| Summary: | Introduction The study of changes in precipitation and temperature components provides the basic information needed to assess the frequency and severity of climatic and hydrological processes. Therefore, the present study investigates the temporal changes of precipitation and temperature at different time scales in Karkheh Watershed. The present results are applicable in regional planning and decision makings.Materials and Methods Karkheh Watershed with an area of 51337 Km2 located in the west and southwest of Iran includes Kermanshah, Hamedan, Kurdistan, Ilam, Lorestan and Khuzestan provinces. The average annual rainfall in Karkheh Watershed varies from 150 mm in the southern regions to more than 1000 mm in the eastern regions. To do this study, daily minimum and maximum precipitation, temperature and sunny hours of six synoptic stations of Ahvaz, Dezful, Hamedan, Khorramabad, Kermanshah and Sanandaj in Karkheh Watershed with the statistical period 1980-2010 were used. The trend of changes in time series of precipitation and temperature was evaluated using Mann-Kendall statistical test and age estimator.Results and Discussion The results show that in the case of precipitation, the changes in most stations are decreasing and without a significant trend, and in a limited number of stations it has a significant decreasing trend, which is generally related to the cold months of the year (-0.09 and -0.07 at the station). Khorramabad and Sanandaj in February). The most changes in precipitation trend were observed in annual time series (-0.02 in Sanandaj Station). Thus, in most stations, a significant decrease in precipitation is observed. In all time series, the minimum and maximum temperatures in most stations have an increasing trend. The rate of this increasing trend is higher at minimum temperature (slope of changes 2.77 and 2.75 in Sanandaj and Ahvaz stations, respectively).Conclusion Analysis of daily, monthly, seasonal and annual trends of precipitation and minimum and maximum temperatures in the period 1980-2010 are very important due to their high impact on the hydrological response of watersheds as well as environmental issues. Also, the role of climate change and its effects on the trend of marginal phenomena in different regions can have many consequences. The present study was conducted to investigate the trend of precipitation and temperature in different time series. The results of the analysis of the monthly rainfall trend in the study period showed a decreasing and insignificant trend in most stations and in most months. Also, in most of the study stations, the monthly minimum temperature, with the exception of January, November and December, has an increasing trend and indicates that the monthly minimum temperature in the Karkheh Watershed has been increasing in recent years.
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