Although water pollution can thus be reduced dramatically, this comes together with a great yield reduction, and a much lower water productivity (larger green plus blue WF) as well. The grey WF can be further reduced to 75 m³ t⁻¹ by shifting the management package to manure N and deficit irrigation (with crop yield of 3.5 t ha⁻¹). For this reference, the grey WF at a usual N application rate of 300 kg N ha⁻¹ (with crop yield of 11.1 t ha⁻¹) is 1100 m³ t⁻¹, which can be reduced by 91 % towards 95 m³ t⁻¹ when the N application rate is reduced to 50 kg N ha⁻¹ (with a yield of 3.7 t ha⁻¹).
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As a reference management package, we assume the use of inorganic N (nitrate), conventional tillage and full irrigation. The water and nitrogen balances of the soil and plant growth at the field scale were simulated with the Agricultural Policy Environmental eXtender (APEX) model. As a case study, we consider irrigated maize grown in Spain on loam soil in a semi-arid environment, whereby we simulate the 20-year period 1993–2012. The objective of this study is to explore the effect of the nitrogen application rate (from 25 to 300 kg N ha⁻¹), nitrogen form (inorganic N or manure N), tillage practice (conventional or no-tillage) and irrigation strategy (full or deficit irrigation) on the nitrogen load to groundwater and surface water, crop yield and the N-related grey water footprint of crop production by a systematic model-based assessment. Fertilizer application can contribute significantly to the grey WF as a result of nutrient leaching to groundwater and runoff to streams. Grey water footprint (WF) reduction is essential given the increasing water pollution associated with food production and the limited assimilation capacity of fresh water.