This work undertaken is an attempt to quantify water budget elements and determine existing irrigation efficiency in a large irrigated area with plant patterns showing large variability. The study was carried out in the Lower Seyhan Plain irrigation area in the Southern Mediterranean coastal region of Turkey. During the two-year work realized in 2006 and 2007, the existing planting pattern consisted of corn, wheat, cotton, citrus and limited acreages of vegetables. The research findings showed that the irrigation efficiency in the study area was rather low, as 20.1% in 2006. Crop water use efficiency and drainage fraction was 28.7% and 56%, respectively, at the catchment level in 2006. In the second hydrological year (in 2007), crop water use efficiency and drainage fraction were found 42.0% and 46.6%, respectively. Because flow rate in the irrigation channels is based on the principle of continuous availability of water, the farmers are encouraged both day- and night-time irrigation. However, they commonly irrigate during daytime. Therefore, water flow in the channels was wasted as drainage water during night. The results imply that both irrigation management of water user association and irrigation methods adopted by the farmers in the area require urgent improvements if we are to decrease wasteful use of available fresh water resources. To this effect, the use of sprinkler and drip irrigation methods should be promoted by water authorities.