## Conditions determining type flux across the soil surface

A discussion forum for Hydrus-1D users.
hydrogal
Posts: 2
Joined: Fri Nov 02, 2018 4:54 pm

### Conditions determining type flux across the soil surface

Hi,

I saw in the manual that infiltration occurs as either a prescribed head or prescribed flux condition, depending on these two equations (2.72 ans 2.73 in the manual)

|-K (dh/dx) -K | <= E
ha <= h <= hs

Where ha and hs are the min and max head at the soil profile, respectively and E is the max potential rate of infiltration. However, I am still confused about when each condition would be implemented according to these conditions.

I am looking for further explanation of this than what I have seen in the manual, or if there is a section about this I may have missed?

Thanks.

Jirka
Posts: 5268
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Conditions determining type flux across the soil surface

The lower limit, ha, is applied during evaporation and the upper limit, hs, during infiltration.

Check out our newest posting at: https://www.pc-progress.com/en/Default. ... filtration. Here we post examples demonstrating the use of HYDRUS-1D to simulate infiltration from ponding and rainfall and issues associated with this process.

J.

snirpardo
Posts: 2
Joined: Tue May 11, 2021 5:52 am
Location: Israel

### Re: Conditions determining type flux across the soil surface

Hi,
Saw the posting you showed as well as read the relevant papers (Feddes 74 & Hanks 69) about the limitation on the surface flux, but it's still not clear to me what is the mathematical procedure that limits the flux - how do you calculate E? it seems that as for transpiration in the simulation we don't give all the relevant data in order to use Feddes's 8th equation [X(t) = delta/(delta + gamma) * Rn * exp(-0.39* LAI)], and as for infiltration I haven't saw any equation that represents the maximal infiltration rate in the current soil status.

Snir.

Jirka
Posts: 5268
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Conditions determining type flux across the soil surface

Once the critical pressure (e.g., hCritS for infiltration and hCritA for evaporation) is reached, we switch from the Neuman (flux) BC to the Dirichlet (head) BC. The code then calculates the flux for a given flow field. We do not use any other equations than the Richards equation. This is described in the manual. J.

snirpardo
Posts: 2
Joined: Tue May 11, 2021 5:52 am
Location: Israel

### Re: Conditions determining type flux across the soil surface

Thanks,
must have missed it in the manual.