## Transpiration calculation

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kalenrob
Posts: 2
Joined: Mon Jun 07, 2010 2:03 pm
Location: USA

### Transpiration calculation

I ran a very simple simulation on a 5' deep X 5' dia lysimeter and I'm trying to figure out how I'm getting bottom free drain flux when precip =0.274 cm, transpiration= 0.44 cm, root uptake = 0.33cm, evap = 0 cm, s-shape model with 0.5 reduction factor. The bottom flux is equal to about 60 cm. However, when I set evap=0.44cm and transpiration=0cm, bottom flux will = 0cm. Why is this?

Jirka
Posts: 5226
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA
You must have something wrongly set up in your project. I would expect larger drainage for the case with evaporation than with transpiration.

However, I can not ask you to send me the project as I do not have the old version of HYDRUS installed on my PC anymore.

Jirka

Jirka
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Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA
BTW, did you specify root spatial distribution in the BOUNDARY module? J.

rvang
Posts: 46
Joined: Wed Apr 24, 2002 7:22 pm
Location: USA
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I think you need to revisit your numbers. Precipitation appears less than your root water uptake, so perhaps you should not expect much if any drainage from the column.

What are the units in space and time? Having leaching of 60 cm (per day?) when precipitaiton is only 0.274 cm (per day?) during all of the experiment?)does not make sense also. Unless the lysimeter (150 cm high?) was completely saturated at t=0. Was drainage 60 cm3 (volume?) or 60 cm (i.e., per unit cross-section)?

What was the experimental bottom bundary of the lysimeter? Is it a seepage face condition (free gravity outflow under atmospheric conditions), or an imposed pressure head? Look at the pressure head profiles during the simlations to see how the profiles behave in time.

Regards. --Rien van G.

kalenrob
Posts: 2
Joined: Mon Jun 07, 2010 2:03 pm
Location: USA
I apologize, it's 60 cm3 cumulative drainge at the end of 365 days. So it's actually a small amount. The boundaries are no flow on the sides, atmospheric at the top and free drainage at the bottom. The intial condition is 14% moisture. It's axisymmtric flow with root uptake of 0.33cm/day in the upper 30 cm.