Quantifying trade-offs between productivity and local water quality in the food-energy-water nexus

The food-energy-water nexus refers to the complex set of physical and causal links between food production, energy production, and the availability and quality of fresh water. Some of these links exist because of increased... [ view full abstract ]

The food-energy-water nexus refers to the complex set of physical and causal links between food production, energy production, and the availability and quality of fresh water. Some of these links exist because of increased renewable energy production, which, like food production, requires arable land.  Increasing demands for both food and energy create incentives for agricultural decision-makers to use high intensity farming practices involving extensive tillage, irrigation and chemical fertilizer application.  Such practices have a detrimental effect on water quality through increased erosion and runoff containing nitrogen and phosphorus compounds.  Producing less food and energy is not a viable solution to water quality degradation, but continuing current practices will negatively impact local supplies of potable water as well as local ecosystems.  

In this work  a food and energy co-production system is designed according to  three conflicting criteria: food production, energy production and water quality regulation. Both  land use options - agricultural activities - and several biomass-to-energy conversion processes are included in the design problem.  Possible food products are corn and soybeans, and possible energy products range from corn ethanol to electricity from cellulosic biomass combustion.  The objective is to determine if the co-production system can achieve high food and energy production without contributing to local water quality degradation.  Towards that end, the agricultural activities considered include a variety of cropping systems under standard, high intensity farming practices as well as conservation practices that involve less tillage and smaller quantities of fertilizer.  Each agricultural activity  by itself contributes to water quality degradation, but to different degrees.  In order to mitigate this degradation, an ecological land use option is included in the design problem: land can be used for energy and production, or to establish a wetland that removes nitrates from the farm runoff before it re-enters the watershed.  Including the wetland as a competing land use option allows for the trade-offs between water quality, food production and energy production to be quantified in terms of the amount of food and/or energy production that must be sacrificed to maintain local water quality.

Results indicate that including the wetland in the co-production system enables the system to reach high levels of simultaneous food and energy production, with no significant impact on water quality.  Without the wetland, a small increase in productivity is achieved, but with a large negative water quality impact.  These results indicate that a small-to-insignificant sacrifice in productivity yields large ecological benefits.  The co-production systems are also compared to conventional renewable energy pathways, corn-to-ethanol and soybean-to-biodiesel.  With or without the wetland, the co-production systems have higher food and energy productivity than either conventional pathway; with the wetland, the co-production system also outperforms the conventional pathways in water quality degradation.