## Need help

A discussion forum for HYDRUS users. HYDRUS is a software package for simulating water, heat and solute movement in one-, two- and three-dimensional variably saturated media. Happy Posting!
medAb
Posts: 6
Joined: Thu Mar 28, 2024 9:34 pm

### Need help

I am a student and I am working on my graduation project.

I'm working on a 2D project for subsurface drip irrigation and I want to make sure what he did is correct. I draw 8 drippers, and the distance between 2 drippers is 100cm. To calculate the variable flux, I did this; The discharge rate for each dripper is 7,2 liter/day, and therefore the variable flux I set is: q=(7,2×1000)/2pi×100=11,46cm/day. Please, is this variable flux correct or should I Multiply by the number of drippers and whether the circles drawn are drippers or are considered like laterals.
2) I simulated evapotranspiration, and when I compared it to the evapotranspiration measured by eddy covariance, there is some similarity, and I want to know how I will explain this matter, knowing that I only set 800 cm as an surface associated with transpiration (in HYDRUS, I simulated only one tree and it can extract water(RWU) from 8 emetters, and the distance between each two is 100 cm), but for eddy covariance, it is calculated at different distances and depending on several factors
3)I want to know the water balance and therefore I want to calculate the amount of irrigation in millimeters, HYDRUS gives me the Cum variable flux in cm², and when I divide it by THE BOUNDARY length associated with the variable flux it gives me a very large number 114mm/day(image below)
I hope you will answer me please, as I no longer have much time before the presentation of my project. Thank you ver much!
Attachments
Water balance
Capture d'écran 2024-06-09 194829.png (30.13 KiB) Viewed 331 times
Shape with 8 drippers
Capture d'écran 2024-06-08 233344.png (60.19 KiB) Viewed 331 times

Jirka
Posts: 6078
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Need help

Ad 1. No, you should not multiply it by the number of drippers. You define the flux, which is then applied at a particular boundary (a dripper). If you have multiple of such boundaries (drippers), it will be automatically integrated.
Ad 2: The total ET flux from simulated domain will be E*A (Atmospheric BC) + T*B (Length associated with transpiration). You need to make correct conversions so that it reflects the ET measured by Eddy covariance. I guess, proportionally (A/B) increase T.
Ad 3. I’m not sure why you would devide it by “THE BOUNDARY length associated with the variable flux”, which is the boundary of drippers. You may consider divide it by the width of the domain.

J.

medAb
Posts: 6
Joined: Thu Mar 28, 2024 9:34 pm

### Re: Need help

1)I understand, thank you very much. Do each of the circles represent a dripper or a section of drip tape? I want to represent only the drippers, not the drip tape. For the flow rate of the dripper, I used this formula: q = discharge rate / (2π × 100).
2)I have a problem is that the potential transpiration is equal to the actual transpiration and the same for evaporation even if the area is not irrigated for 39 days, I don't know where is the problem, I have tried to change hcritA but without any difference.
For the inputs: potential evaporation and potential transpiration I used;
Ep=ETc*exp(-k*LAI)
Tp=ETc-Ep
ETc=Kc*ET0
And: Kc=ET(measured by eddy covariance)/ET0.

Jirka
Posts: 6078
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Need help

1. If you simulate in 2D. then each circle represents an infinite line sourse perpendicular to the transport domain (and you need to adjust the flux so that it correctly represents the flux from individual drippers along a drip line).
2. You need to make sure that you fluxes are correctly defined, that you use correct units, and assign them correctly on the boundaries.
J.

medAb
Posts: 6
Joined: Thu Mar 28, 2024 9:34 pm

### Re: Need help

Thank you very much Professor Jirka.
1)I think what I did is correct so that I put the flux of every single dripper is q=Q/2pi*r*100.
2)I have checked everything (the units and position of each flux) but the actual transpiration and evaporation are still equal to the potential.
3)The issue I'm facing is that I only place the atmospheric conditions on 800cm (100cm between two drippers) corresponding of the emplacement of one tree( boundary length associated to one tree) in a position such that its roots absorb water from 8 drippers, and I want to compare the evapotranspiration simulated(evaporation+transpiration) with the evapotranspiration measured by eddy covariance but I'm unsure how to effectively carry out this comparison and interpret it.
Thank you very much !

Jirka
Posts: 6078
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Need help

I will not be able to answer in the next couple of weeks. This week, I’m traveling in Africa, and next week, I’m giving a HYDRUS short course in Prague. J.