退耕还林背景下黄土高原蒸散量时空演变特征及归因

受气候变化和人类活动影响,近几十年黄土高原地表环境和水碳通量发生显著变化,对区域水资源和生态系统格局产生深刻影响。基于植被界面过程(VIP)遥感蒸散发模型,对退耕还林(草)工程实施以来黄土高原蒸散量(ET)时空变化格局进行模拟研究,揭示了近20年黄土高原水碳通量时空演变特征及其原因。结果表明:(1) 2000-2019年黄土高原ET总体呈显著上升趋势(P < 0.05),倾向率为3.77 mm/a,其中黄河中游黄土丘陵沟壑区ET增长最为显著,而陕西关中平原东部、宁夏银川平原南部等农业区则呈显著下降趋势,倾向率为-5.68 mm/a;(2) 气候变量和归一化植被指数(NDVI)对ET显著上升区的区域平均贡献分别为14.7%和78.6%(其中人类活动贡献为70.5%),而对ET显著下降区的区域平均贡献分别为-58.4%和-31.5%(其中人类活动贡献为-31.6%),表明人类活动和气候变化分别主导了ET显著上升区和ET显著下降区的蒸散变化;(3) 在气候变化主导区域,气温和降水量分别为能量受限区和水分受限区ET增加的主导气象因子,而气溶胶浓度升高导致的日照时数和地表风速下降对作物碳同化和蒸腾具有显著的抑制作用,成为农业区ET下降的主导气象因子。

Spatio-temporal variation of evapotranspiration and its attribution over the Loess Plateau since the implementation of the Grain for Green Project

Affected by climate change and human activities, the surface environment and water-carbon fluxes on the Loess Plateau have significantly changed in recent decades, profoundly impacting regional water resources and ecosystems. Understanding the spatio-temporal variations of water-carbon fluxes in relation to climate change and human activities is crucial for the sustainability of water resources and ecosystems, especially in such a region strongly influenced by human activities. In this study, the remote sensing-based Vegetation Interface Processes (VIP) model was employed to reproduce spatio-temporal patterns of evapotranspiration (ET) over the Loess Plateau since the implementation of the Grain for Green Project. Validated with eddy covariance fluxes, GRACE Terrestrial Water Storage Anomaly (TWSA) and stream discharge, the model predictions were proved to be reliable. Results showed that the ET increased significantly (P < 0.05) in most parts of the Loess Plateau with a rate of 3.77 mm/a. The spatial pattern of ET trend was basically consistent with that of the Normalized Difference Vegetation Index (NDVI) trend. The most prominent increase of ET was observed in the hilly-gully areas along the middle reaches of the Yellow River, while a pronounced downward tendency was detected in agricultural areas such as the southern Yinchuan Plain of Ningxia Autonomous Region and the eastern Guanzhong Plain of Shaanxi Province, with a rate of -5.68 mm/a. The attribution analysis based on partial least squares regression (PLSR) approach showed that there were significant differences in the contributions of each driving factor to ET changes in different areas. The regional average relative contributions (RC) of climate drivers and NDVI in areas where ET significantly increase were 14.7% and 78.6% (RC of human activities was 70.5%), while those in areas where ET significantly decline were -58.4% and -31.5% (RC of human activities was -31.6%), respectively. It indicated that human activities and climate change dominated ET changes in ET significantly increase and ETsignificantly decrease areas, respectively. In climate-dominated areas, air temperature and precipitation were the dominant meteorological factors for the increase of the ET in energy-limited regions (mainly in middle to high mountain areas) and water-limited regions (mainly in desert grassland areas), respectively. The decline in sunshine duration and wind speed owing to the increase of aerosol concentration notably inhibited crop carbon assimilation and transpiration, accounting for the decrease of ET in agricultural areas. This study highlights the divergent responses of ET and the varying drivers in different regions of the Loess Plateau.