Severe drought events inducing large decrease of net primary

productivity in mainland China during 1982-2015

Jun Lia, Zhaoli Wanga,b, Chengguang Laia,b*

a School of Civil Engineering and Transportation, South China University of Technology,

Guangzhou 510641, China;

b State Key Lab of Subtropical Building Science, South China University of Technology,

Guangzhou 510641, China.

Corresponding author: Chengguang Lai (laichg@scut.edu.cn)

1 Introduction on an example of drought event identification

We have given a case to specifically descript the method to identify drought event. The left

panel of Fig. S5 shows three SEDI maps from January to March 2014 in a specific region.

First, drought patch identification is conducted for the each monthly SEDI map. The first grid

with SEDI < -1 is defined as the starting point and the neighborhood grids are then checked. The

neighborhood searching process is repeated until no neighborhood grids remain under drought.

The current drought patch consist of the grids under drought (SEDI < -1). The first drought patch

in the current time step is achieved. The right panel of Figure S5 shows the drought patch in each

month.

Second, drought patches are selected according to the threshold area. As suggested by previous

studies (Xu et al. 2015; Guo et al. 2018), 1.6% of the study region was used as area threshold in

this study. The area of this region was 412500 km2, the threshold area is thus 6600 km2. The area

of the each drought patch from January to March 2014 was calculated. The area of the three

patches from January to March 2014 are 315000, 182500, and 73125 km 2, respectively. The area

of the each drought patch are greater than the threshold area, and these three patches were thus

selected.

Third, the drought events were identified. The overlap area between patches is calculated for

two consecutive months. Two drought patches are considered to belong to the same event if the

overlapping area is larger than the specified threshold, otherwise, they are considered as two

independent drought events. An overlapping area threshold of 6400 km2 is specified, following

Herrera-Estrada et al. (2017) and Guo et al. (2018). The overlap area of drought patches between

January and February was 182500 km2, and the overlap area of drought patches between February

and March was 73125 km2. Therefore, three drought patches from January to March 2014 were

identified a drought event.

The month drought severity (MS) is defined as the cumulated SEDI value of all grids within

the nth drought patch. MS can be defined as:

MSn=∑i∑js (i , j) (1)

s (i , j )=SEDI (i , j )×area (i , j )×1month (2)

where MSn is the monthly drought severity of the nth drought patch, with a unit of the km2month, i

and j indicate the longitude and latitude of a grid cell within a drought event. area is the area of

the grid cell(i , j ).

2. Supplementary table

Table S1. Categorization of dry and wet grade according to the SEDI and the corresponding

cumulative probability relative to the base period.

Categorization SEDI Cumulative probability

Extremely drought Less than -2 0.02

Severe drought -1.99 to -1.5 0.07

Moderate drought -1.49 to -1.0 0.16

Normal -1.0 to 1.0 0.50

Moderate wet 1.0 to 1.49 0.84

Severe wet 1.5 to 1.99 0.93

Extremely wet Larger than 2 0.98

3. Supplementary figures

Fig. S1. The land use and cover in each sub-region in 2015

Fig. S2. The spatial pattern of the land use and cover in 2015.

Fig. S3. The land use and cover change between 1990 and 2015 in each sub-region.

Fig. S4. A flowchart for drought event identification

Fig. S5. An example for drought event identification. The left panel indicates the spatial pattern of SEDI maps from January to March 2014; the right panel indicates the drought patch based on the SEDI maps from January to March 2014.

Figure S6. The severity-extent map for the cumulated SEDI and migration path for the worst

drought events in each sub-region. The black circle represents centroids of the drought cluster in

each month, and the red arrows between connecting centroids are the drought track path.

Figure S7. The boxplot of the monthly NPP anomalies during all drought events. The upper and

lower edges of the box represent the quantiles of 75% and 25% respectively; the horizontal line in

the box represents the median value; the upper and lower horizontal lines out of the box represents

the maximum and minimum respectively.

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