trends analysis of reference evapotranspiration rates by using the mann-kendall and spearman tests in arid regions of iran

abstract in this research, temporal trends of reference evapotranspiration (et0) values were investigated in arid regions of iran. for this purpose, the meteorological observations collected from 11 high quality meteorological sites for a 41-year period (1965-2005) were used and statistically significant et0 trends in the monthly, seasonal and annual time basis were detected by the non-parametric mann-kendall and spearman tests at confidence level of 95 %. for eliminating the effect of serial correlation on test results, the trend free pre-whitening (tfpw) approach was applied. the results indicated that the et0 trends for some sites were increasing (positive) but for some sites showed decreasing (negative) trends. the most significant et0 trends on the monthly time scale occurred at birjand but no significant trend was observed for bandarabbas, sabzevar and semnan sites. on the annual time scale, mashhad revealed the highest positive et0 trend (7.5 mm per year). on the contrary, esfahan showed the highest decreasing (negative) et0 trend of about -6.38 mm per year. in general, the performances and capabilities of mann-kendall and spearman tests were consistence at the verified confidence level.