Remotely-sensed evapotranspiration for informed urban forest management

With higher evapotranspiration (ET) rates compared to other urban land covers, urban forests can play an important role in stormwater flood reduction. Effective incorporation of urban forests into stormwater planning and green infrastructure design requires methods that can quantify ET across mixed-land use landscapes but with sufficient spatial resolution for parcel-specific rates. We used Landsat-derived ET from 2000 to 2018 to estimate 30-m annual ET rates across the City of Virginia Beach, USA, a large urban landscape with increasing flood concerns. Our objectives were to compare ET rates across land covers and then identify land attributes and models to explain spatial ET variation. Upland and wetland forests had higher ET compared to urban areas, where wetland forest had annual ET rates 3?4 times that of urban and contributed ca. 40% of total landscape ET despite covering only 20% of the area. These quantified ET rates highlight the disproportional role that urban forests may play in stormwater runoff reduction, and can inform scenarios of land use change to prioritize forest conservation efforts. Relationships between ET rates and aggregated, higher resolution land attribute data indicated key drivers, where ET increased with canopy cover and decreased with impervious cover and water table depth. A regression model combining these drivers explained approximately 70% of ET variation, providing means to downscale ET estimates to further guide stormwater planning at finer spatial scales. Our findings emphasize the importance of urban forests as green infrastructure elements and point to potential approaches to better incorporate them in stormwater planning decisions.