Role of exchange flow in salt water balance of Urmia Lake

•Urmia Lake is a large hyper-saline lake with ecological significance located in north of Iran.•A built causeway has divided the lake into two parts connected by an opening.•A salt water balance model of the lake is developed to simulate the salinity, water level, and exchange flow between the two parts.•We show the exchange flow through the opening is an important factor in balance of the water levels and salinities between the two parts.

In this paper we examine how exchange flow in Urmia Lake plays a crucial role in dynamics of the lake. Urmia Lake, a very large hyper-saline lake of high ecological significance, is located in northwest of Iran with a 15-km causeway dividing it into north and south lakes. A 1250-m opening in the causeway near the east coast links the two lakes. The differences in mean water levels and densities of the two lakes increase in spring due to large freshwater inflows into the south lake. High evaporation dominates the lake in summer. By incorporating the results of a two-layer hydraulics theory into a mixing model of the lake, we show that the exchange flow through the opening diminishes the water level and salinity differences of the two lakes and hence helps restoration of the pre-construction state of the lake by the end of fall. The model results are confirmed by the field data. The north-south and south-north flows through the opening are equal in most of the year, except in spring when the south-north flow is greater due to the head difference of the two lakes.