Consequences of declining snow accumulation for semiarid intermountain ecosystem water balance

Background/Question/Methods

The western United States and many temperate regions worldwide are experiencing warmer winter temperatures, declining snowpack, and earlier spring snowmelt, raising concerns about the potential impacts for regional water resources. A topic that has not been addressed with respect to declining snowpack is the effect that the shift from snow to rain will have on ecosystem water balance. Our objective was to evaluate the consequences of declining snowpack on the ecohydrology of sagebrush ecosystems that dominate a large fraction of the western US. We used a soil water simulation model, coupled with a snow simulation module based on SWAT2K, to assess the sensitivity of the temporal and spatial distribution of soil water and subsequent water balance. In particular, we focused on mountain big sagebrush ecosystems and quantified the sensitivity of water cycling to variations in snow accumulation, precipitation seasonality, B1 and A2 climate change scenarios, vegetation phenology, and snowmelt runoff regimes.

Results/Conclusions

Snow, precipitation seasonality, and temperature predictions exerted the greatest influence over the temporal and spatial distribution of soil water and ecosystem water balance. Of secondary importance were vegetation phenology, the interactions between snow and temperature seasonality, and precipitation predictions. Of least importance was snowmelt runoff. In intermountain ecosystems, GCMs predict an average increase in temperature resulting in decreasing snowpack, which may be at least partially offset by an increase in winter precipitation at relatively constant annual means. However, reduced snowpack shifts peak soil water from April to February and summer dry periods, in particular, for deeper soil layers tend to start earlier and become more severe. This decreases the correlation between monthly potential evaporation and soil water potential causing the system to become even more soil water limited during the growing season. This could impact vegetation composition (deep-rooted vs. shallow-rooted species) and productivity drastically. We conclude that snow accumulation dynamics can substantially alter ecosystem water balance of semi-arid intermountain ecosystems and that the magnitude of the impact depends on the combination of local weather, the existing seasonality of precipitation, the ability of vegetation to track change phenologically, and future changes in the amounts and seasonality of both temperature and precipitation.