## Solute concentration on day 1

A discussion forum for old Hydrus-1D users.
brittanydove23
Posts: 6
Joined: Thu Dec 07, 2023 3:58 pm
Location: South Africa

### Solute concentration on day 1

Dear Ms./Mr.,

I am modeling the transport of a growth inhibitor applied once via drip irrigation in HYDRUS-1D. To simulate this application, I set a concentration (10) on day 1 under Ctop (variable boundary conditions) and no concentration for the rest of the simulation period. I also set the upper boundary to concentration BC. On day 1, I set the precipitation to 1, evapotranspiration to zero, and plant uptake to zero.

I have two different orchards, and the concentration vs. depth graph shows very different total concentrations on day 1. This discrepancy is puzzling because the initial conditions should be identical in both orchards. At depth 0, both graphs show a concentration of 10, which decreases to zero at about 10 cm. However, when calculating the entire concentration on day 1 from all depths, one orchard appears to have double the concentration of the other. How is this possible, given that I used the same concentration under Ctop with the same precipitation and evapotranspiration?

How can I ensure they are equal since I have applied the same initial conditions? Can I assume that the applied concentration is the same even if it appears differently on day 1?

Additionally, if a chemical has a lower Kd in a certain soil, will HYDRUS simulate higher plant uptake as a higher concentration is in solution and thus can easily be taken up through the roots, compared to a soil with a higher Kd where most of the concentration is sorbed? By the end of the season, will the soil with the lower Kd have a lower concentration because more of the chemical was removed through plant uptake (assuming leaching is not possible below 1m)? Does HYDRUS account for variation in plant uptake as a function of the Kd value, or must I input this manually?

Warm regards,
Brittany

Jirka
Posts: 6079
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Solute concentration on day 1

I will not comment on your simulation results as I have not seen the projects. People always claim that they do something and then it turns out (most often than not) that it is not the case. There are surely reasons for any observed differences.

Kd: In the short term, yes. In the long term, that would depend on many other conditions (and could be the same). HYDRUS certainly accounts for Kd; RWU uptake occurs from the liquid phase solute, which depends on Kd.

J.

brittanydove23
Posts: 6
Joined: Thu Dec 07, 2023 3:58 pm
Location: South Africa

### Re: Solute concentration on day 1

I’ve realised that the difference in concentration on day 1 is heavily influenced by the Kd value. If the Kd values are the same in each orchard, the concentration on day 1 is identical. However, the Kd values are quite different in each orchard. My understanding is that the same concentration should be present on day 1 regardless of the Kd value since it is the day of application. Please help me clarify this.

Additionally, the initial conditions are specified as total concentrations, so it shouldn’t only reflect the concentration in the liquid phase. Even when I input liquid concentrations, the concentration on day 1 remains unchanged.

Here’s how I’m calculating the concentration:

Total Concentration=(conc1+conc2)/2×height
where the height is 1.5 cm intervals (using the area formula for a right-angled trapezoid).

In the soil with a lower Kd, the concentration on day 1 is much higher because the concentration gradient goes from 10 mg/cm3 at the surface to 5.3 mg/cm3 at the next interval, and continues to decrease. This results in a higher total concentration. Conversely, in the soil with a higher Kd, the concentration decreases from 10 mg/cm3 at the surface to 2.88 mg/cm3, resulting in a much lower total concentration.

However, when I set the Kd values in the topsoil to be the same, the concentration on day 1 becomes identical in both orchards.

Please help me understand why this happens and how I can accurately model the same concentrations on day 1 with differing Kd values.

Warm regards,
Brittany

Jirka
Posts: 6079
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Solute concentration on day 1

I'm teaching this week a HYDRUS short course in Prague. I will answer questions in the discussion forum later in the week when I'm less busy with the course. J.

Jirka
Posts: 6079
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Solute concentration on day 1

a. Your assumption is incorrect. If you have the same solute mass inflow and different distribution coefficients, then you clearly will have different initial liquid phase concentrations, since the inflow mass will be distributed differently between the solid and liquid phases. To have the same liquid phase concentrations in different orchards with soils with different Kds, you would need to adjust the solute inflow (and not use the same one).
b. If you specify initial conditions in terms of the total concentration, this solute mass will be distributed by the code between different phases (solid, liquid, gas), and the code will determine the initial liquid phase concentrations (based on Kd, Henry, etc).

J.

brittanydove23
Posts: 6
Joined: Thu Dec 07, 2023 3:58 pm
Location: South Africa

### Re: Solute concentration on day 1

Thank you!

So even though I selected total concentrations in the initial conditions, my concentration vs. depth graph only shows liquid phase concentrations, even though it is also distributed between the solid and gas phases?

I have now adjusted the Kd values within the same orchard to compare differences due to this one parameter, as all other parameters are the same and excluding volatilization. My assumption was as the water content and solute flow rates etc. were identical, I could determine the total concentrations by working backwards from the Kd value using the concentration vs depth graph using the following equation:

𝐾d=Concentration (solid)/Concentration(liquid)
Total concentration=Concentration (liquid)+Concentration (solid)...... no gas phase

However, this approach has not resulted in the same total concentration on day 1; the values are significantly different. How do I figure this out?