流域蒸散耗水率对气候和下垫面变异响应关系的稳定性研究

Time stability in response of evapotranspiration ratio to variation in climate and watershed surface characteristics

气候条件的变异和流域下垫面特征的改变是影响流域蒸散耗水的重要因素。本文聚焦于19002008年间全球83个典型流域数据,基于Budyko水热耦合平衡方程,探究100多年间不同条件下流域蒸散耗水率(AET/P)对气候和下垫面特征变异响应关系的稳定性。结果表明:①从长时间尺度看,大部分流域蒸散耗水率与气候干燥指数(PET/P)和流域特征参数(n值)变异的响应关系呈现较强的时间稳定性。从短时间尺度而言,半湿润流域内蒸散耗水率对干燥指数的响应系数∂(AET/P)/∂(PET/P)在20世纪内持续降低。不同气候条件下蒸散耗水率对流域特征参数的响应系数∂(AET/P)/∂(n)的变化差异显著。分不同下垫面特征来看,低n值(n<2)流域内AET/P对n值的变化更为敏感;②气候条件(PET/P)是大多数湿润区内蒸散耗水率的主导因素,在干旱与半干旱流域内,下垫面特征参数(n值)对AET/P贡献最大。在湿润区内,PET/P对AET/P的贡献程度随时间小幅提升;半湿润区内PET/P对AET/P的贡献度呈下降趋势。在低n值(n<2;流域持水能力较弱)流域内,n值对AET/P的贡献更多。在高n值(n>2;流域持水能力较强)流域内,PET/P是蒸散耗水主导因素,且时间演变趋势较为稳定。基于2个变量对蒸散耗水率的相对贡献率,将83个典型流域划分为下垫面主导区、气候-下垫面共同作用区、气候主导区;③蒸散敏感性及其相关变量在19101920、19201930、19301940、20002008年等时间节点出现较明显的变异。

Climate variability and changes in watershed surface characteristics are strongly affecting the watershed evapotranspiration associated with the water loss.Based on Budyko-based coupled water-energy balance model,this paper focused on the evapotranspiration ratio(i.e.,the ratio of actual evapotranspiration to precipitation)in 83 typical basins worldwide during 1900-2008,exploring the time stability and scale effects in the response of evapotranspiration ratio to variation in climate and watershed surface characteristics.By analyzing relative contributions of each variable to evapotranspiration ratio at different time stages,time stability or variability of evapotranspiration response mode and its influencing mechanism were examined,respectively.The results indicated that:(1)At a long-term scale,there was a strong time stability in the response of evapotranspiration ratio to variation in aridity index(i.e.,the ratio of potential evapotranspiration to precipitation)as well as watershed surface characteristics parameters(the n value)over most watersheds worldwide.Yet,from a perspective of short-term time scale,it was clear that the response coefficient of evapotranspiration ratio to aridity index in semi-humid watershed had been decreasing continuously in the past century.Variations of response coefficient of evapotranspiration ratio to watershed surface characteristics under different climatic conditions were significantly departing.According to various watershed surface characteristics of the basins,AETs in low-n basins(i.e.,whose n is lower than 2)were more sensitive to the changes of underlying surface conditions.(2)PET/P played a dominant role in evapotranspiration in most humid basins while n contributes most to AET/P in arid and semi-arid basins.Contributions of PET/P to AET/P increased slightly in humid regions while it showed a decline in semi-humid basins.In basins with low n values(n<2),the contribution of n to AET/P was dominant.In high-n basins(n>2),evapotranspiration was dominated by the PET/P.Based on relative contributions of two variables to water loss,83 typical watersheds were divided into watershed-dominant areas,climate&watershed-coefficient areas and climate-dominant areas.(3)Between 1900 and 2008,global evapotranspiration sensitivity and its related variables showed obvious variations in some periods including 1910-1920,1920-1930,1930-1940 and 2000-2008.By seeking some universal laws globally,the results of the study can be helpful to manage water resources in river basins,especially to provide scientific reference for strategy on rational and effective vegetation and ecological restoration in basins with different climates and underlying surface conditions.