The current global distribution of megacities includes Africa, Eurasia, North and South America, located both along coasts and inland. There is also heterogeneity in the number of sources that comprise the different megacity watersheds, with some comprised of a single watershed (e.g. Cairo, Kinshasa), while others draw water from many different watersheds (e.g. Los Angeles, São Paulo). The watersheds depicted in Fig 2 became the sink regions used in our moisture tracking analysis.

There are 29 megacities globally, based on 2015 population data [ 3 ] ( Fig 2 ). Of these, 26 cities were categorized as dependent on surface water for their water supplies (with water provided by direct rainfall runoff, snowmelt, or glacial melt), and the remaining three were primarily dependent on actively recharged groundwater. Several of the megacities obtained water from multiple watersheds, and we combined them into a single watershed. For the three groundwater-dependent megacities we identified the corresponding surface watersheds that contribute water to these groundwater capture zones, by overlaying the groundwater outflow points (from [ 3 ]) with the previously described database of global watersheds.

Precipitationshed identification

The precipitationsheds for all 29 megacities were calculated, and the moisture recycling details of these are summarized in Table 2. We depict a selection of the megacities in Fig 3, choosing cities that are (a) located on four different continents, (b) experience different rainy seasons, (c) have a range of watershed areas (with correspondingly, a range of precipitationshed areas), and (d) span the economic spectrum from low to high income. Note that in each figure the sink region (i.e. the megacity’s watershed) is indicated with a yellow line. The precipitationsheds for the remaining 25 megacities are included in the S1 Appendix.

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larger image TIFF original image Download: Table 2. Summary of moisture recycling results for each of the 29 megacities, for neutral years only (i.e. not dry or wet years). The contribution columns indicate the amount of precipitation falling in the sink region (i.e. megacity watershed) that comes from that region, in terms of both the depth of precipitation falling in the sink region, and the fraction of annual precipitation that comes from that contributing region. Note, the ‘Watershed contribution’ column refers to internal moisture recycling within the sink region. https://doi.org/10.1371/journal.pone.0194311.t002

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larger image TIFF original image Download: Fig 3. Megacity precipitationsheds, based on a core boundary (ranging from 1 mm/yr). Yellow lines enclose the sink regions, and the prevailing winds are indicated to illustrate the average direction of the winds throughout the year. https://doi.org/10.1371/journal.pone.0194311.g003

In a general sense, there are several features that are obvious from looking at the four panels in Fig 3. First, precipitationsheds can be very extensive in reach, in particular the precipitationsheds for Karachi and Chicago. Even for these two extensive precipitationsheds, however, the regions that contribute a lot of evaporation (relatively speaking) are much more concentrated near the sink region. Additionally, precipitationsheds can include regions that are a great distance from a sink region, while excluding areas that are very close to the sink region. For example, Chicago’s precipitationshed, includes contributions from the quite distant Pacific Ocean, while excluding regions that are nearby in Canada and the Northeastern USA.

The spatial patterns of the precipitationsheds are largely driven by the prevailing wind patterns. This is quite clear in the Chicago precipitationshed where we can see the flow of moisture from the southeastern United States and from the Gulf of Mexico. In São Paulo, we see the flow from the Atlantic, and how it piles against the Andes Mountains. We include the mean annual wind patterns in Fig 3 for reference.

Table 2 presents the full moisture recycling data for all of the megacities (rows), with columns for total annual precipitation, moisture recycling in the sink region (i.e. the megacity watershed), and the core precipitationshed (i.e. the land areas that contribute 1 mm/yr or more of evaporation to the precipitation in the sink region). There are several interesting details revealed in the summary table. First, there are a large number of cities that experience high terrestrial moisture recycling. Eight megacities receive around 50% or more of their watershed’s precipitation from upwind land areas, including Beijing, Buenos Aires, Chongqing, Karachi, Kinshasa, Lagos, São Paulo, and Wuhan. These could reasonably be considered “terrestrial moisture recycling-dependent” megacities, given their reliance on upwind land for sustaining their water supplies. There are also, four megacities that receive around 20% or more of their precipitation from internal moisture recycling, including Buenos Aires, Cairo, Dhaka, and Karachi. In other words, about 20% of the rain falling within each of these city’s watersheds originates as evaporation within that watershed. Finally, 8 of the 29 megacities receive nearly half of their precipitation from their core precipitationshed. Put differently, these 8 megacities are reliant on the land areas in their core precipitationshed (including the sink region itself) for providing evaporation to sustain their precipitation.

