Urban Evapotranspiration In Beijing

With urbanization developing more and more widely in the whole world, human activities are changing the natural underlying surface to urban area, which influences hydrologic cycling as well as evapotranspiration(ET)significantly, the most important process in water cycling. Urban ET is of great scientific significance, and the study of urban ET will contribute to understand the mechanism of urban water cycling and high intensity of water use in urban area. However, the complexity and diversity of urban underlying surface increase the difficulty to study urban ET and some of the effective ET research methods fit for uniform surface can not be used in urban area.Generally, urban ET is considered lower than natural ET because of impervious part in the city, but this opinion is difficult to explain the high intensity of urban water use and where water goes. This study took into account the heterogeneity of urban surface and estimated Beijing ET by three different methods.The first method is based on water balance principle, and the research divided Beijing into subzones as mountain zone, plain zone, suburban zone and urban zone. The study phase is from 2003 to 2012 and the results were for different subzones yearly of the ten years. The second method is remote sensing inversion method(SEBS Model) based on surface energy balance. This research added anthropogenic heat input module into the original SEBS Model to improve it into SEBS_QF model, which was applied to Beijing area. Compared the result of SEBS_QF model with that of the original SEBS model, the modified model is closer to the result of the water balance method and the error decreased from 49% to 2% for urban ET. The third method is to estimate ET by different methods according to underlying surfaces, which is applied to representative urban zone, Tsinghua Campus. The study divided THU campus into water surface, grass surface, impervious surface, semi permeable surface and built surface, and then calculated different ETs to estimate the total ET of THU campus. The last two methods were only for 2011.The ET results of different methods match each other very well. According to water balance method, the average result of Beijing ET is 517mm/a 2003-2012, which is close to average precipitation. This indicates that water resource in Beijing area has a limited contribution to the Haihe river basin. The result of SEBS_QF model is 528mm/a for Beijing ET in 2011, which is quite close to 521mm/a, Beijing ET in 2011 of water balance method. These two method have the same result when comparing different zone ETs: urban ET>suburban ET>mountain ET. Results of the last two methods for THU campus ET are 683mm/a and 681mm/a, representatively, and they are much larger than the average ET in Beijing, 521mm/a. This indicates that ET of urban surface is higher than natural surface, which is contrary to the general opinion. The reason why this difference comes may be that this research took comprehensive consideration into anthropogenic heat, building interior ET and pipeline leakage. And this result seems more realistic.