3.1 Determination and Verification of Freezing Damage Indices
The principal component analysis method was used to establish the coefficient of freezing damage index Y, and then obtain the Y expression in the three regions (I, II, and III).
YI = 0.33307*X1 + 0.258827*X2 + 0.3044*X3 + 0.00058*X4 + 0.3248*X5 + 0.2301*X6
YII = 0.2459*X1 + 0.1703*X2 + 0.35453*X3 + 0.21615*X4 + 0.3068*X5 + 0.2639*X6
YIII = 0.27802*X1 + 0.2049*X2 + 0.2747*X3 + 0.3043*X4 + 0.2491*X5 + 0.0634*X6 (2)
In Eq. (2), the Y expressions were positively correlated with the six disaster factors, which physically indicated that the greater the negative accumulated temperature, the average temperature, the maximum cooling, the snow depth, the extreme minimum temperature, and the average wind speed during the winter, the more severe the freezing damage. The higher the freezing damage indices, the more severe the degree of freezing damage. With a weight coefficient > 0.3, YI showed that snow depth had the least influence on freezing damage in the overwintering period, while negative accumulated temperature, maximum cooling, and extreme minimum temperature in the overwintering period had the largest influence. YII only analyzed the wheat observation stations and found that all six disaster factors had an impact on the freezing damage indices, among which the maximum cooling and extreme minimum temperature in the overwintering period had the greatest impact. YIII analyzed the freezing damage from 2016 to 2020 and found that wind speed had the least influence on the freezing damage index, while snow depth in the overwintering period had the largest influence. The results showed that the weight of the disaster factors affecting freezing damage in northern Xinjiang were different for the three different regional indices.
Combined with the observed disaster situation in northern Xinjiang, the YI value could reflect the occurrence of freezing damage in northern Xinjiang, with the results approximately in accordance with the recorded historical disaster situation. In the winter of 1966, 27.5% of the winter wheat in the 150th regiment of the Corps Eighth Agricultural Division died, and some winter wheat in Mosuowan and Xiayedi, Shihezi City, also died. More than 80,000 animals died of hunger and cold in Changji Autonomous Prefecture, with a mortality rate of 34%. Four field workers froze to death and several got frostbite in Tacheng City, Emin County, Tacheng. In the winter of 1968, severe cold and a thin snow layer in Wener County, Bortara Autonomous Prefecture, was responsible for a large area of winter wheat death. All the winter wheat in Mosuowan, Shihezi City and the 150th regiment of the Corps Eighth Agricultural Division was lost to freezing damage. In the winter of 1976, Balikun in Hami region had an early snowfall, and all the wheat at the northern foot of the Tianshan Mountains was frozen before harvest, resulting in a decrease of more than 5 million kg in the regions winter wheat harvest. All the winter cabbage in Hami County suffered freezing damage, leading to a loss of about 10 million kg. In Changji Autonomous Prefecture, heavy snow killed 14,000 livestock, and at the beginning of the following year, 10 thousand mu of winter wheat were frozen to death in Emin County.
In the winter of 1984, seven people froze to death in Altay, Yining City in Yili region due to sustained low temperatures (-30˚C) accompanied by strong wind (force 7), which was a rare event. Fruit trees suffered a wide range of freezing damage, power plants shutdown, and 121 vehicles of the transportation department broke down. From January to March of the next year, the Bortala Autonomous Prefecture suffered freezing conditions that killed 34,500 mu of wheat, accounting for 50.8% of the total crop. Winter wheat was frozen to death over an area of 7300 mu in Emin County, Tacheng. Seven cold air intrusions killed 180,000 livestock and affected 11,927 people in 1560 households in Buerjin County, Altay Prefecture. In the winter of 1987, more than 10,000 livestock died of starvation during freezing conditions in Altay. More than 31,300 animals died in the Tacheng area. In the winter of 2009, there was a snowstorm of a once-in-60-years magnitude in Xinjiang, which exceeded the historical maximum in the same period. This snowstorm caused severe freezing damage to winter wheat in northern Xinjiang. Since the beginning of winter in 2010, the Xinjiang Altay region has been hit by three severe cold events and snowstorms, and the snow cover on Mt. Altay had a depth of more than 180 cm, which greatly affected local agricultural and animal husbandry production, road transportation, and the lives of people of all ethnic groups.
In the winter of 2012, 1 million mu of winter wheat in Changji Autonomous Prefecture suffered from freezing damage. The severity of freezing damage was related to the lack of overwintering irrigation water and the weak frost resistance caused by the early sowing date and strong growth. In the winter of 2018, a cold air mass intruded into northern Xinjiang, causing a low temperature of -30˚C in many areas. The low temperature caused severe frostbite in the roots of winter wheat and grapes, which was very harmful to the crops. During the overwintering period of 2020, winter wheat on the Hongqi farm of the Sixth Division of Xinjiang Production and Construction Corps suffered from freezing damage and a lot of crop death occurred. In general, the freezing damage indices reflected the actual historic freezing damage situation well and had a strong adaptability. They can therefore be used to represent the overwintering freezing damage of winter wheat in northern Xinjiang.
