Effect of mountain pine beetle kill on streamflow generation mechanisms

2016 Summer.Includes bibliographical references.The mountain pine beetle (Dendroctonus ponderosae) is an endemic species to Colorado, but a recent epidemic resulted in the mortality of millions of acres of lodgepole pine forest in Colorado since 2002. This study examined the effect of the mountain pine beetle kill on streamflow generation mechanisms using different tracer methods. Eleven nested watersheds with varying level of beetle-killed forest area (47.1% to 97.4%) were chosen for study. Groundwater, surface water, and precipitation samples were taken and analyzed for stable isotope composition (2H and 18O), specific conductivity, and chloride concentrations. Four methods were employed to partition sources of streamflow, or streamflow generation mechanisms (SGM), in beetle-killed watersheds. Stable isotopes (2H and 18O) were used to determine mean fractional contribution of each source (groundwater, rain, and snow) to streamflow. Rain and snow contribution were negatively correlated with beetle-killed forest area (p=0.08 and p=0.35 respectively). Groundwater was positively correlated with increasing beetle-killed forest area (p=0.23). Specific conductivity and chloride were each used in a 2-component (groundwater and precipitation) hydrograph separation. Using specific conductivity, beetle kill was negatively correlated with average groundwater contribution (ρ = -0.13), but the result was not significant (p = 0.71). Using chloride, the results were correlated (ρ=0.19), but not significant (p = 0.58). Specific conductivity and chloride measurements were then coupled in a 3-component (groundwater, rain, and snow) end member mixing analysis (EMMA). Beetle-killed forest area and fractional groundwater contribution were positively correlated (ρ=0.26), but not significant (p = 0.43). Watershed characteristics were examined to determine potential metrics of groundwater contribution. Mean watershed elevation displayed a significant negative correlation with mean groundwater contribution (p = 0.08)