Observed Changes in Climate and Streamflow in the Upper Rio Grande Basin

Abstract

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 modelbased 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. (KEY TERMS: climate variability/change; runoff; snow hydrology; water supply.) Chavarria, Shaleene B. and David S. Gutzler, 2018. Observed Changes in Climate and Streamflow in the Upper Rio Grande Basin. Journal of the American Water Resources Association (JAWRA) 54(3): 644-659. https://doi.

Changes in Climate and Land Cover Affect Seasonal Streamflow Forecasts in the Rio Grande Headwaters

“…Trend analysis indicated increasing monthly mean streamflow during March ( p = 0.04, slope = 4.5 ft 3 /s/yr) and April ( p = 0.001, slope = 24.2 ft 3 /s/yr); no other months showed statistically significant trends, but the slopes of the time series regressions for June and July were strongly negative at −15.3 and −14.9 ft 3 /s/yr, respectively. These results indicate a shift in the timing of runoff toward earlier in the year, continuing a pattern observed in the URG (Chavarria and Gutzler 2018) and other southwestern U.S. basins (Robles et al 2017), and consistent with a previous analysis of snowmelt timing in Colorado (Clow 2010).…”

“…These conditions, however, are not present in the RGHW. Previous studies have shown that SWE and SCA are changing in the URG due to changing climate (Chavarria and Gutzler 2018). This is illustrated by decreasing linear regression slopes for both SnowModel‐simulated basin peak SWE (−4.3 mm/yr, p < 0.01) and SnowModel‐simulated mean SCA during the melt period April–June (−0.005 SCA/yr or −0.5%/yr, p < 0.01) (Figure 5; Figure ; Sexstone 2020).…”

“…Streamflow at the outlet of the RGHW is measured by the Colorado Division of Water Resources near Del Norte, Colorado as part of the Rio Grande Compact and provided to the U.S. Geological Survey (USGS site ID 08220000; https://doi.org/10.5066/F7P55KJN). There are several trans‐basin ditches and reservoirs in the RGHW, but previous analysis has shown only minor differences between gauged streamflow and naturalized streamflow, that is, streamflow that would occur in the absence of anthropogenic water impoundments, trans‐basin water imports, and water diversions (Blythe and Schmidt 2018; Chavarria and Gutzler 2018), and for the purpose of this study gauged streamflow is referred to as observed streamflow. The typical RGHW hydrograph exhibits a large snowmelt peak between mid‐May and mid‐June.…”

“…In a historical context, the Rio Grande region and southwestern U.S. has experienced long droughts, as observed in paleoclimate reconstructions, that correlate with increased temperatures and decreased precipitation (Woodhouse et al 2010; Woodhouse et al 2012; Lehner et al 2017a). The more recent instrumented record (approximately post‐1950) indicates a susceptibility to warm and dry snow droughts (Dierauer et al 2019), increasing air temperatures, and decreasing fraction of precipitation derived from snow in the URG (Chavarria and Gutzler 2018). There has been a negative trend in annual mean streamflows in the northern branch of the Rio Grande throughout the 20th Century (Blythe and Schmidt 2018), and a severe drought in the 2000s led to water shortages in the URG region (Patterson and Doyle 2019).…”

Strengthened scientific support for the Endangerment Finding for atmospheric greenhouse gases

“…1, middle column). On the Rio Grande, warming temperatures are contributing to reductions in the fraction of precipitation that becomes river flow (141,142).…”