تأثیر تغییر اقلیم بر طول مراحل رشد و نیاز آبی گندم و جو (مطالعه موردی: دشت بیرجند)

Introduction
The global climate is changing and, despite efforts to reduce greenhouse gas emissions, weather variation is inevitable. Meanwhile, agriculture as a major water consumer will require adaptation to these variations, along with other challenges, to guarantee its persistence and sustainability. Given the arid and semi-arid climate of Iran, water, as a main limiting factor for agricultural production, plays an important role in determining the type of farming activities (Osamu et al., 2005). Crop water requirements and evapotranspiration are the main cause of water consumption in agricultural sector, the both accepted to face a dramatic increase in future under influence of increasing temperatures resulting from climate change (Koocheki et al., 2001). In this regard, the foreknowledge about future changes in climate and its effects on agricultural water use can be helpful for farmers and decision-makers. This study aimed to evaluate the climatic conditions of Birjand plain in the next two decades, and to investigate the effects of climate change on water consumption of wheat and barley as two main crops in this region.

Materials and methods
In the present survey, the effects of estimated climate change in Birjand plain on water requirement of wheat and barley and wheat yield in the 2010-2039 period were studied. Based on average weather data for the last thirty years from Birjand synoptic stations, climatic parameters of temperature and precipitation for the time period of 2010-2039 were simulated with LARS-WG5 using A1 scenario confirmed by the IPCC. Wind speed and relative humidity also were estimated for the future period. Common planting and harvesting dates were obtained from local farmers and Birjand Agriculture Organization and duration and crop coefficients (Kc) for early, developmental, middle and final crop growth stages in the current period were extracted from FAO Irrigation and Drainage Paper No. 56. For the future period, these dates and growth stages duration were estimated by matching the average daily temperature at the current time with the time of their occurrence in the future. The reference and crop evapotranspiration were calculated based on Penman-Monteith equations and effective rainfall was determined with USDA method. Crop water requirement was also calculated using the CropWat software and effects of climate change on crop yield was evaluated using DSSAT model. The genetic factors were taken from the study of Mansouri (2012) in Birjand. Other required input data (edaphic and crop data) obtained from Agricultural Organization and local farmers.

Results and discussion
The results showed that the minimum and maximum temperatures of the future years will increase, and there would be an approximately 3.5 percent more annual rainfall in the future decades. The largest temperature increase will occur in cold months, with a maximum rise in temperature of 2 °C in February. Overall, the average maximum temperature for 2010-2039 would be up to 0.6 °C higher than current period in Birjand. These higher temperature will result a significant decrease in the length of crop growth period, so that the time of planting to maturity will be reduced from 240 and 219 days (in current terms) to 227 and 193 days (in the future) for wheat and barley, respectively. Koocheki et al. (2001) has estimated 26 days shorter growing season for wheat in Mashhad due to higher temperatures in future. Despite higher reference evapotranspiration, crop water requirement will drop in the future between 19.5 percent (wheat) to 22.5 percent (barley), due to earlier planting of these crops and their shorter growing period. Indeed, due to the shorter duration of crop growth under climate change condition, the wheat yield will be reduced by about 30 percent. It was also previously estimated that despite an increase in carbon dioxide, wheat yield will be lower between 13 and 28 percent towards year 2050, which depends to location and employed general circulation models. Obviously, the agricultural practices and management should improve to comply with the inevitable climate changes. Increasing tolerance of wheat cultivars grown in this area to higher temperatures, especially during flowering time, can be effective in order to adapt to climate change.

Conclusion
Climate change is one of the main challenges facing the agricultural sector, especially in arid and semi-arid regions, and simulation of its effects in the future, can be useful for the planning and policy making to ensure food security for the community and welfare for the producers. The models which used in this study have already been successfully used in different studies.Our results showed that the use of these models to predict climate change and its effects on crops water requirements in Birjand plain will be possible with a good accuracy.

Keywords: Crop coefficient, Evapotranspiration, General atmosphere circulation model, LARS-WG

References
Koocheki, A., Nassiri Mahallati, M., Sharifi, H.R., Zand, E., and Kamali, G. 2001. A simulation study for growth, phenology and yield of wheat cultivars under the doubled CO2 concentration in Mashhad conditions. Desert 6(2): 117-127. (In Persian with English Summary)
Mansouri, E. 2012. Predict of wheat cultivars yield impact of climate change in Southern Khorasan province by using DSSAT model. MSc Thesis in Water Resources Engineering. Faculty of Agriculture, University of Birjand. (In Persian with English Summary)
Osamu, T., Yoshida, K., Hiroaki, S., Katsuhiro, H., and Hajime Tangi, H. 2005. Estimation of irrigation water using Cropwat model at KM35 Project Site, in Savannakhet, LAO, PDR. Proceedings of the International Symposium on Role of Water Sciences in Transbounday River Basin Management. 10-12 March. Ratchathani, Thailand.