Boulder, Sierra, INVESTIGATOR
In many mountainous regions snowmelt is a primary driver of hydrological and ecological processes. The timing and magnitude of snowmelt influences the partitioning of water into hydrological pathways such as evapotranspiration, overland flow, and ground water recharge. This partitioning dramatically impacts ecosystem services by altering water availability in the root zone during the spring transition and throughout the growing season. To improve knowledge of the relationships between snowmelt timing, hydrological flowpaths, and vegetation response, we analyzed the effect of snowmelt timing on runoff production and vegetation greenness in montane environments of the Western United States. The analysis combines yearly peak NDVI derived from the Global Inventory Modeling and Mapping Studies (GIMMS), MODIS-based NDVI observations, and estimates of snowpack distribution from ground stations and remote sensing. In this regard, we show that the strongest temporal relationships between snowmelt timing and vegetation greenness are obtained for the middle elevation range between 2000 m and 2600 m (R-squared values greater than 0.5) with significantly weaker relationships above this region. This finding suggests a switch from water limitations at the lower elevation to energy limitations at the highest elevations. Complimentary spatial analyses indicate that the timing of snowmelt also shows a statistically significant relationship with the spatial distribution of forest greening for individual years (R-squared ~ 0.3). These results indicate that runoff production from middle elevations may be significantly lower than higher elevations as longer growing seasons and higher potential evapotranspiration at lower elevations maintain soil moisture deficits. These results indicate that the sensitivity of runoff generation, forest greening, and carbon uptake to changes in climate may exhibit tipping points associated with elevation.
Molotch, N.P., Guan, B., and Trujillo, E. (2012): Elevation-dependent controls on snowmelt partitioning and vegetation response inferred from satellite observations (Invited). Fall Meeting, American Geophysical Union, December 2012. Abstract H21B-1179..