Increased water yield and altered water partitioning follow wildfire in a forested catchment in the western United States

Abstract

As wildfires in much of the western United States increase in size, frequency, and severity, understanding the impact of these fires on water yield from forested headwater basins is essential to successful management of water resources. The current study examines the changes in partitioning of the hydrologic cycle in the Mill Creek Basin that follow the Chippy Creek Fire in Montana, USA, due to alterations to the vegetative regime. The analysis utilizes remote sensing‐based vegetation indices and evapotranspiration, a model‐interpolated precipitation product, and discharge data to assess annual water budgets and vegetative regimes in the Mill Creek Basin. After being almost 90% burned in the Chippy Creek Fire, vegetation in the catchment has shifted from almost exclusively mixed conifer forest to sagebrush scrubs and grasses. This shift in vegetation is accompanied by abrupt shifts in partitioning of the water budget, resulting in an altered ecohydrologic regime. Post‐fire, evapotranspiration decreases annually by 250 mm (46%), and evaporative fraction decreases by 0.53. However, evapotranspiration product biases may overestimate this decrease from pre‐ to post‐fire. This decrease in evapotranspiration results in an annual increase in streamflow of 136 mm, a 21% increase in the runoff ratio, and a 140% increase in water yield. These changes to the water budget are consistent for 10 years post‐fire and show no trend towards pre‐fire values during the study period. Results will help inform planning and management of water resources downstream of forested catchments that have been impacted by wildfire.

Wildfire severity and vegetation recovery drive post‐fire evapotranspiration in a southwestern pine‐oak forest, Arizona, USA

“…Our results differ from earlier studies of the impacts of high‐severity burning on forest ET in the western U.S. In these earlier studies, high fire severity resulted in low plant canopy cover and decreased ET, primarily due to decreased vegetation transpiration (Blount et al., 2020, Dore et al., 2010, Ha et al., 2015, Ma et al., 2020, Montes‐Helu et al., 2009, Poon & Kinoshita, 2018). Vegetation recovery at these high fire severity sites was slow, with persistent low cover and high soil exposure resulting in high levels of soil E but low canopy T and overall ET, sometimes lasting for 15 years or more.…”

Wildfire controls on evapotranspiration in California’s Sierra Nevada

“…The mean bias error in our study was -36 mm/year in unburned sites, and 29 mm/year in burned sites, which could underestimate the ET reduction after wildfires by 65mm/year. These magnitude of bias are smaller than modeled results in Blount et al (2020); Poon and Kinoshita (2018b), which overestimated the ET reduction by approximately 111 mm/year. Quantifying runoff changes associated with reductions in ET is critical for predicting water supply.…”

“…This method has been successfully applied across the Sierra Nevada in previous studies (Goulden et al 2012;Goulden and Bales 2019;Roche et al 2018), but limitations nonetheless exist in predicting ET. The RMSE (108 mm/year) of our ET estimation, was not large compared to the errors in model simulated ET reported by Chen et al (2016) and Blount et al (2020), which varied from 11 to 27 mm/month depending on vegetation types of flux-tower sites. The variations among vegetation type was consistent with Chen et al (2016) in that forest sites (175 mm/year) have larger errors than grassland (45mm/year) and shrubland (50 mm/year), as less of the ET calibration data are from dense forest sites (Fig.…”

Change in fire frequency drives a shift in species composition in native Eucalyptus regnans forests: Implications for overstorey forest structure and transpiration

“…These compound disturbances of drought and fire have the potential to drive widespread disruption to eco‐hydrologic functioning (Mirus et al, 2017), resulting in a shift in eco‐hydrologic regime (Blount et al, 2020). There are growing numbers of studies focused on ecological aspects of species replacements such as changes in climate‐fire‐vegetation interactions, succession pathways, carbon stocks and ecological resilience in response to perturbations.…”