Wildfire burn severity affects postfire shifts in evapotranspiration in subalpine forests

Authors: Mankin, Kyle; Patel, Rhythm

Submitted to: Journal of Natural Resources and Agricultural Ecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/2023
Publication Date: 9/1/2023
Citation: Mankin, K.R., Patel, R.P. 2023. Wildfire burn severity affects postfire shifts in evapotranspiration in subalpine forests. Journal of Natural Resources and Agricultural Ecosystems. 1(1): 1-11. https://doi.org/10.13031/jnrae.15438.
DOI: https://doi.org/10.13031/jnrae.15438

Interpretive Summary: Wildfire in subalpine forests removes canopy vegetation and changes surface soil condition. These impacts cause major changes in hydrologic function in the burned area. Our goal was to see if fire, and particularly burn severity, caused changes that we could measure using fine-resolution actual evapotranspiration (ETa) estimated from satellite imagery. We analyzed prefire and postfire ETa in four wildfire burn areas (2002 Million Fire, CO; 1996 Hondo Fire, NM; 2002 Montoya Fire, NM; 2000 Cerro Grande Fire, NM). Before fire, ETa was higher in areas that would become high-severity burn areas. After fire, ETa dropped dramatically in all burned areas, likely in response to reduced canopy cover and transpiration. The ETa decrease was greater in high-severity burn areas (42-63%) than in low-severity burn areas (24-44%). None of the four burn areas showed recovery, as measured by ETa, even 17 to 23 years after wildfire. This research shows that prefire satellite-based ETa images could be useful for identifying areas where high-severity fire would be likely to occur, and thus serve as a useful tool to target areas for forest management to reduce fire risk. The postfire ETa images could be useful for identifying areas where active forest restoration is needed after wildfire.

Technical Abstract: Ecohydrologic response to fire is complex, with devastating consequences on source-water hydrology. Postfire canopy structure, leaf area, infiltration, and soil-water storage all may act to reduce evapotranspiration (ET), a key hydrologic variable and indicator of ecohydrologic function. This study explored the use of 30-m resolution Landsat-based SSEBop-model estimates of actual ET (ETa) to assess the effects of fire in the Upper Rio Grande Basin (2002 Million Fire, CO; 1996 Hondo Fire, NM; 2002 Montoya Fire, NM; 2000 Cerro Grande Fire, NM). SSEBop ETa data from 1985-2019 were analyzed to better understand postfire response and recovery in several sub-alpine burned areas. Results show step reductions in ETa after fire across all burn severities in all four fire areas. Prior to fire, ETa was generally higher in high-severity burn areas, indicative of higher fuel loads, although high variability indicated the influence of additional factors. All areas had greater response (decreased ETa) to fire in high-severity areas and lesser response as burn severity decreased. The decrease in ETa after high-severity fire ranged from 42% to 63%, compared to after low-severity fire, which ranged from 24% to 44%. None of the four burn areas demonstrated postfire ETa recovery after 17 to 23 years. This study demonstrates clear utility for remote-sensing ETa estimates as a system-level measure of susceptibility to fire, impact of fire and, potentially, recovery after fire. However additional research is needed to account for the covariate effects of other fuel, topographic, weather, and climate factors.