Global water scarcity: the monthly blue water footprint compared to blue water availability for the world's major river basins

Conventional blue water scarcity indicators suffer from four weaknesses: they measure water withdrawal instead of consumptive water use, they compare water use with actual runoff rather than natural (undepleted) runoff, they ignore environmental flow requirements and they evaluate scarcity on an annual rather than a monthly time scale. In the current study, these shortcomings are solved by defining blue water scarcity as the ratio of blue water footprint to blue water availability – where the latter is taken as natural runoff minus environmental flow requirement – and by estimating all underlying variables on a monthly basis. In this study we make for the first time a global estimate of the blue water footprint of humanity at a high spatial resolution level (a five by five arc minute grid) on a monthly basis. In order to estimate blue water scarcity at river basin level, we aggregated the computed monthly blue water footprints at grid cell level to monthly blue water footprints at river basin level. By comparing the estimates of the monthly blue water footprint with estimates of the monthly blue water availability at river basin level, we assess the intra-annual variability of blue water scarcity for the world’s major river basins. Monthly blue water footprints were estimated based on Mekonnen and Hoekstra (2011a). Natural runoff per river basin was estimated by adding estimates of actual runoff from Fekete et al. (2002) and estimates of water volumes already consumed. Environmental flow requirements were estimated based on the presumptive standard for environmental flow protection as proposed by Richter et al. (2011), which can be regarded as a precautionary estimate of environmental flow requirements. Within the study period 1996-2005, in 223 river basins (55% of the basins studied) with in total 2.72 billion inhabitants (69% of the total population living in the basins included in this study), the blue water scarcity level exceeded one hundred per cent during at least one month of the year, which means that environmental flow requirements were violated during at least one month of the year. In 201 river basins with in total 2.67 billion people there was severe water scarcity, which means that the blue water footprint was more than twice the blue water availability, during at least one month per year. Global average blue water scarcity – estimated by averaging the annual average monthly blue water scarcity values per river basin weighted by basin area – is 85%. This is the average blue water scarcity over the year within the total land area considered in this study. When we weight the annual average monthly blue water scarcity values per river basin according to population number per basin, global average blue water scarcity is 133%. This is the average scarcity as experienced by the people in the world. This population-weighted average scarcity is higher than the area-weighted scarcity because the water scarcity values in densely populated areas – which are often higher than in sparsely populated areas – get more weight. Yet another way of expressing water scarcity is to take the perspective of the average water consumer. The global water consumption pattern is different from the population density pattern, because intensive water consumption in agriculture is not specifically related to where most people live. If we estimate global blue water scarcity by averaging monthly blue water scarcity values per river basin weighted based on the blue water footprint in the respective month and basin, we calculate a global blue water scarcity at 244%. This means that the average blue water consumer in the world experiences a water scarcity of 244%, i.e. operates in a month in a basin in which the blue water footprint is 2.44 times the blue water availability and in which presumptive environmental flow requirements are thus strongly violated. The data presented in this report should be taken with care. The quality of the presented blue water scarcity data depends on the quality of the underlying data. The estimates of both monthly blue water footprint and monthly blue water availability per river basin can easily contain an error of ± 20 per cent, but a solid basis for making a precise error statement is lacking. This obviously needs additional research. Furthermore, improvements in the estimates can be made by including the effect of dams on the blue water availability over time, by accounting for inter-basin water transfers, by distinguishing between surface water, renewable groundwater and fossil groundwater, by improving estimates of environmental flow requirements, by looking at water scarcity at the level of sub-basins, and by considering inter-annual variability as well. Despite this great room for improvement and bringing in more detail, the current study is a milestone in global water scarcity studies by mapping water scarcity for the first time on a monthly basis