hooking east and forming the border between Texas and Mexico. Snow piles up on the peaks surrounding the headwaters throughout the winter, and in spring the snowpack begins to melt and feed the river.
The resulting streamflow is used by farmers and cities, including Albuquerque, New Mexico, and El Paso, Texas. 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.
Current operational streamflow forecasts depend on estimates of the amount of snow and rain that have fallen in the basin, and they assume that a particular amount of precipitation and snowpack will always yield a particular amount of streamflow.
In recent years, those forecasts have tended to over-predict how much water will be available, leading to over-allocation of the river. 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.
The scientists used tree ring-derived streamflow data from outside of the Upper Rio Grande basin to reconstruct estimates of precipitation within the watershed stretching back to 1571.
Then they combined this information 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 converted into streamflow.
They found the runoff ratio varies significantly from year to year and even decade to decade. The biggest factor associated with this variation was precipitation: a smaller percentage of the snowpack becomes streamflow during drier years.
But the scientists also found temperature affected the runoff ratio. Over the last few centuries, the runoff ratio was reduced when temperatures were warmer. They found the low runoff ratios seen in dry years were two and a half to three times more likely when temperatures were also warmer.
“A warmer year can make an already bad situation much worse,” Lehner said.
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.
This new understanding of how temperature influences runoff ratio could help improve water supply forecasts, which do not currently consider whether the upcoming months are expected to be hotter or cooler than average. The authors are now assessing the value of incorporating seasonal temperature forecasts into water supply forecasts to account for these temperature influences.
—Laura Snider is a Senior Science Writer and Public Information Officer at NCAR. This post originally appeared as a press release on the NCAR website.