3.1 Data sources and preprocessing
The precipitation data used in this study were from the Climate Forecast
System Reanalysis (CFSR) (https://globalweather.tamu.edu), which
provides daily meteorological indicators for specific latitudes and
longitudes, selected from 1 January 1979 to 31 December 2013. The soil
data were obtained from the Harmonized World Soil Database (HWSD)
(http://webarchive.iiasa.ac.at/Research/LUC/), which was in a grid
format with an original resolution of approximately 30 arc seconds. The
topography data were provided by Geospatial Data Cloud site, Computer
Network Information Center, Chinese Academy of Sciences
(http://www.gscloud.cn). ASTER GDEM V2 version of the original elevation
data was selected, and the original spatial resolution was 30 m.
Elevation data can be used to extract slope in ArcGIS 10.2. The land use
data were derived from the interpretation of remote sensing images.
According to the research needs, Landsat 5 TM images in 2000 and Landsat
8 OLI_TIRS images in 2015 were selected as the base map for
interpretation. The source of remote sensing images data was the same as
topography data, and the original spatial resolution was 30m. Land use
status in 2000 and 2015 were obtained through manual interpretation
(Zhang et al., 2018). The enhanced vegetation index (EVI) data from 2000
to 2016 were obtained from synthetic vegetation index product MOD13Q1 V6
with an original resolution of 250m (Didan, 2015), and the average EVI
from the 97th to 289th days (plant growth season) of each year is
considered the year’s EVI (Xu et al., 2020). The population data were
from the Statistical Yearbook of Yan’an (http://data.cnki.net), and the
resident population of each county in 2015 were selected as the
calculation standard.
It should be noted that, (1) all data were presented in grids with a
resolution of 90m × 90m, (2) the area with a slope below 6° was ignored
because the GFG was hardly carried out in such an area (Zhang et al.,
2019).