Identifying the heat wave leading to a decrease in cotton yield in Isfahan County in the summer of 2013

Introduction

Climatic changes result in extremes of weather. Increase in temperature at global land surface leads to increase in frequency, duration, and intensity of heat waves, consequently imposing thermal stress on cultivation systems. Heat waves and the resulting thermal stress damage the produce during growth and consequently reduce its yield (Moore et al. 2021), posing potentially serious threats to food security in the future (Miralles et al. 2019). The importance of examination of the properties of heat waves and their effects on agriculture, therefore, has kindled the interest of many researchers in this subject. For instance, the effect of climatic changes on agricultural produce in India has been studied by Kumar and Gautam (2014) and Patel et al. (2019). The effect of heat waves on plant growth and yields of agricultural produce has also been the subject of numerous studies; for example Fontana et al (2015), Chakraborty et al. (2019), and Hatfield and Prueger (2015) have studied the effect of heat waves on wheat and corn growth and yield. Acknowledging the effect of heat waves on the flowering stage as critical, Gusso et al. (2014) reckoned that these waves could strengthen the drought and, as a result, damage the produce. Deryng et al. (2014) and Chung et al. (2014) used climatic change models to study properties and consequences of heat waves on agricultural produce in future decades. The issue of adaptation to the consequences of climatic changes such as heat waves is also an important issue in management of the effects of heat waves on agricultural produce, which has been studied by Wreford and Adger (2010) and Pathak et al. (2018). Considering the catastrophic damage to cotton fields of Isfahan County during the heat wave of July and August 2013, the aim of this study is to identify the statistical properties and synoptic patterns of this heat wave in order to prevent the damage and plan for adaptation to such hazards in the future.

Materials and Methods

In order to fulfill the objective of this study – to identify the heat wave of July and August 2013 – two definitions have been used. In the first definition, the maximum daily temperature in July and August in weather stations of Isfahan County (stations of Isfahan, East Isfahan, and Kabutar Abad) was compared with the long term mean of maximum daily temperature; also, the heat wave was identified with two days or more in a row the maximum daily temperature was higher than the long term mean of maximum daily temperature. In the second definition, since thermal stress in cotton occurred at 40 degrees Celsius, days with the temperature of maximum 40 degrees Celsius or more in weather stations of Isfahan County, which continued for two days or more, were considered as heat waves.

In order to synoptically analyze the patterns that caused heat waves in July 2013 and to compare them with the long term mean, the data of mean sea level pressure, mean geopotential height of July 2013, and the long term mean of the above-mentioned data in July from 1991 to 2020 were retrieved from https://psl.noaa.gov; using these data, the maps of sea level pressure and geopotential height at the level of 500 hPa were drawn in grads software.

Results and Discussion

By comparing the maximum temperature at the time of occurrence of heat wave with the long term mean and by considering the temperature at which thermal stress in the plant took place, findings of the present study verify the occurrence of the heat wave in July and August 2013. Also, results indicate that the establishment of Saudi Arabia cyclone at ground level and Saudi Arabia anticyclone at 500 hPa level and the expansion of its tongue to the central parts of the country led to the spread of heat wave to the Isfahan County in July 2013. In this study, examination of maximum daily temperatures and comparing them with long term mean of daily maximum temperature indicated that in July and August 2013, the Isfahan County had 29 days with temperatures higher than the long term mean, from July 12st to August 9th. In fact, long-term duration was one of the important properties of the heat wave in July and August 2013. Even when considering 40 degrees Celsius for the occurrence of thermal stress in cotton, the long-term heat waves duration which last about 6 or 7 days also occurred in Isfahan in July and August 2013. According to the second finding of this study, the mean sea level pressure and 500 hPa pressure level patterns in July 2013 were not significantly different from the long term mean sea level pressure and 500 hPa pressure patterns in July from 1991 to 2020. However, the strengthening of the Saudi thermal low pressure pattern on the earth's surface and the strengthening of the Saudi Arabia high height at the level of 500 hPa compared to the long-term average, which causes more subsidence of the air and as a result the impossibility of movement and also warming of the air in the lower layer of the atmosphere, caused the entry of warm air from Saudi Arabia to the center of the country and it has been an effective pattern in creating a heat wave in Isfahan County.

Conclusions

This study demonstrates the properties of the heat wave of July and August 2013 in Isfahan County, including the difference in temperature, compared to the long term mean temperature, daily and hourly duration, and synoptic patterns that contributed to the formation of the aforementioned heat wave. Considering the detrimental consequences of heat waves, especially in the agricultural sector, precise identification of the heat wave which damages the produce is necessary in order to plan for adaptation to the conditions resulting from heat wave, and for crisis management in order to reduce the damage to the produce in the future.