Energy analysis of crop irrigation: Role of water reclamation and water exportation
Freshwater availability is the major constraint to agriculture in arid and semi-arid regions. Worldwide, agricultural irrigation is the leading sector in the overall water consumption. As the largest agricultural region in the United States and the leading exporter of many commodities, California was chosen as a spatial domain to model the carbon footprint reduction and resource savings (water and energy) when applying reclaimed water to crop irrigation. An extensive compilation of the most recent publicly available datasets was used to calculate the energy intensity for each water supply source, associated carbon footprint reduction and monetary savings for employing reclaimed water versus traditional groundwater application. Furthermore, a quantification of water exported through agricultural trades was performed. Exported water is defined as the physical water content contained in crops plus the associated induced evapotranspiration due to their irrigation. Exported water differs from virtual water in that the former is the physical water exported outside of a geographical boundary and the latter is cumulative water footprint required to reach the final product. Therefore, the exported water is permanently lost and is no longer available for the natural hydrologic cycle from its origin.
Our calculations indicate that on an average basis for the time domain 1998-2010, the fractional water use for agriculture, and urbanized consumption in California was 0.81 and 0.19 respectively. Annually, crop irrigation consumed an average of 4.2 x 1010 m3 of fresh water, of which 1%, 46.8% and 52.2% came from reclaimed water, groundwater, and surface water, respectively. Each of these three main water sources is associated with a range of energy intensity (in kWh m-3), depending on the process and environmental characteristics of the end-use location. The analysis of multiple process and environmental configurations produced a detailed energy intensity database, with the associated carbon intensities (in kgCO2,eq kWh-1). The overall exported water (i.e., contained in and evaporated/transpired from crops) in California's agricultural commodities was 2.88 x 1010 m3 yr-1, equivalent to 68.3% of the total water used in irrigation. The majority of the exported water was in the form of induced evapotranspiration, amounting to 67.7% of the irrigation water use, whereas approximately 2.32 x 108 m3 yr-1 or 0.6% of the water used for irrigation leaves the agricultural spatial domain as content of the crops. Our results show that the physical water content contained in crops is minor relative to the associated evapotranspiration of the irrigated crops, confirming the hypothesis that for each unit of water exported, the loss of water via evapotranspiration induced by each crop far outweighs the crop water content.