The Role of Climatic Factors in Determining the Start Date of Planting and Growing Period of Colza with Application of CropSyst Model, Case Study: Coastal Provinces of Caspean Sea in Iran

Introduction
The use of climatic and natural variables in the regulation of agricultural activities has a particular importance. Of the important characteristics of climate are the onset and retreat dates of main rainy season which have a determining role in the agricultural activities such as seeding time, cultivation period and other agricultural programs and strategies like irrigation. Iran residing in the arid and semiarid region has a variety of climates and consequently experiences high irregularities of spatial and temporal distribution of precipitation and other climatic elements. Southern coasts of Caspian Sea in Iran as a region with the highest precipitation in the country, has good potentials for agricultural activities. Among the most important crops for the development of cultivation in this plain is colza (canola). Since colza can be cultivated under rainfed conditions in high precipitation areas, therefore the analysis of precipitation characteristics in the region on the one hand, and onset and retreat dates of main rainy season in that plain on the other hand, can play an important role in the development of cultivation area.

Materials and Methods
Average onset and retreat dates of main rainy season were calculated at selected stations of the region using an index called “cumulative percentage of mean daily rainfall during the year in 5-day periods (pentads)” with Instat software, and the relevant graphs were plotted. Average onset and retreat dates of main rainy season are the dates of year when 10% and 90% cumulative mean annual rainfall during pentads are obtained respectively. The length of the main rainy season is taken as the time interval between the rainfall onset and retreat. The cumulative rainfall based on pentads is used to decrease the daily rainfall fluctuations during the year, so that the detection of onset and retreat dates of main rainy season is much more convenient. Based on the dates obtained from the software and using the daily data of climatic elements including rainfall, minimum temperature, maximum temperature, minimum relative humidity, maximum relative humidity, solar radiation and wind speed in a 26-year period (1984-2009), and also with the aid of soil properties at the stations, the cultivation period and potential yield values of colza were estimated by CropSyst model. Finally, based on the highest yield obtained from the dates entered to the model, the most suitable planting date and duration of cultivation were determined.

Results and Discussion
The results of this study showed that the onset dates of main rainy season in the stations vary between 30 Aug to 1 Nov, and these dates have had less fluctuations in comparison with the retreat dates; since the retreat dates have had differences about 50 days, i.e. from 17 Mar in Bandar Anzali in the west to 6 May in Gorgan in the east of the region. The low differences among onset dates of main rainy season in the stations prove the regularity of rainfalls which result from the regularity of rainfall-producing mechanisms in the region. This occurs especially in September as the onset of rainy season in the region and the onset of advection rainfalls originating from Siberian high pressure, particularly around the Rasht and Bandar Anzali. It seems that determining the onset and retreat dates of main rainy season using rainfall amounts leads to more reasonable results than the use of number of the rainy days. Also the length of the main rainy season in the stations varies from 185 days in Manjil to 234 days in Ghaemshahr, which represents a few weeks difference in comparison with the graph illustrated by Sedaghat (2007: 36) regarding the cumulative mean monthly rainfall in Iran. The most appropriate planting dates for colza in the selected stations of the region were suggested from 5 Oct in Manjil to 20 Nov in Bandar Anzali. Relevant cultivation durations in the stations vary from 173 to 209 days. Potential yield of the product in the region shows significant direct correlation with amount of rainfall, and significant inverse correlation with distance from shore.

Conclusion
Recognition of characteristics of main rainy season and its onset and retreat dates has a determining role in the various activities especially agricultural activities. In this research average onset and retreat dates of main rainy season were calculated at selected stations of coastal provinces of Caspean Sea in Iran using an index called “cumulative percentage of mean daily rainfalls during the year in 5-day periods”. The results showed that the onset dates are more regular than the retreat dates due to regularity of rainfall-producing mechanisms in September. Differences in suggested planting dates in this study and the report of the Ministry of Agriculture can be related to different varieties of colza used in the studies, and different calibrations of models based on the climatic and natural circumstances at the various parts of the country; Hence our special offer for respected researchers in the future studies is to calibrate the CropSyst model with regard to the climatic and natural circumstances of the study area.