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Why does poor drainage make soil salinization worse in irrigated fields?

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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.

Poor drainage exacerbates soil salinization in irrigated fields because it prevents the necessary downward movement of water through the soil profile.1

In a healthy, well-drained system, irrigation water can flush excess salts downward and away from the root zone. However, when drainage is poor, this flushing process cannot occur. Instead, the following cycle leads to the progressive destruction of the soil:1

Salt Accumulation: All irrigation water contains at least small amounts of dissolved salts. When water cannot drain downward, it remains in the soil or rises toward the surface as saline groundwater.21
Evaporation: As water evaporates from the soil surface or is transpired by plants, the water molecules escape into the atmosphere, but the dissolved mineral residues are left behind.21
Surface Concentration: Without drainage to carry these salts away, they accumulate and concentrate at the surface.13

Over time, this buildup inhibits plant growth by making it difficult for roots to absorb water and, in severe cases, by introducing toxic levels of salt. This process eventually reduces agricultural yields and can force the abandonment of farmland.1423