concentration and RWU problems

[nadiakarima]

Dear all,

I hope you've been well.
I am a Master student from Indonesia, and very new at using this HYDRUS software. I've been trying to model root water uptake, water flow and solute transport with wetland (CW2D). There are quite a few questions regarding the result I obtained.



1. As attached, I saw that there is a jump in the outlet part for pollutants, while the only input is from the upside of the wetland. I am not sure how it happened and would like to know if I had done something wrong in the model.



This is from the observation point I made in the outlet.



This is from the simulation animation. It happened very briefly, only for 0.05 days and it was gone by 0.075 days. The concentration also goes in the opposite direction of my assigned outlet. I made the outlet on the lower right corner of my model, while the pollutants moved to the other side. I wonder if I had done something wrong in the simulation.



2. Regarding the root water uptake, I noticed that when I run the simulation, the result is a bit different from the files examples in the program, whereas the root water uptake doesn't change over time, while mine changes over time.



This is the initial condition, with the root 20 cm deep into the soil.



This is by the time of 0.3 days



This is by the time of 0.625 days, and it ceases to zero by the time of 0.675 days. Does this mean the root uptake has reached maximum capacity since it ceases to zero, and all the solutes are moving with the water?



That would be all my questions for now, as I'll be trying to figure them out again. I hope my question is not too shallow.
Thank you in advance for your assistance.

Sincerely,
Nadia

[Jirka]

Answers:
1. Infiltrating water has NO3 concentrations of 270 and more, and thus I do not find it suprising that the outflow concentrations reach this value as well.
You have water entering into the domain at the top right corner (during the entire simulation), but initially also from the bottom left corner (since you impose a BC of zero head, which is larger than the initial condition (h=-0.4 m) there). You use the same solute concentrations to enter the transport domain on both sides. After a while, water starts leaving the domain at the bottom right corner (but that happens only after you develop a saturated zone at the bottom of the domain), and then you likely get what you expect.
2. The root water uptake will reflect changes in the saturation status in the domain. You are using the Feddes stress response function, which reduced uptake both at full saturation and when the soil becomes dry. You fully saturate the domain after some time and then root water uptake completely stops.

Recommendation:
You should start with the standard HYDRUS code before starting to use the Wetland module. You need to first understand water flow in your project, and that is the same in the standard code and the Wetland module. Only once you understand the flow (and you clearly do not at present), then you should move on to the Wetland module with its complicated biogeochemical reactions.

J.