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

Abstract

The Grain for Green Project (GFGP) in Chinas 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 ( Q mod) based on the water balance equation with three sets of ET products against the observed mean annual streamflow ( Q obs) at seven catchments over the Loess Plateau. The results indicated that Q mod estimated from GET shows the best agreement with Q obs in terms of both spatial ( R 2=0.97, P 0.01) and temporal ( R 2=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.