退耕还林还草工程加剧黄土高原退耕区蒸散发

The Grain for Green Project intensifies evapotranspiration in the revegetation area of the Loess Plateau in China

黄土高原退耕还林(草)工程显著影响着区域植被生长及水文循环过程,对陆地蒸散发(ET)年际变化有着很大影响.本文基于全球通量估算和模型模拟蒸散发产品、格点气候数据及遥感植被特征参量数据,利用趋势分析、逐步回归分析及广义线性模型等方法,研究了退耕还林(草)工程实施前后黄土高原退耕区ET的时空变化特征.研究结果表明:(1)与退耕前相比较,退耕区在退耕还林(草)后ET的升高趋势明显,但其变率存在较大的空间差异,其中,以年降水量在400～600mm区域内变化趋势最大;(2)相较于生长季平均温度和生长季总降水量,退耕区土壤含水量及植被NDVI是ET变化的主要影响因子;(3)退耕后ET变化趋势增大更多地归因于退耕区植被生长的加速,并且退耕后土壤含水量对ET贡献较退耕前降低,而NDVI的贡献度增加.本研究针对黄土高原退耕区在退耕还林(草)工程前后ET的变化趋势进行的探究,为工程实施对该区域水文过程的影响评估提供了科学依据.

The Grain for Green Project(GFGP) in China’s Loess Plateau significantly influences regional vegetation growth and water cycles, especially the inter-annual variability of terrestrial evapotranspiration(ET). In this study, we analyzed the spatial-temporal variations of ET and its driving factors before the period of(1982-1997) and after implementing of the GFGP in the revegetation area of the Loess Plateau during 2001-2015. Firstly, three sets of ET products are collected herein, including global flux product evapotranspiration(FLUXNET ET, FET), model simulation WB-MTE-ET(WET) and GLEAM ET(GET). To evaluate the performance of the ET products in the Loess Plateau, we calculated the mean annual streamflow(Qm od) based on the water balance equation with three sets of ET products against the observed mean annual streamflow(Qo bs) at seven catchments over the Loess Plateau. The results indicated that Q mod estimated from GET shows the best agreement with Qo bs in terms of both spatial(R2=0.97, P<0.01) and temporal(R2=0.94, P<0.01) variations. Thus, we selected GLEAM ET(GET) products in the following analysis. Secondly, we analyzed the trend of annual NDVI and ET in the growing season before and after implementing of the GFGP in the revegetation area using the linear regression. Furthermore, we conducted the stepwise regression analysis to identify the controlling factors that affect the ET changes in the revegetation area during different period. The environmental factors included soil moisture, air temperature, precipitation and averaged Normalized Difference Vegetation Index(NDVI) during the growing season. The results showed that:(1) The inter-annual variability of ET in the revegetation area was significantly increased after implementing of the GFGP compared with that before the GFGP. The inter-annual ET variability also exhibited great spatial differences which showed the highest peak in the regions with annual precipitation ranging from 400 to 600 mm a-1.(2) Compared with the mean air temperature and total precipitation in growing-seasons, the variation of NDVI contributed the most to the changes in ET followed by the soil water content.(3) After implementing of the GFGP, the contribution of the soil water content to ET variation has generally declined, while the contribution of NDVI has increased. This study reveals the influence of the GFGP on the regional water cycle in terms of ET variation, which is of guiding importance to the ecological engineering construction, as well as the rational water resources allocation.