蒸散发与饱和水汽压差及净辐射间滞回现象的机理及应用研究

Hysteresis response of evapotranspiration to vapor pressure deficit (VPD) and net radiation (Rn) have been observed in different ecosystems, indicating the highly nonlinear controls of VPD and Rn on evapotranspiration, however, the mechanisms of ET-VPD and ET-Rn hystereses are yet to be known. The phase angle difference between VPD and Rn, soil hydraulic transport, plant water storage are all considered to be responsible of the hysteresis phenomena, however, discrepancies still exist among different results. The response of stomatal conductance to environmental variables is also supposed to induce these hystereses, but research on the mechanism remains lacking. The uncertainty associated with evapotranspiration simulation in land surface models, is related to the degree to which the model can capture the measured ET-VPD hysteresis, however, research remains rare on reducing evapotranspiration simulation error through incorporating mechanisms of the hysteresis response of evapotranspiration. To address these issues, this proposal combines measurements and models to: 1) Investigate factors and mechanisms controlling ET-VPD and ET-Rn hystereses in different ecosystems; 2) Investigate the role of stomatal conductance in relating ET-VPD and ET-Rn hystereses; 3) Finally assess the causality between hysteresis response of evapotranspiration and its simulation error, and further investigate the method of reducing evapotranspiration simulation error.

蒸散发（ET）与饱和水汽压差（VPD）及净辐射（Rn）关系的滞回现象在不同生态系统内均有发现，说明了VPD和Rn对蒸散发控制作用的高度非线性，然而目前对ET-VPD和ET-Rn滞回现象产生的机理尚不完全清楚。VPD与Rn间的相位差、土壤水力传导、植物体内储水均被认为是产生滞回现象的原因，但研究结果仍存在分歧。气孔导度对环境因子的响应亦被认为是产生这些滞回现象的原因，但对其机理研究仍不足。陆面模型对蒸散发模拟的不确定性，与模型能否准确重现ET-VPD滞回曲线相关，但仍缺乏研究尝试通过考虑滞回曲线机理来降低蒸散发模拟误差。基于此，本项目拟基于野外观测和模型研究，1）确定不同生态系统内控制ET-VPD和ET-Rn滞回曲线的因素及机理；2）研究气孔导度在联系ET-VPD和ET-Rn两类滞回曲线间的作用；3）最后研究蒸散发滞回曲线与蒸散发模拟误差间的关系，并探讨降低蒸散发模拟误差的方法。

蒸散发是水文循环中重要的分量，但当前对蒸散发的模拟精度仍较低。因此本项目从研究蒸散发滞回曲线的机理入手，首先开发了耦合植物根系吸水和储水过程的SPAC模型，以该模型为基础研究发现：1）植物储水过程对蒸散发滞回曲线影响较小；2）通过模型数值试验分析发现，气孔导度对气象因子的响应，对辐射与饱和水汽压差之间的相位差有较大的贡献作用；3）蒸散发滞回曲线的存在，对Penman-Monteith蒸散发模型参数的可识别性具有一定影响，主要表现为在辐射与饱和水汽压差相位差越小的情况下，优化算法对模型参数的优化效果越好，反之亦然。通过该项目的研究，主要揭示了蒸散发与气象因子间滞回关系的机理，这为进一步研究提高蒸散发模拟精度打下基础。