Numerical experiments to explain multiscale hydrological responses to mountain pine beetle tree mortality in a headwater watershed

Abstract

The effects of mountain pine beetle (MPB)‐induced tree mortality on a headwater hydrologic system were investigated using an integrated physical modeling framework with a high‐resolution computational grid. Simulations of MPB‐affected and unaffected conditions, each with identical atmospheric forcing for a normal water year, were compared at multiple scales to evaluate the effects of scale on MPB‐affected hydrologic systems. Individual locations within the larger model were shown to maintain hillslope‐scale processes affecting snowpack dynamics, total evapotranspiration, and soil moisture that are comparable to several field‐based studies and previous modeling work. Hillslope‐scale analyses also highlight the influence of compensating changes in evapotranspiration and snow processes. Reduced transpiration in the Grey Phase of MPB‐induced tree mortality was offset by increased late‐summer evaporation, while overall snowpack dynamics were more dependent on elevation effects than MPB‐induced tree mortality. At the watershed scale, unaffected areas obscured the magnitude of MPB effects. Annual water yield from the watershed increased during Grey Phase simulations by 11 percent; a difference that would be difficult to diagnose with long‐term gage observations that are complicated by inter‐annual climate variability. The effects on hydrology observed and simulated at the hillslope scale can be further damped at the watershed scale, which spans more life zones and a broader range of landscape properties. These scaling effects may change under extreme conditions, e.g., increased total MPB‐affected area or a water year with above average snowpack.

Climate-driven disturbances in the San Juan River sub-basin of the Colorado River

“…As in other studies, we found that at the size of the basin and the land cover variability can obscure the signal of change (Biederman et al, 2015;Penn et al, 2016). When we consider forested regions only, we are able to understand how and why streamflow is projected to change under the disturbed conditions.…”

“…Streamflow decreases were previously reported for the upper CRB at Lee's Ferry using the Variable Infiltration Capacity (VIC) hydrologic model (Liang et al, 1994(Liang et al, , 1996 forced with future climate change scenarios alone and reported decreased future projected streamflow (Christensen and Lettenmaier, 2002). A separate study reported that the CRB streamflow is highly sensitive to precipitation and temperature shifts, with large reductions in streamflow estimated for small increases in precipitation and temperature (Nash and Gleick, 1991) resulting from enhanced rates of evapotranspiration.…”

“…Recent studies examining climate change and extreme wildfire on runoff erosion found that peak streamflow sediment yield will increase with climate change and fire severity due to the lack of spatial heterogeneity in land cover types (Gould et al, 2016). A study by Penn et al (2016) compared hillslope to watershed-scale responses and found a muted effect on the watershed scale in a headwater basin of the Colorado Rocky Mountains.…”

“…Climate impact studies from the southwestern USA highlight the strong influence of changing temperature and precipitation on forest distributions (Dale et al, 2001), forest mortality , and streamflow (McCabe and Wolock, 2007;Nash and Gleick, 1991). Specifically, the strong interaction between forests and water (Anderson et al, 1976) means that forest disturbances will have a large impact on water volume and the timing of streamflow.…”

Forest Disturbance Feedbacks From Bedrock to Atmosphere Using Coupled Hydrometeorological Simulations Over the Rocky Mountain Headwaters

“…Recent hydrologic research, however, has shown no identifiable change in water yield in response to infestations (Biederman et al, ). Heightened moisture availability, along with less canopy shading and understory regrowth, serves to increase ground evaporation and partially offset transpiration losses (Penn et al, ), which dampens MPB signal at larger scales (Biederman et al, ; Brown et al, ; Penn et al, ). Other field (Bearup et al, ) and modeling (Bearup et al, ) studies suggest that while overall runoff may remain largely dampened by scaling behavior and compensating factors, groundwater contribution to streamflow significantly increases following outbreak.…”