Earth’s Water Resources

Water’s been making the news lately — specifically the lack of fresh water for human consumption. Many agencies, both government and NGOs, have programs to provide developing countries access to fresh water. They’ve tried digging canals, drilling wells, extracting water from the atmosphere, building desalination plants, even “harvesting” icebergs. Some of these solutions are more feasible than others, some are energy intensive, and most are expensive. Capetown, South Africa is in its third year of drought. Their reservoirs are close to running dry and rationing is already in force. There are two reasons for this crisis (which is happening in more places than Capetown): Global rainfall patterns are changing, and the population keeps increasing.

So I thought this was a good time to share a model I created that puts our water resources into perspective. The graphic is based on data widely available online, but I pulled mine from the USGS website. Each form of water on Earth is shown as a separate cube scaled accurately to its volume.

The sphere representing annual rainfall is, of course, not part of the total — at any given time it could be in any of the cubes. But I was just curious how large it was compared to the “permanent” cubes. Interestingly, the average time a water molecule spends as vapor in the atmosphere before falling as some type of precipitation is estimated to be only 9 days. So the blue sphere is also a good approximation for the amount of water that evaporates each year from our oceans, lakes, and rivers.

Here’s the data on which my graphic is based:

Water   Volume (km3)   Percent of Total   Notes
oceans   1.3×109   96.5   including connected inland seas
ice   2.4×107   1.78   polar caps, glaciers, snow, permafrost
groundwater   2.3×107   1.71   to a depth of 2 km
lakes   1.8×105   0.013   average seasonal volume
atmosphere   1.3×104   0.00096   to an altitude of 100 km
rivers   2.1×103   0.00016   average seasonal volume
rainfall   5.1×105   0.038   annual global rainfall total

The total volume of water in all forms = 1.35×109 km3. Values in the percent column do not add to 100 because I rounded insignificant figures. This total volume is being slowly increased by volcanic gas emissions (typically 60% water vapor), and “precipitation” from space in the form of ice grain micrometeorites and the occasional comet impact. Some water is being lost from the upper atmosphere via photodissociation of H2O and subsequent escape of H2 into space. I was unable to find numbers for these perturbations, but I suspect they’re far too small to show up in my graphic.

I considered adding one more cube to the model representing the solid (non-water) volume of the Earth, but discovered the cube would be too large to fit at the same scale as the water. If I changed the scale to fit the Earth cube, everything smaller than the groundwater cube shrunk to less than a pixel and disappeared. Then I found this great visualization online, courtesy of the USGS. The graphic artists removed all the water from Earth leaving dry land topography, and “merged” all that water into a single sphere at the same scale. You can see why the Earth cube wouldn’t fit with the scale of my graphic.

So all I wanted to do with this post is provide another, more visual, way to understand the relationship between Earth’s different water resources. No equations, laws of science, or reasoning required. Yeah, sure, I could have done my graphic as a simple pie chart, but this kind of data just has more visual impact in 3D.

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