Application of Precipitation Evenness and Effective Time-Rain Indicators to Estimate Rainfed Wheat Yield and Its Fluctuations in Semi-Arid Climate of Tabriz

Annual and seasonal rainfall distributions are the main determinants of yield in rainfed conditions, of which seasonal rainfall is more important. Therefore, it is necessary to provide precipitation indicators that have a significant conceptual relation with crop performance fluctuations. In the present study, the relation between rainfall uniformity indices and rainfed wheat yield in Tabriz plain was evaluated in 19-years period. Also, in order to examine the effect of precipitation more accurately, the Rain-Time Index [M1] [M2] (RTI) was used, which could indicate the effect of simultaneous changes of the two factors of rainfall time and rainfall amount on crop yield fluctuations. The results showed that the correlation between rainfed wheat yield and slope of uniformity line (ratio of cumulative precipitation per length of growing period) was significant at 1% level. However, there was no significant correlation between the uneven distribution index and yield. Also, according to the obtained results, the first effective rainfall event is the most effective rainfall event on the yield of rainfed wheat. It seems that the inadequacy of effective spring rainfall, which is one of the main reasons for the low yield of rainfed lands, prevented the explanation of the positive effect of late season rainfall on yield. Therefore, in order to evaluate the effect of intermittent precipitation and Rain-Time Index [M3] [M4] on yield more accurately, a single supplementary irrigation should be applied at the stage of grain filling and its effect on the studied indicators and yield should be investigated.