Coniferous Forest Change in the Great Salt Lake Basin 1986–2025: Snow Sublimation with Water-Resource Implications

The Great Salt Lake in northern Utah, the USA, is at record-low levels, causing interest in factors that have reduced inflow. One potential factor is the expansion of coniferous forests, which intercept snow that sublimates to the atmosphere rather than reaching streams. We evaluated the increase in coniferous cover in the lake tributary basins and quantified the potential sublimation water loss. In Google Earth Engine we built annual summer–winter Landsat composites (14 features: six reflectance bands and NDVI for each season, 1986–2025) and classified coniferous forest using a 100-member Random Forest ensemble, giving an annual uncertainty envelope with a median run-to-run spread of 4.3%. We assessed trends with the Mann–Kendall test and Sen’s slope and estimated canopy sublimation as a fraction of snowfall. Coniferous cover in the Bear, Weber, and Jordan River tributary basins increased significantly over 1986–2025 (Sen’s slope +27 km2 yr−1, p = 0.00064; trend-based increase 1059 km2, about 35% of the 1986 extent). Applying Utah canopy-sublimation fractions (5–11% of snowfall) and a mean snowfall water equivalent of 0.71 m, the additional forest sublimates an estimated 30,000–67,000 acre-feet per year. While this is roughly 1.4–3.2% of mean annual river inflow, only a portion of the snowfall contributes to river flow, so the actual impact is lower, although loss is linear with snowfall.