# The Net Influence of Drought on Grassland Productivity over the Past 50 Years
## 2 Materials and Methods
### Biome-BGC Model  
With climate, soil, and vegetation types as input data, the Biome-BGC model can simulate any scale from 1 m\({}^{2}\) to a regional, or even world, space and can simulate the daily value data of ecosystem variables to the annual value data of the NPP and other parameters in time, which has been widely used in the world. The Biome-BGC model is developed from the forest dynamic model, based on photosynthetic reaction and soil water balance, calculating the photosynthetic intensity and primary productivity. Based on the principle of energy and substance conservation, the Biome-BGC mode mainly simulates the substance fluxion and circulation process in the ecological system, such as energy, carbon, nitrogen, and water. The energy and substance, which are subtracted from the import and export of the energy and substance in the ecosystem, are allocated to different pools through the vegetation's physiological and ecological processes, and interrelate each pool by fluxes at the same time. Solar shortwave radiation is a driving power source of the whole process of ecology, using the albedo and the bill law to calculate the absorbed radiation of the canopy. Precipitation in the form of rain and snow, are saved in snowdrifts, soil, and the canopy when they fall into the ecosystem, and they leave the ecosystem by evaporation, evapotranspiration, runoff, and seepage. The evaporation and evapotranspiration can be estimated using the Penman-Monteith equation. Carbon and nitrogen are involved in plant photosynthesis, growth, and decomposition processes. The canopy is divided into the sun part and lunar part, simulating photosynthesis by [PERSON], the carbon is used firstly for autotrophic breathing and allocated to each part of vegetation using the differences in growth rate, as shown in Figure 2. \begin{table}
\begin{tabular}{c c} \hline
**Test Designing** & **Test Content** \\ \hline CO\({}_{2}\), N deposition, climate change (CLM) & The real value of CO\({}_{2}\) concentration and N deposition in 1961 levels + the real historical value of the climate factor \\ Only change the precipitation (PREC) & The real value of CO\({}_{2}\) concentration and N deposition in 1961 levels + the average of other climate elements (temperature, vapor pressure deficit, solar radiation) \\ + the real historical value of precipitation & + the real historical value of precipitation \\ Only change the temperature (TEMP) & The real value of CO\({}_{2}\) concentration and N deposition in 1961 levels + the average of other climate elements (precipitation, vapor pressure deficit, solar radiation) + the real historical value of temperature \\ The other variables & Longitude and latitude, elevation, available depth of soil, the composition of soil particles, vegetation type \\ \hline \end{tabular}
\end{table} 
Table 1: Experimental design considering the sensibility of different factors. The simulated step length is one day, while the ecosystem is separated into four carbon pools. The emphasis on the water cycle and water availability for the control function of the absorption and storage of carbon, with consideration of the effects on organic matter decomposition of soil temperature, water content, and lignin content of branches and leaves, the model mechanism is perfectly suited for studying the effect of drought on the carbon cycle. It is suitable for carbon cycle simulation on a regional scale because of the diversified analog scale and flexible output form. [PERSON] et al. evaluated the influence of climate change and increased atmospheric CO\({}_{2}\) concentration on the Chinese terrestrial ecosystem carbon cycle based on the Biome-BGC process [42]. Therefore, in this study, we chose the Biome-BGC model to depict the grassland productivity response to drought.