n_stress_expand and swdef_expand ranges and meanings in wheat

[JJguri]

Could some one explain which is the difference between x_stress_expand and xdef_expand? I am using swdef_expand to test water stress on wheat simulations. However when I want to use the same parameter for N it does not exist. Therefore, I am using n_stress_expand. Although, they have the same value range 0-1, I can deduce that stress is higher close to 1 in n_stress_expand, which is the contrary for swdef_expand. While I can see values of n_stress_expand during a wet year, it is 0 during dry years. Is it because there was not a N stress on the crop under the dry year?

[peter-devoil]

Each of those stick functions has a x_ABC and y_ABC parameter. The x_ABC parameters are the x coordinates of the stick function, y_ABC similarly for y coords Sometimes ABC relates supply:demand ratio to some stress process (eg leaf expansion here). You may not see N stress if there's no water flow to transport N?

[JJguri]

Thanks @peter-devoil . Could you please let me know where they are defined? Particularly swdef_expan and n_stress_expan. I can see in one the stress is close to 1 and in the other close to 0, so I want to understand the ratio and the numerator and denominator of the ratios.

[peter-devoil]

It's implemented here. Parameters read here. Wheat parameters here.

If you want to explore it; do a 100 year (dryland) run with daily outputs of water supply (esw) & demand (Eo). Plot their ratio against swdef_expan, and verify it's the same slope as the parameters describe.

You'll find N calcs in the same file; but remember that N uptake only happens when water is being sucked out - ie by mass flow. If there's no water flow the calculation is irrelevant.

[JJguri]

thanks @peter-devoil, is there a variable/parameter which specified if there is enough mass flow or not for N uptake?. Otherwise, I need to classify seasons when the crop was under (or not) water stress and assume that when it was, the N uptake was not happening. I will explore the calculations of the N factors, so it they are 0 probably it is because there was not flow that day.

[JJguri]

@peter-devoil do you think the calculation of the NNI could be useful to assess the level of N stress during the growing season? I output this daily and as an average of the growing season. It was calculated as follows (Cte is only for comparison purposes):

if (biomass_wt > 0) then
    Na = biomass_n/biomass_wt*100
    NNI = Na/n_conc_crit
    Cte = 1
endif

[peter-devoil]

I'd leave that Q to somebody more qualified. There has been much discussion on the merits of using a target n concentration vs SLN. If I understand correctly, n_conc targets vary through the plant's lifecycle, where SLN doesn't. @uqghamme and @hut104 may have more detail, and certainly more reading material.

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