Identification and characteristics of combined agrometeorological disasters caused by low temperature in a rice growing region in Liaoning Province, China

Characteristics of the single agrometeorological disaster scenarios

SAD-f occurred in 49 out of 57 years at different spatial scales, with a maximum IOC of 0.519 in 2013; SAD-d occurred in 33 years with a maximum IOC value of 0.808 in 1995; SAD-s occurred in 5 years, with a maximum IOC value of 0.115 in 1977 (Fig. 4). SAD-d showed a declining trend over the past 57 years, but the SAD-d frequency was higher than SAD-f and SAD-d. Since the mid-1980s, the frequency of SAD-f has increased, while the frequency and scale of SAD-s were relatively small.

A large-scale grade SAD-f event occurred in 2013 in Liaoning Province, and regional SAD-f occurred in 17 years. Five years showed a large-scale SAD-d and 11 years had regional SAD-d. There were no large-scale and regional years for SAD-s (Table 5).

The occurrence of SAD-f was recorded at 15 sites with a frequency greater than 20%, 13 sites with frequency in the 10%–20% range, and 24 sites with a low frequency (P ≤ 10%) in Liaoning Province from 1961 to 2017 (Fig. 5). SAD-d occurred at 12 sites with a frequency higher than 20%, 22 sites with frequency between 10% and 20%, and 18 sites with a low frequency (P ≤ 10%). In the three scenarios, the occurrence frequency and distribution of SAD-f was the highest and SAD-s was the lowest.

Comparison of the characteristics of single agrometeorological disasters and combined agrometeorological disasters

The maximum IOC of SAD was 0.808 in 1995 and the mean value was 0.294 for the 57 years of the study; the maximum IOC of CAD was 0.654 in 1987, and the mean value was 0.180 over the past 57 years; SAD and CAD occurred in all 57 years (Fig. 6). Both SAD and CAD showed declining trends from 1961 to 2017. The IOC was lower for CAD than for SAD for 42 years and higher than SAD for 14 years.

This paper analysed the mean IOC of SAD and CAD over six decades and found that the interdecadal mean value of the IOC in CAD was lower than that of SAD over five of the periods, but the IOC of SAD was lower than that of CAD in 1971–1980 (Fig. 7). The IOC of SAD showed a decreasing trend from the 1970s to the 2010s but showed an increasing trend after 2011. The IOC of CAD showed a decreasing trend from the 1970s to the 2000s, but showed an increasing trend after 2001 (Fig. 7).

There was one site (Fushun) with a SAD frequency of more than 50% in Liaoning Province from 1961 to 2017, 42 sites with a frequency between 20% and 50%, and nine sites with a frequency lower than 20%. There were four sites (Xinfeng, Jianping, Xinbin and Caohekou) with a CAD frequency higher than 50%, 13 sites with a frequency in the range 20%–50%, and 35 sites with a frequency lower than 20% (Fig. 8). The frequency and range of CAD were less than those of SAD.

There has been little research into the temporal or spatial distribution of CAD for rice and its occurrence characteristics: most research has been on SAD. For example, studies have examined the characteristics of SCD, DCD, FD for rice in northeast China^{26, 27, 29}, and the risk of multiple disasters for rice in northeast China^{30, 31}. Han et al.³¹ analysed the risk of disaster using the reduction rate of rice yield in Liaoning Province from 1980 to 2011, and found that the high-risk areas were distributed in the west and northeast of Liaoning Province; higher rates of yield reduction in lean years were mainly found in western Liaoning and its surrounding areas. In this study, a higher frequency of CAD was mainly distributed in the northwest of Liaoning Province, while that of SAD occurred in the northeast of Liaoning Province. The median frequency of CAD occurred in the northwest and northeast of Liaoning Province, while that of SAD covered most areas in Liaoning Province. The range of medium and higher frequency occurrence in CAD was consistent with the distribution of high-risk and high yield reduction areas in the study of Han et al.³¹. Therefore, it can be speculated that the CAD scenarios might magnify the effect of each single disaster, and, therefore, CAD would more easily lead to a higher reduction in the rice yield.

Comparison of the occurrence of single agrometeorological disasters and combined agrometeorological disasters

During the rice growing season in Liaoning Province, there were three scenarios of SAD and six scenarios of CAD. Compared with SAD, CAD had more scenarios and more complex processes, and its effect on rice was more difficult to evaluate. In SAD, the occurrence frequency and distribution of SAD-f and SAD-d were both high, when FD and DCD occurred alone in only one rice growth stage. In CAD, the occurrence frequency and distribution of TD-1, when FD and DCD occurred simultaneously, was the highest in the six scenarios. A single or combined occurrence of FD and DCD was most common disaster for rice in Liaoning Province. The occurrence frequency and distribution of OD-1 were both smaller than that of SAD-f, indicating that the occurrence was lower when FD happened at both the seedling and milk stages. SAD-s and OD-2 had the lowest frequency and range in all scenarios, indicating that DSD rarely appeared in SAD and CAD. The occurrence of SCD was not major disaster in the growth and development of rice in Liaoning Province, but the occurrence of DCD or FD, or both, was.

In this study, the occurrence frequency and range of SAD and CAD for rice showed declining trends in most sites over the past 57 years, which was consistent with the results of other studies. Studies on rice DCD and SCD concluded that cold damage events of rice in most areas of northeast China showed decreasing trends^{26, 27}. Because of events such as climate warming, earlier warming in spring, delaying first frost dates and fewer low temperature days in summer, the trend of disasters was lower in rice planting areas³⁰. However, although rice disasters showed a decreasing trend, local disasters may increase because of the frequent occurrence of climate anomalies. SAD-f and OD-1 scenarios in this study showed no significant decreasing trend, and even a partial increasing trend. Jiang et al.²⁹ believed that the possibility of frequent SCD in north-east China was still high. According to Xi et al.³², cold periods would still occur in the growing season of rice in northeast China. Hu et al.³³ concluded that the increase of SCD in northeast China was mainly because of the increase of climate variability, and most of the sites with increases were located in areas with decreasing temperature or no obvious trend of temperature increase.

