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Why does much of the Tigris-Euphrates river water not reach the Persian Gulf?

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Map showing the Tigris and Euphrates river basin and its drainage network across Turkey, Syria, and Iraq, with color shading for elevation, major dams, canals, lakes, and boundaries with neighboring countries. The green areas indicate lower elevations along the river valleys, while higher elevations are shaded in brown and orange. A small locator map in the corner shows the basin's position within the Middle East. — Tigris-Euphrates river system The Tigris and Euphrates river basin and its drainage network.

A significant portion of the water from the Tigris-Euphrates river system does not reach the Persian Gulf due to a combination of natural environmental factors and extensive human intervention.12

Approximately 90 percent of the water and silt that passes Baghdad and Al-Ramādī is lost before reaching the gulf. The primary reasons for this include:1

Evaporation and Seepage: High temperatures in the region, which can exceed 120 °F (49 °C) in the summer, lead to significant evaporation rates. Additionally, water is lost through seepage into the ground.132
Irrigation: Extensive irrigation practices, which are essential for agriculture in the region, divert large volumes of water from the main channels.412
Marshlands and Reservoirs: Water is absorbed by extensive marshlands and diverted into large reservoirs and depressions, such as Lake Al-Tharthār and Lake Al-Ḥabbāniyyah, which are used for water control and storage.152

Consequently, the Tigris and Euphrates contribute only a small portion of the water that eventually flows into the Shatt al-Arab, with the majority of that river's flow instead deriving from tributaries like the Kārūn River.12