Collaborative Research: RUI: Recovery trajectories of the hillslope green water cycle after rapidly repeated wildfires

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As temperatures rise and rain and snowfall become more intermittent, wildfire recurrence is expected to increase in forested areas. Although wildfires that re-burn an area after only a short time (10 years) have been unusually common over the last 30 years, the mechanisms governing these ?rapid reburns? are incompletely understood. It may be that decreasing water availability, i.e., an altered hydrological cycle, on forested hillslopes is changing the controls on plant regrowth after fire, and so on subsequent wildfires. However, there are almost no data on the soil water resources that plants need following fires, the spatial and temporal changes in soil moisture in burned areas over multiple years after fires, or moisture feedbacks with vegetation regrowth. This research addresses this critical gap in data on post-reburn hillslope hydrology and ecohydrologic system recovery over time and will also help identify whether reburns might mitigate or worsen the downstream flood and landslide risks that often accompany higher post-fire water flows. The new knowledge generated by this research will be produced in close coordination with federal and state water and forest managers, including a series of research-management workshops to improve research communication with management practices. The project will also engage with the nonprofit community via a volunteer organization related to the field area and with the public via creation of a temporary science exhibit for display at a local museum and also for portable education at local preschools and elementary schools during ?meet-a-scientist? visits and will expand research a opportunities for undergraduate and graduate researchers.This research aims to understand (1) how hillslope hydrological response differ with increasing numbers of natural reburns and with time since last fire; (2) how the patterns and feedbacks between the recovery of hillslope soil moisture and evapotranspiration fluxes are linked to the recovery of vegetation, and how they vary with number of reburns, seed-source distances, and time since fire; and (3) how the gradual change in the hillslope water balance relates to downstream river flow dynamics after a fire or reburn under variable climate influence. This research will test multiple components of an overarching hypothesis that there are both spatio-temporal (correlative) and biophysical (causative), linking soil moisture and soil hydraulic process changes following reburns. In addition, it addresses how the vegetation regrowth provides the fuel load for subsequent fires, and how downstream flows and flood risks recede over time following multiple wildfires. Soil moisture, matric potential, texture, water repellency, infiltration, and hydraulic property measurements, live vegetation, and woody debris will be mapped across six 0.25 ha field sites and monitored over time throughout the study, building on an additional two seasons of preliminary data. Field methods include in situ sensors and data loggers, 3D geophysical surveys of soil moisture, micrometeorological monitoring, and detailed vegetation ecology surveys including tree seedling presence and survival. The six exemplary field sites on the southern flank of Mt. Adams, WA, are of similar ecology, climate, slope, elevation, aspect, distance to unburned forest, and past severity per fire, but differ in the number of past fires (1 to 3 in the last 13 years, and an unburned control) and time since fire. Modeling of evapotranspiration will be conducted for major vegetation and bare land cover classes and applied in a spatially and temporally extensive manner across two relevant watersheds with the aid of remote sensing data and long-term climate, drought, and streamflow time-series. This modeling analysis will seek to better understand the spatiotemporal dynamics of the post-fire hillslope water cycle and its relation to streamflow dynamics after different numbers of wildfire reburns and different recovery intervals after fire.