Investigating the Effect of Changes in Frequency and Intensity of Daily Rainfall on its Annual and Seasonal Trends in Kurdistan province

Most of the studies that have been conducted in order to investigate the pattern of temporal and spatial changes of rainfall in rainfed areas have finally resulted in determining the trend of total annual, seasonal, and monthly rainfall in the target areas. However, changes in the frequency and intensity of rainfall are among the important parameters that have a great impact on the trend of rainfall and the planning of rainfed areas. The purpose of this study is to investigate the effect of changes in the frequency and intensity of daily rainfall on the trend of annual and seasonal rainfall in Kurdistan province.

For this purpose, the rainfall data of 30 years (1988 to 2017) of four synoptic stations of Sanandaj, Bijar, Qorveh, and Saqez, located in Kurdistan province, were used. After the data homogeneity test, the trend of changes in time series of frequency and amount of rainfall was calculated through the non-parametric Mann-Kendall test and Sen's slope estimator.

 The obtained results indicated a decreasing trend between 25 and 31% of total annual rainfall compared to the average of the studied period in four stations and this decrease was related to spring, summer, and winter seasons in most cases. The rainfall trend did not change significantly in the autumn season. Even though the trend of the rainy days in Sanandaj and Qorveh stations had a significant increase of 0.54 and 0.63 days per year, respectively, the results showed that this increase was related to spring rains which have had less intensity than five millimeters per day and cannot be very effective in providing water requirement. The noteworthy point was that the intensity of rainfall has decreased in all stations which is the main reason for the more than 40% decrease in the amount of rainfall during the spring and winter seasons of Sanandaj, Qorveh, and Saqez stations.

In general, increasing the frequency of low-intensity rains and decreasing the intensity of rains in rainfed areas can be a serious threat to reducing yield. Therefore, it is necessary to investigate the changes in the frequency and intensity of rainfall, along with its values, in order to determine the distribution pattern of annual and seasonal rainfall in rainfed areas.