By Matt Hildner
San Luis Valley water officials have spent over a decade building on past work to measure snowpack and refine streamflow forecasting. During that span, they’ve added more measuring devices, a new Doppler radar for the region, and the incorporation of an additional stream forecasting model. The aim of those efforts, which are ongoing, is to improve the accuracy of predicting Rio Grande Compact Delivery requirements.
While accurate streamflow forecasts matter across regions for a variety of reasons, the nature of the Compact gives them added importance in Colorado.
The state has an annual delivery obligation under the Compact that varies according to the yield of streamflows. In wet years, Colorado is required to send more water down the Rio Grande and the Conejos River and its tributaries to New Mexico. In dry years, the state keeps a greater share of the flows.
If forecasts overestimate a wet year and the Colorado Division of Water Resources sets a large curtailment on surface water diversions, irrigators are left watching runoff surge past their headgates during early season high flows. Conversely, if forecasts underestimate flows, water users are curtailed late in the season when water is already limited.
Division Engineer Craig Cotten said his office traditionally used streamflow forecasts from the Natural Resources Conservation Service and the National Weather Service. He suspects factors such as fire and beetle kill have impacted the accuracy of the forecasts over the last 15 years.
Coombs, who also manages the Conejos Water Conservancy District, said the model has the ability to take in a greater range of factors that might influence snowpack and, hence, streamflow. “WRF-Hydro contemplates beetle kill, soil moisture, fires — all of that,” he said.
Cotten said if all three forecasts vary widely, settling on one streamflow figure to meet delivery requirements is difficult. If two of the forecasts align and one remains an outlier, his office goes with the former.
“That’s been helpful to have different viewpoints, he said.
Local water managers have also worked to increase the data available to forecasting models.
Prior to 2015, local snowpack had been measured by the 17 Snow Telemetry snow gauges, known as SNOTEL sites, and nine snow courses where NRCS and DWR officials take and record measurements by hand.
By 2015, the CWCD and Coombs had added six snow gauges and five stream gauges to increase the local data available to WRF-Hydro.
The CWCD, the Rio Grande Water Conservation District and the San Luis Valley Water Conservancy District (SLVWCD) then pushed to add radar to the region. Previously, the basin had been covered by radars in Pueblo and Grand Junction.
With catalyzing support from CWCB and added funding from Alamosa County and the Colorado Department of Transportation, both of which wanted the radar for travel management, the $1.1 million radar was installed in 2019. The data from the new radar is used by both the National Weather Service and scientists working on the WRF-Hydro model.
SLVWCD Manager Heather Dutton said the new radar, which sits at San Luis Valley Regional Airport, provides a fuller picture of storms, especially up the Conejos drainage and the low-laying country in the Rio Grande mainstem. “The radar has helped immensely because it allows us to fill in the holes between the SNOTELs, the snow courses, and the added SNOWLITEs,” she said.
Efforts to increase the types and amount of data and forecasting accuracy are not limited to local action. Cotten said he just met forecasters and water managers from outside the state, and he’s hopeful it could lead to enhanced forecasting for the entire Rio Grande Basin.
In November, the U.S. Bureau of Reclamation submitted a report to Congress focused on new snow-measurement technologies that could be deployed over the next five years. The report noted new technologies can aide better water supply forecasts, spur research and development and, in some instances, be used directly by water managers.
In the meantime, water managers in the Valley are looking to increase coverage and density of snowpack measurement data.
The Conejos and SLV conservancy districts plan to add six to eight more SNOLITE gauges, depending on construction costs. While specific sites still need to be nailed down, the south end of the Conejos drainage, the upper Saguache Creek drainage, and areas in the upper Rio Grande above Rio Grande Reservoir and near Snow Mesa are all candidates.
Lastly, water officials across the state are looking for funding for statewide Light Detection and Ranging flights. LiDAR, as the technology is known, uses lasers to measure the topography from the air in the dry season and to provide a snapshot of snowpack during a subsequent winter or spring flight.
“We need everything,” Dutton said. “I don’t think that LIDAR is a silver bullet since it only shows snowpack at a point in time, but it’s part of the picture.”