#RioGrande: Changes in winter temperature, increases in springtime temperature, and decreased streamflow

Map of the Rio Grande watershed, showing the Rio Chama joining the Rio Grande near Santa Fe. Graphic credit WikiMedia.

From The New Mexico Political Report (Laura Paskus):

For more than a decade, researchers have explained that warming will affect water supplies in the southwestern United States. Now in a new paper, hydrologist Shaleene Chavarria and University of New Mexico Earth and Planetary Sciences Professor David Gutzler show climate change is already affecting the amount of streamflow in the Rio Grande that comes from snowmelt.

“We see big changes in the winter and early spring,” said Chavarria. “Big changes in winter temperature, increases in springtime temperatures and decreases in streamflow.”

The paper, recently published in the Journal of the American Water Resources Association, is based on her graduate work in the Earth and Planetary Sciences Department.

Snowpack is the main driver of the Rio Grande’s flows in the Upper Basin of New Mexico, explained Chavarria, who examined annual and monthly changes in climate variables and streamflow volume in southern Colorado for the years 1958 through 2015.

She found that flows have diminished in March, April and May. And not only is snowpack decreasing, snowpack melts earlier as temperatures continue to rise.

This means flows have increased in the late winter and early spring and decreased later in the season, when farmers need it most for irrigating crops and orchards. The paper points out that as temperatures continue warming and streamflows drop during the growing season, more people will rely on groundwater pumping, further depleting already-stressed aquifers.

“One thing that we that we really need to keep in mind is that conditions in the basin are changing—we know from the paleoclimatic records, and stories from the past—and we have projections of what could happen in the future,” she said. “We need to take those together and we need to prepare because ultimately what we do today and in the coming years is going to affect future generations—mainly our children and our grandchildren, so we need to take care of what we have now.”

Like many in New Mexico, Chavarria’s community—and its forests and watershed—have been affected by climate change…

Her co-author and graduate adviser, David Gutzler, has worked on climate change issues in the southwestern United States for more than 20 years.

The state is now about three degrees warmer than it was during the 1970s, he said, and those changes are seen in extremes. Winters aren’t as cold, and more summer days are very hot, even by New Mexico standards. That warming trend over time is different from variability, or year to year changes.

“New Mexico is a variable climate so for millennia we have seen wet periods and dry periods on the order of decades,” he said. “What’s not so normal, by historical standards, is how warm it’s been.”

The takeaway for policymakers, he said, is that snowpack is becoming a less reliable source of streamflow in New Mexico’s rivers. Even in snowy years, the region’s warmer springs melt the snowpack faster and earlier than in the past.

Here’s the abstract from the paper:

Observed streamflow and climate data are used to test the hypothesis that climate change is already affecting Rio Grande streamflow volume derived from snowmelt runoff in ways consistent with model‐based projections of 21st‐Century streamflow. Annual and monthly changes in streamflow volume and surface climate variables on the Upper Rio Grande, near its headwaters in southern Colorado, are assessed for water years 1958–2015. Results indicate winter and spring season temperatures in the basin have increased significantly, April 1 snow water equivalent (SWE) has decreased by approximately 25%, and streamflow has declined slightly in the April–July snowmelt runoff season. Small increases in precipitation have reduced the impact of declining snowpack on trends in streamflow. Changes in the snowpack–runoff relationship are noticeable in hydrographs of mean monthly streamflow, but are most apparent in the changing ratios of precipitation (rain + snow, and SWE) to streamflow and in the declining fraction of runoff attributable to snowpack or winter precipitation. The observed changes provide observational confirmation for model projections of decreasing runoff attributable to snowpack, and demonstrate the decreasing utility of snowpack for predicting subsequent streamflow on a seasonal basis in the Upper Rio Grande Basin.

Here’s a report about Santa Fe’s preparation for this season from Julie Ann Grimm writing for the Santa Fe Reporter:

Groundwater wells that have mostly been resting on the city’s west side since the construction of a Rio Grande diversion are likely to get put back into action this summer.

Dismal snowpack and low rainfall so far this spring mean water in the river is scarce.

These have long been the plans in Santa Fe, where officials decided in the early 2000s to build a massive infrastructure project to draw water off the river. The Buckman Direct Diversion went online in 2011, becoming a fourth source in the water supply portfolio for Santa Fe’s homes and businesses, along with two well fields and the Rio Grande. Since the reservoirs on the smaller Santa Fe River are also low due to the drought conditions, that leaves the wells.

The city’s policy is to “minimize groundwater use in years when surface water availability is limited—like this year,” reads a memo from water utility staff to diversion board managers late last month explaining the plan.

City Water Division Director Rick Carpenter tells SFR he doesn’t anticipate a supply problem this season.

“Those wells have all been resting and the water levels are coming up. Two examples that I can give are two Buckman wells that have recovered so much that they have gone artesian, which means that the water is coming out of them, we don’t even have to pump them,” he says. “So the aquifer is recovered. That is always what we had hoped for. If we do have to pump the wells all summer, we should be fine.”

Most of the water drawn each year at the Buckman is technically water piped into the Rio Grande over the continental divide from the San Juan River in Colorado to the Chama River in New Mexico. And that—coupled with availability of other water to carry it, and additional water from local reservoirs—has been able to meet the majority of demand in recent years. Not so for this summer.

The memo contemplates two operational scenarios: one in which the diversion is shut down for three months, and one in which it draws just enough water to stay open.

Conservation education and restrictions in the city have led to water use generally declining over the past two decades. Officials say the city would likely need about 10,000 acre-feet this year, and given the surface water shortage, just over half that is expected to now come from the wells. During July and August, more water would be produced from wells than any other source.

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