Relationship of Synoptic Patterns of Effective Precipitation with Planting Date and Yield of Rain-fed Wheat in Kermanshah

Introduction
Understanding climate parameters and their effects on crop growth is one of the most important agricultural issues. This understanding can help increase crop yield and thereby increase production. The position of Iran and its natural geographic features have led to a variety of climate and seasonal variations. These variations in each season have created different conditions for agricultural products. Among the climatic elements, the amount and temporal distribution of rainfall and temperature fluctuations in different stages of crop growth have the greatest impact on the yield of agricultural products, especially rain-fed wheat cultivation. In the growth stages of each crop, especially in rain-fed agriculture, the planting date with the first effective rainfall is important environmental-managerial factor in effective yield. The main goal of current study is to investigate the relationship of synoptic patterns of effective precipitation with planting date and yield of rain-fed wheat in Kermanshah County. 
Materials and methods  
In this study, we gathered data from growth stages (phenology) of wheat, Azar-2 variety, and precipitation data of agrometeorological research station of Sararood, Kermanshah,  from 2004-2005 to 2015-2016. In the next step, the data of geopotential heights, wind speed and direction and specific humidity from 850 and 500 hPa levels were taken from NCEP / NCAR reanalysis dataset. The planting date of rain-fed wheat has been calculated based on 2 methods. In the first method, the starting dates for the first effective rainfall were determined. Accordingly, the date of cultivation was when the total precipitation of 5 mm within one or two consecutive days occurred and after that, the precipitation occurred by 10 days. The second method was to determine the planting date on the basis of climatic data of sowing and germination dates recorded in Sararood agrometeorological research station during the years of study. The effective rainfall was defined as 5 mm rain to wet 5 cm of soil depth in loamy soil. Moreover, the staring date of sowing wheat was calculated according to Weibull formula with a 75-percent probability of success. In the following, taking into account the first days of precipitation and based on the number of rainy days, the synoptic patterns of the occurrence of effective rainfall have been derived using a correlation-based method.
Results and discussion
According to the results, suitable dates for Rain-fed wheat cultivation in Kermanshah Province were considered from October 25 to November 5. Determining the threshold of correlation coefficient (0.57), we found 4 general synoptic patterns. The features of each pattern were as follows: pattern 1 had the highest share in severe rainfall. This pattern in the pre-germination stage (sowing) had the highest frequency in the occurrence of effective rainfall of 5 mm and more. Pattern 2 was in contrast with Pattern 1, and it was most frequent occurrence in rainfall. Patterns 3 and 4 had characteristics such as limited rainy days and extreme daily rainfall. In addition, synoptic patterns of effective precipitation more than 5 mm were minor trough, Mediterranean trough, Omega Block and cut-off low. The results of synoptic studies of atmospheric patterns also showed that the northern seas of Indian Ocean (Red and Arabian Seas) are the main sources of moisture for all synoptic patterns.
Conclusion
The main goal of current study was to investigate the relationship of synoptic patterns of effective precipitation with planting date and yield of rain-fed wheat in Kermanshah County. The effective precipitation of the examined synoptic patterns provided helpful conditions for germination of plant in the next priority after the first pattern. The results indicated that the lowest wheat yield as compared to average yield (2113 kilograms per hectare) occurred in the years with the forth precipitation pattern. On the other hand, the highest wheat yield observed when the synoptic patterns of 2nd and 3rd were occurred simultaneously within a year. For example, the years of 2006-2007, 2014-2015 and 2015-2016 had wheat yield of 2888, 2486 and 2700 kilograms per hectare, respectively. Moreover, the effects of synoptic patterns on planting date showed that the commence of first effective rainfall with patterns 1, 2, 3 and 4 delayed germination stage abour nearly 4, 10, 3 and 5 days, respectively. It should be noted that to take more precise results, it is reasonable to work with long-period data.