Can evaporation from urban impervious surfaces be ignored?

Urban evapotranspiration is an important component linking the urban hydrological cycle and energy balance. Previous studies on urban evapotranspiration (ET) mainly focused on evaporation from the soil, vegetation, and water surfaces. However, the urban Ei process was overlooked mainly due to its low amount. In this study, the urban Ei was determined by eddy correlation (EC) observations combined with a flux footprint model and urban land use information for two urban EC sites. The evaluation of flux data quality indicated that more than 76 % of observed flux data can be used for Ei analysis at the two stations during rainy periods. The daily urban Ei exhibited high intermittent temporal patterns. The Ei value was 0 during the non-rainy period and peaked on the following 1-2 days after the rainy event, then gradually decreased to 0. The average daily Ei accounted for 11-14 % of the total ET for the two EC stations during the rainy period. Further analysis indicated that urban Ei significantly altered the urban energy balance and turbulent transport processes. The urban Ei reduced sensible heat flux (H) and Bowen ratio (BR) during the rainfall period, thereby playing a vital role in mitigating urban heat land effect. Moreover, urban Ei improved the turbulent transport efficiency of latent heat flux (rwq) and restored turbulent transport similarity in an urban area. The temporal characteristics of urban Ei exhibited a large discrepancy among different impervious surface materials, which mainly depend on the water retention and thermodynamic properties of the impervious surface. Gravel had the best water retention capability and resulted in the highest spatially averaged Ei, while the concrete surface had the best thermal storage capability and resulted in the longest duration time of Ei. The spatial average of urban Ei and its duration time were positively correlated with the water retention and heat storage capabilities of impervious surface materials. This study gained insight into urban Ei, including its determination method, temporal characteristics, controlling factors, and impacts of urban Ei on the regional energy balance and turbulent transport process.