Snowpack drives greenup in the Sierra

Year to year variation in climate has been extreme in California's Sierra Nevada. In particular, low winter snowpack raises concerns about municipal water supplies as reservoirs are fed by spring meltwater. However, low winter snow cover has implications for the region's forests as well. Less snow (and warmer temperatures) often mean an earlier onset of spring and more extended and dryer summer than would otherwise occur. With climate change, warming temperatures are likely to bring earlier snowmelt and altered seasonal conditions, so it is important that this dynamic is well understood across the landscape.

Snow cover presents a significant challenge for disturbance detection as snow cover masks evergreen vegetation that may be healthy. Where vegetation is a mix of trees and understory, only emerging trees provide a winter NDVI signal in many cases. When compared to just the prior year's baseline condition, snow conditions during that prior year may complicate interpretation. Therefore, longer term baselines are preferred as they capture the maximum NDVI or average NDVI of the site which is more likely to be closer to a snow-free condition.

ForWarn distinguishes most snow cover through a separate processing step which overrides the need for change calculations entirely. Areas with heavy snow cover are shown in pink or lilac.