湿热地区城市乔木蒸腾降温观测及计算模型优化

Observation and Model Optimization for Transpiration Cooling of Urban Arbor in the Hot-Humid Region

作为植被区别于其他地表和景观设计要素的关键,蒸腾是其调节微气候的重要作用途径,而气孔导度是影响蒸腾作用最重要的生理参数.通过秋冬季长时间连续的树干液流和植被微气候参数测试,研究广州典型城市绿化乔木(杧果树,树高9.38 m,胸径0.621 m,冠幅8.60 m)的蒸腾和气孔导度变化特征,检验和优化不同计算模型的性能.结果表明,乔木蒸腾与其显著的降温(叶片与不透水面温差最大-14.6℃)增湿效应相对应.蒸腾速率呈扁平的"单峰型"日变化特征,不同天气下的最大值及排序为晴天(6.7×10^(-5)kg/(m^(2)·s))、多云及阴天(3.6×10^(-5)kg/(m^(2)·s))、小雨天(3.3×10^(-5)kg/(m^(2)·s))、中到大雨天(2.3×10^(-5)kg/(m^(2)·s)),蒸腾速率与太阳辐射和空气水蒸气压亏缺(空气距离饱和状态的程度)成正比,相关性最强.冠层气孔导度的变化趋势与蒸腾速率一致,太阳辐射和空气水蒸气压亏缺分别是冠层气孔导度最重要的促进和抑制因素,二者联合作用,使得晴天、多云天和阴天的冠层气孔导度在0.5×10^(-3)~7.0×10^(-3)m/s之间变化,具备一致性.优化后的Noilhan-Planton模型与Penman-Monteith模型相结合,可准确预测城市乔木在无雨和小雨天(降雨量<3.2 mm)的逐时蒸腾速率(R^(2)=0.80,RMSE=0.9×10^(-5)kg/(m^(2)·s),d=0.94).研究结果为湿热地区的城市植被微气候评估、城市绿化和风景园林设计提供数据支持和重要参考.

Transpiration,as the key to distinguish vegetation from other land surfaces and landscape design elements,takes an important role in microclimate modulation of vegetations,and stomatal conductance is its most important physiological parameter. This study investigated the transpiration and stomatal conductance characteristics of a typical urban arbor( mango tree,9. 38 m high,trunk diameter 0. 621 m,crown diameter 8. 60 m) in Guangzhou through long-term consecutive observations of trunk sap flow and microclimate parameters in the dry seasons of autumn and winter,and tested and optimized the related predictive models. The results show that the arbor tree’s transpiration corresponded well to its significant cooling( with a maximum temperature difference between leaves and impervious surface of-14. 6 ℃) and humidification effects. The transpiration rate presented a flat"single peak"diurnal variation,with the peak values in various sky conditions in the order of sunny( 6. 7 × 10-5 kg/( m2·s)),cloudy and overcast( 3. 6× 10-5 kg/( m2·s)),light rain( 3. 3 ×10-5 kg/( m2·s)),and moderate to heavy rain( 2. 3 ×10-5 kg/( m2·s)) days.The positive and strong correlations were confirmed between transpiration rate and solar radiation and water vapor pressure deficit( VPD). The same diurnal trend was found in the canopy stomatal conductance. Solar radiation and VPD were two important factors for promoting and inhibiting stomatal conductance,respectively,and their combined effects resulted in the consistent canopy stomatal conductance performances( 0. 5 × 10-3-7. 0 × 10-3 m/s) on sunny,cloudy,and overcast days. The optimized Noilhan-Planton model combined with the Penman-Monteith model could accurately predict the hourly transpiration rate of urban arbor in rainless and light rain days( rainfall < 3. 2 mm)( R2= 0. 80,RMSE = 0. 9 × 10-5 kg/( m2·s),d =0. 94).