Using the historical disaster data of winter wheat freezing damage in northern Xinjiang, literature data (Mo et al., 2013; Zhang et al., 2016), and the corresponding freezing damage index values calculated here, the overwinter freezing damage of winter wheat was divided into the four levels given in Table 2. The corresponding relationships between freezing damage indices and disaster levels were established.
Table 2
Levels of winter wheat freezing damage during the overwintering period
Freezing damage indices | Disaster level |
Freeze-free | Mild | Moderate | Severe |
Y | Y < 0.6 | 0.6 ≤ Y < 0.7 | 0.7 ≤ Y < 0.8 | Y ≥ 0.8 |
Mortality rate (%) | < 8 | 8 − 15 | 16 − 30 | > 30 |
3.2 Time Series Analysis of Freezing Damage Indices
According to Fig. 2, in the YI and YII regions, the KMO values were 0.681 and 0.666 respectively, the significance (P < 0.01) of Bartlett's test of sphericity for both regions, and the degree of common factor variance of each factor was > 0.7. The cumulative variance contribution rates were > 85%. By analyzing the first three principal components, the cumulative variances were 86.960% and 86.688% respectively. The indices decreased significantly with increasing time. The index ranges of regions YI and YII were 0.36 − 0.72 and 0.44 − 0.93, respectively, with average values of 0.51 and 0.64, and their tendency rates were − 0.0021/a and − 0.0027/a, respectively. The years with YI > 0.7 in region YI included 1966, 1968, and 1976, and the years with YI < 0.4 included 2006, 2008, and 2019. In Region YII, the years with YII > 0.8 included 1960’s, 1970’s and 1980’s .The index regions YI and YII were divided into three periods: 1960 − 1989, 1990 − 2009, and 2010 − 2020, whose tendency rates were − 0.0023/a and − 0.0038/a, 0.0013/a and 0.0024/a, and − 0.0115/a and − 0.0182/a, respectively. According to the above analysis, in the past 60 years, the most serious periods of frost damage were concentrated in 1960’s, 1970’s,and 1980’s. The average annual freezing damage index values were higher at wheat observation stations than at the 49 meteorological stations in northern Xinjiang. Over the whole study period the index values in regions YI and YII significantly decreased, but there was an increasing trend from 1990 to 2009.
The Mann–Kendall mutation test was used to analyze the change trend of freezing damage indices (YI, YII) from 1960 to 2020 in northern Xinjiang. According to the intersection point of UF (statistical series calculated in time series order) and UB (statistical series calculated in reverse of the time series order) curves in each region, the decrease in index values was an abrupt change phenomenon. According to Fig. 3, in the YI region, the coincidence points of the freezing damage indices, as determined by the intersection of the two curves, were 1982, 1984, and 1985, and the mutation year in the YI region was 1985. The coincidence point in the YII region was 1981.
In the YI and YII regions, the average freezing damage index values before the mutation were 0.56 and 0.71, while after the mutation the index values were 0.48 and 0.60, respectively. Therefore, the degree of frost damage in northern Xinjiang decreased after the 1980s compared with the situation before the 1980s, displaying a significant downward trend, although an obvious upward trend was observed from 2008 to 2012.
3.3 Spatial Distribution Characteristics of the Freezing Damage Indices
3.3.1 The average freezing damage index values in the YI region from decade to decade
As shown in Fig. 4, the analysis was conducted region by region in six time periods over the whole 1960 − 2020 study period (YI region). During 1960 − 1970, the average YI value was 0.57, with values > 0.7 mainly distributed in the Altay region, and values > 0.8 mainly distributed in Fuyun and Qinghe counties. Values of YI between 0.6 and 0.7 were mainly distributed in the middle and eastern sections of the Tianshan Mountains northward to Altay, including Balikun Kazakh Autonomous County, Qitai County, Jimsar County, Midong District, Shihezi City, Wusu City, Emin County, Wulhe District, and Fuhai County.
During 1971 − 1980, the average YI value was 0.56, and areas with YI values > 0.6 were distributed in the same areas as in the 1960s, with the addition of Hutubi County in the middle of the Tianshan Mountains. The observation station in Balikun County had an average YI value > 0.7.
During 1981 − 1990, the average YI value was 0.51, with a significant reduction in the area with values > 0.6, which were distributed in the Altay area, as well as Shihezi City, Midong District, Qitai and Balikun stations.
During 1991 − 2000 and 2001 − 2010, the average YI values were 0.48 and 0.49, respectively. Areas with YI > 0.6 were located in the Altay area as well as the stations at Qitai, Midong, and Shihezi. During 2011 − 2020, the average YI value was 0.46. Areas with YI > 0.6 were concentrated in Altay, with areas with YI > 0.7 distributed in Qinghe and Fuhai counties.
