The evaluation of rainfall duration variability in Kurdistan Province

Introduction
The consequences of climate change, changes in precipitation characters, including the amount, time and it’s duration are expected. Considering that the rain provides the water resources on the planet, change in regime, amount and duration of rainfall, caused a disturbance in the ecosystem of the Earth. It also affects the environmental conditions. Kurdistan province has an agricultural economy, thus variation in the rainfall duration can affect agricultural activity and other activities. To achieve success in the environmental management planning and efficient use of water resources over an area, it is essential to have information about rainfall variation.
An important parameter of atmospher is precipitation.It has a lot of changes over the time and space. It is a basic element in the formation of the activities and prospects of the environment. Several studies have done by researchers on the properties of precipitation in the different regions of the world and also in Iran. In general, precipitation showed a negative trend in many regions (Gorgio,2002: 675). For example, an assessment of summer rainfall in eastern China showed a positive and a negative trend in the north (Gemmer et al, 2004: 39; Gong et al, 2004: 771).Annual rainfall has decreased in southern parts of Italy and the decrease in winter precipitation was larger (Marco et al, 2004: 907). An increase in summer rainfall, especially, in June and July has been reported in the Yangtze River basin (Tong et al, 2007: 1016). A decrease in winter rainfall and an increas for other seasons has been showed in Turkey (Kahya and Partal, 2007: 43). Evaluation of maximum daily rainfall at the global scale showed an increasing trend in rainfall (Sethwestra et al. 2013: 3904). Negative anomalies of precipitation was reported for the most stations in southern west Ethiopia (Girma et al. 2016: 3037). Based on Iran's annual rainfall, positive and negative trends in annual rainfall have been showed (Asgari and Rahimzadeh, 2006: 67). A decrease in rainfall, especially in the decade of 1995-2005 revealed in Iran using annual rainfall (Asakereh and Razmi,2012: 159). Assessment of changes in seasonal patterns of rainfall in Hamedan, showed that the beginning of the rain tend to the summer and the end of winter (Movahedi et al.,2013: 23). The results of precipitation extreme indices on Iran showed a positive trend in  the west and the south west and a negative trend in the north (Masoodian and Darand, 2013: 239).
 
Methodology
For this study, the daily precipitation obseravtions obtained from synoptic stations in Kurdistan province during 01.01.1989 to 31.12.2014 were anlayzed. A database with dimensions of 9526 * 8 was created. The time was set on rows (9526 days) and the rainfall was set on columns. Homogeneous and heterogeneous monthly rainfall data were assessed by apply cumulative deviations test and Vercelli maximum of likness. Mann-Kendall approach was implemented to extraxt the trend at the significant level of 90%, 95%, and 99 %. The significant differences in the mean of time series data before and after a mutation year by Mann-Whitney test were evaluated. The statistical calculations were done in the Matlab software.
              
Results and Discussion                
The results showed that during the study period, duration of rainfall for autumn, winter and spring, in most of the stations, has been reduced. The results indicated that the rainfall duration for summer showes an increases in rainfall.Which is in line with the result of many previous studies.The reduction in the rainfall in the rainy season and an increase in rainfall in summer were obsorved. As a result the duration rainfall also has been  changed. Annual rainfall has decreased in southern Italy and decrease in precipitation in winter is more (Marco et al, 2004: 907). Movahedi et al. 2013, By studying the seasonal rainfall in Hamadan, They found that the rainfall began to ward the winter and their end to the summer have changed.

Conclusion                                 
     Evaluation of duration time series of rainfall over different months of the year showed that in the rainy months of autumn, winter and spring rainfall duration has decreased. For example, Baneh station showed a decline of 0.3 day in December, and Marivan showed a decline of 0.6 day in January. The average rate of decline in rainfall duration in March for the Qorveh station was 0.4 day per decade. In addition, a decline in spring rainfall duration was observed as well. Bijar station showed a decline of 0.2 day in May. However the rainfall duration in summer showed an increase. For example, Zarinah station obtained an increase of 0.2 day per decade in August.