Fig. S3 The linear trend of (a) surface soil moisture (0-10cm
soil moisture content, unit: kg m-268yr-1), (b) severe drought occurrence (unit: mon
68yr-1), (c) extreme drought occurrence (unit: mon
68yr-1) for 1951-2014 derived from GLDAS dataset. To
minimize the geophysical discrepancy of soil moisture, we standardized
the surface soil moisture anomalies and then use the standardized
results to characterize drought. A severe and extreme drought event is a
month with the surface soil moisture less than -1.5 and -2.0 standard
deviation, respectively. This approach adopted here is similar to
Sohrabi et al. (2015), but only employs surface soil moisture.