Winter · drought & snowpack
Mountain snow is the West's largest natural reservoir, and it is shrinking. This is where we project how much water a basin can count on, through 2050.
Why winter matters
In most Western river basins, snowpack stores more water than human-built reservoirs. About 80% of the Colorado headwaters' supply arrives as snow. But Western snowpack has fallen roughly 18% since 1955 as more precipitation falls as rain and melt comes earlier, shrinking the buffer that mines and every other user draw on.
CAM6 coupled to CLM5, plus CMIP6 SSP370
CAM6 runs coupled to the CLM5 land model, with an 8.6 m soil column, so soil moisture, snow water equivalent, snowmelt timing, precipitation (P) phase, evapotranspiration (ET), and runoff come straight from the model rather than being inferred. CMIP6 SSP370 supplies the long-term forced trend and its spread. Scenarios come from crossing PDO and AMO phases with the forced drying trend: the best case pairs a positive PDO and negative AMO (the wetter-favorable combination for the Southwest) with the weaker end of the forced drying, the worst case pairs a negative PDO and positive AMO with the stronger end, and the mid-range takes neutral phasing at the ensemble median. We separate the Colorado headwaters into a snow-accumulation season (SONDJF) and a snowmelt season (MAM) to track melt timing directly.
Metrics: self-calibrated Palmer Drought Severity Index (scPDSI), standardized soil moisture index (SSI), snow water equivalent, runoff, P minus ET. Models used: CAM6, CMIP6
Extended CAM6 simulations to 2050
The historical runs are extended to 2050 with new sea-surface-temperature-forced ensembles, exploring the Colorado headwaters region. These runs are the raw material behind the trajectories, and are released openly on the Data and Code page.
Note on resolution
CAM6 is a coarse-grid model (1.9° x 2.5°). Results are reported at basin scale with the residual uncertainty that coarse resolution imposes in complex terrain stated plainly.