Hydrologic cycle

Behold how this drop of seawater
has taken so many forms and names;
it has existed as mist, cloud, rain, dew, and mud,
then plant, animal, and Perfect man;
and yet it was a drop of water
from which these things appeared.
Even so this universe of reason, soul, heavens, and bodies,
was but a drop of water in its beginning and ending.

from The Secret Rose Garden: Mahmud Shabistari, Translated by Florence Lederer / Edited by David Fideler

Lecture index: Nature of H2O as a compound, Importance of water, Standard depiction of the water cycle, Hydrologic cycle step by step, How much H2O is where??, Residence times.

Nature of H2O as a compound

Water is a simple compound, one whose chemical formula you are highly likely to know. It has at least two somewhat unusual traits.

The above is a map of phases for pure H2O. Add some dissolved material to the water and the boundaries change. As common experience confirms these transformation are of significance to how the hydrologic cycle operates on earth.

Water's importance to humans

What is the importance of water in geology and human endeavors?

Standard depiction of the water cycle

This diagram from the USGS shows a fairly standard and useful depiction of the water cycle. Note how it informs the reader not only with words, but visually. Which elements of the picture represent reservoirs? Which represent transfer processes? What elements might be added for more complete understanding of the hydrologic cycle? This is a good place to start in learning about the water cycle, but it is just a start.

Hydrologic cycle step by step using system approach

There are several strategies for constructing system diagrams. The one we will use is to first brainstorm about the various components without considering how they are linked. First, reservoirs can be listed, then transfer processes, and then variables. Then one can start linking them together in a system diagram, starting with what is most obvious and clear and building out from there. Revisions are to be expected along the way, and new components may be identified.


Image of Smith Falls in Nebraska. What are the reservoirs and transfer processes evident here?

Transfer processes

Variables associated with transfer processes.

What might some rules look like?

Powerpoint demonstration/exercise - construction of hydrologic diagram.

How much H2O is where??

size, cubic miles
% of total
Oceans 317,000,000 97.24%
Ice caps, Glaciers 7,000,000 2.14%
Ground water 2,000,000 0.61%
Fresh-water lakes 30,000 0.009%
Inland seas 25,000 0.008%
Soil moisture 16,000 0.005%
Atmosphere 3,100 0.001%
Rivers 300 0.0001%
Total volume 326,000,000 100%

Source: Nace, U.S. Geological Survey, 1967

How would you make such estimates? Which one of these is perhaps most surprising? Which one fluctuates the most? Which one has the most variability?

Residence times

One of the outcomes of a system model can be an idea of the residence time of the entity being modeled in a certain reservoir. How long can water be 'locked' up in a glacier before it melts and moves along other paths of the hydrologic cycle? Another possibility is that the residence time for a reservoir is known or can be estimated, and this constraint can help build a better system model. Of course, individual molecules of water will spend different amounts of time in a given reservoir, and so averages are being considered.

Which reservoir of the ones listed in the table above do you think has the longest residence time and which the shortest (let your intuition be your guide to start)?

How might you estimate residence times?

Example: If a mountain glacier persists through time at 10 km long, and moves 10 m a year, then a snow flake that falls on the upper most part and becomes part of the ice will on average arrive at the glacier front 1000 years later to melt and become surface water. So the residence times of water in mountain glaciers is something on the order of centuries to millennia, and the larger the glacier the longer the average residence time. Some ice in Antarctica is hundreds of thousands of years old.

You can also 'date' aquifer water using cosmogenically produced radioactive material and other tracers. It is a challenging task, but provides very useful information. The way this works is that solar radiation is constantly producing cosmogenic nuclides in our atmosphere and at the earth's surface, or it was introduced by nuclear testing. Surface water incorporates these nuclides. When the water goes subsurface, it stops incorporating any of the nuclide, and what it inherited from the surface starts to decay away. With time the level of the nuclide decreases, and so by measuring the amount you can estimate the time since the water was at the surface. Of course, because water mixes, in practice it is more complicated than described above, but hopefully this conveys the gist of the approach.

Why is knowledge of residence times useful in environmental geology, and specifically in understanding groundwater?

Summary diagram for south Florida aquifers taken from USGS web site: http://sofia.usgs.gov/publications/sir/2004-5069/ . Note that, as expected the deeper you go in the aquifers, the older the water gets. The younger the water is the more easily that aquifer can be contaminated by surface sources, and the older the water, the longer it will take to replace any water that is taken out. (U.S. Department of the Interior U.S. Geological Survey SIR 2004-5069 Surface-Water and Ground-Water Interactions in the Central Everglades, Florida By Judson W. Harvey1, Jessica T. Newlin1, James M. Krest1, Jungyill Choi1, Eric A. Nemeth1, and Steven L. Krupa2 1U.S. Geological Survey 2South Florida Water Management District, West Palm Beach, Florida).

Water also permeates art, religion and other human endeavors.

The Negro Speaks of Rivers

Langston Hughes

I've known rivers:
I've known rivers ancient as the world and older than the
flow of human blood in human veins.

My soul has grown deep like the rivers.

I bathed in the Euphrates when dawns were young.
I built my hut near the Congo and it lulled me to sleep.
I looked upon the Nile and raised the pyramids above it.
I heard the singing of the Mississippi when Abe Lincoln
went down to New Orleans, and I've seen its muddy
bosom turn all golden in the sunset.

I've known rivers:
Ancient, dusky rivers.

My soul has grown deep like the rivers.

Source: http://www.poets.org/viewmedia.php/prmMID/15722

The Rime of the Ancient Mariner
Samuel Coleridge -

And now there came both mist and snow,
And it grew wondrous cold:
And ice, mast-high, came floating by,
As green as emerald.
And through the drifts the snowy clifts
Did send a dismal sheen:
Nor shapes of men nor beasts we ken --
The ice was all between.
The ice was here, the ice was there,
The ice was all around:
It cracked and growled, and roared and howled,
Like noises in a swound!
All in a hot and copper sky,
The bloody Sun, at noon,
Right up above the mast did stand,
No bigger than the Moon.
Day after day, day after day,
We stuck, nor breath nor motion;
As idle as a painted ship
Upon a painted ocean.
Water, water, every where,
And all the boards did shrink;
Water, water, every where,
Nor any drop to drink.

For the entire poem: http://etext.lib.virginia.edu/toc/modeng/public/Col2Mar.html