Tuesday, June 15, 2010

Watersheds in a nutshell in a nutshell

Hello!  I would like to describe to you the issues that are paramount to my watershed in such a way that you learn about watershed management principles.  First of all, what is a watershed?

A watershed is a piece of land that drains water to an outlet.  An outlet could be a river mouth or an aquifer*.  There is no scale requirement for the definition, and there can be watersheds within watersheds.  They can be super-big or tiny.  For example, the Fort Branch watershed is neighborhood-sized and is a subwatershed of the Austin watershed (city-sized), which is a subwatershed of the Colorado River Basin (about Texas-sized).  Live in the U.S.?  You can find your watershed here (go to the water section and click "Hydrologic Units," then on "Redraw map") and here (scroll over the names to see the watersheds light up).  But technically, you can make a watershed while building your sand castle.  
OK, so technically this isn't a watershed since it isn't "land," but close enough.  Look, there is an outlet!

So, the watershed, like all things, participates in the hydrologic cycle.  As in, when it rains on the land (and you inhale the sweet scent of actinomycetes) some water…
  • Infiltrates into the soil and is either stored in place or percolates further down to the water table; or 
  • Becomes overland flow, runs off, and maybe ends up in a river down the hill; or 
  • Transpires.  "Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. Transpiration is essentially evaporation of water from plant leaves (reference)." Some water
  • Evaporates up from the land to the clouds again. Evaporation is "the process by which water changes from a liquid to a gas or vapor (reference)."
There are other parts of the water cycle not mentioned here, like water storage in glaciers.  Want more details? Click here.

Here is a picture courtesy of the USGS that helps the visual learners.
Thanks, USGS.

So, what is going on with the hydrologic cycle in my watershed? Well, there are some complaints.  If you read this report, you’ll see that aquatic habitat in the creek has been rated very poorly.  Why? Well, if aquatic habitat isn’t even in the creek (for the most part), you can’t rate that category very highly, can you?  And what does aquatic wildlife need, but …water (aquatic, water - you see where I am going with this)?  There isn’t enough water in our creek to support aquatic habitat.  Now, the cynics out there might ask, well… so? Is not having aquatic wildlife bad?  Well, I guess it is up to us to decide. But let’s discuss why there is not that much water in our creek, and then we can decide what to do about it. 
Here we are upstream, a day after a big rain.

Remember, oh so long ago, when we discussed how rainwater either soaks into the ground or runs off, down slope? OK, well when you cover up your watershed with impermeable stuff like cement and asphalt, the rain is more likely to runoff.  Since it will drain off all at once instead of soaking in, the river draining this watershed gets a lot more water at one time.  So with the same amount of rain, the water flowing in a river that drains a "pavement watershed' will be high all at once, and then a lot lower later on (drip, drip).  A watershed covered in permeable stuff like soil will contribute water to the river flow much more slowly, so the river flow would be more constant.  River water flow in a "soil watershed" wouldn't reach the high peak it reaches for a “pavement watershed.”

So maybe, we don’t have so much aquatic habitat or wildlife because our watershed is partially covered in pavement, which is preventing the soil from absorbing rainwater and helping to keep the river flow at a more constant level. Gasp!!!  There is SO much more to talk about, so stay tuned! Coming up we'll learn why the Austin watershed experts don't want clean rocks in their creeks (?).

Aquifer: a water-bearing rock that readily transmits water to wells and springs (which can form rivers)  (U.S. Geological Survey, 2010).
Water budget: the study of the water that goes into a watershed versus what water leaves the watershed.  It is assumed that the water budget of my watershed has changed due to patterns in development.   


  1. Very well done! I will have to make a trip to my local watershed, Tannehill Creek.