Dear Prof. Simunek

Currently i am doing an experiment to simulate water and heat transport in a soil column and i want to use surface energy balance at bottom BC but it seems this boundary only apply at top (if i'm not wrong).

I tried to simulate such condition with HYDRUS-1D in two direction. One with using surface energy balance BC at top (set decline from vertical axes to 1) and the other one with using surface energy balance at bottom (set decline from vertical axes to -1 and entered the values some factors negative and also changed the graphical editor parameters exactly upside down).

i got exact same results compared to normal situation.

It seems to be working!

With this method or trick, I am able to use boundaries that not exist in one side in another side. Another example is zero gradient BC (for heat) that exist only at bottom. But when i use this method (set -1 for decline from vertical axes), What will happen to gravity? will it also change upside down? or it won't change? also i read the manual and it says only i should use 1 or 0 for direction. Why? Can i use this method for inverse modeling too?

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Also, I noticed when using the surface energy BC, I can not import evaporation in atmospheric boundary condition (this option will not appear)

What type of water BC can i use with surface energy balance BC ?

## Using HYDRUS-1D Upside down!

### Re: Using HYDRUS-1D Upside down!

1. Indeed, the decline from vertical axes is equal to 1, 0, -1 for vertical downward, horizontal, and vertical upward flow. It is the magnitude of gravity. You should be able to run in the inverse mode a project that has gravity=-1.

2. Evaporation is the solution of the surface energy balance equation (see eqs. 2.95 and 2.97). That's why you cannot enter it as input.

J.

2. Evaporation is the solution of the surface energy balance equation (see eqs. 2.95 and 2.97). That's why you cannot enter it as input.

J.