Significant increase in gray water footprint enhanced the degradation risk of cropland system in China since 1990

Cropland intensification is one of the main ways to achieve food growth; however, it also leads to the degradation of the water quality. The gray water footprint (GWF) is an important index for evaluating the degradation of the water quality caused by the overuse of cropland. Therefore, evaluating the GWF and analyzing the degradation risk caused by the rapid growth of crops are important ways to measure the sustainable intensification of cropland use. In this study, by using agricultural production panel data at the national level from the China National Agricultural Production database and nitrogen, and phosphorus cycle parameter data from previous studies, we adopted the gray water model and gray water stress to evaluate the GWF and related degradation risk of cropland systems in China from 1990 to 2018. We found that the GWF in China increased fourfold from 1990 to 2018. The GWF increase in southern China was generally lower than that in the northern provinces. GWFs of rice, wheat, and maize increased rapidly by 200%, 258%, and 741%, respectively, whereas for soybean, potato, and other cereals, the increases in the GWF were relatively small. GWF intensities of rice, wheat, and maize were higher than those of other crops, ranging from 1.71 × 104 to 5.5 × 104 m3/hm2. Chemical fertilizers were the main source of GWF from 1990 to 2018. Moreover, the degradation risk caused by the GWF significantly increased. Thirty-five percent increased to seventy percent of the provinces had serious gray water stress and were at a severe risk of degradation from 1990 to 2018. To reduce the GWF and related degradation risk in cropland systems, the intensity of cropland use should be reduced, and the spatial distribution of cropland should be improved and optimized.