Tuesday, November 2, 2010

The Hydrological Cycle Now

In thinking about drought, it's useful to have an understanding of the water cycle now.  This morning, I've been studying a 2006 review published in Science: Global Hydrological Cycles and World Water Resources, by Oki and Kanae (subscription required).  The above figure (click for a larger version in a new window) summarizes the present situation.  The flows are in cubic kilometers per year, while the storage quantities are in cubic kilometers.  Some things that seem noteworthy:
  • The net transfer of about 45 km3/yr from ocean to land, returned to the ocean via river flow.
  • Human usage of about 10% of that - 2.7 km3/yr for agriculture, 0.8 km3/yr for industry, and 0.4 km3/yr for domestic use.
  • Globally, water input to cropland is still 80% from rain, with only 20% from irrigation.
Of course, while total usage is still a modest fraction of the total available resource, huge limitations are placed by both geographical and seasonal inhomogeneities.  In particular, the distribution of global runoff is as follows:

The blue areas are where the water is.  Obviously, in a lot of cases, this is not where the people are.  A measure of water stress is the total amount of human withdrawals as a fraction of the available river flow.  When this goes over 0.4, water stress is considered high (because at least in the dry season, usage is likely to be almost total with rivers failing to reach the sea).   The map of this index is as follows:

Again, click for larger version in a separate window.  The western US, southern Europe, the Middle East, northern India/Pakistan, and northern China are the epicenters of water stress.  These regional variations lead people to undertake large engineering projects to move water from the unstressed places to the stressed.  Perhaps the most famous is the South-North Water-Transport Project in China.

It's interesting to compare the water stress map above to the first principal component of the PDSI:

Apparently, the dry will be getting drier.

This post is part of a series on the future of drought.


Alexander Ac said...

An wet areas will be getting wetter.

BTW there is a link between (increasing) atlantic hurricane activity and (increasing) drought in Amazonia.

See also this interview in Nature:

Drought strikes Amazon again -


Going Green said...

The thing that I've never understood about the the greenhouse gas warming is that there is little feedback into the oceans of the increased temperature. Or, at least not enough to cause the oceans to warm enough for further evaporation which then would counteract the expected drought. I can understand warmer and wetter. I'm struggling with warmer and drier--particularly since the oceans are over 70% of surface area.

Is there an easy way to understand how the ocean evaporation is suppressed in a carbon-rich atmosphere?

Mike Aucott said...

Going Green,

The oceans warm more slowly than the atmosphere, but they are warming nevertheless; thermal expansion due to this warming is one reason why sea level rise is accelerating.

There's no reason why warmer oceans would not evaporate more water. My understanding of the reason for the drought predictions in a hotter world is that hotter land surfaces evaporate much more water; therefore in between rain events some areas will dry out more than they do now.