LB 19-194 - How does water yield respond to the interactions between bark beetle outbreaks and wildfire?

Background/Question/Methods:

Disturbances such as wildfire and insect outbreaks play a key role in structuring ecosystems and watersheds worldwide. However, climate change has intensified disturbance regimes, which can have compounding negative effects on ecosystem processes and services such as water yield and drinking water supply. Water yield is often predicted to increase after vegetation is killed or removed by disturbances due to decreases in evapotranspiration. However, water yield responses to wildfire and insect outbreaks are not always evident in field observations. Furthermore, because wildfires often occur after beetle outbreaks, it is difficult to understand the specific mechanisms driving water yield changes. We applied a novel model framework that couples ecohydrology, fire regimes, and beetle effects (RHESSys-WMFire-Beetle) in two semiarid basins in the western US to assess the effects of forest disturbances on water yield. We prescribed three beetle outbreak scenarios: no mortality, 30% mortality, and 60% mortality, and two fire scenarios, with fire and without fire in a factorial design, resulting in six modeling scenarios.

Results/Conclusions:

We found that the water yield responses depended on the level of vegetation mortality, local aridity, and time since disturbances. Water yield increased when mortality was high but showed no change or decreased when mortality was low. In less water-limited regions, water yield mainly decreased after disturbances, but it sometimes increased in water-limited regions due to higher evapotranspiration of surviving plants. Water yield increased over the short term following disturbances (less than ten years), but sometimes decreased over the long term due to increases in water consumption from younger plants. Moreover, post-fire water yield changes were less obvious if there was a beetle outbreak before the fire than when the fire occurred in isolation. This research can help scientists and land managers isolate the main mechanisms that drive water yield changes after multiple disturbances.