LAUSR

Rain-fed agriculture, which is highly dependent on rainfall, is one of various types of farming method. Therefore, the occurrence of drought can play a significant role in the yields of rain-fed plants. Therefore, the present study tries to examine the most important effective time period on the changes of the annual yield of rain-fed winter wheat (AY) under the influence of drought changes by using the correlation between calculated SPEI drought index in different time scales and simulated AY using AquaCrop model based on Backward Multiple Generalized Estimation Equation (BMGEE) method in the north-west of Iran. In this study, climatic data of 8 synoptic stations from 1967 to 2016 were used to calculate SPEI indices and AY values. According to the results, at Arak station time period from Jan to Mar (with |B| coefficient equal to 0.688), at Babolsar station time period from Nov to Jan (with |B| coefficient equal to 3.747), at Ghazvin, Ramsar and Rasht stations time period from Mar to May (with |B| coefficients equal to 1.155, 1.35 and 2.185, respectively), at Zanjan and Tabriz stations time period from Apr to Jun (with |B| coefficients equal to 0.249 and 1.33, respectively) and at Tehran station time period from Feb to July (with |B| coefficient equal to 4.018) were the most important effective time periods on the changes of AY under the influence of drought changes. It seems steam extension and heading growth stages of rain-fed winter wheat are the most sensitive stages of plant growth to drought occurrence.