تأثير دوره هاي بارندگي و شاخص SPI به عنوان شاخص تامين رطوبتي بر عملكرد جو ديم (مطالعه موردي شهرستان تبريز)

Effect of Precipitation Period and SPI Index as an Indicator of Moisture Supply on Rainfed Barley Crop Yield (Case Study: Tabriz County)

در كشت دیم عوامل اقلیمی متعددی موثر هستند كه از بین آن‌ها بارندگی به عنوان یك عامل بسیار تأثیرگذار شناخته می¬شود. هدف این مطالعه تعیین موثرترین دوره بارندگی در طی دوره رشد محصول جو دیم با استفاده از متغیرهای حاصله از شاخص تامین رطوبتی و بارندگی در شهرستان تبریز می¬باشد. شاخص بارندگی استاندارد شده (SPI) به عنوان شاخص تامین رطوبتی محصول در مقیاس¬های هفتگی لحاظ شد. نتایج نشان داد بیشترین همبستگی عملكرد با مقدار بارندگی در مقیاس زمانی ده هفتگی، بین 6 اسفند ماه تا 16 اردیبهشت ماه رخ می¬دهد. داده¬های این دوره زمانی بر اساس اینكه مقادیر شاخصSPI در مقیاس 28 هفتگی در هفته اول كاشت در حالت مرطوب (SPI28≥0) یا خشك (SPI28 <0) باشند به دو رابطه خطی بین مقدار بارندگی و عملكرد تفكیك شد. در صورتی‌كه در هفته اول كاشت SPI28≥0 باشد، مقدار بارندگی بین 6 اسفند تا 16 اردیبهشت 78 درصد تغییرات عملكرد را توجیه می¬كند. مقایسه شرایط تامین رطوبتی اوایل كاشت نشان می¬دهد كه به ازای دریافت مقدار یكسان بارندگی در دوره ششم اسفند الی شانزده¬ام اردیبهشت ماه، در صورت وجود رطوبت در اوایل كاشت، عملكرد محصول تا بیش از60 درصد نسبت به شرایط خشك افزایش می‌یابد. از اینرو تنها یك دوره خاص بارندگی در طی دوره رشد بر عملكرد محصول تأثیر¬گذار نبوده و برای توجیه تغییرات عملكرد نسبت به بارندگی بایستی مقدار بارندگی در دوره ششم اسفند الی شانزده¬ام اردیبهشت ماه و شرایط تامین رطوبتی اوایل كاشت را همزمان در نظر گرفت.

Introduction: Many researchers studied and emphasized on determining the importance of climatic factors that affect crop yield. As the most source of moisture in rainfed cultivation, precipitation is the most important climate factor. Spatial and temporal change of this factor effects crop yield. Standardized Precipitation Index (SPI) is useful to characterize the condition of the moisture supply before and during the growing season of crops. Studies have shown that in some areas there is little correlation between spring wheat yield and SPI, while in other areas there is significant relationship between wheat yield and SPI. This difference indicates SPI as an indicator of moisture supply, depend on the study area .The purpose of this study was to determine the most effective period of precipitation during growing season for rainfed barley using variables obtained from moisture supply and precipitation periods in Tabriz. The most effective period of precipitation can be used for the management of rainfed cultivation. Materials and Methods: Daily temperature and precipitation data of Tabriz station were collected from Iran Meteorological Organization for the years 1955 to 2013. In addition, barley yields data were collected for the years 1977 to 2013. In this study, the occurrence of phenological stages (germination, tillering, anthesis, ripening and harvesting) were estimated using growing degree days (GDD). The SPI value for 28-week time scale of the first week after planting (SPI28) was considered as an indicator of the moisture supply during growing season. SPI28 values less than zero and greater than zero representing different classes of drought and humidity respectively. For correlation analysis, 128 weekly variables were defined at different time scales of daily precipitation data (Table 2). The relationship between the crop yield and precipitation variables were analyzed by linear correlation. Results and Discussion: The correlation coefficient (r) between precipitation and annual rainfed barley yield were presented in Table 2. The highest correlation between yield and precipitation occurred during the 10-week period between 25 February and 6 May, which was mostly observed at the end of April to mid-May that was coincide with the beginning of anthesis. So it can be concluded that the anthesis stage was the most critical stage to water stress in barley. Based on the SPI28 value greater than zero (wet conditions) or less than zero (dry conditions), the amount of precipitation (between 25 February and 6 May) was divided into two groups. The amount of precipitation between 25 February and 6 May explained 78% of the yield variations when SPI28 was greater than zero (wet conditions). One mm increase in precipitation in this period increased the yield with the rate of 2/76 kg / ha. If early planting conditions is dry (SPI 28 <0), one mm increase in precipitation between 25 February and 6 May will increase the yield at the rate of 1/65 kg /ha. Sufficient moisture during planting resulted in establishment of the plant, taking tillering plant and reducing the cold damage, consequently improving crop yield. Studies showed that the lack of sufficient moisture at the planting and anthesis stages strongly affects crop yield. Therefore single irrigation down at planting and anthesis stages increased the barley yield significantly. Conclusion: Barley yield in Tabriz is influenced by different factors. In this study the effects of variables such as soil type, temperature changes, pests and diseases and crop management practices were not considered. The results showed that the amount of precipitation in ten weekly scales explained yield variation better than other time scales. Sufficient moisture during planting, increased yield up to 60% by receiving same amount of precipitation between 25 February and 6 May rather than dry conditions. Therefore, if moisture supply is available during planting, the plant will have higher potential to produce using the amount of precipitation between 25 February and 6 May. Therefore, a specific period of precipitation during the growing season had no effect on barely yield. To better explain the yield variations simultaneously both amount of precipitation between 25 February - 6 May and early planting moisture supply conditions must be considered simultaneously. Farmers can consider the moisture supply conditions before planting, and then in the first week decide on planting crop, crop insurance and plant irrigation.