Comparison of detrending methods for the temperature and precipitations time series

AbstractAssessment of meteorological variables time series can indicate the effect of climatic factors on water resources and hydrological conditions. In this case, statistical methods are useful and effective tool to be considered. But to evaluate and quantify level of this impact, it is necessary to remove trends in the series and detrended series be used. In this paper, two detrending methods based on slope of line and Fourier series were used for deterending the annual temperature and precipitation time series in Ajichai basin. Performance of this methods was evaluated based on compare average long-term precipitation and temperature before and after detrending. Results of trend analysis for precipitation did not show the same general behavior, but the general trend of temperatures in the basin was rising. Detrending results also indicate the improper performance of both methods. In the first method differences in the data before and after detrending was high however, in the second method no difference was seen. To solve this problem, in this study a mixed approach was proposed to modify the first method. This method detrends time series based on the Theil–sen’s slope line exactly in the years after starting the trend that shows more reasonable results compared to the other methods. In this method, average of long-term precipitation and temperature in the basin changed from 328 to 339 mm and 11 to 10. 6 oC, respectively.