# The Net Influence of Drought on Grassland Productivity over the Past 50 Years
## 2 Materials and Methods
### Experimental Design and Assessment Methods  
The sensitivity between land ecosystem carbon flux and single environment factors (such as historical CO\({}_{2}\) concentration, precipitation, temperature, and land use) have been analyzed to describe the net influence of drought as a single factor on grassland productivity [30; 39; 40]. Currently, the multifactor sensitivity simulation test has become an important method for studying the effect of a single factor and multiple factors. To eliminate the influence of historical CO\({}_{2}\) concentration changes on carbon flux, according to the CO\({}_{2}\) concentration data released by NASA from 1959 to 2012, this study took the CO\({}_{2}\) concentration level of 317.419 PPM in 1961, the starting year of the model operation, and the nitrogen deposition adopted the regional level of the 1980s which was 0.000411 kgN/m\({}^{2}\)/yr [41]. Using the Biome-BGC model as a tool and the NPP as an evaluation index of carbon balance, and based on multiple scenario simulation experiments, we quantified the effects of single climate factors on grassland productivity by analyzing the effects of temperature and precipitation changes on grassland carbon fluxes, identified the quantitative effects of drought on grassland productivity patterns in Inner Mongolia over the past 50 years, and revealed the differences in the responses of different grassland types to drought. Through designing different factors to simulate drought conditions, the datum such as different climate factors (precipitation and temperature); the historical levels, at a specific time, of CO\({}_{2}\) concentration and nitrogen deposition; precipitation change (using the average of all the other factors); and temperature change (using the average of all the other factors) in different situations were inputted to the Biome-BGC model for simulating the NPP change in the Inner Mongolia grassland over the past 50 years and evaluating the carbon balance changes on the Inner Mongolia grassland under different situations. The experimental design is shown in Table 1. Under the CLM test mode, the different climate factors (precipitation and temperature) and the historical level, at a certain time, of CO\({}_{2}\) concentration and nitrogen deposition are executed using control test groups, to simulate the NPP change from 1961 to 2012 in every grid cell at a regional scale. In the precipitation change test group (using the average of all the other factors), the grassland NPP change respond to precipitation during 1964-2012 is analyzed while only the precipitation changed. In terms of the temperature change test group (using the average of all the other factors), the grassland NPP change respond to precipitation during 1964-2012 is analyzed, while only the temperature changed. The net effect of drought, over the past 50 years, on grassland productivity features the following: The net effect of drought on the NPP analysis equals the whole climate factor and the historical average CO\({}_{2}\) concentration and nitrogen deposition scene simulation minus the influences of the temperature and precipitation change scene simulations, to obtain the interaction influences, coupled with the precipitation change scene simulation of the NPP [42]. Since drought effects could be induced both by precipitation and temperature changes, we added the interactive effects between precipitation and temperature scenarios to the drought effects from precipitation change to reflect drought effects induced by both precipitation and temperature changes. This implies that the drought effect is the sum of the PREC and interactive effect. The main processes were as follows:  
1.