UC Merced

The work presented here was motivated by a need to better understand components of the water balance in the Sierra Nevada with respect to a warming climate. I present two case studies preceded by a summary of a climate data set prepared during the course of this work. The first case study examined the implications of climate warming on snowpack storage in the Merced and Tuolumne River watersheds, which encompass Yosemite National Park. The second study focused on estimating the impact of forest treatment and fire on forest evapotranspiration (ET) at the patch to watershed scale in the American and Kings River basins.

Modeled April 1st snowpack storage in the Yosemite area declined 38, 73, and 90% for +2, 4, 6°C dry-year warming scenarios. Seasonal snowpack disappears below 2000 m elevation with 4°C warming in both wet and dry years. Assuming vapor pressure remains constant with increasing temperature resulted in up to 100 mm more late-spring snow water equivalent than the respective constant-relative-humidity scenario with 6°C warming. Reduced snowpack and snow-cover duration will mean less summer melt input to meadow and forest soils, resulting in loss of wetlands and longer forest fire seasons. Other management implications include reduced late-season streamflow for dependent downstream communities and wildlife.

Water-limited forests exhibited smaller changes in the normalized difference vegetation index (NDVI) per respective changes in basal area than less water-limited areas. Intensive thinning projects, which reduced forest density 40-50%, resulted in NDVI reductions of 0.09-0.12 units from control plot values of 0.7-0.8, corresponding to ET reductions of 152-216 mm yr-1. The minimum observed change for treated areas was 0.07 NDVI units from an initial value of 0.7 units, associated with a basal area reduction of 9%. Net ET reduction due to forest fires in 1990-2008 in the American River watershed was about 5 times that in the Kings River (65 vs. 14 million m3 yr-1). In addition to reducing the multiple impacts of catastrophic forest fire, forest thinning and managed fire offer the potential to offset water losses due to increased ET in a warming climate.