Posted by: shipbright | October 19, 2009

Basics: The hydrologic cycle…the engine of the world’s water

The last couple of posts have reflected emerging mainstream awareness that there is an issue with freshwater scarcity on our planet – all steps in the right direction.  But right now, I want to come back to some of the basics needed to understand how the Earth works in regards to water.

Most of you readers have studied the hydrologic cycle in science class some time ago.  It’s important to revisit this natural phenomenon because it creates our freshwater.  Reacquainting oneself with the natural forces at work will also help gain insight as to why a longterm change in the world’s climate can dramatically effect the cycle of water.  The Earth is resilient, but not bullet-proof, and can take a lot of abuse but there is a tipping point.  Climate change is a controversial issue and I’ll tackle that head on in upcoming posts but for now let’s just revisit the hydrologic cycle as a starting point:


We’ll take a starting point with evaporation and evapotranspiration [see my earlier blog on evapotranspiration and NASA satellites] and so it begins.  Leaving salt and minerals behind the water travels into the atmosphere as newly transformed freshwater and returns to earth in the form of rain or snow.  There the water will run off into streams, rivers and lakes to head back to the ocean.   The freshwater might instead become “historical” water residing for months, years, decades or centuries by seeping into groundwater aquifers or remain as snow or ice in glaciers and polar ice caps before eventually returning to the oceans.

I’ve been scouring the internet and my library to  present this information in a way that is a bit more interesting than just a static graphic like above.  Fortunately, NASA has put together a short video that has some cool graphics [but corny music] that make it a bit more interesting than what I had back in High School science class.  I particularly like the graphic of how the sun generates evaporation and evapotranspiration of water into the atmosphere around the globe:

The question we need to ask ourselves is how the world’s hydrologic cycle is being effected by population growth, food production, urbanization, deforestation and climate change. 

I ask those who are skeptical of climate change to suspend that skepticism for a moment and restart the conversation.  The hydrologic cycle is something we should all be able to agree upon and we should be able to agree that changes to the variables that are part of  the natural cycle can cause changes in the cycle and outcomes.  The controversy of climate change is “is it a natural cycle of climate change” or is it “man made”? 

We’ll talk about that in another post but for now as part of the civil discourse in addressing the fundamental and critical need for freshwater, which all non-marine biota depends upon for life, let us agree that life is about balance and renewal…


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