بررسی رخداد تغییر اقلیم و تأثیر آن بر زمان کاشت و طول دورۀ رشد گندم دوروم (دیم) مطالعۀ موردی: ایستگاه سرارود کرمانشاه

Introduction
Climate change generally affect all economic sectors, but the agricultural sector is the most sensitive and vulnerable sector, because crops are highly dependent on climatic resource. According to the scientific evidences, climate change in the future, especially the combined effects of rising temperatures and rising CO2 concentrations in the atmosphere and increase the likelihood of some events may have significant impacts on agricultural products.
This paper is trying to explore the past and future trends in climate parameters, their outcomes on the sowing date and length of the growing season in rainfed wheat in the Kermanshah region.
Materials and methods
The synoptic station of Sararood, Kermanshah, has geographical position of 47 degrees 20 minutes western with height of 1351.6 meters above sea level.
Simulation of climate parameters i.e., maximum and minimum temperatures and rainfall in coming decades was carried out using the results of output the CCSM4 model under the scenario RCP4.5.
The outputs of the above mentioned model is low. Thus for produce the climatic data of temperature in the studied area, these outputs statistically were small-scaled in the period 2013-2039 and consequently the simulated data were used for next stages.
In order to study the impact of climate change on the displacement of sowing date and change the length of growth period in the future, firstly to estimate sowing date of rainfed wheat, initial rainfall dates were extracted from the synoptic station of Sararood. Then, according to this definition that the sowing date can be considered when the total rainfall by early October reach to five mm, provided that fifteen days after that should not be dry, sowing date for both past and future climates was determined. In regard to the correlation between the each growth stage of wheat with the heating temperature factor, the length of growth period of wheat using the index GGD was calculated. But the remarkable thing here is that due to the lack of data on the growth period before 1988, length of growth period for base period was considered from 1988.
Results and discussion
Changes in temperature and rainfall in the past
Results of Kendall and sen 's Estimator slop indicated that changes in rainfall in most months of the year has been decreased. This trend was significant (P<0.01) in March and annually; and temperature variables significantly has been increased in most months of the year. Thus it can be concluded that the temperature of the region has been inflounced by the factors in the past been that has increases the temperature.
The results of climate change on the studied region in the period of 2013-2039 in which that climate behavior of base period is compared with the future period indicated that average maximum temperature
with the except for the years 2013 and 2022, and minimum temperature with the except for the years 2015 and 2018 was lesser than average long-term (2010-1970) minimum and maximum temperature in the base temperature. The rainfall during the years under study in the future was more that the average long-term in the base period, which result in increasing temperature and rainfall during the future year.
Appropriate time for sowing date of wheat in the present and future conditions
To investigate the changes related to initiation of sowing in the coming periods, the long-term average initiation dates of sowing of wheat based on distance from the source (first of October) of base climate were compared with the initiation dates of sowing of wheat based on future years. The results indicate, on average, sowing date of wheat in past climate started from second decade of December, where as sowing date in future climate will be started from third decade of October. Furthermore, temperature in future climate will be more than past climate, thus the only reason for this could be the start of earlier rainfall, in the words start of earlier rainfall in future years. Based on these, on average, rainfall in the past climate started from the second decade of December and will be started in future climate from the second decade October of.
Changes of growth period length
To investigate the changes of growth period length of wheat in the future periods, mean of growth period length in base climate was compared with the growth period length in future years. The results indicated that mean of growth period length in past climate was 209 days, while in future climate was 184 days. Thus, it can be concluded that mean of growth period length of wheat in future climate 25 days will be shorter which. The reason of decrease in growth period length can be due to increasing of temperature in future climate, where the mean temperature in the future climate will be 15.8 oC and in past climate 14.5 oC.
Conclusion
Studies showed that in Kermanshah region, rainfall of past periods has decreasing trend, while temperature has increasing trend in most months of year especially in cold months. In the future period, the temperature in all months of the year will be increased between 1.7 to 2.5 C º until the end of 2039. Also appropriate sowing dates for wheat will be the second decade of December month for past period, whereas for the future period will be the third decade of October and later. Displacement of rainfall in future period toward early cool-season has caused that sowing dates of wheat start earlier in future climate compared to past one. Comparison of growth period length in future and past periods indicated that in spite of the initiation of sowing date of wheat in past period start later than future period, but the length of growth period of wheat in future climate will be shorter 25 days relative to past climate that is the result of increasing temperature in future periods relative to past.