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

Abstract

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 < 0.05). Second, statistically significant positive trends (p < 0.001) in the annual mean WUEc for 1982–2015, with a value of 7.11 × 10−3 g C kg−1 H2O yr−1, indicate that the rate of carbon sequestration in the agricultural ecosystem was faster than that of water loss. Finally, the GGP was conducive to the improvement of WUEc in this arid and semiarid area, with interannual trend values of WUEc from 1982 to 2001 of 4.26 × 10−3 g C kg−1 H2O, and 9.88 × 10−3 g C kg−1 H2O (p < 0.05) after the GGP. Our analyses explored how the changes in WUEc after the GGP were more sensitive to the NDVIc (the subscript c stands for cropland) trends, which means that NDVIc was an important controller influencing GPPc and ETc tendency and thus further influencing WUEc in response to the GGP. This study contributes to a better understanding of the variations of agricultural carbon-water coupling and provides references and theories for the development of water-saving agriculture in an ecological restoration area.