## Concentration units

A discussion forum for Hydrus-1D users.
simratpalsingh
Posts: 9
Joined: Thu Aug 20, 2015 5:07 pm

### Concentration units

Hi,

I am trying to simulate the concentration of NO3-N in the soil profile at different depths due to the application of liquid hog manure. I know that hydrus gives out the concentration of solutes in the units mmol/cm3. However, I have the observed data in the units mg/kg which is the concentration of solute in soil. Is there any way I can compare the observed and simulated data ?

Simrat

Jirka
Posts: 5216
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Concentration units

In HYDRUS you can actually use any concentration units for concentrations in the liquid phase you want, e.g., dimensionless, g/L, mmol/L, mmol/cm3, etc. Since the governing convection-dispersion equation is linear (unless you consider nonlinear sorption), the final results are the same no matter what units you use, as long as they are consistent between initial and boundary concentrations, and (zero-order) reaction constants. If you have observed concentration data in mg/kg(of soil), you need to convert them to mg/Volume(of soil), i.e., in total concentrations, and then you can use them as well, providing that indicate that (i.e., in Solute Transport Boundary Conditions window you indicate that Initial Conditions are in total concentrations).Hydrus will then distribute solute mass between different phases based on the distribution coefficients.

J.

simratpalsingh
Posts: 9
Joined: Thu Aug 20, 2015 5:07 pm

### Re: Concentration units

Hello Jirka,

I've an another question regarding the units for root solute uptake. Since the concentration units of input and results are consistent in Hydrus 1D. Lets assume if I input the liquid phase initial concentrations of nitrate in mg/l and if I also consider the root solute uptake, then what would be the units of root solute uptake ? I can see it in mg/cm2 in my results. So shall I just multiply the root solute uptake by 100 to get the results in mg/ha ? I'm a bit confused.

Jirka
Posts: 5216
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Concentration units

Root Water Uptake (nodal), S: [L3L-3T-1]= [T-1]
Transpiration, S*depth: [L3L-3T-1][L]= [T-1] [L]= [L T-1]
Cumulative root water uptake (cumulative transpiration), S*depth*time: [L3L-3T-1][L][T]= [L3L-2]=[L]

Concentration, c: [ML-3]
Solute Uptake (nodal), Sc: [L3L-3T-1][ML-3]=[ML-3T-1]
Solute Uptake (profile), Sc*depth: [ML-3T-1][L]= [ML-2T-1]
Cumulative solute uptake (profile) , Sc*depth*time: [ML-2T-1][T]= [ML-2]

lotbaert
Posts: 2
Joined: Wed Jun 17, 2020 12:28 pm
Location: Belgium

### Re: Concentration units

Dear Hydrus users

I would like to insert an initial concentration of a pollutant in the profile graphical editor (Hydrus 1D). My measured concentrations are in mg/kg. Am I right if I do this using the following procedure:
- Solute transport general information: change mass units to mg
- Transport boundary conditions: change initial condition to 'in total concentrations mass solute/volume soil'
- Soil profile graphical editor - initial conditions concentration: now I can insert my initial concentration in mg/l soil ?

Lotte

Jirka
Posts: 5216
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Concentration units

The default input for initial concentrations is “liquid phase concentrations”, which is in [mass/volume of water, which are standard concentration units]. Alternatively, one can enter as an initial condition “total concentrations”, which are in units of [mass of solute per volume of soil]. The latter option needs to be selected in the “Solute transport – Boundary Conditions” dialog window. If the latter option is selected, the total concentration is then divided into different phases (liquid, solid, gas).

If your measured concentration is in [mg/kg], you need to convert this value into units use by Hydrus using either dry or wet bulk density (depending on your measurements).
J.