ABSTRACT
It is inevitable that people have an environmental footprint on the Earth. People use natural resources like land, water and energy and undertake various kinds of activities that bring along pollution. Whether the total footprint of all activities together is sustainable, fully depends on the size of the footprint. Consider for example the carbon footprint. The fact that human activities add greenhouse gases into the atmosphere has become a problem because of the total amount of greenhouse gases added. The carbon footprint of one activity does not have an impact. It is the total carbon footprint of humanity that has become too big: the average global temperature is likely to increase substantially, with secondary effects on evaporation and precipitation patterns and sea level and tertiary effects on ecosystems and societies (IPCC, 2007). We have a similar size-issue with our ecological footprint, or our ‘land footprint’. The ecological footprint is the total bioproductive area required to sustain the various components of our consumption pattern. In 2008, humanity’s ecological footprint exceeded the Earth’s biocapacity by more than 50 per cent (WWF, 2012). This means that we would need 1.5 Earths to sustain our present way of living. We currently survive on this one planet by overexploiting it, which can be done temporarily but cannot be maintained in the long term. The concern is not directly the ecological footprint that can be associated with any specific human activity, but our total ecological footprint. With the water footprint, we have the same issue of size. In the case of the water footprint, the typical unit for evaluation is the river basin (Hoekstra et al., 2011). As we will see later in this chapter, there is also an important global dimension to the water footprint, but let me start with the river basin. Within a river basin, water resources availability is constrained by the amount of precipitation. The precipitation that adds to the water in a river basin will leave the basin again by evaporation or runoff to the ocean. The evaporative flow (green water) can be made productive in crop fields or production forests. In this way, the
evaporative flow is not ‘lost’ to the atmosphere but productively used. The runoff flow (blue water) can be made productive as well, by withdrawing water from aquifers and rivers and using it in industries or households or for irrigating fields. In this way, the runoff flow is not ‘lost’ to the ocean, but consumed for useful purposes. We can use all the green and blue water available in a river basin in a certain period. Temporarily, we can even use more than that, by depleting groundwater and lake reservoirs but, in the longer term, from a sustainability point of view, we cannot use more than the rate of replenishment. The upper limit to consumptive water use within a river basin is the precipitation within the basin. However, this is really an upper-upper limit, because there are two reasons why the actual upper limit lies substantially lower. One is that we cannot use all green water; the second is that we cannot use all blue water. The ‘loss’ of water to the atmosphere through non-beneficial evapotranspiration and the ‘loss’ of water to the ocean are not real losses. These flows are essential for the functioning of ecosystems and of societies depending on those ecosystems.
