Green, blue and grey water footprint reduction in irrigated crop production
In the face of increasing freshwater scarcity, reducing the consumptive and degradative water use associated with crop production, the world’s largest water user, is indispensable. The thesis explores the potential for reducing the green, blue and grey water footprint (WF) in irrigated crop production by a systematic model-based simulation for a large number of field management practices and different cases. The research has been set up in four subsequent studies, whereby the first two focus on green and blue WF reduction, the third on grey WF reduction and the fourth on the trade-off between blue and grey WF in crop production.
Green and blue water footprint reduction in irrigated agriculture: effect of irrigation techniques, irrigation strategies and mulching. This study aims to explore the potential for reducing the green and blue WF per tonne of crop production by considering four different irrigation techniques, four irrigation strategies, and three mulching practices for various cases, including three crops, four different environments, three hydrologic years (dry to wet), and three soil types. The AquaCrop model is applied to simulate the effect of different combinations of field management practices on evapotranspiration, crop yield, and thus WFs per tonne. WF reduction is calculated by comparing the WF associated with a certain field management package with a reference (furrow technique with no mulching and full irrigation). The result shows that the average reduction in the consumptive WF is 8–10% if we change from the reference to drip or subsurface drip, 13% when changing to organic mulching, 17–18% when moving to (subsurface) drip in combination with organic mulching, and 28% for (subsurface) drip in combination with synthetic mulching. Reduction in overall consumptive WF always goes together with an increasing ratio of green to blue WF. The WF of growing a crop for a particular environment is smallest under deficit irrigation, followed by full irrigation, supplementary irrigation and zero irrigation (rain-fed). Growing crops with sprinkler irrigation has the largest consumptive WF, followed by furrow, drip and subsurface drip.
Marginal cost curves for green and blue water footprint reduction in irrigated agriculture: guiding a cost-effective reduction of crop water consumption to a permit or benchmark level. This study aims to develop marginal cost curves (MCCs) for WF reduction in crop cultivation. MCCs present information on the cost-effectiveness of various field management practices, and can be used to estimate the cost associated with a certain WF reduction target (WF permit or benchmark). AquaCrop is used to estimate the effect of different management packages on evapotranspiration and crop yield and thus on WF of crop production, for three crops. The annual average cost for each management package is estimated as the sum of the annualized capital cost, and the annual operation and maintenance costs. The WFs and annual costs associated with the management packages are used to develop alternative WF reduction pathways, after which the most cost-effective pathway is selected to develop the MCC for WF reduction. It is shown that the most cost-effective way to reduce the WF of crop production is to change the irrigation strategy, followed by the mulching practice and finally the irrigation technique. The application of MCC for WF reduction to a certain WF permit level is shown using a hypothetical example.
Grey water footprint reduction in irrigated crop production: effect of nitrogen application rate, nitrogen form, tillage practice and irrigation strategy. The study aims to explore the potential for reducing the grey WF per tonne of crop production by assessing the effect of different combinations of nitrogen (N)-application rate, from 25 to 300 kg N ha-1 y-1, inorganic-N or manure-N, conventional or no-tillage and full or deficit irrigation, for irrigated maize on loam soil in a semi-arid environment. The APEX model is applied to estimate the N loads and crop yield for different field management practices, and thus to calculate the grey WF per tonne. Compared to the reference case of an N-application rate of 300 kg N ha-1y-1, with inorganic-N as fertilizer, conventional tillage and full irrigation, the grey WF can be reduced by 91% by reducing the N-application rate to 50 kg N ha-1y-1. It can be further reduced by applying manure-N and deficit irrigation. Water pollution can be reduced dramatically, but this comes together with a great yield reduction, and a much lower water productivity as well. The overall (green, blue plus grey) WF per tonne is found to be minimal at an N application rate of 150 kg N ha-1, with manure, no-tillage and deficit irrigation (with crop yield of 9.3 t ha-1).
Trade-off between blue and grey water footprint of crop production at different nitrogen application rates under various field management practices. The study explores the trade-off between blue and grey WF by changing N-application rates from 25 to 300 kg N ha-1y-1 under a reference management package (inorganic-N, conventional tillage, full irrigation), or by changing the field management practices for irrigated maize on loam soil and in semi-arid environment. The APEX model is applied to simulate the effect of field management practices (seven N-application rates, two N forms, two tillage practices and two irrigation strategies) on evapotranspiration, N load to freshwater and crop yield, and thus blue and grey WFs per unit of crop are calculated. The result shows that increasing N application from 25 to 50 kg N ha-1y-1, is a no-regret move, because crop yield is increased by a factor 2, and blue and grey WFs per tonne are reduced by 40% and 8%, respectively. Decreasing the N application from 300 to 200 kg N ha-1 y-1 is a no-regret move as well, with a grey WF per tonne reduced by 72%, while the blue WF and yield remain the same. Increasing the N application from 50 to 200 kg N ha-1 y-1 involves a trade-off between blue and grey WF, because crop yield is increased by a factor 3, and the blue WF per tonne declines by 60% but the grey WF increases by 210%. The minimum blue WF per tonne is found at an N application of 200 kg N ha-1y-1, while the minimum grey WF per tonne is at 50 kg N ha-1 y-1.
Conclusion: The thesis contributes to the advancement of the field of water footprint assessment in numerous ways. First, a shadow water-balance method was developed and applied to explicitly distinguish between the green and blue WF of crop production. Second, the APEX model was applied to estimate the grey WF of crop production for the first time in the thesis. Third, the thesis offers the first comprehensive and systematic study of the potential for reducing the green, blue and grey WF per unit of crop production by changing field management practices under various cases. Fourth, the study shows the trade-off between the blue and grey WF, and between WFs and crop yield at different N-application rates and under various field management practices (N-forms, tillage practices and irrigation strategies). Finally, the thesis shows how one can develop and apply a model-driven MCC for irrigated crop production.