Temporal and spatial distribution of extreme precipitation indices over the lake Urmia Basin, Iran

Climate extremes have more direct and significant impact than average state on social and ecological systems. In this study temporal and spatial distribution of extreme precipitation events were analyzed based on the daily precipitation data of seven meteorological stations in the Lake Urmia Basin in Iran from 1987 to 2014. Eleven indices of precipitation extremes were selected and calculated using the RClimDex software. The Mann-Kendall test was employed to assess trend in extreme precipitation indices, and the Sen's Estimator test was used to determine the slope of the significant trends. Results demonstrate that consecutive dry days (CDD) exhibit an increasing trend. The other indices of precipitation extremes, Consecutive wet days (CWD), heavy precipitation days (R10mm), heavier precipitation days (R20mm), heaviest precipitation days (R25mm), maximum 1-day precipitation (RX1day), maximum 5-day precipitation (RX5day), very wet day precipitation (R95), extremely wet day precipitation (R99), simple daily intensity index (SDII), and wet-day precipitation (PRCPTOT) show decreasing trend and time of change in most indices starting from 1995-1996. However, all the linear trends for each index are not statistically significant. Decreasing trends in precipitation days were greater than those in precipitation indices. The slope of trends in extreme precipitation indices showed that the highest slope of the decreasing trend occurred in wet-day precipitation, consecutive dry days and very wet day precipitation. Spatial distribution for precipitation extremes exhibited a declining trend in most regions in the Lake Urmia Basin. Furthermore, the extreme precipitation indexes had positive correlations with the annual total precipitation, and their correlation coefficients were statistically significant at the 1% significance level, except for consecutive dry days.