The drawdown trend of the water level in Urmia Lake poses a serious problem for northwestern Iran which has had negative impacts on agriculture and industry. This research investigated likely causes of the predicament by estimating trends in the time series of hydroclimatic variables of the basin. Three non-parametric statistical tests, the Mann-Kendall, Spearman rho, and Sen's T, were applied to estimate the trends in the annual and seasonal time series of temperature, precipitation, and streamflow at 95 stations throughout the basin. The Theil-Sen method was also used to estimate the slopes of trend lines of annual time series. The results showed a significant increasing trend of temperature throughout the basin and an area-specific precipitation trend. The tests also confirmed a general decreasing trend in the basin streamflow that was more pronounced in the downstream stations. The annual trend line slope was found to be from 0.02 to 0.14 A degrees C/year, -7.5 to 3.8 mm/year, and -0.01 to -0.4 m(3)/s/year for temperature, precipitation, and streamflow, respectively. The homogeneity of the monthly trends was also evaluated using the Van Belle and Hughes tests as confirmation. Temporal analyses of the trends for the temperature and streamflow of the basin detected significant increasing trends beginning in the mid-1980s and 1990s. The correlations between streamflow and climate variables (temperature and precipitation) were detected by Pearson's test. The results showed that the streamflow in Urmia Lake basin is more sensitive to changes in temperature than precipitation. In general, the decline in the lake water level can be related to both the increase of temperature in the basin and an improvement in over-exploitation of the water resources.