Comparison of winter wheat GDD values calculated using daily and hourly temperature data in two climatic regions of Iran

Temperature is one the most important meteorological variables affecting plant growth and development.that influence on alive.In this study the heat requirement of winter whaet in to climatic region of Iran namely Pars Abad and Darab were estimated using 3 hourly tempeareure data and hourly data generating models.The results revealed that climate type,merthod of GDD calculation and temperature thresholds are have significant effect in GDD calculation.In the regions with maximum temperature above the fixed thereshold,maxmum-minimum method was superior to mean temperature method.The results showed that models using daily data are almost coincide,but a significant difference were observed between hourly and daily models.In warm climate, the calibrated values of maximum temperature was slightly above the given threshold but in cold the climate the difference between calibrated and fixed thereshold was lower.In general,the results of study showed that in early , phenological stages, mean temperature mehod is preferable for GDD calculation,in the other two stages (mid and late) due to higher daily range of temperature, the Max-Min method can be recommended.