Rice is a higher temperature-loving crop, which is mainly restricted by temperature conditions during its growing season. Liaoning Province is in the south of the rice planting area of the colder regions in China. Because of the relatively low latitude, heat conditions during the rice growing season were better than those in Jilin and Heilongjiang to the north of Liaoning Province. The climatic risk of cold damage in the rice growing season was lower than other regions in northeast China³⁴. The occurrence of CAD was generally caused by low temperatures, which were the dominant factor. When two or more disasters occur together, there is a coupling or amplifying effect on rice growth compared with a single disaster.

A comparison of the rice yield reduction rates in the years when CAD or SAD occurred in more than 50% of stations in Liaoning Province revealed that the former happened in 5 years, 1969, 1974, 1976, 1980 and 1987, whereas the latter happened in 7 years, 1972, 1973, 1985, 1986, 1990, 1995 and 2013. When CAD was the major occurrence, the average yield reduction rate in the five years was 10.6%. The yield reduction rate in 1969 was 34.6%, which was the highest in the past 57 years. When SAD was the major occurrence, the average yield reduction rate in the seven years was 9.8%. The average yield reduction rate in the years when CAD dominated was greater than in the years when SAD dominated. Therefore, it can be speculated that CAD has a greater effect on rice growing than any single disaster within CAD. However, it is difficult to quantify the effect on rice yield of CAD, and further controlled field experiments should be conducted to verify these. It is difficult to control field experiments that are limited by conditions and facilities.

Comparison of the occurrence of agrometeorological disasters in years having rice yield reductions

On the basis of the rice yield reduction rate in calculations Liaoning Province from 1961 to 2017, a total of 10 years (Table 6) were screened. Six years had large-scale disasters (including SAD and CAD) and four years had regional disasters. In 1969, which showed the highest yield reduction rate (34.6%), 30 sites had TD-1 disasters and the other 22 sites had SAD-f disasters. In 1972, the second highest reduction year (29.1%), 11 sites had MD-1 disasters, i.e. three kinds of disasters occurred, seven sites had TD-1 disasters, one site had a TD-2 disaster, 31 sites had SAD-d disasters, one site had a SAD-f disaster, and only one station had no disaster. The TD-1 disaster, i.e. delayed cold damage and frost injury, was the most frequent CAD over the years, and SAD-d, i.e., delayed cold damage, was the most frequent SAD. The occurrence of single and combined agrometeorological disasters in different regions strongly affected the rice yield. Generally, the larger the disaster range, the higher the yield reduction. However, some years were not completely consistent with this conclusion. The yield reduction rate was also related to the type, severity, occurrence period and geographical location of the disasters.

In every year from 1961 to 2017, CAD or SAD occurred in Liaoning Province, and the rice yields declined in 23 of the 57 years owing to meteorological disasters (Fig. 9). Although meteorological disasters occurred in the other 34 years, there was no reduction in rice production, which may be related to the gradient of the disaster or the spatial distribution of the rice planting areas. The rice yield reduction rates in 1969 and 1976 were 34.6% and 15.6%, respectively. In these two years, CAD occurred at 30 stations and SAD occurred at 22 stations, and TD-1 was the main type of CAD, whereas SAD-d was the main type of SAD. Using statistical data, on the rice planting area of each city in Liaoning Province, the provincial area can be divided into four regions. The first region was Shenyang City, which has the largest rice planting area, accounting for 20%–25% of the total rice planting area; the second region was Panjin City, accounting for 15%–20% of the total rice area; the third region encompassed Tieling and other six cities, accounting for nearly 50% of the total rice area, with each city representing 5%–10%; and the fourth region encompassed Jinzhou and five other cities, accounting for 10%–15% of the total, with each city representing 0–5%. As shown in Fig. 10a,b, TD-1 occurred in the first region in both 1969 and 1976 and in the second region in 1969. SAD-d occurred in the second region in 1976. In the third region, TD-1 occurred at more stations of 1969 than in 1976. The rice area in the first three regions accounted for nearly 80% of the total rice area, and CAD occurred more often than SAD in these regions. Thus, there was a greater yield reduction rate in 1969 than in 1976.

The occurrence characteristics of single disasters or the risk of yield reduction were analysed in previous studies, but the quantitative effect on rice production was rarely evaluated. Ji et al.²⁶ reported that the delayed cold damage in 1961, 1962, 1969, 1972, 1976, 1989 and 1995 was so severe that there was a large reduction in rice production. In our paper, we examined the occurrence of not just one disaster, i.e. delayed cold damage, over time, but also other types of disasters including SAD and CAD. For example, in 1972 and 1976, the disaster scenario affecting the largest number of stations was TD-1, i.e., both delayed cold damage and frost damage occurred in the growing season of rice. In 1961, the most widespread damage came from a single disaster—frost damage. According to the records³⁵, Liaoning Province experienced frost damage in 1961, 1962, 1969, 1972, 1976 and 1995, and the rice yield was seriously reduced. Most regions of Liaoning Province experienced both delayed cold damage and frost damage in 1976 and 1995. There was a low temperature during the critical period of rice growth (mid-July to mid-August) in 1995. In 1985, the growing season in most areas was characterized by unusually persistent low temperature and little sunshine. These statistics were basically consistent with the conclusion of this study. In the process of rice production, a variety of disasters occurred caused by low temperature, such as delayed cold damage, frost damage and sterile cold damage.

Source: Ecology - nature.com