Wildfire controls on evapotranspiration in California’s Sierra Nevada

Abstract

We used Landsat-based measures of annual evapotranspiration (ET) to explore the effects of wildfires on vegetation water use across California’s Sierra Nevada. Wildfires decreased ET relative to unburned and pre-fire controls, in many areas this reduction persisted for at least 15 years. The ET reduction averaged 265 mm yr−1 (36% of pre-fire ET) during the first year after fire, and 169 mm yr−1 (23%) over the first 15 years after fire. The ET reduction varied with burn severity, pre-fire canopy density, and hydro-topographic environment. In areas burned at low severity the ET reduction in the first year after fire averaged 224 mm yr−1 (31% of pre-fire ET) whereas high severity were reduced a 362 mm yr−1 (50%) for the first year. Forest stands that were denser pre-fire had a larger ET reduction across all burn severities. Evapotranspiration reduction following moderate-to-high-severity burns was greatest at 900–1300 m asl elevation. The combination of pre-fire canopy density and burn severity explained 70% of the spatial variation in first-year ET reduction. Forest restoration and a reintroduction of low-intensity fire have been proposed as management practices to mitigate fire risk and improve ecosystem health. Our findings illustrate that restoration and fire reintroduction may reduce the current total ET by up to 9%, with potential benefits for downstream water supply in a globally important food-producing region.