GIS-based analysis of spatial distribution patterns of growing degree-days for agricultural applications in Iran

The geographical distribution of growing degree-days (GDDs) within Iran was studied using GIS-based maps. GDDs were calculated using daily thermal parameters (daily maximum and minimum air temperature). Based on the purpose of the study and climatic conditions of Iran, the average value of 5?C was chosen for GDD calculation. The calculations were carried out using daily weather data of 113 meteorological stations throughout Iran. The clustering of the 113 point stations’ data were achieved by using statistical models combined with a digital terrain model. The final outcomes were five GDD maps (two for start and end of heating period, one for January, one for cold season and the last one annual growing degree days). The relationships between calculated GDDs and key variables that may affect temperature were carefully studied. (e.g. geographical data of latitude and longitude, elevation, nearest grid distance from the sea) Geographical distributions of GDDs were found to be correlated closely with the climatic types of different regions that are mainly based on relief. The distribution of the GDDs presented in the maps showed that the calculated values were close to the measured ones, which confirmed the validity of the GIS-based approach. In few cases significant differences between predicted and calculated values were observed, mostly in the regions with less number of weather stations. Consequently, the highly accurate generated maps can be used as a useful tool for prediction of crops and pests phonological events and crop modeling.