3.1. Drought intensity and frequency
In the main grape-growing regions of China, CWDI-based drought intensity during bud break-flowering showed a gradual decrease from the north to the south and from the west to the east from 1981 to 2016. Exceptional drought (D4) during bud break-flowering mainly occurred in Xinjiang Uygur Autonomous Region, northern Gansu province, Ningxia Hui Autonomous Region, Hebei province, western Jilin province, northwestern Shandong province, northwestern Yunnan province, and southwestern Sichuan province. Abnormally dry (D0) was mostly in the Southeast China (Fig. 3(a)). During the 36-year study period, drought intensity during flowering-veraison showed a gradual decrease from the north to the south of the study regions. Exceptional drought (D4) during flowering-veraison mostly occurred in Xinjiang Uygur Autonomous Region, northern Gansu province, Ningxia Hui Autonomous Region, and central-south Hebei province (Fig. 3(b)). Drought intensity during the later study growth stages (i.e., veraison-berry maturation and berry maturation-leaf fall) was generally at lower levels than drought intensity during the earlier growth stages of grape (i.e., bud break-flowering and flowering-veraison). During the growth stages of veraison-berry maturation and berry maturation-leaf fall, most of the study regions experienced abnormal dryness (D0) or moderate drought (D1), and exceptional drought (D4) was focused in Xinjiang Uygur Autonomous Region and Gansu province (Fig. 3(c) and (d)).
During all four study growth periods of grape in the study regions, the subtotal frequency of abnormally dry (D0) and exceptional drought (D4) was generally higher than the subtotal frequency of moderate, severe, and extreme droughts (D1, D2, and D3) from 1981 to 2016. Overall, abnormally dry (D0) during the study growth periods of grape occurred in more than 60% of the study years (local farmers referred to it as ‘three times per five years’) in southern China. The areas where exceptional drought (D4) existed in more than 60% of the study years differed from growth period to growth period. During bud break-flowering, exceptional drought (D4) existed in more than 60% of study years in Xinjiang Uygur Autonomous Region, northern Gansu province, Ningxia Hui Autonomous Region, Hebei province, northwestern Shandong province, western Jilin province, southwestern Sichuan province, and Yunnan province. During flowering-veraison, exceptional drought (D4) existed in more than 60% of study years in Xinjiang Uygur Autonomous Region, northern Gansu province, Ningxia Hui Autonomous Region, and southern Hebei province. During veraison-berry maturation and berry maturation-leaf fall, exceptional drought (D4) existed in more than 60% of study years in Xinjiang Uygur Autonomous Region and northern Gansu province. During all four study growth periods of grape, extreme drought (D3) existed in fewer than 10% of the study years in most of the study regions. Meanwhile, severe drought (D2) existed in 20%–30% of the study years in North China and the western part of Northeast China. During veraison-berry maturation and berry maturation-leaf fall, moderate drought (D1) existed in 20%–30% of the study years in North China, Northeast China, and Southeast China regions (Fig. 4).
3.2. Drought coverage and trend
The annual percentages of stations indicate the drought coverage in a certain area. During the 36-year study period, more than 33% of the study stations showed exceptional drought during bud break-flowering in 20 of the study years, more than 25% of the study stations showed exceptional drought during flowering-beraison in 21 of the study years. During veraison-berry maturation (berry maturation-leaf fall), 10%–25% of the study stations showed moderate drought in 35 (31) of the study years. Overall, drought was more spread out and intense during bud break-flowering and flowering-veraison stages than other study growth stages (Fig. 5).
From 1981 to 2016, the annual percentage of stations showing moderate drought during all four study growth periods of grape unanimously displayed an increasing trend. During bud break-flowering, the annual percentage of stations showing severe drought displayed a decreasing trend while the annual percentage of stations showing exceptional drought displayed an increasing trend. During the other three growth periods of grape, the annual percentage of stations showing severe drought displayed an increasing trend but the annual percentage of stations showing exceptional drought displayed a decreasing trend. During berry maturation-leaf fall, the annual percentage of stations showing extreme drought displayed a decreasing trend. By contrast, during bud break-flowering, flowering-veraison, and veraison-berry maturation, the annual percentage of stations showing extreme drought all displayed an increasing trend (Fig. 5).
