Assessment of grey water footprint in paddy rice cultivation: Effects of field water management policies

Excessive use of fertilizers and manure has detrimentally affected surface and ground water resources due to high rates of nitrogen (N) deposition. Reducing the nitrogen-related grey water footprint (GWF) is crucial, otherwise it may lead to severe food production-related water pollution and a reduction in freshwater absorption capacity. This study explored the feasibility of farmland water management policies (WMPs) for GWF regulation in paddy rice fields. Four WMPs-frequent shallow irrigation (FSI), wet and shallow irrigation (WSI), controlled irrigation (CI), and rain-catching and controlled irrigation (RCCI)-were analyzed based on daily field water balance and nitrogen transportation in Nanjing, China. The results showed that GWF varied among the four WMPs. During 2015-2017, the GWF values under FSI, WSI, CI, and RCCI policies were 303.6, 366.4, 369.5, and 218.3 mm, respectively. The GWF under RCCI was the smallest for all the years, that under CI was the highest in the wet year (2016), and that under FSI was the highest in the dry year (2017). GWF increased with an increase in total field water inflow and precipitation. Additionally, changes in the irrigation and drainage methods could regulate and decrease GWF. Pollutant leaching risk increased with rising precipitation, which could be mitigated by reducing drainage in the early stages of growth. In East China, RCCI was observed to be the best policy for managing field water in paddy production with regard to GWF reduction. The findings of this study can aid in agricultural water resource management.