近40年来青藏高原典型高寒草原和湿地蒸散发变化的对比分析

Comparison of Variations in Land Surface Evapotranspiration Between Typical Alpine Steppe and Wetland Ecosystems on the Tibetan Plateau over the Last Four Decades

青藏高原地表蒸散发是决定亚洲水塔水储量变化的关键要素。在快速升温背景下,长时间尺度的青藏高原地表蒸散发如何响应气候变化亟需深入探讨。以青藏高原两种典型高寒生态系统(草原和湿地)为研究对象,以野外观测和互补蒸散发模型为研究手段,利用常规气象资料驱动互补蒸散发模型,应用于青藏高原的典型资料稀缺地区,并就模拟结果进行验证评估,揭示了两种典型高寒生态系统近40年的蒸散发变化特征。结果表明,校正参数后的非线性互补蒸散发模型可较为准确地模拟两种下垫面的蒸散发,亦即该模型在青藏高原资料稀缺区具有较好的应用潜力。1973—2013年,青藏高原典型高寒草原蒸散发呈不显著的增大趋势,而高寒湿地则以2.0 mm/a的速率显著增大。相关分析表明,高寒草原和湿地蒸散发的年际变化主要与水汽压(即空气湿度)有关。阶段性分析发现,1970s至1990s末期,两种生态系统蒸散发皆在波动中逐渐增大;而1997年以后,高寒草原和高寒湿地蒸散发的变化模式表现出明显差异:前者在波动中逐渐减小,后者则持续增大至2000s中期。造成这种差异的原因可归结为高寒湿地受冰川融水的影响,土壤含水量可维持在较高的水平,加之2000s高寒湿地的水汽压和日照时数增大,使得该时段内地表蒸散发仍呈增大之势,亦即上游的冰川融水对下游的湿地蒸散发有重要影响。结果表明,空间距离较近的两种典型高寒生态系统,由于所受水源补给不同,局地蒸散发对气候变化的响应模式可能有较大差异。

Land surface evapotranspiration(ET_(a))is a key element that determines the terrestrial water storage of the Asian Water Tower.However,the long-term variations in ET_(a)and its response to the ongoing climate change remain largely unknown.Here,this study used both in-situ observation and complementary relationship-based(CR)modeling technique to investigate the changes in ET_(a)from typical alpine steppe and alpine wetland on the Tibetan Plateau during the last four decades.The results showed that the CR model was able to accurately simulate ET_(a)once its parameters could be locally calibrated,suggesting that this model has a great potential for understanding the long-term variations in ET_(a)over such a sparsely-instrumented but hydrologically-important region.During 1973-2013,both alpine steppe and alpine wetland showed increasing trends in ET_(a),but such an increase was only significant for the alpine wetland in which ET_(a)increased with a rate of 2.0 mm/a.Further correlation analysis suggested that the changes in ET_(a)over these two ecosystems was primarily controlled by changes in the vapor pressure over the last 40 years.The ET_(a)consistently increased in both alpine ecosystems before the late 1990 s,but their changes in ET_(a)became contrasting after the late 1990 s because ET_(a)decreased significantly over the alpine steppe but continued to increase over the alpine wetland until the mid-2000 s.The main reason for the increase in ET_(a)at the latter ecosystem was the increase in vapor pressure and sunshine hour during this period.Moreover,the soil moisture of the wetland could be replenished from the glacier melting,which could provide enough water for surface evapotranspiration process.This study shows that while the geographical distance is short,the response of ET_(a)to climate change in these two alpine ecosystems might differ obviously because of the different hydrological cycle regimes.