variations in temporal occurrence of discharge in urmia lake basin

Urmia lake has experienced a critical environmental condition in recent years. this paper discusses changes in various hydrological measures in the urmia lake basin (ulb) and its tributaries. the mann-kendall test with trend-free pre-whitening (tfpw) approach was used to estimate the trend in annual time series (1978-2011) of average and peak values of discharge recorded in 65 and 55 stations of ulb, respectively. accordingly, the cumulative sum (cusum) control chart test was used for change detection analysis and t-statistic to compare the discharge in two periods (before and after 1995 as the year in which the urmia lake water level started to decline). the results revealed that 55 stations (~86%) had a descending trend in recorded average values of discharge data. in 38 stations (~59%), this diminishing trend was significant at 95% confidence level. the trend of peak data recorded in 42 out of 55 stations was declining. this falling trend was significant at 95% confidence level in seven stations. the average value of t-statistic for all stations (2.8) represented a significant difference between discharge values in two periods at most of the studied stations. the cusum results indicated that in more than 50% of the stations, which recorded discharge series of 11 rivers, the existing jump was significant at 95% of confidence level. the trend of 0-1 m³/s interval was ascending, but the trend of other discharge ranges (more than 1 m³/s) was descending. the results of this study also revealed that the amount of peak and discharge volume entering the lake in 1998-2011 decreased in comparison with 1978-1997.