Not all Precipitation That Falls in a Watershed Flows Out
After reading through the comments on the previous post - I did a little digging and found this article from the USGS regarding watersheds. Here is the full article: What is a Watershed? I like how the USGS specifically calls out Georgia below and its famous clay soil (bolded below).
Infiltration: When rain falls on dry ground, some of the water soaks in, or infiltrates the soil. Some water that infiltrates will remain in the shallow soil layer, where it will gradually move downhill, through the soil, and eventually enters the stream by seepage into the stream bank. Some of the water may infiltrate much deeper, recharging ground-water aquifers. Water may travel long distances or remain in storage for long periods before returning to the surface. The amount of water that will soak in over time depends on several characteristics of the watershed:
- Soil characteristics: In Georgia, clayey and rockey soils of the northern areas absorb less water at a slower rate than sandy soils, such as in Georgia’s Coastal Plain. Soils absorbing less water results in more runoff overland into streams.
- Soil saturation: Like a wet sponge, soil already saturated from previous rainfall can’t absorb much more … thus more rainfall will become surface runoff.
- Land cover: Some land covers have a great impact on infiltration and rainfall runoff. Impervious surfaces, such as parking lots, roads, and developments, act as a “fast lane” for rainfall - right into storm drains that drain directly into streams. Flooding becomes more prevalent as the area of impervious surfaces increase.
- Slope of the land: Water falling on steeply-sloped land runs off more quickly than water falling on flat land.
Tags: clay, precipitation, soil, watershed
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November 29th, 2008 at 5:09 pm
but there IS some infiltration nonetheless, and the loss of groundwater that the paving over of Atlanta (thus absolutely preventing rain from soaking into the ground) is a significant factor in the weak water levels in the Chattahoochee. More infiltration would mean a higher water table, which would mean better streamflow in dry weather, which would mean less stress on Lanier.
December 1st, 2008 at 3:15 pm
I’m not sure I agree with you.
A paved area returns more, not less, water to the Chattahoochie. The water goes from the road to the sewer to the water plant to the river, with small losses from start to finish.
Atlanta isn’t entirely paved over. There is still plenty of ground for infiltration. And the water table would most likely be lower, not higher if you took away the paved roads - because plantlife would grow in place of pavement.
Instead of water coursing through the system and back to the river, it would be sucked up by thirty plants. In dry weather, only moreso.
That’s why as the article says, a paved (impervious) surface promotes flooding - because more water gets into the stream.
If you didn’t have paved areas, you wouldn’t have MORE water in the river necessarily - just a less volatile water level.
December 2nd, 2008 at 11:35 am
Wrong wrong and wrong again! The runoff means the water gets into the chattahooch immediately, but then is gone in a couple of days. By contrast, groundwater formation means that the water seeps into the river gradually, even during periods when there is no rain. Same thing with plants. They act as storage, and keep the soil moist in dry weather. Atlanta has literally paved its way into drought. The problem is not rainfall, its that the rainfall disappears down the storm sewer immediately never to be seen again.
December 2nd, 2008 at 12:50 pm
“By contrast, groundwater formation means that the water seeps into the river gradually, even during periods when there is no rain.”
Sounds a lot like…
“If you didn’t have paved areas, you wouldn’t have MORE water in the river necessarily - just a less volatile water level.”
Stop rephrasing my comments and using them as your own.
Water will seep into the river only when the water table is higher than the level of the river. During a huge dry spell like this, that may not always be the case. There is a reason that natural riverbanks are almost always lush even in a dry spell - during a dry spell the river tends to donate to the surrounding water table.
1/5 of an inch of rain amidst a multi-year drought isn’t going to alter that.
Plants keep soil moist in dry weather? I’m pretty sure that plants conserve water during drought conditions, and uptake water whenever it is provided under those conditions. Certainly the Royal Horticultural Society seems to think so. Plants are storage devices for water - but they store it for their own uses; they don’t re-release it into the ground when it’s dry out to be… what, magnanimous? Charitable? Suicidal?
Atlanta has not paved it’s way into the drought - Atlanta’s pavement has nothing to do with this drought. If Atlanta were razed to the ground, the drought would persist. Take a look at Atlanta on Google Maps sometime - it’s not a giant parking lot. Heck, the river itself is surrounded by forests and parks throughout most of it’s trek through the city.
December 2nd, 2008 at 7:59 pm
OK, let’s try to take it slowly. Say you have three areas with similar topography. One is paved with cement. One is grassy, one is forested. On the same day, an inch of rain falls one each. Then seven days pass. Which one will have the most moistest in the soil?
December 2nd, 2008 at 11:18 pm
Josh, you make an interesting point, but rkolter is certainly correct on the major points. You also have to remember what a tiny portion of the acf watershed is covered by concrete. Any effects will be localized and short duration.
