# The Observed Impacts of Wind Farms on Local Vegetation Growth in Northern China
## Materials and Methods
### Data Processing and Methods  
The MODIS data used in this paper are all the sinusoidal equal-area projections of 1 km and then re-projected onto a 0.01\({}^{\circ}\) resolution, which are coarser than the 1 km data to avoid spatial gaps [14]. Finally, there are 4617 pixels (81 columns, 57 rows), covering 1747 wind turbines. Vegetation growth exhibits strong spatial variability at the pixel level, which is mostly related to changes in topography or land surface types. Although the EVI varies from one wind farm to another, and between WFPs and NNWFPs, the mean areal EVI difference between WFPs and NNWFPs is very small (Figure S2). The effects of surface heterogeneity, such as land surface type and topography, are minimized by using average anomalies [11]. First, the average summer (June-August) average values of LAI and GPP for every year are calculated, and are then used to create summer anomalies by subtracting  
Figure 1: (a) Surface elevation (m) at \(\sim\)1 km spatial resolution for the study region. Plus symbols represent pixels with one or more wind turbines. (b) Number of individual wind turbines built annually over the period 2003–2014. The numbers of annual (black) and accumulated (red) turbines built for each year are shown on the top of the bar. The location of the study region is plotted within the map of northeast China. The data is based on “2012–2015 Research Report of China wind power market and being built in the construction of wind farm project.” The location of study area is plotted (red) within the map of China. the climatological data at every pixel during 2003-2014. The NPP, LST, albedo, and ET data are processed in the same way. According to the longitudes and latitudes of the wind turbines' locations, the corresponding pixels for the soil moisture data are chosen. The summer averages and the anomalies of soil moisture are then calculated. Pixels containing at least one wind turbine are defined as wind farm pixels (WFPs) and pixels located 6 to 9 km away from the WFPs are defined as nearby non-wind-farm pixels (NNWFPs) [9; 28]. The transitional zone between WFPs and NNWFPs avoids the effect of turbulence caused by blade rotation on the NNWFPs [29]. Finally, a total of 484 WFPs and 690 NNWFPs are obtained (Figure 2). To explore the downwind effect of the WFs, the 537 pixels that are 3 to 5 km away from the WFPs are divided into the following two groups: upwind wind farm pixels (UWFPs, 245 pixels) and downwind wind farm pixels (DWFPs, 292 pixels) (Figure 2). Since the wind direction in the study area during the growing season is mainly from the southwest to northeast (see the discussion), the pixels to the southwest of WFPs are designated as UWFPs, and the pixels to the northeast of WFPs are designated as DWFPs. As the background LAI varies substantially from one year to another, in order to highlight the spatial variability at the pixel scale, for each year the regional mean LAI anomalies (one constant value) are removed from the original anomalies as proposed by [PERSON] et al. [9]. Then, the differences of average LAI anomalies, during the post- and pre-turbine construction periods, are calculated to analyze the spatial coupling between the differences and the layout of wind turbines. Since the wind turbines built during 2006-2011 in the study area account for 94.3% of the total, two time periods are selected to represent pre- and post-turbine construction periods, 2003-2005 and 2012-2014. In addition, two single years, 2004 and 2012 (the regional mean anomalies are similar), are chosen to calculate the differences in the LAI anomaly for two individual years. Note that the LAI changes shown in the figures represent changes relative to regional mean values rather than absolute changes. The study area is very small (0.6\({}^{\circ}\)\(\times\) 0.8\({}^{\circ}\)) and, therefore, it can be reasonably assumed that the WFPs and NNWFPs have the same climatic background. If the development of wind turbines can affect the LAI, the LAI difference between WFPs and NNWFPs will change over time.