Time trend analysis of some agroclimatic variables during the last half century over Iran

We assessed the trends of precipitation, maximum and minimum temperature (Tmax and Tmin), diurnal temperature range (DTR), water requirement of autumn-planted wheat and summer-cropped corn (CWRwheat and CWRcorn), and reference evapotranspiration (ET0) under climate change over Iran using the Mann-Kendall test in 1967-2012. The presence of change-point in the series was also studied by the Pettitt test. In order to determine the length of growing season of autumn-sown crop (GSAC) and growing season of summer-sown crop (GSSC), the temperature-based and soil water-based approaches were employed. The analyses for Tmax, DTR, precipitation, and ET0 series were carried out over the entire year, GSAC, GSSC, mid-GSSC (MGSSC), early GSAC (EGSAC), and late GSAC (LGSAC). Further, the Tmin analysis was performed during the January-February-March (JFM). Precipitation had a negative trend in EGSAC, LGSAC, and GSAC at most stations. However, a positive trend in precipitation was found during MGSSC and GSSC at approximately 74% of cases. An increasing trend was detected for Tmax (except for LGSAC and EGSAC) and Tmin in most investigated locations. Moreover, DTR exhibited a downward trend for most regions except in GSAC. Increased Tmin and reduced DTR, unlike elevated Tmax, seem to have positively influenced autumn-sown crops. At the majority of cases, there was an upward trend in ET0, CWRwheat, and CWRcorn. Such increase in CWR and ET0 appears to have placed pressure on water resources through enhancing irrigation demand. Unlike for precipitation, ET0, and Tmax, a significant change-point occurred within Tmin, DTR, and CWR series for most sites.