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Perrot, Danielle O. ¹ ; Molotch, Noah P. ² ; Musselman, Keith N. ³ ; Pugh, Evan T. ⁴

¹ University of Colorado at Boulder
² University of Colorado at Boulder
³ University of California, Los Angeles
⁴ University of Colorado at Boulder

The recent mountain pine beetle epidemic in the Colorado River Basin has resulted in widespread tree mortality in lodgepole pine stands across the Colorado Plateau. The mountain pine beetle (MPB) infests trees over large areas at a fairly rapid rate, resulting in a loss of nearly all canopy biomass within three to four years. One of the most significant issues resulting from this epidemic is the hydrologic impact of changes in vegetation distribution. In particular, the complex interactions between vegetation and snow largely determine the effects of changing vegetation on water yield, as snow represents the dominant input of water into these semi-arid mountain ecosystems. We hypothesize that the affected stands will experience a change in sub-canopy hydrolometeorological fluxes and surface albedo, which will influence snowmelt rates. The result of these impacts on the basin scale hydrology is largely unknown given the complexity of these micro-scale interactions. We have developed a mechanistic approach toward understanding these impacts using distributed hydrologic instrument clusters, hyperspectral snowpack characterization techniques, a detailed distributed snowpack model (SNTHERM), and hemispherical photography. This measurement and modeling approach is able to resolve the spatio-temporal evolution of snowmelt and snowpack characteristics (such as density, grain size, and temperature) at the micro-scale (i.e. < 10 cm) for green, red, and grey phase stages of beetle-related tree mortality. Using SNTHERM, we model the snowpack along a transect between two trees over the course of the melt season (February 28-June 30) under the three stages of mortality to assess changes in snowpack characteristics due to changes in canopy structure. The results of our modeling show that the canopy conditions of red and grey phase stands are associated with earlier dates of isothermal conditions and melt-out. SNTHERM predicts a red phase melt out date 6 days earlier than green phase, and a grey phase melt out date 9 days later than that of green phase. A loss of canopy not only seems to result in greater melt rates in the grey phase stand, but also in more homogenous snowpack morphology across the transect (i.e. spatial variability in density, temperature, and grain size is reduced under conditions of progressed canopy mortality compared to the green phase). With the aid of remotely sensed snow and vegetation information, these results will provide the basis for larger scale simulations of the hydrologic impacts of beetle infestation across the Colorado River Basin.