Urban water demand will increase by 80% by 2050, while climate change will alter the timing and distribution of water. Here we quantify the magnitude of these twin challenges to urban water security, combining a dataset of urban water sources of 482 of the world’s largest cities with estimates of future water demand, based on the Intergovernmental Panel on Climate Change (IPCC)’s Fifth Assessment scenarios, and predictions of future water availability, using the WaterGAP3 modelling framework. We project an urban surface-water deficit of 1,386–6,764 million m³. More than 27% of cities studied, containing 233 million people, will have water demands that exceed surface-water availability. An additional 19% of cities, which are dependent on surface-water transfers, have a high potential for conflict between the urban and agricultural sectors, since both sectors cannot obtain their estimated future water demands. In 80% of these high-conflict watersheds, improvements in agricultural water-use efficiency could free up enough water for urban use. Investments in improving agricultural water use could thus serve as an important global change adaptation strategy. Rising population and changes in water supply under climate change affect cities globally. This study finds that in 27% of cities studied, water demand is likely to exceed availability by 2050, with many other cities competing with agriculture on water needs.

Water competition between cities and agriculture driven by climate change and urban growth

Assessing reliability of global models is critical because of increasing reliance on these models to address past and projected future climate and human stresses on global water resources. Here, we evaluate model reliability based on a comprehensive comparison of decadal trends (2002-2014) in land water storage from seven global models (WGHM, PCR-GLOBWB, GLDAS NOAH, MOSAIC, VIC, CLM, and CLSM) to trends from three Gravity Recovery and Climate Experiment (GRACE) satellite solutions in 186 river basins (∼60% of global land area). Medians of modeled basin water storage trends greatly underestimate GRACE-derived large decreasing (≤-0.5 km3/y) and increasing (≥0.5 km3/y) trends. Decreasing trends from GRACE are mostly related to human use (irrigation) and climate variations, whereas increasing trends reflect climate variations. For example, in the Amazon, GRACE estimates a large increasing trend of ∼43 km3/y, whereas most models estimate decreasing trends (-71 to 11 km3/y). Land water storage trends, summed over all basins, are positive for GRACE (∼71-82 km3/y) but negative for models (-450 to -12 km3/y), contributing opposing trends to global mean sea level change. Impacts of climate forcing on decadal land water storage trends exceed those of modeled human intervention by about a factor of 2. The model-GRACE comparison highlights potential areas of future model development, particularly simulated water storage. The inability of models to capture large decadal water storage trends based on GRACE indicates that model projections of climate and human-induced water storage changes may be underestimated.

https://www.pnas.org/content/pnas/115/6/E1080.full.pdf

Global models underestimate large decadal declining and rising water storage trends relative to GRACE satellite data

High-resolution global maps of annual urban land coverage provide fundamental information of global environmental change and contribute to applications related to climate mitigation and urban planning for sustainable development. Here we map global annual urban dynamics from 1985 to 2015 at a 30 m resolution using numerous surface reflectance data from Landsat satellites. We find that global urban extent has expanded by 9,687 km2 per year. This rate is four times greater than previous reputable estimates from worldwide individual cities, suggesting an unprecedented rate of global urbanization. The rate of urban expansion is notably faster than that of population growth, indicating that the urban land area already exceeds what is needed to sustain population growth. Looking ahead, using these maps in conjunction with integrated assessment models can facilitate greater understanding of the complex environmental impacts of urbanization and help urban planners avoid natural hazards; for example, by limiting new development in flood risk zones. The world keeps urbanizing. This study finds that since 1985 global urban lands have expanded four times faster than previously recognized and faster than population is growing.

High-spatiotemporal-resolution mapping of global urban change from 1985 to 2015

The studies that quantify the human and economic costs of increasing exposure of cities to various natural hazards consider climate change together with increasing levels of population and economic activity, but assume constant urban extent. Accurate estimates of the potential losses due to changing exposure of cities, however, require that we know where they will grow in the future. Here, we present the first-ever estimates of the changing exposure of urban infrastructure to floods and droughts due to urban land expansion from 2000 to 2030. The percentage of the global urban land that lies within the low elevation coastal zone (LECZ) increases only slightly to 13% by 2030; nonetheless, this corresponds to a 230% increase in the amount of urban land within the LECZ (from 71,000 km 2 to 234,000 km 2 ). In 2000, about 30% of the global urban land (i.e., nearly 200,000 km 2 ) was located in the high-frequency flood zones; by 2030, this will reach 40% (i.e., over 700,000 km 2 ). The emerging coastal metropolitan regions in Africa and Asia will be larger than those in the developed countries and will have larger areas exposed to flooding. The urban extent in drylands will increase by nearly 300,000 km 2 , reaching almost 500,000 km 2 . Overall, without factoring in the potential impacts from climate change, the extent of urban areas exposed to flood and drought hazards will increase, respectively, 2.7 and almost 2 times by 2030. Globally, urban land exposed to both floods and droughts is expected to increase over 250%. There are significant geographical variations in the rates and magnitudes of urban expansion exposed to floods or droughts or both. Several policy options exist to safeguard urban infrastructure from flood and drought hazards. These range from directing development away from flood- or drought-prone zones to large-scale adoption of “green infrastructure” (or “eco-efficient infrastructure”). Decisions, taken today on managing urban growth in locations exposed to these hazards, can make a big difference in mitigating likely losses due to floods and droughts in the near future.

