Zonation and Estimation of the Trend of the Thermal Unit of Growing Season due to Temperature Changes, Iran

Introduction  
One of the challenges in the 21st century is the issue of climate fluctuations. The increased average global temperature and its pursuing changes are part of the climatic changes that have been addressed in all ecological theories. In addition to this global increase in the air temperature, many of the phenomenological, meteorological and satellite-based studies have reported a surge in the length of the growing season caused by the augmented temperature in northern areas during the twentieth century. Any crop for its germination and growth requires a minimal temperature below which the growth process is impeded or stopped. This temperature is specific for each plant variety. One of the critical effects of this phenomenon (global warming) is a change in the degree-days of plants. A plant growth is a variable of the temperature. The growing degree day index and its availability for a crop plays a vital role in the process of the growth and enhanced productivity of inputs, including cultivation date and greater yield. Thus, the growing degree-days vary relative to the temperature fluctuations. Investigating the effects of temperature in different time periods on the growth and development of organisms, especially plants, is of paramount importance.  
The objective of this paper is to investigate whether there are specific trends in the number of heat units during the growing season using temperature indices to determine indicators, capacities and limitations of the agricultural climate in planting varieties with different adaptation styles.
Materials and methods
In this paper, daily temperature data (minimum, maximum and average) reported by 31 Iranian synoptic meteorological stations over a 25-year common statistical period were used to estimate the trend of heat units during the growing season for 1985-1986 and 2009-2010 periods.
To calculate heat units, the length of growth season was initially extracted for temperature thresholds of 5 °C and 10 °C using Julian coding (January 21st with code 1 and January 20th with code 365) and then heat units of the length of growth season were calculated by subtracting average day temperature from base temperature. Maps of heat units have been prepared in GIS. The statistical defects have also been reconstructed using autocorrelation method, and the randomization of the data has been tested by the Mann-Kendall test. The series that had specific changes or trends at the confidence level of α=0.05 were identified. Then, using Mann-Kendall test, it was determined how and when these trends were started and variations in heat units were estimated.  
Results and discussion
To calculate the base heat units at 5 °C, the start and end dates of 5 °C and the length of growing season have been first extracted and then based on the length of growing season, the number of heat units during each season has been calculated for the stations. The average length of the growing season at a temperature of 5 °C is varied in different stations ranging from 147 days at Shahrekord station to 365 days at Bandar Abbas and Bushehr stations. The heat units during the growing season also varied from 2218 to 4174 degree-days at stations under study, with the lowest number belonging to Shahr-e-Kord Station and the highest to Gorgan Station. The length of growing season at 10 °C was variable from 67 days at Shahrekord Station to 348 days at Bandar Abbas Station. The number of heat units during the growing season for this base was 861 degree-days for Shahrekord station and 5664 degree-days of the Bandar Abbas station. The results of the Mann-Kendall test indicated that the average heat units in most stations under study over the past 25 years followed a specific trend throughout the growing season. That is, for a temperature threshold of 5 °C, changes in most stations had an increasing trend, with the exception Sanandaj and Shahrekord stations, which pursued a negative or decreasing trend. For a temperature threshold of 10 °C, most stations displayed a positive and incremental trend, except for Abadan and Ahwaz stations that had a negative and declining trend.  The results of Mann-Kendall's graphic test on heat unit data with a temperature threshold of 5° C revealed that changes in all stations followed an incremental trend, except for Abadan and Bandar Abbas stations that had a dramatic and decreasing trend. Among the series of heat units at 10 °C, unlike Abadan, Ahwaz and Bam stations, which have a dramatic and declining trend, the rest of the stations followed an incremental trend.
Conclusion
The analysis of the change trends of heat units during the growing season for temperature thresholds of 5 °C and 10 °C indicated that most of these changes were dramatic except few of them following a progressive and steady trend. For a 5 °C temperature threshold, the changes in all stations had an incremental trend with the exception of Bandar Abbas station, which demonstrated a sudden and declining change. Considering the temperature threshold of 10 °C for the analysis of heat unit series, it was observed that Abadan and Ahwaz stations had a sudden and decreasing trend whereas other stations such as Gorgan, Arak, Zahedan, Ramsar, Mashhad, Qom, Sanandaj, Anzali and Tehran followed a steady and incremental trend.  Among the series of heat units for 10 °C, unlike the Bam station, which displayed a sudden declining trend, the stations of Urmia, Bushehr, Birjand, Khoy and Sabzevar followed a sudden incremental event.  The results of the research exhibited more significant changes in the series of heat units at a threshold of 10 °C in comparison with heat units at a threshold of 5 °C. At the base temperature of 5 °C, the number of heat units for the length of growing season is increased from south to north and from west to east, and at the base temperature of 10 °C, it revealed an increasing trend from south to north and from west to east of the country. According to the results, it can be suggested that in the areas where the length of the growing season declines, given that farmers struggle with low yields and crops that are not grown adequately, farmers would be better off cultivating crops with the low growth period or early plants to provide the heat energy needed by the plant during the growing season.