Negative nitrate concentrations

[Maria Eliza Turek]

Dear DAISY community,

I am working with DAISY to simulate nitrate leaching in a small region of Switzerland and encountered several examples where the model is simulating negative leaching, although the rest of the simulations seem reasonable. Some values are close enough to zero, but I get extreme combinations of soil/crop rotations where the values go as low as -400kgN/ha. In the attached example, I have a NO3-Leak-Matrix value of -318.27kgN/ha at 2025-8-21.  Has anybody experienced such situations? How did you deal with this?

Thanks a lot for your feedback,

Maria Eliza 

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Maria Eliza Turek

[Kiril Manevski]

Hi Maria,

Isn't it an upward flux? 

You better check the simulated water transport first, then simulated nitrate leaching. 

Many greetings,

K.

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[Per Abrahamsen]

You are using deep groundwater and logging at the bottom of the soil columns, which mean water should only move downwards.  I think NO3-Matrix-Leak should never be negative in such a system, and certainly not like the three big events.

My best guess is that the advection-dispersion equation fails to solve correctly, and Daisy give negative leaching to maintain mass balance.

I suggest trying the 2D solver, which is slower, but uses an entirely different solution to the transport equations.  It is the "Movement rectangle" stuff below:

;; soil column definition
(defcolumn ES_profile default
;(Surface (EpFactor 0.9))
 (Soil
 (MaxRootingDepth 120 [cm])
 (horizons
( -30 [cm] A_soil_20_sample1)
( -90 [cm] dummy_soil_20_sample1)
( -120 [cm] B_soil_20_sample1)
 ))
 (Groundwater deep)

         ;; This switches to the 2D solver:
 (Movement rectangle
   (Geometry (xplus 1 [cm])
     (zplus -2.5 -5 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120)))
;;  (Groundwater lysimeter)
  (Chemistry original
      (combine (N (reaction nitrification
                           (denitrification
                           (water_factor (0.721 0.000) (0.978 0.110) (1.0 1.0)))))))
)

[Maria Eliza Turek]

Thank you Per,

I will try the solution and get back to you!

Have a nice day,

Maria Eliza 

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Maria Eliza Turek

[Kiril Manevski]

Hello, but the negative N flow corresponds to a negative water flow of also large magniture (against a large precipitation event), so it looks like a water balance issue, isn't it better to check the water transport first under "Lysimeter bottom" before going to 2D simulation?

Kiril

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[Per Abrahamsen]

You are right, the problem is on the water. Richards' equation can't solve it,and this gives negative percolation when it should be impossible.

The 2D code may still be worth trying, it will not give negative percolation but maybe a mass balance error instead.

2016-6-15 we get 25 mm infiltration in a very dry soil.  This causes the soil water content to increase from 154 mm to 540 mm, which 1D Daisy interprets as 330 mm must have come from below.  If the 2D solver gave the same result, it would interpret it as a mass balance error.

Kindly 

Per 

[Kiril Manevski]

Hello,

Some good arguments in the emails to write a nice sentence why 2D was selected,

Many greetings,

Kiril

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[Maria Eliza Turek]

Dear Per and Kiril,

The 2D solution solved the problem indeed. No negative leaching and no upward water movement at the bottom of the profile. 

Thanks a lot for the suggestions!

Maria Eliza 

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Maria Eliza Turek