Temporal and spatial variation and correlativity of water productivity indexes in irrigated land of China

In order to quantify the water utilization in grain production from different perspectives, an analysis is performed on multiple water productivity indexes including the gross inflow water productivity (P_wg), the irrigation water productivity (P_wI), the evapotranspiration water productivity (P_wET) and the generalized water productivity (P_wu). The spatiotemporal variation of the four productivity indexes and the correlations among them for irrigated land in China are examined. Results show that the national values of the four indexes being 0.694, 1.361, 1.314, and 0.860 kg/m³, respectively. Moreover, these water productivity indices had increased between 1998 and 2010 in almost all of the provinces studied, although their spatial distributions remained similar in 1998, 2005, and 2010. The highest and lowest values are found for the provinces in and around Huang-Huai-Hai Plain and for the provinces in northeastern China, northwestern China, and south of the Yangtze River, respectively. The degree of spatial difference decrease as follows: P_wI >P_wET >P_wu > P_wg. We have found significant linear correlations between each pair of indexes, with correlation coefficients greater than 0.950 between P_wu and all other indexes. Despite considerable differences between the four indexes in terms of their applicability and numerical performance, all of them can be used to measure differences in the water productivity of grain crops between provinces.