4) Potential net primary productivity (NPP-p)
We used the Thornthwaite model to calculate the potential NPP (NPP-P) (Lieth and Box, 1972) as follows:
\(\text{NPP}_{p}=3000\times[1-\exp^{-0.0009695(V-20)}]\),
where V is the annual actual evapotranspiration (mm), and NPP-P is measured in units of g·C·m−2·yr−1.
The annual actual evapotranspiration data were retrieved from the MOD16A3 dataset of the NTSG, in which the evapotranspiration values are determined using the Penman–Monteith equation (Mu et al., 2007). The parameter used to estimate the NPP-P; i.e., the actual evapotranspiration, depends on solar radiation, temperature, precipitation, air pressure, and wind speed. Blank spaces on the evapotranspiration map (Figure 2) indicate an absence lack of data.