Changing global patterns of urban exposure to flood and drought hazards

. To sustain growing food demand and increasing standard of living, global water withdrawal and consumptive water use have been increasing rapidly. To analyze the human perturbation on water resources consistently over large scales, a number of macro-scale hydrological models (MHMs) have been developed in recent decades. However, few models consider the interaction between terrestrial water fluxes, and human activities and associated water use, and even fewer models distinguish water use from surface water and groundwater resources. Here, we couple a global water demand model with a global hydrological model and dynamically simulate daily water withdrawal and consumptive water use over the period 1979–2010, using two re-analysis products: ERA-Interim and MERRA. We explicitly take into account the mutual feedback between supply and demand, and implement a newly developed water allocation scheme to distinguish surface water and groundwater use. Moreover, we include a new irrigation scheme, which works dynamically with a daily surface and soil water balance, and incorporate the newly available extensive Global Reservoir and Dams data set (GRanD). Simulated surface water and groundwater withdrawals generally show good agreement with reported national and subnational statistics. The results show a consistent increase in both surface water and groundwater use worldwide, with a more rapid increase in groundwater use since the 1990s. Human impacts on terrestrial water storage (TWS) signals are evident, altering the seasonal and interannual variability. This alteration is particularly large over heavily regulated basins such as the Colorado and the Columbia, and over the major irrigated basins such as the Mississippi, the Indus, and the Ganges. Including human water use and associated reservoir operations generally improves the correlation of simulated TWS anomalies with those of the GRACE observations.

Global Modeling of Withdrawal, Allocation and Consumptive Use of Surface Water and Groundwater Resources

A B S T R A C T Urban growth is increasing the demand for freshwater resources, yet surprisingly the water sources of the world’s large cities have never been globally assessed, hampering efforts to assess the distribution and causes of urban water stress. We conducted the first global survey of the large cities’ water sources, and show that previous global hydrologic models that ignored urban water infrastructure significantly overestimated urban water stress. Large cities obtain 78 � 3% of their water from surface sources, some of which are far away: cumulatively, large cities moved 504 billion liters a day (184 km 3 yr � 1 ) a distance of 27,000 � 3800 km, and the upstream contributing area of urban water sources is 41% of the global land surface. Despite this infrastructure, one in four cities, containing $4.8 � 0.7 trillion in economic activity, remain water stressed due to geographical and financial limitations. The strategic management of these cities’ water sources is therefore important for the future of the global economy.

https://www.eoas.ubc.ca/~mjelline/212_website/212_live/resources/Required-Reading/Mcdonald_gleeson_2014_Water-on-an-urban-planet--Urbanization-and-the-reach-of-urban-water-infrastructure.pdf

Water on an Urban Planet: Urbanization and the Reach of Urban Water Infrastructure

Urban water systems are impacted by land use within their source watersheds, as it affects raw water quality and thus the costs of water treatment. However, global estimates of the effect of land cover change on urban water-treatment costs have been hampered by a lack of global information on urban source watersheds. Here, we use a unique map of the urban source watersheds for 309 large cities (population > 750,000), combined with long-term data on anthropogenic land-use change in their source watersheds and data on water-treatment costs. We show that anthropogenic activity is highly correlated with sediment and nutrient pollution levels, which is in turn highly correlated with treatment costs. Over our study period (1900–2005), median population density has increased by a factor of 5.4 in urban source watersheds, whereas ranching and cropland use have increased by a factor of 3.4 and 2.0, respectively. Nearly all (90%) of urban source watersheds have had some level of watershed degradation, with the average pollutant yield of urban source watersheds increasing by 40% for sediment, 47% for phosphorus, and 119% for nitrogen. We estimate the degradation of watersheds over our study period has impacted treatment costs for 29% of cities globally, with operation and maintenance costs for impacted cities increasing on average by 53 ± 5% and replacement capital costs increasing by 44 ± 14%. We discuss why this widespread degradation might be occurring, and strategies cities have used to slow natural land cover loss.

https://www.pnas.org/content/pnas/113/32/9117.full.pdf

Katherine F. Weber

Papers

Water on an Urban Planet: Urbanization and the Reach of Urban Water Infrastructure

Estimating watershed degradation over the last century and its impact on water-treatment costs for the world’s large cities