Formation mechanism and step effect analysis of the crop gray water footprint in rice production

Gray water footprint (GWF) is a simple and efficient indicator to quantify water pollution that has received extensive attention. Previously, an agricultural GWF was estimated by assuming the nitrogen loss rate throughout the crop cycle, while the field chemicals accompanying water movement during crop growth were not considered. In this study, using field observations of water and nitrogen transport in Nanjing, China, the GWF was calculated at the time steps of a day (GWFd), sub-growth period (GWF(s)), and whole crop season (GWF(w)), and its formation mechanism and step effect in rice production were explored. Field drainage and leakage caused nitrogen loss and generated a GWF, with the latter occurring intermittently. The values of GWFd, GWFs, and GWFw from 2015 to 2018 were 0.42, 0.38, and 0.31 m(3)/kg, respectively, and the step effect exists temporally for the rice GWF. The GWF of paddy rice growth decreased as the measurement period increased and the time step was calculated using GWFd as a reference. Herein, it was determined that the GWFw used in previous studies may have greatly underestimated the adverse impact of the GWF on water quality, particularly in humid regions or years. Therefore, it is essential to identify the formation mechanism and appropriately select the time steps for GWF evaluation during rice production. This research provides methodological references for GWF assessment and reduction in crop production systems. (C) 2020 Elsevier B.V. All rights reserved.