# The Net Influence of Drought on Grassland Productivity over the Past 50 Years
## 2 Materials and Methods
### Model Applicability Evaluation
carbon and water fluxes passed the test of significance level 0.001, in which the slopes of GPP, NEP, ET, and NPP are 0.80, 0.60, 0.65, and 0.78, respectively, and the simulated values are all relatively close to the 1:1 line and evenly distributed on both sides. The GPP, NEP, ET, and NPP root mean square error values were 5.36 gC/m\({}^{2}\)/8 d, 0.89 gC/m\({}^{2}\)/d, 0.62 mm/d, and 36 gC/m\({}^{2}\)/yr, respectively, which were within a reasonable range of the simulation errors. The coefficients of determination of GPP, NEP, ET, and NPP were 0.59, 0.30, 0.46, and 0.79, respectively, and the regression effects were significant, indicating that the Biome-BGC model could better simulate meadow grassland carbon and water fluxes with high simulation accuracy, and had strong simulation performance and adaptability. The typical grassland carbon and water flux validation mainly used the data of the Xilinhot station from 2003-2007, as shown in Table 3. The GPP and Re of the Xilinhot flux station are the daily value data from 2006-2007, and the NEP and ET are the daily value data from 2003-2007. Based on the conversion relationship between aboveground and belowground biomass (belowground biomass = 4.25 \(\times\) aboveground biomass) and the conversion relationship between NPP and biomass (carbon gC\(\cdot\)m\({}^{-2}\) = biomass \(\times\) 0.45 g\(\cdot\)m\({}^{-2}\)) for typical grasslands in temperate zones, the NPP was calculated for the Xilinhot grazing weather station from 1980 to 2006 data. As can be seen from Figure 4, all carbon and water fluxes passed the test of significance level 0.001, where the slopes of GPP, Re, NEP, ET, and NPP were 0.79, 1.11, 0.73, 0.96, and 0.77, respectively, and the simulated values were all relatively close to the 1:1 line and evenly distributed on both sides. The GPP, Re, NEP, ET, and NPP root mean square error values were 0.69, 0.67, 0.53 gC/m\({}^{2}\)/d, 0.61 mm/d, and 45.6 gC/m\({}^{2}\)/yr, respectively, and the simulation errors were within a reasonable range. The coefficients of determination of GPP, Re, NEP, ET, and NPP were 0.70, 0.81, 0.55, 0.64, and 0.71, with significant regression effects, indicating that the Biome-BGC model could better simulate typical grassland carbon and water fluxes with high simulation accuracy, and had strong simulation performance and adaptability. Figure 3: Different carbon and water flux validations of meadow steppe in Inner Mongolia: (**a**) GPP; (**b**) NEP; (**c**) ET; (**d**) NPP. The verification of the carbon and water fluxes in the desert grassland mainly used the data of the Sunit Zuqi station from 2008-2009, as shown in Table 3. The NEP and ET of the Sunit Zuqi flux station are the daily value data from 2008-2009. Based on the conversion relationship of aboveground and belowground biomass of temperate desert grassland (belowground biomass = 7.89 \(\times\) aboveground biomass) and the NPP and biomass conversion relationship (carbon gC\(\cdot\)m\({}^{-2}\) = biomass \(\times\) 0.45 g\(\cdot\)m\({}^{-2}\)), the 1982-2006 NPP data were calculated for the pastoral Ulaatzhongqi meteorological station NPP data. As can be seen from Figure 5, all carbon and water fluxes passed the test of significance level 0.001, where the slopes of NEP, ET, and NPP were 0.75, 0.87, and 1.0, respectively, and the simulated values were all relatively close to the 1:1 line and evenly distributed on both sides. The NEP, ET, and NPP root mean square error values were 0.66 gC/m\({}^{2}\)/d, 0.54 mm/d, and 21.7 gC/m\({}^{2}\)/yr, respectively, which were within a reasonable range. The coefficients of determination of NEP, ET, and NPP were 0.77, 0.56, and 0.83, respectively, with significant regression effects, indicating that the Biome-BGC model could better simulate the typical grassland carbon and water fluxes with high simulation accuracy, and had strong simulation performance and adaptability.