通过对城市单株乔木蒸散发特征及其冷岛响应的三维观测和数值分析,定量解析植被冷岛效应的成因.研究结果表明:1)小叶榕各部位冷岛强度绝对值基本上在3.0℃ 以上,其中前部最强能达到-5.19℃,顶部能达到-3.57℃;2)早上,东侧(前部和右部)的叶片蒸散发率先升高,正午顶部蒸散发达最强,西边(左部,后部)的叶片蒸散发较晚达到峰值,夜间各部位的蒸腾量非常微弱,不超过0.05 mm/h;3)城市乔木整体的冷岛效应强度与其蒸散发的相关系数为0.70,显著性低于0.01,蒸散发每升高1 mm/h,整树可降低3.56℃,顶部的蒸散发对顶部的冷岛效应贡献最明显(Spearman相关系数为-0.61),顶部、前部和右部的蒸散发对整树的冷岛效应贡献最大(相关系数绝对值均大于0.60),各部位蒸散发对整树冷岛效应贡献顺序为前部>顶部>右部>左部>后部>底部,即东侧和顶部>西侧>底部.

Mitigation of the urban heat island (UHI) effect by regulating the transpiration rate of urban vegetation is one of the most important aspects for adapting to global warming and urbanization. Accurate estimation of the transpiration rate of urban trees is critical for it. Urban trees are usually geographically dispersed (e.g., street trees) and the only feasible method to measure the transpiration rate is the sap flow method, which is seriously limited owing to its destructiveness and lack of representativeness. To overcome this challenge, this study developed an advanced approach based on the three-temperature (3T) model by extending its energy balance from two-dimensional (2D) to three-dimensional (3D). The advanced approach was thus named as the 3D-3T model and verified by the sap flow method. The results showed that: (1) there was a good agreement between the results of the two methodologies in the daytime, with a determination coefficient (R-2) of 0.88 and root mean square error (RMSE) of 0.01 mm h(-1). This is the first result worldwide to successfully measure the transpiration rate for a large tree without contact or damage. (2) The leaf transpiration rate measured by a LI 6400XT and the plant transpiration transfer coefficient of Ficus concinna from the 3D-3T model also showed a good agreement at nighttime (R-2 = 0.97). (3) The urban F. concinna could consume not only most of the net radiation, but also the sensible heat from the surrounding air, thereby helping to reduce the ambient temperature and mitigate the UHI effect. These results provide a feasible way to accurately estimate the transpiration rate of individual urban trees and may be useful for urban environmental management and urban planning.

参考蒸散发ET0是水文气象研究中关键变量,其在不同海拔区域对气象要素的响应特征不尽相同.基于四川省境内不同海拔区域的38个气象站点的基础数据,利用FAO-56 Penman-Monteith方法计算其1970—2016年共47年逐日的参考蒸散发ET0,通过滑动T检验识别ET0序列存在的突变点,并分析ET0以及相关气象要素在四川省不同海拔的分布特征,采用敏感度分析法以及贡献率法分析气象要素在不同海拔对ET0变化的响应.结果表明:(1)ET0序列随海拔升高呈先增加后减少的趋势,低海拔(低于1500 m)、中海拔(1500～3000 m),高海拔(高于3000 m)ET0年均值分别为745、1001、964 mm·a?1;(2)1998年是序列的突变点,在1970—1998年有显著的下降趋势,而后序列显著增加;突变点前后相对湿度、日照时间和风速趋势发生改变,而日平均温度在1998年前后均保持增加趋势;(3)敏感度分析结果表明,相对湿度对ET0响应最大,日照时间最小;(4)贡献度分析表明,相对湿度、风速和日照时间对于ET0的变化的贡献率在1998年发生转折,而温度对于ET0的变化均为正贡献;(5)在中海拔和低海拔区域,参考蒸散发变化主要驱动要素为风速,而在高海拔则主要为温度及相对湿度.该研究结果将有助于深入了解气象要素对ET0变化的响应,为四川省水资源规划与管理提供科学依据.