| | 206 | === Land cover change (with age classes) === |
| | 207 | Describes r6614. Land cover change now accounts for age classes. It is controlled by '''veget_update'''. Set '''veget_update''' = 0Y if land cover change should be disabled. The wood pool and its subsequent fluxes were moved from the land cover change routine to a separate routine. Furthermore, land cover change also deals with the change of biological land uses to non biological land uses (of which the most important change is probably urbanization). If urbanization happens, all the carbon an nitrogen are stored in a series of variables '''burried_xxx''' where xxx stands for a different pool, e.g., litter, soil, .... Burried_xxx are cumulative variables thus increasing over time . There is a place holder in sapiens_lcchange.90 to also develop the release of the buried carbon and nitrogen following de-urbanization (see ticket #616). The series of the burried_xxx variables are not yet written to an output file but this could be easily added (they are already defined in the xml files). |
| | 208 | |
| | 209 | An interesting parameter is '''min_vegfrac'''. When reading in a land cover map, PFTs with a fraction below min_vegfrac are removed. Likewise the fraction cover of a PFT after a land cover change should not be less than min_vegfrac either. This requirement seems to have been solely established to avoid ending up with too many PFTs with very small fractions. Because the the non-biological and biological fraction covers of each pixel should sum up to one, removing even these very small fractions implies that these fractions need to be added to one of the remaining PFTs. First it is tried to add the fraction to the bare soil (this will only be accepted if the new fraction of the bare soil exceeds min_vegfrac), then the code tries to allocate the residual fraction to the largest vegetated fraction. If age classes are used this should be the largest vegetated fraction in the first age class of a PFT. If all of the above failed, the residual fraction is added to frac_nobio irrespective of whether frac_nobio exceeds min_vegfrac. Everytime this happens, the failure to meet the min_vegfrac criterion is registered in the variable '''failed_vegfrac'''. This variable is not yet added to an output file. |
| | 210 | |
| | 211 | Note that the min_vegfrac criterion could be the reason of why very small land cover changes occur. Another consequence is that the land cover fractions in the model are not exactly the same as those read in from the maps. Deviations should remain small and should not accumulate over time. Assume that in y0 the fraction of PFT2 = 0. In y1 the map tells us the fraction is half of min_vegfrac. The model will keep the PFT fraction to zero. The model and the map will no longer be in line with each other. In y2 the map tells us the fraction is twice min_vegfrac. The model will now accept the change. The model and the map will be in line with each other. |
| | 212 | |
| | 213 | |
| | 214 | === Leaf area === |
| | 215 | Describes rXXXX. There is no longer a parameter for maximum LAI in the trunk. Given that LAI is a key variable that links the biogeochemistry to the biophysics in ORCHIDEE it deserves some background information. LAI is now calculated as a prognostic variable and is the net result of growth, allocation and turnover. There are few parameters in stomate that do not affect the LAI but three parameters stand out: |
| | 216 | * Growth is largely determined by GPP and Ra. (a) GPP is mainly driven by the LAI so this results in an interesting feedback. If the LAI is very low, GPP will be low, growth will be low, LAI will remain low, etc. When GPP is too low to make the LAI, the vegetation will die. This could be rather quick, i.e., within a year, or it could be rather slow, i.e., several years. Given this is a feedback loop there is no single parameter that can be used to control this feedback. (b) The ratio of NPP/GPP or the fraction of GPP that is used in autothropic respiration and especially maintenance respiration will to a large extent determine how much C is availble for growth. If the LAI is very high or low, always check the NPP/GPP ratio. Ra and thus the NPP/GPP ratio can be tuned through the '''MAINT_RESP_SLOPE_xxx''' variables and the '''COEFF_MAINT_INIT'''. |
| | 217 | * Allocation distributes the growth across the different plant components. The parameter that controls the fraction that ends up in the leaves is '''K_LATOSA_xxx'''. This variable can be used to tune LAI but the effect of increasing K_LATOSA_xxx decreases for larger values (possibly due to N-limitation). The range of observed K_LATOSA_xxx is quite large. The model shows a nice sensitivity within the observed range. Note that allocation is a "zero sum game", hence, allocating less C to other components will result in more C going to the leaves. Note that the leaves (GPP) and roots (N-uptake) are necessary to grow in the first place (see section '''Leaf and roots'''). |
| | 218 | * Turnover will decrease the leaf area. The parameter that controls the turnover is '''LONGEVITY_LEAF'''. For evergreen PFTs the relationship between longevity_leaf and turnover is straightforward. For deciduous PFTs the relationship is more confusing (see section '''Leaf longevity and turnover'''). Turnover also affects the leaf age and young leaves have a higher VCMAX than old leaves. The age-effect on vcmax is strong enough to make that a small young canopy may have a higher GPP than a large old canopy. |
| | 219 | |
| | 220 | |
| 214 | | === Land cover change (with age classes) === |
| 215 | | Describes r6614. Land cover change now accounts for age classes. It is controlled by '''veget_update'''. Set '''veget_update''' = 0Y if land cover change should be disabled. The wood pool and its subsequent fluxes were moved from the land cover change routine to a separate routine. Furthermore, land cover change also deals with the change of biological land uses to non biological land uses (of which the most important change is probably urbanization). If urbanization happens, all the carbon an nitrogen are stored in a series of variables '''burried_xxx''' where xxx stands for a different pool, e.g., litter, soil, .... Burried_xxx are cumulative variables thus increasing over time . There is a place holder in sapiens_lcchange.90 to also develop the release of the buried carbon and nitrogen following de-urbanization (see ticket #616). The series of the burried_xxx variables are not yet written to an output file but this could be easily added (they are already defined in the xml files). |
| 216 | | |
| 217 | | An interesting parameter is '''min_vegfrac'''. When reading in a land cover map, PFTs with a fraction below min_vegfrac are removed. Likewise the fraction cover of a PFT after a land cover change should not be less than min_vegfrac either. This requirement seems to have been solely established to avoid ending up with too many PFTs with very small fractions. Because the the non-biological and biological fraction covers of each pixel should sum up to one, removing even these very small fractions implies that these fractions need to be added to one of the remaining PFTs. First it is tried to add the fraction to the bare soil (this will only be accepted if the new fraction of the bare soil exceeds min_vegfrac), then the code tries to allocate the residual fraction to the largest vegetated fraction. If age classes are used this should be the largest vegetated fraction in the first age class of a PFT. If all of the above failed, the residual fraction is added to frac_nobio irrespective of whether frac_nobio exceeds min_vegfrac. Everytime this happens, the failure to meet the min_vegfrac criterion is registered in the variable '''failed_vegfrac'''. This variable is not yet added to an output file. |
| 218 | | |
| 219 | | Note that the min_vegfrac criterion could be the reason of why very small land cover changes occur. Another consequence is that the land cover fractions in the model are not exactly the same as those read in from the maps. Deviations should remain small and should not accumulate over time. Assume that in y0 the fraction of PFT2 = 0. In y1 the map tells us the fraction is half of min_vegfrac. The model will keep the PFT fraction to zero. The model and the map will no longer be in line with each other. In y2 the map tells us the fraction is twice min_vegfrac. The model will now accept the change. The model and the map will be in line with each other. |
| 220 | | |
| 221 | | === Leaf area === |
| 222 | | Describes r6908. There is no longer a parameter for maximum LAI in the trunk. Given that LAI is likely the key variable that links the biogeochemistry to the biophysics of ORCHIDEE it deserves some background information. LAI is now calculated as a prognostic variable and is the net result of growth, allocation and turnover. |
| 223 | | |
