Improving grey water footprint assessment: Accounting for uncertainty

The grey water footprint (GWF) refers to the amount of freshwater required to dilute pollutants to meet water-quality standards. The aim of this paper was to estimate the GWF and its uncertainty for crop production at the basin scale. The proposed approach was tested in the Rio Mannu Basin (Sardinia, Italy) for durum wheat production. The fraction of nutrients flowing into the river and groundwater was evaluated using the Soil and Water Assessment Tool model that was calibrated with in-stream monitoring data. A bootstrap technique coupled with Monte Carlo simulations was used to estimate the uncertainty of the GWF due to the variability of the primary input data and the unknown natural background level of nutrients in the waters. The GWF for total phosphorus (TP) input (3284 m 3 t −1 ) was higher than that for dissolved inorganic nitrogen (DIN) (275 m 3 t −1 ), despite the lower rate of phosphorus fertiliser application. The uncertainty was found to be relevant for both DIN (60%) and TP (18%). The environmental sustainability of durum wheat production was assessed throughout the water pollution level. This showed that the TP load exceeded the assimilation capacity at the reach scale, and that further analyses are needed to assess the environmental sustainability at the basin scale.