Assessing the impact of urbanization on urban evapotranspiration and its components using a novel four-source energy balance model

Urbanization substantially changes many aspects including the regional hydrological cycle, energy balance, and microclimate. However, the degree to which urbanization alters urban evapotranspiration (ET) and its components (soil evaporation (E), vegetation transpiration (T), impervious surface evaporation (I) and water body evaporation (W)) remains unclear. A significant obstacle is the absence of a multi-source energy balance model for an urban area. To solve this issue, a customized four-source energy balance model for urban areas (FSU model) is proposed that differentiates between urban E, T, W, and I. The performance of the FSU model was verified using the eddy correlation (EC), stable hydrogen and oxygen isotope observations in a mega city: Tianjin, China. Long-term urban ET and its composition changes were reconstructed using the Landsat image during the period of 1986-2021 in Tianjin. Trend analyses demonstrate that urban ET, E, and T exhibit significant decreases of trend, while urban W, sensible heat flux (H), and Bowen ratio (BR) exhibit significant increases in trends with urbanization. Urban ET decreased at a rate of 1.41 mm/yr, corresponding to a ~ 13% decrease below the long-term mean value of total urban ET during the period 1986-2021. Correlation analyses revealed a declining trend of urban ET, E, and T primarily caused by urban land use changes, while the increasing trends of urban W, H, and BR were mainly due to the urban microclimate changes. The proposed FSU model aids in assessment of the urban heat island (UHI) effect and facilitates scientific water resources management in urban areas. This research improves the in-depth understanding of the impact of urbanization on urban ET and its components.