3. IMPACTS OF CLIMATE CHANGE ON MOUNTAIN SNOW 3.1 INTRODUCTION The Modeled Area Defined for The SRM Implementation While Chapter 2 covered the history and potential future of Aspen’s climate in terms of temperature and precipitation, this chapter focuses on how climate change could affect snow conditions. The study team developed and applied snowpack models and determined relationships with key climate variables to analyze how snowpack in the four area ski mountains could be affected under the climate scenarios. Our objectives were to estimate the length of the ski season, the timing of snowpack buildup and melt, the snow depth and coverage at specific times, and the snow quality at different locations and times. Snow accumulation at the base area begins approximately 1 week later by 2030 and anywhere from 1.5 to 4.5 weeks later by 2100. All model runs show a substantial impact on early season snow depths at the top of the mountain. This is because of October snowfall melting off and the precipitation coming as rain rather than snow. Melt at the base area begins 4 to 5 days earlier by 2030 and 2.5 to 5 weeks earlier by 2100. The length of the snow season is about 1.5 weeks shorter by 2030 and 4 to 10 weeks shorter by 2100. By 2100, it is unlikely that a winter snowpack persists at the base area, with the exception of the B1 emissions scenario. In general, a 15% increase in precipitation compensates for a 2.7°F (1.5°C) warming, such that there is little change in snow depth. Snow quality remains high, with less than a 20% increase in the density of the top few inches of snow. 3.2 SNOW MODELS FIGURE 3.1: The modeled spatial extent defined for the SRM implementation. The Snowmelt Runoff Model (SRM), developed and maintained by the U.S. Department of Agriculture, Agricultural Research Service (Martinec, 1975; Martinec et al., 1994; model and documentation available at http:// hydrolab.arsusda.gov/cgi-bin/srmhome) was used as the main model to examine four ski areas in the Aspen area, and the SNTHERM model to obtain detailed information on spatial variability about Aspen Mountain (Jordan, 1991). SRM is focused on surface processes, and is specifically designed to assess snow coverage and snowmelt runoff patterns. The model uses a temperature-index method, which is based on the concept that changes in air temperature provide an index of snowmelt. SRM requires geographic information systems (GIS) information (including a digital elevation model, land use/land cover, and estimates of snow cover) for implementation. Appendix F provides a detailed description of the GIS and remote sensing processes used to generate the topographic, land cover, and estimates of snow cover that Chapter 3 - Impacts of Climate Change on Mountain Snow 31