## Evaporation from Fracture region

A discussion forum for Hydrus-1D users.
igor.andin@gmail.com
Posts: 13
Joined: Sun Feb 20, 2022 12:06 pm
Location: Germany

### Evaporation from Fracture region

Dear Jirka,

Would you mind clarifying 1 thing, since I didn't find specific information concerning this question on the forum or manul.?
How evaporation is implemented in the Dual-Permeability model? I see that calculations can be done either with Penman-Monteith Combination Equation or with Hargreaves Formula. Does this mean that Hydrus assumes 1 value of ET for the complete domain?

My question arises from the mt physical understanding of the evaporation process, where I expect that the fracture will dry out quicker than the matrix, but the overall evaporation,I believe, will come from the matrix region, explicitly, due to the larger store of water and surface area.

Please can you throw some light on this case?

Jirka
Posts: 5648
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Evaporation from Fracture region

First, the dual-permeability model does not allow you to use the meteo module (i.e., the Penman-Monteith Combination Equation and Hargreaves models); you need to specify directly the potential evaporation. HYDRUS will distribute the surface flux based on “Fraction of Surface Flow into Fracture” (qTopF).

rTop=abs(rSoil)-abs(Prec)
rTopM=rTop*(1.-qTopF)/(1.-wFT)
rTopF=rTop*qTopF/wFT

where wFT is the volumetric fraction of the fracture domain and (1-wFT) is the volumetric fraction of the matrix domain.

During infiltration, when matrix becomes ponded, excess water is directed into fractures.
rTopF=rTopF+min(0.,rTopM-vTopM)*(1.-wFT)/wFT
No such redistribution is done during evaporation.
J.

igor.andin@gmail.com
Posts: 13
Joined: Sun Feb 20, 2022 12:06 pm
Location: Germany

### Re: Evaporation from Fracture region

Just 1 thing to figure out. I am talking now only in the water flow context now. Is it true that parameter SF (surface flux in the fracture) is turned off when the matrix is saturated and generates excess water? I have a feeling that excess water overwrites the parameter vTop. Attached you can find plots of the surface flux with the rain intensity lower than hydraulic conductivity and higher( denoted with "high Rain" in title). In the case of SF0 with high intensity, since there is no partitioning at the surface I expect vTopF =0. Could you please clarify it?

Best,

Andin
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Jirka
Posts: 5648
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Evaporation from Fracture region

Potential flux gets directed to the matrix or fracture based on qTopF (0-1). However, once the matrix becomes saturated, the flux (Neuman) BC for the matrix domain is changed into the head (Dirichlet) BC, the actual flux (for this BC) into the matrix is calculated, and the excess flux (a difference between how much is applied to and how much actually infiltrates into the matrix) is diverted into fractures (in addition to what is applied there initially). J.