Study of Trends in Precipitation and Stream Flow of the Lake Urmia Basin During the Past Four Decades

The Lake Urmia basin in North West of Iran is one of the important basins of this country due to its climatological, economical, ecological and environmental condition. In this study in order to discover the likely causes of the lake shrinkage, trends of the two effective factors on its inflows (precipitation and rivers discharge), were investigated. For this purpose, rain gauges and hydrometric stations data at seasonal and annual scale across the basin were collected. Long time-series of variables were prepared and their trends were analyzed using the Mann-Kendall and seasonal Kendall test. According to the results although annual precipitation indicated decreasing or increasing trends at some stations, but most of them were not significant at 0.05 probability level. Despite lack of significant, decreasing trend for annual precipitation across the basin, the majority of hydrometric stations showed significant decreasing trend in the annual stream flow data at 0.05 probability level. Therefore, from statistical point of view, the significant decline at annual discharge of the streams draining into the lake and the shrinkage of the lake cannot be attributed to decline at annual precipitation. Therefore, some other causes such as increase in evapotranspiration due to atmospheric warm-up. Over-extraction of aquifers, extensive landuse changes, impoundment of water behind numerous dams and decline at the lake inflows might be effective on the Lake Urmia shrinkage.