Historical changes of temperature and vapor pressure deficit during the crop growing season in Iran

Climate change variables have rarely been analyzed specifically for the growing season of crops. In this study, changes in minimum (Tmin) and maximum temperature (Tmax), daily maximum saturated vapor pressure (VPsat) and estimated daily VPD weighted for transpiration (VPDtran) were examined at five locations in Iran over 40 years during the growing seasons of chickpea and maize. The results of this analysis showed a general trend of increasing Tmax. The increase in Tmax was especially pronounced during the period when maize is grown. Increasing Tmax resulted in the calculation of increasing VPsat at all locations. Minimum temperature also showed a clear increase at four of the five locations. The one exception in increasing Tmin was a location (Isfahan) leeward of the Zagros mountains where the air may be dried as it traverses the mountain, likely resulting in the observed stability of the dew point at this location. City growth encroached in the vicinity of the weather stations so some of the warming might be attributed to the expansion of city boundaries. At the four locations where both Tmax and Tmin increased, no increase in VPDtran was identified. Therefore, no increasing trend for increasing water requirement by the crops was identified. On the other hand, the lack of an increase in Tmin at Isfahan resulted in an increase in atmospheric VPDtran. The increased VPDtran results in higher water requirement in growing crops. Using the genotypes with breakpoint in their transpiration rate at high VPD might alleviate the deleterious effect of high VPDtran on growth of rainfed crops at Isfahan.