The catchment of the Karun River of the Zagros Range has on area of more than 23000^km2 . The climate of the catchment is hot with dry summers and mild winters. The majority of precipitation in this region occurs in the winter. Rainfalls in this region are generally categorized based on three types of air masses:

• Indian Ocean seasonal air mass, which does not affect much in the Karun catchment area rainfall.

• Polar Continental air mass, which causes a little humidity in the region.

• Mediterranean air mass, which results in severe periodic showers in the region.

The average annual precipitation in the Karun catchment area, at the location of Pol-e Shaloo (about five kilometers upstream of the Dam site) is 620^mm , and the snow covering mountains on the location of the catchment is 2000^m.a.s.l. with an area of 16000^km2 .