青藏工程走廊多年冻土下垫面遥感蒸散发时空异质性研究

The Qinghai-Tibet Engineering Corridor (QTEC) refers to the Qinghai-Tibet Highway and Railway that pass through the permafrost regions in the central Qinghai-Tibet Plateau (QTP). The in-situ observations suggested that permafrost disturbance occurred obviously by the engineering activities, altering the heat conditions of permafrost and the energy-water exchange processes between land and atmosphere, which in turn affect the spatiotemporal variations of regional evapotranspiration. Thus, examing the spatiotemporal variations of evapotranspiration and the effect of permafrost on evapotranspiration along the QTEC is of great importance to elaborate the effects of engineering activities on permafrost dynamics and the surface heating fields over the QTP. In addition, the repeated freezing-thawing cycles in surface soils in permafrost area of QTP can impact the hydrothermal characteristics in soils, resulting in large uncertainties in estimating evapotranspiration using conventional methods. In this study, the meteorological data and hydrothermal observations of active layer will be firstly used to assess and improve the remote sensing data in the QTEC regions for reducing the calculation error of the surface energy balance system (SEBS). Then, the hydrothermal characteristics (e.g. soil moisture, unfrozen water, soil temperature, etc.) of the active layer during freezing and thawing cycles and their effects on evapotranspiration processes will be analyzed. Based on these results, we will establish and improve the forcing dataset and parameterization schemes of SEBS model that are suitable for permafrost regions. Finally, we will analyze spatiotemporal variations of evapotranspiration under different underlying conditions in permafrost regions using the simulated dataset from the revised SEBS model. This project will help to understand the effects of permafrost on evapotranspiration in the context of engineering activities.

青藏工程走廊是青藏公路、铁路等大型工程通过而形成的南北向贯穿多年冻土区的廊道区域。定点观测表明多年冻土受到明显扰动，多年冻土热状况以及地气能水交换发生变化，影响区域蒸散发的时空变化。研究青藏工程走廊多年冻土对蒸散发的影响及其时空变化对理解工程活动对多年冻土影响及青藏高原地表加热场效应都具有重要意义。青藏高原冻土区表层土壤反复冻融，影响到冻土区土壤水热变化特征，导致传统方法的蒸散发估算存在较大误差。本项目拟利用青藏工程走廊多年冻土区的气象观测和活动层水热观测数据评估并提高遥感再分析数据质量，研究冻融过程中活动层水热变化特征（土壤含水量、未冻水含量、土壤温度等），分析土壤水热因素对蒸散发过程的影响，构建适合多年冻土区的遥感蒸散发模型的驱动数据集和参数化方案，提高地表蒸散发反演精度，并分析不同冻土下垫面特征下地表蒸散发的时空变化特征。项目结果将有助于理解工程影响下的多年冻土对蒸散发的影响机制。

青藏工程走廊定点观测表明多年冻土受到明显扰动，多年冻土热状况以及地气能水交换发生变化，影响区域蒸散发的时空变化。研究青藏工程走廊多年冻土区蒸散发的影响因素及其时空变化对理解工程活动对多年冻土影响及青藏高原地表加热场效应都具有重要意义。针对次问题，项目开展以下三个方面的研究：1）结合观测数据评估遥感蒸散发在青藏工程走廊的适应性；2）基于蒸散发遥感模型分析蒸散发变化对气象要素的敏感性和贡献率；3）结合MODIS遥感数据和气象要素利用遥感蒸散发模型分析青藏工程走廊蒸散发的时空变化特征。研究结果显示改进后的遥感蒸散发模型能够较好的估算青藏工程走廊的蒸散发；气象要素的敏感性和贡献分析结果说明气象数据的准确性直接影响蒸散发的模拟的结果；近18年的蒸散发模拟结果显示青藏工程走廊蒸散发呈现显著的下降趋势。这些研究结果有助于理解工程影响下的多年冻土对蒸散发的影响机制。