Using water footprints to estimate nations’ carrying capacities and demographic sustainability

April 9th, 2013

Dr. Samir Suweis, Prof. Andrea Rinaldo, Prof. Amos Maritan & Prof. Paolo D’Odorico

The super-exponential growth of the human population is critically testing the Earth’s ability to meet mankind’s most basic needs, including food security. In fact, most of the water we use is to produce the food we eat.1 By relying on food imports from other nations,2 several countries already indirectly consume more freshwater resources than they have access to within their boundaries. Thus, the trade of commodities is associated with a virtual transfer of a volume of water, corresponding to the amount of water used for their production.3 Studies on the virtual water trade network4 stress how effective water management needs to account for global – and not only for local – water budgets, as well as to include the effect of trade on the virtual water balance.2

Suweis and colleagues5 recently took a step in this direction by estimating the maximum sustainable population of each country around the world based on their available water resources (i.e. the nations’ carrying capacity), and accounting for both local and “virtual” water resources. Their results highlight the existence of a serious global water imbalance.

The carrying capacities in the study were estimated on the basis of water footprint calculations. Consider, for example, a country with a given population. Each inhabitant consumes a given amount food which corresponds to a particular level of water consumption Wc which typically varies depending on the type of diet, age, cultural, and social conditions. In the absence of trade, people rely on local water resources and the local carrying capacity can be calculated as Kloc = WFloc/Wc, where WFloc is the sum of the water footprints2 of all food commodities that can be produced in that nation. To take into account the entire water budget of a country, you also have to consider the net virtual water import WFtrade, i.e., the sum of the water footprints of all imports minus the footprints of all exports. Using this approach, Suweis et al. estimated the number of people that can be sustained by each country’s local and virtual water supplies, or the virtual carrying capacity, KV = (WFloc +WFtrade)/Wc.

It is important to note that Kloc is the maximum population that is sustainable on the basis of the locally available freshwater resources, and therefore, if the actual population of a country is less than Kloc, the country is “water rich”, i.e. the locally available freshwater resources can sustain more people than are present. On the other hand, if the actual population is less than KV but larger than Kloc, the country is virtual water dependent, i.e. its population is sustained by importing food from other countries.

Using these quantitative estimates of the carrying capacities of each country based on available local and virtual water resources, Suweis et al. looked at the relationship of demographic growth to water availability. By expressing population growth using a classic model known as a logistic equation6, they found that in water rich countries the population grows according to a logistic law in line with the local carrying capacity Kloc – in these countries population grows by relying on the local water resources, as if there was no (virtual) water export. Conversely, population dynamics in water poor countries exhibit a logistic growth in line with virtual carrying capacity KV – their demographic growth strongly relies on the importation of virtual water. The authors also investigated the impact of the topology of the virtual water trade network on the long-term sustainability of the world population. Using three popular random graphs and a graph that resembles the existing global virtual water network, they found that the real network topology appears to be the least efficient in terms of sustaining large populations over long times.

These results highlight a serious global water imbalance and point to the long-term unsustainability of the food trade system. Both water-rich and trade dependent populations are relying for their long-term growth on the same pool of resources. As a consequence it is expected that at some point the volume of virtual water traded in the global market will have to decrease so that water rich nations hold larger amounts of local freshwater resources to meet their own demand leading to a reduction of their exports, and thereby causing the emergence of water limitations in trade dependent countries. Unless new freshwater resources become available or investments in more water-efficient agriculture are made, these trade dependent populations will have to decrease.

To test this hypothesis, Suweis et al. studied global population dynamics using the logistic model for each nation and accounting for the coupling existing among nations through the virtual water trade network. To do this they used local carrying capacities for water rich nations and virtual carrying capacities for trade dependent countries. Based on these model simulations, they indeed found that, in agreement with other studies7, the world’s population would have to start decreasing around the half of this century.

Finally, Suweis et al. also investigated some potential strategies to mitigate this alarming scenario. They concluded that, in order to sustain the current rate of demographic growth, the efficiency of agricultural production needs to increase continuously through technological innovation. A positive impact on the global water balance could also be obtained through a cooperative “contract” between water rich and trade dependent nations, whereby water rich countries devote a relatively small fraction of their local resources to export to trade dependent nations.

Even though these model-based predictions of the future world population are based on a variety of assumptions on different socio-economic scenarios, the results of this study highlight the important role that the global virtual water balance is expected to play on demographic growth in the near future. The constraints on growth estimated by the model suggest that it is important the global allocation of freshwater resources is accounted for in future policies and management strategies.

References:

Falkenmark, M. J., J. Rockstrom, and H. Savenjie (2004). Balancing Water for Humans and Nature, Earthscan, London. Hoekstra, A., and A. Chapagain (2008). Globalization of Water, Wiley Blackwell, Malden, Mass. Allan, J.A. (1998). Virtual water: a strategic resources. Global solutions to the regional deficits. Ground Water, 36: 545-546. Suweis, S., Konar M., Dalin C., Hanasaki N., Rinaldo A. and Rodriguez-Iturbe I. (2011). Structure and controls of the global virtual water trade network. Geophys. Res. Lett. 38, L10403 Suweis, S., Rinaldo, A., Maritan, A. and D’Odorico P. (2013). Water-controlled wealth of nations PNAS early edition, doi:10.1073/pnas.1222452110 Kingsland S (1982) The refractory model: the logistic curve and the history of population ecology. Q Rev Biol 57:29-52 Johansen A, Sornette D (2001) Finite-time singularity in the dynamics of the world population, economic and financial indices. Physica A 294:465-502.

Samir Suweis completed his Ph.D. in Environmental Engineering at the EPFL (Ecole Polytechnique Fédérale Lausanne), and is currently a post-doc in the LIPh laboratory at the Physics Department of the Padoua, University (Italy). His research is at the interface of ecology, hydrology and complex systems under a theoretical framework provided by statistical mechanics. Andrea Rinaldo is professor of Hydrology and Water Resources, and the director of the Laboratory of Ecohydrology at the EPFL. His research focuses on a wide range of topics including transport phenomena in the hydrological cycle, hydrogeomorphology, ecohydrology, stochastic modeling of natural phenomena, and networks in nature. Amos Maritan is professor of Statistical Physics at the University of Padua, Italy and director of the LIPh laboratory. His main research interests are in the statistical mechanics of out-of-equilibrium systems, with interdisciplinary applications ranging from ecology to environmental science. Paolo D’Odorico is professor at the University of Virginia in the Department of Environmental Sciences. His research focuses on the understanding and modeling of hydrological processes and their interaction with ecological and land surface processes, climatology and sustainability science. The full version of the article was published in the Proceedings of the National Academy of Sciences and can be found here.

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.