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

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.