The average characteristics presented above (Table 2) and the 36-year core precipitationshed (Fig 3), do not communicate the seasonal variation within a year. Fig 4 depicts the average monthly distribution of both precipitation in the megacity watersheds (bars) and the monthly average terrestrial moisture recycling (TMR) ratios (lines), for neutral, dry, and wet years.

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larger image TIFF original image Download: Fig 4. Summary of monthly average precipitation and terrestrial moisture recycling (TMR) during neutral, dry, and wet years. Note that the y-axis corresponds to both meters per month of precipitation (represented by bars), and the fraction of precipitation originating from upwind land surfaces (represented by lines). The dots indicate significant differences for either dry or wet years during that month; see Methods for further details. https://doi.org/10.1371/journal.pone.0194311.g004

In general, there is a wide range in the types of annual precipitation patterns we see among the four featured megacities (bars in Fig 4). There are three apparent types of annual cycles: relatively constant (e.g. Chicago), very wet season with very dry season (e.g. São Paulo, Kinshasa), and multiple rainy seasons (e.g. Karachi). The comparison of neutral, dry, and wet rainfall years (corresponding to black, red, and blue bars, respectively), indicates that in some locations there is very little difference between wet and dry years (e.g. Chicago, Karachi), whereas there are much bigger differences in others (e.g. São Paulo, Kinshasa).

The terrestrial moisture recycling ratios, referring to the fraction of rainfall coming from land versus ocean, are depicted as lines in Fig 4. An important note is that the three lines for neutral-, dry- and wet-year terrestrial moisture recycling do not follow the same patterns for each of the megacities. In some regions there is a peak in terrestrial moisture recycling (e.g. Chicago, Karachi), a relatively steady rate of moisture recycling (e.g. São Paulo), or increased variability in moisture recycling during the dry part of the year (e.g. Kinshasa).

The TMR ratios presented in Fig 4, suggest there may be significant differences between dry and wet year TMR. We found that 20 of 29 megacities had significantly higher TMR ratios during dry years, during two or more months, and ten of those experienced significant differences during four or more months. Conversely, eight megacities had significantly higher TMR during wet years for two or more months, and two of those had significantly higher TMR ratios during four months.

To explore the dry and wet year dynamics spatially, we calculated the difference in evaporation contribution during dry and wet years, and weighted each gridcell by its importance to megacity watershed rainfall (see Eq 3 in Methods). Fig 5 depicts this calculation for Chicago, Karachi, Kinshasa, and São Paulo. Chicago’s differences indicate more contribution from northern latitudes during dry years and more from lower latitudes in wet years. The relatively higher contributions from the Mexico-California region suggests wetter years may be associated with wet years in the desert southwest, and potentially with tropical storm activity in the Gulf of Mexico. Karachi’s differences are more heterogeneous than Chicago’s, but we still see the marked importance of land areas during dry years and oceanic sources during wet years. The Tibetan Plateau, Himalaya, and western Russia are key dry year sources of rainfall. Meanwhile, key wet year sources include the Indian subcontinent, parts of Iran, and southern Pakistan itself.

Kinshasa’s wet-year sources include much of the core of the Congo river basin. The dry-year sources originate in the Atlantic Ocean, as well as directly south from Angola and Zambia. São Paulo’s dry and wet year dynamics are quite complex, including ocean to land tele-connections. Dry year sources are dominated by relatively more contribution from just off the coast of São Paulo in the Atlantic Ocean, as well as just over São Paulo province itself. Wet year sources, however, are related to strong transport from the middle Atlantic Ocean, which then flows over the Amazon, piles against the Andes Mountains, and sweeps south to São Paulo. This tele-connection from the oceans to the Amazon is what makes wet years have significantly higher TMR ratios (Table 2), since moisture sources are predominantly in the southern Amazon, and central Brazil.