三江源蒸散发遥感估算及其时空分布特征研究

蒸散发是地表水热平衡的基本变量，也是衡量植被生长水分适应性的重要指标。针对三江源地面实测资料匮乏的现状，以MODIS系列产品为主要数据源，通过对地表温度—植被指数特征空间法的改进，在日尺度实现了该地区2011～2019年蒸散发的连续遥感估算，并进一步解析其时空变化特征与影响因子，揭示不同土地覆被类型的蒸散发差异，以期为三江源畜牧业可持续发展与生态环境保护提供支撑。对比分析表明：蒸散发的估算结果达到了现有遥感蒸散发产品的精度要求，可用于分析三江源地区蒸散发的时空变化特征。近9年，三江源蒸散发总体呈现先减少后增加趋势，多年平均值为420.04 mm；受海拔与降水控制，蒸散发空间分布异质性明显，从东南向西北逐渐减少；3 194～4 620 m海拔范围内，蒸散发随海拔高度增加呈单峰型变化，站点尺度年蒸散发与降水量之间的相关系数为0.71。虽然不同土地覆被分类系统下蒸散发的统计结果存在差异，但单位面积蒸散发具有林地>灌丛/灌木林>草地/草甸>裸土地/无植被区的明显特征，像元尺度多年平均蒸散发与植被覆盖度的相关系数高达0.77。

Remote Sensing Estimation of Terrestrial Evapotranspiration and Analysis of Its Temporal-spatial Distribution Characteristics over the Three-River Headwater Region

Terrestrial Evapotranspiration （ET）， defined as the sum of water lost to atmosphere from soil through evaporation and plant transpiration， is a primary process driving the energy and water exchange among the atmosphere， hydrosphere and biosphere. Facing a significant warm-wet change in the Three-River Headwater Region （TRHR）， accurate ET information is of great importance for a wide range of applications including water resources management， hydrometeorological predictions and ecological protection. However， due to the complex topography and sparse distribution of ground-based meteorological observations， the accurate estimation of ET over the TRHR is always not easy. The traditional surface temperature-vegetation index triangular/trapezoidal characteristic space was transformed from regional to pixel scale based on land surface energy balance principle， so daily ET over the TRHR from 2011 to 2019 could be retrieved continuously from a series of MODIS （Moderate-resolution Imaging Spectroradiometer） products. Then we analyzed the temporal-spatial distribution characteristics of ET and its influencing factors over the study region with special focus on ET difference over a variety of land cover types. Comparison between our estimation with other remote sensing-based ET products shows that the accuracy of our algorithms has reached a comparable level， which lays a good basis for further analysis. Results show that ET in recent nine years over the whole TRHR decreased first and then increased with annual average value of 420.04 mm. Controlled by altitude and precipitation， the distribution of ET varied significantly in space with the high values in the southeast and low values in the northwest. ET with the elevation between 3 194 m and 4 620 m increased first and then deceased with altitude. The Pearson correlation coefficient （r） between annual precipitation and ET at site scale was 0.71. The ET statistics of natural ecosystems varied with different land use/cover maps， but all statistics show clearly that ET per unit area followed the order： forest land > shrubland > grassland > bare land. The r between vegetation coverage and annual ET was as high as 0.77 at pixel scale.