Vuichard's page on MergeOCN

This page describes the work of inclusion of the N cycle in the trunk of ORCHIDEE based on the work of S. Zaehle. It is based on two versions of OCN. The "original" one as given by S. Zaehle in 2009 (rev 19, source:branches/orchidee-N@19) and the one modified by B. Guenet, N. Viovy which corresponds to the head of the same branch (source:branches/orchidee-N@19).

We first aim to implement the N-cycle in a version closed to the trunk version of ORCHIDEE but that accounts for the re-formulation of the aloccation (making use of labile C pool of reserve) as done by S. Luyssaert in the DOFOCO branch. To do so, we started from a "hybrid" version where the sechiba component is from the trunk version of ORCHIDEE and the stomate component is from the DOFOCO branch. The work of integration of the N specifities has started from this version that is stored here : source:branches/orchidee-N

Here below, are described the differences in between the original code of OCN and the papers that have been used as a reference by S. Zaehle and the questions raised by N. Vuichard when checking the code of OCN.

SOM decomposition - Routine : som_dynamics

• fraction from passive to slow pool. OCN contains the version of Century published in 1993 while the ORCHIDEE is more based on the publication of 1987. The fraction towards the passive pool of the carbon flow that goes out of the slow pool is set to 0.03 in Parton 1987, instead of 0.003-0.009*clay_fraction in Parton, 1993. Soenke kept the parametrization of 1987. There is no clear motivation on the reason why.

N flows in soil - Routine : nitrogen_dynamics

• Calculation of the anaerobic balloon. Nitrification and denitrification are varying (oppositely) depending of the anaerobic condition in soils as done in Li et al., 2000. This is done by using a variable named anvf (for ANaerobic Volumetric Fraction). In Li et al. (2000), anvf is calculated using the oxygen diffusion coefficient (Ds) which makes use of the air-filled porosity (afps). In OCN, the calculation of afps is defined with the following equation but not used later in the calculation of Ds. (See, here)

   d_ox(:) = 1.73664 * ( 0.15 * (exp(-(soilhum_av(:)**3.)/0.44)-exp(-1./0.44))) * & 
            (1.+0.007*tsoil_av(:))
  

  So, in OCN, diffusion is not function of afps/afps_max ratio but accounts for soilhum and soil temperature according to Monteith & Unsworth, 1990.
  

We keep here the original formulation of Li et al., 2000

• Calculation of the oxygen partial pressure. For both the calculation of the gradient of oxygen partial pressure (d(pO2)/dz) and the change in oxygen partial pressure (d(pO2)/dt), we need dz, the depth over which one makes the calculation. It is not clear to me which depth one should use. For information, OCN used z_decomp ([see here)

• Calculation of the oxygen partial pressure. Still in the calculation of the change in oxygen partial pressure (d(pO2)/dt) in OCN, it seems that "dz" term (z_decomp) is missing in the following equation (here, see here]) when we compare with the reference formulation (Li et al., 2000, Eq. 4 of Table 2)

• Calculation of the oxygen partial pressure. The conversion factor here used for converting the respiration term in O2 pressure accounts for the air-filled porosity (see Li et al., 2000 eq. 3 table 2). In OCN, afps_max is used instead of afps. We keep the original formulation

• Calculation of the anaerobic balloon - anvf. The two constants a and b are not specified in Li et al., 2000. OCN uses a=0.85 and b=1 without mention to any publication (see here). As mention in the code of OCN these parameters have been estimated temporarily and need to be tuned. Might be good to find the values used in DNDC...