Observed Changes in Mountain Hydrology Following a Mountain Pine Beetle Epidemic in the Snowy Range of Wyoming

Abstract:

A mountain pine beetle epidemic in the Snowy Range Mountains of Wyoming peaked in 2008 coinciding with changes in climate. The combination of the two effects have potentially changed hydrologic response in mountain watersheds. Shorter snowmelt duration and an earlier onset of snowmelt are hypothesized to occur as results of both mountain pine beetle epidemics and global climate change, while beetle effects likely point to increased total flows, baseflows, and peak flows. We used statistical analysis to identify changes in hydrologic response over the past four decades by comparing hydrograph components from 2012–2014 water years to hydrograph components from the 1960’s–1980’s water years using analysis of variance (ANOVA) and analysis of covariance (ANCOVA) including a precipitation covariate. The 2012–2014 group was found to be associated with (1) shorter snowmelt duration, (2) earlier onset of snowmelt, and (3) increased baseflows. No differences in total discharge, snowmelt discharge, stormflow discharge, peak discharge, or day of peak discharge were detected.

Pearson’s correlation coefficients between watershed and runoff characteristics for six mountain watersheds were calculated for the 2013 and 2014 water years. Watershed characteristics include percent green conifers, percent red phase conifers, and percent grey phase conifers derived from a Random Forest land classification map. For the 2013 water year, watershed area expressed as percent red phase conifer was found to be significantly correlated to watershed discharge expressed as percent baseflow with a Pearson’s Correlation Coefficient of +0.95 (alpha level = 0.05). The positive correlation between red phase conifer and baseflow may be considered corroborating evidence of a mountain pine beetle induced change on mountain hydrology detected in the ANOVA/ANCOVA analysis. No significant correlations between beetle phase and either snowmelt duration or onset of snowmelt were detected.