## Time variable boundary conditions and solute transport (hydrus 2D)

A discussion forum for users of the new HYDRUS 2D/3D. HYDRUS is a software package for simulating water, heat and solute movement in two- and three-dimensional variably saturated media. Happy Posting!
Ann
Posts: 2
Joined: Tue Sep 27, 2022 1:13 pm
Location: Italy

### Time variable boundary conditions and solute transport (hydrus 2D)

Hi
In my simulation I consider a vegetated soil substrate (with certain characteristics) and I want to simulate water flow, solute transport (nitrogen decay chain) and root water uptake. My doubts concern:

'Time variable boundary conditions'
I have entered precipitation in cm/hour, but I have doubts about other variables so I would like to know if the following assumptions are correct:
- for evapotranspiration calculation I used the Penman-Monteith equation (cm/hour)
- for partitioning of evaporation and transpiration, I used Beer's law to calculate the required Tp (cm/hour) using an appropriate k, kgr and LAI
- For h crit I used the formula given in the program help: Hr=exp(hMg/RT) but the values are really huge (order of magnitude 10^8 cm)
- I didn't enter any values in other columns

Solute transport
I would like to know if the following assumptions are correct:
- Neglecting transition to NO2 and considering only NH4+ and NO3-, I entered 2 as number of simulated solutes
- Referring to initial concentrations of NH4+ and NO3- (obtained from preliminary tests on a soil sample), I inserted them as constants throughout the soil domain
- in the nitrogen chain, if I don't consider denitrification and therefore NH4+ is fully converted to NO3- and considering that nitric acid is not adsorbed, is it correct to enter no reaction parameters for NO3- and instead enter kd, SinkS1' and SinkL1' values for NH4+?

Jirka
Posts: 5614
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Time variable boundary conditions and solute transport (hydrus 2D)

Time variable boundary conditions: Seems OK. If hCritA is too large, you can use lower values (on the order of 100-500 m depending on the texture). The results are not particularly sensitive to this variable.

Solute transport:
- Neglecting NO2: That’s commonly done as nitrification of NO2 to NO3 is much faster than nitrification of NH4 to NO2.
- Parameters: Seems OK. Nitrification requires aerobic conditions, denitrification anaerobic conditions. You are unlikely to have both.

J.