Variability of growing season indices in northeast of Iran.

Accurate use of precipitation can be considered as one of the best options to decrease the amount of underground water extraction for agriculture in arid and semi-arid areas such as northeast of Iran. For this reason, characteristics of the growing season such as onset, cessation, and length of the growing period should be analyzed. In this paper, we have calculated growing season characteristics of five locations in northeast of Iran using 45 years historical daily weather data and employed four approaches with different calculation methods. As temperature is one of the limiting factors in irrigation-based agriculture, the first approach has been based on this factor. The three remaining approaches were based on joint rainfall and temperature approach, rainfall, evapotranspiration, and temperature approach, and the final approach was based on availability of adequate water in 0.25 m of soil profile. The calculated onset dates using second and third approaches have been based on soil water balance model and relative evapotranspiration rate, and both were evaluated also to find whether the onset is a false start occurrence or not. The results showed that, when temperature was the only limiting factor, Bojnourd station with 197 days showed the longest growing season, however, when precipitation was used along with temperature, longest growing season (124 days) was obtained for Sabzevar station. The third approach which benefits from a water balance model and is similar to rainfed conditions showed the longest growing season with 147 days for Mashhad station. When adequate soil water approach was used, Bojnourd station with 255 days showed the longest growing season. Evaluation of false start of the growing season indicated the lowest probability of false start occurrence for Mashhad compared with other locations. [ABSTRACT FROM AUTHOR]