But rkolter - just since I don’t want you to get a big head about being right and all, I decided to dredge up an old thread from this spring regarding where you thought the level would be around now -
http://www.atlantawatershortage.com/20080326/weve-improved-to-extreme-drought-status/
rkolter Says:
“My purely speculative guess is that we will breech the 1030’s, but not by much, and stay above dead pool.”
And not to toot my own horn but in that same thread…
RichS Says:
“Andrew & Rkolter - I think both of your predictions are a little toward the pessimistic side” …… blah, blah, ……
“…..a fair guess might be that 2008 ends up at a similar level as 2007.”
Toot.
December 3rd, 2008 at 10:37 am
Richs- Heh! Go ahead and toot your own horn. That’s one prediction I’m delighted to be wrong about.
Josh - Take a moment and decide what you’re hoping to prove. Plants don’t act as water storage for the land, keeping the soil moist in dry weather. They just don’t. You’re wrong.
A forest might have moister soil a week after a one-inch rain because the land under the canopy is often cooler and out of the direct sunlight, and the air is more humid thanks to plants under the canopy releasing water as they respire.
Now, a grassy patch - grasses have shallow, dense roots. An inch of rain may never make it past those roots - if the ground underneath was dry to begin with, a week later after an inch of rain, it’ll still be bone dry - and your grass will be wanting more water.
A cement slab is a conundrum - any water that gets under it will likely stay there a very long time - water infiltrates homes thanks to this very situation. But, cement slabs are heavy and tend to compress the dirt under them. An inch of rain won’t land on the ground itself of course, but the rain on the ground around the slab will percolate slowly into the soil under it. But, probably not far or for long - it’s just an inch in a drought.
But an inch of water on barren ground? It’ll soak in and make mud. That mud will cake. A week later you might find damp earth just under that caked mud. With nothing to prevent the rain from landing on it and nothing to absorb the water, the land itself does a pretty good job of staying moist.
Winner - barren ground or a forest. Loser - grass or concrete.
But again, the concrete of Atlanta is not a single sheet - there are open spots just about everywhere - it would be akin to glueing 100 1″ square pieces of plastic randomly on your body and hoping not to get wet when you get sprayed with a hose.
December 3rd, 2008 at 10:41 am
“The water table would be lower not higher if you took away the pavement.” - rkolter
OK, lesson 2, lets do a kitchen experiment. Take three kitchen sponges. One is slightly moist and springy, one is hard and dry, and the third is made of concrete. Run the tap on each one for 3 seconds. Describe the results.
December 3rd, 2008 at 10:50 am
wait, RichS, just remember you are defending someone who believes that a forest is the enemy of a well-functioning watershed
December 3rd, 2008 at 8:08 pm
Now you’re just being a troll.
December 4th, 2008 at 8:12 am
here’s a map of your 1″ square pieces of paper. There’s quite a lot of them.
http://www.ucriverkeeper.org/hardscape.htm
December 4th, 2008 at 8:18 am
there’s an ever decreasing percentage of the flow in the Chattahoochee that’s coming from groundwater. This is proven fact, and it means that in order to maintain adequate flow for sewage treatment plant dilutions so that dissolved oxygen levels are maintained to an EPA standard, and to provide the downstream users with their statutorially entitled volumes of flow, a like amount more needs to come from storage. If infiltration into the ground were dramatically improved, then storage releases from Lanier could be reduced. This is the basic meaning of the phrase “Atlanta has paved its way into drought.”
December 4th, 2008 at 9:18 am
Josh,
I took some time out to do some research before responding to you.
Ground water levels have not changed substantially in Georgia over time. Here’s the archives for USGS monitoring:
http://ida.water.usgs.gov/ida/available_records.cfm?sn=02334430
And when there is a change, it is due to the substantial amount of groundwater pumping stations in Georgia. Here’s the report for this year:
http://pubs.usgs.gov/fs/2008/3072/pdf/fs2008-3072.pdf
The root problem is that you are taking something you think is a fact, and something that is a fact, and linking them when there’s no data to do so. Even if ground water levels were being depleted at a growing rate, and even if Atlanta’s being paved over, the two are not clearly linked.
The image you provided seems reasonable - Atlanta is growing. But what you are missing is that the region that is colored - that has pavement, is not entirely paved. It’s not a bowl, but a sieve. Look at modern suburbia - roads, driveways, homes, gas stations… and trees. And yards.
The statement that “Atlanta has paved its way into drought” is deliberately misleading - the drought would exist with or without atlanta or pavement.
March 14th, 2009 at 7:40 am
I must say, I could not agree with you in 100%, but that’s just my opinion, which could be wrong.
p.s. You have a very good template for your blog. Where did you find it?