Evaporation & profile thickness

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MeganMasson
Posts: 2
Joined: Mon Apr 08, 2019 11:56 am
Location: South Africa

Evaporation & profile thickness

Post by MeganMasson » Mon Apr 08, 2019 12:14 pm

Hi Jirka,

I want to simulate the recharge to a backfilled pit.

I've summarised the set up below;
  • The initial water content is assumed to be 0.17
  • The material is comparable with a loam in terms of Ksat conductivity.
  • Precipitation and evaporation are applied as daily data with the atmosphere BC with surface runoff and a hCritA value of -150m. The annual precipitation is 830 mm/a and the potential evaporation 1200 mm/a.
  • The bottom boundary is assigned as a free drainage BC (It is assumed the water table is deep)
  • Simulation period of 10 Years
In order to estimate the recharge into the backfilled pit I considered a profile that was 2 m thick and a second profile that was 5 m thick. I anticipated that the actual surface flux in both setups would be the same and the bottom flux between the two would vary over time.

However what I found was that the actual surface flux for the two scenarios was significantly different. The 2 m profile has a much larger actual surface flux (vTop and sum(vTop)) compared with the 5m profile. On closer inspection, I noticed that the difference in the cumulative surface flux (Sum(vTop)) between the two scenarios is due to the actual evaporation. There is much greater evaporation that occurs in the simulation of the 5 m column compared with the 2m column.

As a check, I evaluated columns of 10 and 20 m thickness and in each case the infiltration is the same but the actual evaporation increases and consequently the overall actual surface flux (vTop and sumvTop) reduces with increasing profile thickness.

Why would the actual evaporation increase with increasing thickness of the soil profile?

Thanks,

Megan

Jirka
Posts: 4651
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Evaporation & profile thickness

Post by Jirka » Mon Apr 08, 2019 2:56 pm

There is more water available for evaporation (until it drains through the bottom) in a 5 m soil profile than in a 2 m soil profile.

Make sure that you have the same spatial discretization in the topsoil layer in both simulations. Actual Evaporation calculated by the code is relatively sensitive to the spatial discretization at the upper boundary. Note that the gradient is calculated from the pressure head at two neighboring nodes J.

MeganMasson
Posts: 2
Joined: Mon Apr 08, 2019 11:56 am
Location: South Africa

Re: Evaporation & profile thickness

Post by MeganMasson » Wed Apr 10, 2019 7:36 am

Thanks Jirka

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