Effects of mountain pine beetle caused tree mortality on streamflow and streamflow generation mechanisms in Colorado

Abstract

The mountain pine beetle (Dendroctonus ponderosae Hopkins) (MPB), an endemic beetle in Colorado forests, saw dramatic population growth in the 1990's. As a result of this epidemic, the mountain pine beetle killed large tracts of forest as it spread. To evaluate the effects of MPB caused tree mortality on streamflow and streamflow generation mechanisms multiple investigative approaches were taken. In north-central Colorado, 21 watersheds representing minimally to highly affected watershed areas were chosen. Physical watershed characteristics were determined through a geographic information system. Long-term streamflow records for each watershed were assessed for data stationarity and change-points in peak flow, date of peak flow and annual water yield. Peak streamflow, date of peak streamflow and annual water yield all had stationarity. Since data were stationary, change-point analyses were not conducted. Streamflow, groundwater and precipitation samples were collected and analyzed for stable isotope concentrations. Isotopes of 2H and 18O partition source water contributions to streamflow from precipitation as snow or rain and groundwater (as a surrogate for groundwater). Annual δ2H and δ18O isotopic signatures for streamflow and streamflow source waters, as snow, groundwater and rain, were determined and used to partition source water contributions to streamflow for each watershed. In general, during the 2012 water year, source water contributions to streamflow were as follows: snow 60%, groundwater 20% and rain 20%. The correlations between snow, groundwater and rain contributions to streamflow and MPB killed area were not statistically significant at α ≤ 0.05 (psnow = 0.582, pgroundwater = 0.543 and p;rain = 0.897). While Colorado has suffered extensive forest kill since the onset of the MPB epidemic, the results of this study suggest that MPB killed watershed area has little to no effect on peak streamflow, date of peak streamflow, annual water yield or streamflow generation mechanisms.