The consideration of time step in calculating grey water footprints of agricultural cropping systems
Citation
Vergé, X., VanderZaag, A., Smith, W., Grant, B., Gordon, R. (2017). The consideration of time step in calculating grey water footprints of agricultural cropping systems. Ecological Indicators, [online] 78 31-36. http://dx.doi.org/10.1016/j.ecolind.2017.03.006
Plain language summary
A water footprint is a way to measure how much water a system uses per unit of product produced. It is calculated using both the quantity of water used in the production processes and the waste water generated by these processes. A grey water (GW) footprint is the amount of water needed to (virtually) dilute pollutants to acceptable concentrations. This concept was proposed by the Water Footprint Network, however it faces many difficulties when applied to agricultural production systems. Problems arise because crop production cannot be fully controlled and it is weather-dependent which greatly affects the calculations of the year’s GW footprint.
This study examines the effect of different time steps on the annual GW footprint of corn and soybean production systems. The time steps studied were daily, weekly, monthly, seasonally and yearly. Data from 30 years of daily average nitrate nitrogen (NO3-N) concentrations in drainage water were used. For each crop year the volume of water needed to dilute NO3-N to an acceptable level (less than 10 mg per litre) was calculated. Daily calculations show concentrations are generally less than the maximum acceptable levels, showing that fields often provide their own ‘dilution’ water. Annual average concentrations for NO3-N were 2.0 mg/L-1 and 0.4 mgL-1 for corn and soybean, respectively, which is well within the acceptable range.
Overall, the GW footprint varied significantly when calculated for different time steps. The greatest annual footprint occurred when calculated daily. The GW footprint for corn ranged from 2700 mm of water when calculated daily, to 0 for the yearly time step. For soybeans it ranged from 500 mm to 0. The range of the GW footprint with the different time steps goes beyond crop production to animal production as well. The GW calculations should be reconsidered and standardized to fix these variations.
Abstract
A water footprint considers both the water volumes involved in production processes and the resulting waste water generated. The grey water (GW) footprint represents the volume of fresh water required to assimilate pollutants to acceptable concentrations—a concept proposed by the water footprint network—but it faces several difficulties when applied to agricultural production systems. Crop production cannot be fully controlled and it is weather-dependent, which greatly affects the year-to-year GW calculations. In this study, we examined the effect of time step on the calculation of annual GW footprints by utilizing 30 years of daily average nitrate-nitrogen (NO3-N) concentrations in drainage water (both leachate and runoff water derived from a process-based model) from corn and soybean production systems. For each crop year, the volume of water required to assimilate NO3-N to an acceptable threshold concentration (i.e. <10 mg L−1) was calculated over different time steps (daily, weekly, monthly, seasonally and yearly), and each case was summed to an annual GW value. Daily average NO3-N concentrations in the effluent water were generally below the acceptable threshold concentrations, with intermittent exceedances. Thus, the fields often provided their own ‘dilution’ water, and annual average concentrations were only 2.0 mg L−1 and 0.4 mg L−1 for corn and soybean, respectively. The GW footprint varied significantly when calculated for different time steps. The greatest annual footprint occurred when calculated daily (shortest time step). The GW footprint for corn ranged from 2.7 × 103 m3 ha−1, or 2700 mm of water, when estimated daily to zero for the yearly time step. For soybean it ranged from 0.5 × 103 m3 ha−1, or 500 mm of water, to zero. The GW footprint results are therefore highly dependent on the time step of calculation. The effect of this issue extends beyond crop production as it is exported and amplified through feed rations to affect the GW footprint from animal production. To be able to reconcile these problems, the GW calculation pathways should be reconsidered and standardized.