3.3. Drought recurrence feature
According to the drought butterfly structure diagram, exceptional drought (D4) recurred every 1yr. during both bud break-flowering and flowering-veraison from 1981 to 2016 in Northwest China. During veraison-berry maturation, exceptional drought (D4) showed recurrences of every 1yr. and 2yr., though the recurrence of 1yr. was more evident, extreme drought (D3) showed recurrences of every 1yr., 2yr., 4yr., and 9yr., with the recurrence of every 9yr. being the most manifest. During berry maturation-leaf fall, exceptional drought (D4) showed recurrences of every 2yr., 7yr., 9yr., and 11yr., with the dominant recurrence being every 7yr., extreme drought (D3) showed recurrences of every 1yr., 2yr., 4yr., 5yr., 6yr., and 7yr., with the relatively more apparent recurrences of every 1yr., 2yr., and 7yr. (Figs. A.2-A.6). Overall, drought was more frequent during the study grape growth stages in northern China (including Northwest China, North China, and Northeast China) than in southern China (including Southwest China and Southeast China). In northern China, different levels of drought (i.e., D1, D2, D3, and D4) showed the recurrence of every 1yr. during various growth stages of grape from 1981 to 2016. In addition, drought recurrence of every 7yr. was dominant during the growth stages of grape in northern China (Table 3).
Table 3. Recurrence features of moderate, severe, extreme, and exceptional droughts (D1, D2, D3, and D4) during the four study growth periods of grape (i.e., bud break-flowering, flowering-veraison, veraison-berry maturation, and berry maturation-leaf fall) in the main grape-growing regions of China (i.e., Northwest China, North China, Northeast China, Southwest China, and Southeast China). The more evident recurrence features are set in bold.
Region
|
Intensity
|
Growth period
|
Bud break-flowering
|
Flowering-veraison
|
Veraison-berry maturation
|
Berry maturation-leaf fall
|
Northwest China
|
D1
|
--
|
--
|
--
|
—
|
D2
|
--
|
--
|
--
|
—
|
D3
|
--
|
--
|
1yr., 2yr., 4yr., 9yr.
|
1yr., 2yr., 4yr., 5yr., 6yr., 7yr.
|
D4
|
1yr.
|
1yr.
|
1yr., 2yr.
|
2yr., 7yr., 9yr., 11yr.
|
North China
|
D1
|
--
|
--
|
1yr., 5yr.
|
1yr., 2yr., 3yr., 6yr., 7yr.
|
D2
|
--
|
3yr., 7yr.
|
--
|
--
|
D3
|
--
|
1yr., 3yr.
|
--
|
--
|
D4
|
1yr., 3yr., 4yr., 5yr., 7yr., 12yr.
|
1yr., 2yr., 4yr.
|
--
|
--
|
Northeast China
|
D1
|
--
|
1yr., 2yr., 3yr.
|
1yr., 7yr.
|
1yr., 2yr., 7yr., 8yr.
|
D2
|
1yr., 4yr., 7yr., 8yr., 11yr., 12yr., 20yr.
|
1yr., 3yr., 4yr., 5yr., 6yr., 7yr.
|
--
|
--
|
D3
|
1yr., 5yr., 6yr., 7yr., 11yr.
|
--
|
--
|
--
|
D4
|
--
|
--
|
--
|
--
|
Southwest China
|
D1
|
5yr., 8yr.
|
1yr., 2yr., 3yr., 5yr., 6yr., 7yr., 8yr., 9yr., 12yr., 15yr.
|
--
|
--
|
D2
|
2yr., 3yr., 5yr., 7yr., 8yr.
|
1yr., 11yr.
|
--
|
--
|
D3
|
4yr., 8yr., 12yr., 16yr.
|
--
|
--
|
--
|
D4
|
--
|
--
|
--
|
--
|
Southeast China
|
D1
|
15yr.
|
--
|
--
|
3yr., 4yr., 7yr., 11yr., 12yr., 15yr.
|
D2
|
--
|
--
|
--
|
--
|
D3
|
--
|
--
|
--
|
--
|
D4
|
--
|
--
|
--
|
--
|
Note: “--”means no recurrence feature was detected in the butterfly analysis.