Evapotranspiration Question

[Scott Olson]

Okay, I got a question for someone who is more familar with evapotranspiration. If you had a lake about twenty acres wide and you also had a twenty acre corn field that was fully watered twice daily (mature plants). Which of these two areas will give off the most moisture for the atmosphere? Also, would the air pressure on a decently sized lake be differant than that of the land around it, if so what effect does this have on the moisture content of the surronding area.

Thanks!
-Scott Olson.

 

[William Scheftic]

Well I am not sure, but there are several factors to consider.
1.) How much water would evaporate of the sfc of the water
- Given, temperature, sky cover, latitude, and humidity this shouldn't be too difficult to figure out.

2.) What is the surface area of the stomata of the leaves of a fully mature cornfield compared to that of a flat surface.

3.) How much water is absorbed into the soil?

4.) How much water is used by the plant?

5.) Since the RH is likely alot more below the canopy of a corn field. Does the stomata below the canopy truely transpire into the air. Through time, due to the vapor gradient between the below canopy and above canopy air, water vapor should continually be released through the stomata.

6.) Of course how much water is used to water the corn field.

I may not be considering some aspects, certainly doesn't seem like a simple question.

 

[Jeff Snyder]

Okay, I got a question for someone who is more familar with evapotranspiration. If you had a lake about twenty acres wide and you also had a twenty acre corn field that was fully watered twice daily (mature plants). Which of these two areas will give off the most moisture for the atmosphere? Also, would the air pressure on a decently sized lake be differant than that of the land around it, if so what effect does this have on the moisture content of the surronding area.

Thanks!
-Scott Olson.

Scott,

I don't know about you, but I'd rather chase central IA/IL (cornbelt area) than chase the shores of Lake Superior or some other large lake. With larger lakes, it seems that much of the incoming solar radiation (insolation) seems to go towards heating the water (and increasing it's mixing depth), what would otherwise be called ground flux, than towards latent heating leading to evaporation. With vegetation, it appears that photosynthesis yeilds more moisture for less insolation, thereby giving more sensible heating (i.e. higher surface temperature) with higher dewpoints (couresy of evapotranspiration) than the lake counterparts... I'm no expert in evapotranspiration or surface heat fluxes near lakes, so hopefully folks to can chime in...

 

[Justin Turcotte]

Extremely complex. A few points not covered. Vapor pressure deficit (the larger the greater ET), plant stress (the greater the less T), wind speed (the larger the greater ET), Plant density, soil conductivity/porosity...

There are some reasonably simple equations to calculate evaporation off a body of water. There are many more variables for ET.

 

[William Scheftic]

Extremely complex. A few points not covered. Vapor pressure deficit (the larger the greater ET), plant stress (the greater the less T), wind speed (the larger the greater ET), Plant density, soil conductivity/porosity...

There are some reasonably simple equations to calculate evaporation off a body of water. There are many more variables for ET.

Yeah it seems hard to answer the question. Fairly simple to figure out what's important. Now we just need somebody to put it together. If I wasn't so lazy, maybe I'd get to it. lol.

 

[Joel Wright]

On some summer mornings I've noticed fog covering both a cornfield, and a body of water, while surrounding areas are free of it. I think the fog are caused by two different reasons when this happens. Over the lake the temperature is cooler, substantially dropping the Td depression. Over the (flat) cornfield, the dewpoints are likely higher than the surrounding ambient readings. This would obviously lower the depressions as well.


As Jeff pointed out, a lot of the incoming solar energy over the lake would go towards heating the water. There's still going to be a great deal of evaporation on a given day though. My parent's lake loses a few inches of lake level a week during the summer with a dry spell. I'm not sure how that would compare to the tremendous release in water vapor the cornfield however.

I would be very curious to see the results of this comparison.

 

[Glen Romine]

For those interested in how operational models approach this problem, you may enjoy reading this link or following the suggested references:

http://meted.ucar.edu/nwp/pcu2/etaveg1.htm

It should be noted that this is a vast improvement over older models - where the ground was treated as a 4" think slab of concrete for all practical purposes.

Glen

 

[Scott Olson]

Wow. Great comments everyone. If I had the means I would certainly do an expirement on this. I know there has been a lot of talk on here about moisture content increasing because of the corn fields, which since there is evidence of this. Seems to imply that transpiration gives of significantly more moisture than just having saturated soil. Does anyone know of any weather instrument or buoy that is located in a lake? Maybe even one located near the coast of the great lakes?

Thanks everyone,
Scott.

 

[David Wolfson]

I'd try taking the demand approach. How much water does corn require vs. how much evaporates from a water surface?

All sorts of links on the subject: http://www.cropproductionbasics.com/irrigation.htm
Here's a web find about state-by-state irrigation in general:
http://water.usgs.gov/pubs/circ/2004/circ1...cs/table07.html
See the column headed, "Application Rate in acre feet per acre". As one would expect, it's quite dependent on climate. SD is 1.18 whereas OK is 1.59 and AZ is 6.21(!)

Here's a map of average yearly US pan evaporation (evaporation off an open body of water): http://www.grow.arizona.edu/images/water/panevap.gif

But what about corn in the growing season specifically?

Bingo... for Iowa, a pretty typical state I'd think:
http://www.agron.iastate.edu/courses/agron...son2/2.1.1.html
... specifically for corn, from this link....
http://www.agron.iastate.edu/courses/agron...es/figure16.gif

Ans.: Open water evaporation is always higher than corn evapotranspiration.

 

[Joel Wright]

Originally posted by David Wolfson
I'd try taking the demand approach. How much water does corn require vs. how much evaporates from a water surface?

All sorts of links on the subject: http://www.cropproductionbasics.com/irrigation.htm
Here's a web find about state-by-state irrigation in general:
http://water.usgs.gov/pubs/circ/2004/circ1...cs/table07.html
See the column headed, \"Application Rate in acre feet per acre\". As one would expect, it's quite dependent on climate. SD is 1.18 whereas OK is 1.59 and AZ is 6.21(!)

Here's a map of average yearly US pan evaporation (evaporation off an open body of water): http://www.grow.arizona.edu/images/water/panevap.gif

But what about corn in the growing season specifically?

Bingo... for Iowa, a pretty typical state I'd think:
http://www.agron.iastate.edu/courses/agron...son2/2.1.1.html
... specifically for corn, from this link....
http://www.agron.iastate.edu/courses/agron...es/figure16.gif

Ans.: Open water evaporation is always higher than corn evapotranspiration.

Thanks. Those were some very interesting reads.