Monthly water stress: spatially and temporally explicit consumptive water footprint of global crop production

摘要

Irrigation is the dominant human activity leading to water stress, with environmental consequences on the local and global level. The relevance of spatial resolution to the assessment of water consumption and to impacts related to crop production has been acknowledged in previous research on water footprint. The temporal aspects of crop cultivation and the related impacts, however, have been neglected in analyses with global coverage. Such aspects are important since different crop options can shift irrigation water consumption within a year, increasing or decreasing the related water stress. Additionally, in some regions, temporal aspects are crucial due to the high variability of water availability. Consequently, an annual assessment might be misleading regarding crop choices within and among different regions. A temporal resolution is therefore essential for proper life cycle assessment (LCA) or water footprint of crop production. For this purpose we develop a water stress index (WSI) on a monthly basis for more than 11,000 watersheds with global coverage. The median and average watershed area are 1327 and 19591 km~2, respectively. The WSI ranges from 0.01 (least water scarcity) to 1 (maximal water scarcity), and quantifies the fraction of water consumed of which other users are potentially deprived of. Moreover, irrigation water consumption for 160 crop groups is calculated on a monthly basis and on a high spatial resolution (＜10 km). Crop water footprints (WFP) are calculated by multiplying monthly WSI with monthly crop irrigation water consumption and by summing the result over the cultivation period. With these results we facilitate a new level of detail for WFP analysis. We estimate global irrigation water consumption in the year 2000 at 1.21~*10~(12) m~3/a, with an average WSI of 0.44. The regional pattern changes considerably with higher temporal resolution and therefore in many regions it is relevant to consider monthly WSI. Changes are also shown to be sensitive to crop types due to different growth patterns, which might lead to increasing or decreasing water footprint. Additionally, we examine the role of different conceptual assumptions for the definition of water footprint characterization factors, which can be expressed as marginal and average figures. WSI is a marginal characterization factor. However, a practitioner may favor an alternative average factor to match impact assessment with the given goal and scope of the study. An average characterization factor allows for calculating WFP of a whole region as well as the global annual WFP of agriculture, which is estimated at 3.5~*10~(11) m~3-equivalents. This number can be interpreted as water consumed in an extremely water-stressed situation and therefore highly depriving others of its use.