淮河流域蒸散发时空变化与归因分析

蒸散发（Evapotranspiration，ET）是联结土壤-植被-大气过程的纽带，对理解地表水热平衡至关重要。因此，量化分析ET时空变化特征、揭示其主要控制因子对区域用水管理和农业生产十分重要。利用遥感数据和气象数据，基于BEPS模型估算了1981-2019年的淮河流域ET，分析了该区域ET时空分布特征，并通过敏感度系数和贡献率方法对该区域的ET多年变化特征进行了归因分析，最后借助数值实验方法深入探究影响特湿润年（2003年）ET较低的主要原因。结果表明：（1）1981-2019年淮河流域多年平均ET为549.83 mm，其中夏季ET占全年ET的比值达到47.63%；1981年以来区域ET整体呈极显著上升趋势（4.41 mm/a，P<0.01）；季节上，除冬季外，其他三个季节的ET增幅均呈显著性增加（P<0.05），四季增幅速率大小依次为：夏季>春季>秋季>冬季；空间上，中东部和南部ET较高，重心模型显示ET高值区域呈显著的由北向南的移动趋势；（2）归因分析结果表明，淮河流域ET对气温变化最敏感，其次为相对湿度、太阳总辐射、叶面积指数（LAI）和降水，但ET对LAI的正敏感性逐渐增强导致LAI的显著升高对流域ET年际变化贡献最大（44.5%），其次是气温的升高（25.93%）；同时，LAI是春、夏、秋三季ET变化的主导因素，气温是冬季ET变化的主导因素；（3）数值实验显示高相对湿度是引起特湿润年（2003年）ET明显偏低的最主要因素，这与导致长时间序列ET变化的原因不同。因此，建议今后加强极端气候条件下ET变化的归因分析，为更有效地应对全球气候变化提供决策服务。研究结果能够为认识淮河流域环境变化对水循环影响及合理分配区域水资源提供科学参考。

Spatiotemporal changes and attribution analysis of evapotranspiration in the Huai River Basin

Evapotranspiration (ET) is a link between soil-vegetation-atmosphere interactions, which is an essential process in understanding both water and energy balances of an ecosystem. Accurate quantitative analysis of its spatiotemporal changes with revealing sound underline forcing factors are of importance to promote the agricultural production and water management of a region. Based on the modified BEPS model, we used the remote sensing data and meteorological data to simulate ET in the Huai River Basin from 1981 to 2019 and analyzed its temporal and spatial changes. We further performed the attributional analysis to quantify its influencing factors with the help of the sensitivity index and contribution rate means. At last, we explored the main reasons for the lower ET in the special humid year of 2003 by numerical experimental methods. Results showed the following:(1) the long-term average annual ET of the Basin was 549.83 mm from 1981 to 2019, with the largest proportion in summer (47.63%). The evolution of the ET was an extremely significant increasing trend (4.41 mm/a, P<0.01) since 1981. Seasonally, the increase rates of ET in the other three seasons except winter exhibited obvious increasing trends (P<0.05). The increase rates of ET in the four seasons, in descending order, was summer, spring, autumn, and winter. Across the Basin, high ET values were observed in the central, eastern and southern regions. The center of gravity model showed that the area with high ET values moved significantly from north to south on the spatial scale. Attribution analysis showed that air temperature appeared the most sensitive driver for ET, followed by relative humidity and total solar radiation, leaf area index (LAI) and precipitation. However, the positive sensitivity of ET to the LAI gradually increased so that the significant increase in LAI contributed most to the interannual variabilities of ET (44.5%) in the Basin, followed by the increase of air temperature (25.93%). Correspondingly, we also found that LAI was the leading factor in ET of the spring, summer and autumn, while air temperature was the most significant variable for the winter ET. Finally, our numerical experiments indicated that the high relative humidity was the most important factor responsible for the Basin''s low ET in 2003- a year with high humid, which is different from the attribution of the long-term ET changes. It is suggested to strengthen the attribution analysis of ET under extreme climate conditions in the future, so as to provide decision-making service for coping with global climate change more effectively. The results could provide scientific guide for understanding environmental influence to water cycle and the rational allocation of regional water resources in the Huai River Basin.