In the context of this study, where the focus is on the water and energy balance, the significance of NPP lies in the fact that it limits leaf growth and thus controls, among other factors, the leaf area index (LAI). This is accounted for by the phenology part, in which the LAI is computed as the minimum of a temperature limited value, Ay, a water limited value, Alv, and a growth limited one, i.e.
The value of A is updated every 10 days.
Temperature limitation of LAI, which has its reason in a frost avoidance strategy of the vegetation, is prescribed as in Dickinson et al. :
with the temperature at 0.5 m soil depth, To.5, and standard values of Amax = 5, = 5°C and 7^max=15°C, with the exception of agriculture, where describes the daily mean temperature at which leaves are formed in spring and shed in autumn. Water limitation is modelled such that is set to the value that maximises NPP as long as the LAI increases. At decreasing LAI, A«/ is set to the LAI value of the preceding time step, and as soon as NPP becomes negative, is set such that NPP equals zero, thus decreasing the LAI just enough to avoid carbon losses. Eventually, AG-accounts for 50% of NPP invested into leaf growth, but cannot be lower than to allow initial leaf growth. The actual LAI, A, computed with this scheme is distributed in a partially "clumpy" fashion, depending on the expected maximum annual leaf area index. The grid area fraction covered by vegetation, is computed from where is the mean temperature of the warmest month, MI is the annual moisture index (annual precipitation devided by annual potential evapotranspiration) and a critical threshold value below which vegetation starts becoming patchy rather than evenly distributed. Especially in semi-arid areas, clumping can have a significant effect on the energy balance, leading to a general increase in NPP [Knorr, 1997].
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