Wednesday, October 23, 2013

All About the World's Water

I heard a report on the radio today about the possibility of cities or regions tapping into the Great Lake's water.  I started to think about what a precious resource the fresh water in the Great Lakes is.  Then I wondered just how much fresh water there is on our planet, and where is it?

Although we call our planet "Earth," it actually is more of a water planet since over 70 percent of the Earth's surface is covered in water.   That really helps out marine organisms in particular since 97.5 percent of the water on our planet is salt water.  For the animals and plants that require fresh water  --  including us humans -- this precious commodity only makes up 2.5 percent of the Earth's water. 

Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, icebergs, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and other total dissolved solids.

The ability to access fresh water to sustain life is made more difficult by the fact that nearly 70 percent of that fresh water is frozen in the icecaps of Antarctica and Greenland.  Much of the remainder of fresh water is trapped as soil moisture or buried and inaccessible in deep underground aquifers.  

Thus, less than one percent of the world's fresh water (0.007% of all water on earth) is accessible for direct human use. This is the water found in lakes, rivers, reservoirs and those underground sources that are shallow enough to be tapped at an affordable cost. Only this amount is regularly renewed by rain and snowfall, and is therefore available on a sustainable basis.

In terms of the available surface freshwater, lakes contain about 87% including 29% in the African Great Lakes, 20% in Lake Baikal in Russia, 21% in the North American Great Lakes, and 14% in other lakes. Swamps have most of the balance with only a small amount in rivers, most notably the Amazon River. The atmosphere contains 0.04% water.
Continental Breakdown of Share of Global Runoff and Population
Total river runoff
Share of global river runoff
Share of global population
Europe 3,240 8.0 13.0
Asia 14,550 35.8 60.5
Africa 4,320 10.6 12.5
N & C
6,200 15.2 8.0
S America 10,420 25.6 5.5
Australia& Oceania 1,970 4.8 0.5
Totals 40,700 100.0 100.0
Table 2

Distribution of global runoff is highly uneven and corresponds poorly to the distribution of the world population (see table 2). Asia has 69% of world population but 36% of global runoff. South America has 5% of world population, 25% of runoff. The Amazon River accounts for 15% of runoff and is currently accessible to 25 million people (0.4% of world’s pop). Unfortunately, between 50 and 95 percent of river water is not accessible for use.    

Water Use and Consumption Estimates on a Global Scale, 1990
(km cubed/year) 
(km cubed/year)
Agriculture* 2,880 1870
Industry 975 90
Municipalities 300 50
Reservoir losses# 275 275
Subtotal 4,430 2,285
Instream flow needs 275 275
Total as a percent
of AR (12,500 km cubed) 
54% 18%
Table 4: *Assumes average applied water use of 12,000 (m cubed/ha) and consumption equal to ~65% of withdrawls. #Assumes evaporation loss equal to 5% of gross reservoir storage capacity.
So how is the world's fresh water used?  More than half of water is used in agriculture. followed by industry, and municipalities.   Evaporation from reservoirs is estimated to average 5% of gross storage capacity of reservoirs. 

Assuming that the average per capita water demand stays unchanged, the total human appropriation in 2025 will be 9830 km3/yr, or 70% of estimated available water (compared to current 54%). Clearly we are approaching the limit of available fresh water supply. 

So what will we do to supply life sustaining water to everyone on our planet?  There are really only two options.  We can either do a better job capturing flood runoff, or we can desalinate sea water.  With present technology, the latter option is too energy-intensive.  As the human population has increased over the last 60 years, we build damns.  Worldwide, new dams were constructed at rate of 885 per year from 1950-80.  New damns are still being built, but at a slower rate of 500 per year.  As we damn all the accessible rivers the options for new damns diminishes.  In the future it is expected that only 350 new damns will be built per year.  

The most significant solution is for us to get better at using water.  Right now around 60 percent of the water used in agriculture is wasted before it gets to the intended crop.  Pollution also destroys significant water sources.

With one in eight people in the world not having access to safe water it is important to use this resource in a prudent manner. So in the decades to come, we as the dominate species on Earth must do a better job managing the fresh water resource.



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