Change in annual precipitation in the northwest of Iran

The detection of changes in precipitation is important in water resources research and hydrological modelling studies. In this study, the Mann-Whitney non-parametric change point model (CPM) was applied to indicate whether a change point has occurred in the mean of annual precipitation in 18 synoptic stations in different regions of Iran for the period 1951-2010. The parametric likelihood ratio test (LRT) and the non-parametric Thiel-Sen slope were also employed to evaluate the detected change points. The precipitation time series (PTS) of the stations, which were serially correlated, were transformed to produce uncorrelated pre-whitened time series. The pre-whitened time series were then used as input file for the CPM, LRT and Thiel-Sen slope. The non-parametric CPM estimated change points in the PTS of Kerman, Oroomieh, Shahroud and Tabriz in 1997, 1994, 1967 and 1994, respectively. The results of the LRT indicated that the PTS before and after these change points follow a gamma distribution but with different means. The difference in annual precipitation means before and after the detected change point for Oroomieh station, located in the northwest of Iran, is 67 mm, as an example. The Thiel-Sen slope estimated a greatly decreasing rate of change for Kerman station after 1997.