Spatiotemporal variations of agricultural water use efficiency and its response to the Grain to Green Program during 1982–2015 in the Chinese Loess Plateau

The interaction between agricultural water use efficiency (WUEc) and change in vegetation dynamics (mainly crops coverage) was analyzed to assess the ecological effect of the implementation of China's Grain to Green Program (GGP) and to further analyze the impact of the policy on agriculture on a long-term scale from an ecosystem perspective. Remote sensing data (e.g., gross primary productivity, GPP; evapotranspiration, ET; and the normalized difference vegetation index, NDVI) were used to investigate the spatiotemporal characteristics of WUEc in the Loess Plateau during three periods: 1982–2015, 1982–2001, and 2001–2015. The results indicated that, first, the average and the trend of WUEc showed strong spatial heterogeneity over 1982–2015, a period that shows a strip distribution. The average value increased from the northwest to the southeast while the multiyear trend value decreased from the northwest to the southeast, and a positive slope prevailed (p