3.3.2 The average freezing damage index values in the YIII region
As shown in Fig. 5, the average freezing damage index values in the last 5 years at 83 meteorological observation stations (YIII region) during 2016 − 2020 were selected for a regional analysis. The KMO value was 0.587, the significance (P < 0.01) of Bartlett's test of sphericity, and the degree of common factor variance of each factor were > 0.7. The cumulative variance contribution rate was ≥ 85%. The first four principal components were extracted for analysis, and their cumulative variance was 92.9%. The average YIII value was 0.50, with YIII values > 0.7 mainly distributed in Fuyun and Qinghe counties in the Altay region, in which YIII values > 0.8 were distributed in northeast Fuyun County. An area of 0.6 < YIII ≤ 0.7 was concentrated in the north of Changji Prefecture and extended to Altay and part of Tori County.
3.3.3 Distribution of high freezing damage index values in the last 20 years
According to Fig. 2, the average freezing damage index values in the overwintering period in 2009 were the highest in the last 20 years, and over the last five years the highest values occurred in 2018. The distributions of freezing damage according to the indices in the overwintering periods of 2009 in the YI region and 2018 in the YIII region are shown in Fig. 6. According to Fig. 6, in the overwintering period of 2009, YI values ≥0.8 were distributed in the whole of the Altay region, and an area of 0.7 ≤ YI < 0.8 was distributed around Altay and the stations in Qitai County, Changji City, and Shihezi City. An area of 0.6 ≤ YI < 0.7 was distributed in the middle and eastern sections of the Tianshan Mountains and extended north to the vicinity of Altay, bit not Urumqi City.
In the overwintering period of 2018, the area with YIII values ≥0.8 was significantly reduced compared to the situation in 2009, and the freezing damage was concentrated in Fuyun and Qinghe counties. An area of 0.7 ≤ YIII < 0.8 distributed in other parts of Altay, northern Changji Prefecture, and part of Qitai County. The area of 0.6 ≤ YIII < 0.7 was significantly larger than in 2009, and was distributed in Emin County, Toli County in the Tacheng area, and the middle and eastern sections of the Tianshan Mountains, extending north to the Altay area, but not Urumqi City. In 2009 and 2018, there were major wheat planting areas with mild and moderate freezing damage.
3.4 Frequency and Number of Winter Wheat Freezing Damage Occurrences
When the indices were compared with the relevant disaster data, YII had the highest coincidence with the wheat freezing disasters in the historical reports. The frequency and number of freezing damage occurrences at wheat observation stations in each 20 year period within the full study period were analyzed in detail.
Table 3
Distribution of freezing damage occurrences at wheat observation stations in the YII region
Period | 1960 − 1980 | 1981 − 2000 | 2001 − 2020 |
Disaster level | Mild | Moderate | Severe | Total | Mild | Moderate | Severe | Total | Mild | Moderate | Severe | Total |
Altay City | 1 | 3 | 17 | 21 | 4 | 4 | 10 | 18 | 4 | 3 | 12 | 19 |
Tacheng City | 4 | 8 | 5 | 17 | 6 | 2 | 3 | 11 | 2 | 4 | 0 | 6 |
Bole | 10 | 6 | 1 | 17 | 5 | 2 | 1 | 8 | 8 | 0 | 0 | 8 |
Wusu | 7 | 6 | 2 | 15 | 4 | 1 | 1 | 8 | 7 | 0 | 0 | 7 |
Shihezi City | 8 | 3 | 2 | 13 | 5 | 3 | 2 | 10 | 7 | 3 | 1 | 11 |
Changji City | 9 | 4 | 7 | 20 | 3 | 1 | 1 | 5 | 4 | 2 | 3 | 9 |
Qitai County | 1 | 6 | 14 | 21 | 3 | 7 | 9 | 19 | 4 | 6 | 7 | 17 |
Yining City | 3 | 3 | 1 | 7 | 2 | 0 | 1 | 3 | 1 | 0 | 0 | 1 |
Xinyuan County | 2 | 0 | 0 | 2 | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 |
Zhaosu County | 8 | 2 | 0 | 10 | 2 | 1 | 0 | 3 | 2 | 0 | 0 | 2 |
Balikun County | 0 | 6 | 15 | 21 | 7 | 5 | 5 | 17 | 7 | 9 | 1 | 17 |
Total | 53 | 47 | 64 | | 42 | 26 | 33 | | 46 | 27 | 24 | |
The observation station in Zhaosu County was 1,851 m above sea level (asl), and the observation station in Balikun County was 1,679 m asl, while the other observation stations were between 460 and 930 m asl. The higher the altitude within the same region, the more frequently freezing damage occurred and more years there were with severe freezing damage. With the abrupt climate change in the 1980s, the frequency of freezing damage decreased significantly from 1981 to 2020, and the number of years with severe freezing damage decreased gradually over the 20 year periods. The number of years with mild and moderate freezing damage from 2001 to 2020 increased slightly compared with the number from 1981 to 2000. Compared with the previous 20 years, the number of years with severe freezing damage in Aletai and Changji stations increased slightly from 2001 to 2020, while the number years with severe freezing damage at the remaining winter wheat observation stations decreased. According to the frequency and number of freezing damage occurrences at winter wheat observation stations in the last 20 years, Shihezi, Changji, and Qitai were the main planting areas of winter wheat, where freezing damage often occurred. The Yili area (Yining City, Xinyuan County, Zhaosu County) was the best area for wheat planting, due to its low freezing damage index values and low frequency of freezing damage.