The Tigris-Euphrates transboundary basin faces growing water scarcity driven by climate change, population growth, and inefficient irrigation, which intensifies political sensitivities among its riparian countries (Turkey, Syria, Iraq, and Iran). However, no comprehensive framework currently exists to quantify agricultural water consumption or to distinguish beneficial from non-beneficial water use across all basin states, thereby hindering evidence-based transboundary management. It is hypothesized that conventional irrigation efficiency metrics underestimate actual water consumption at the basin scale due to return flows, and that irrigated area expansion—rather than farm-level efficiency gains—is the primary driver of increased real water consumption across the basin. This study assesses agricultural water consumption and irrigation efficiency in Turkey, Syria, Iraq, and Iran using an integrated approach that combines a modified Soil and Water Assessment Tool (SWAT-TE) with the Water Accounting Plus (WA+) framework. The SWAT-TE model, calibrated using multiple hydrological indicators (river discharge, evapotranspiration, and soil moisture), simulates river flows at both the farm and basin scales. The WA+ framework is then used to quantify beneficial and non-beneficial water use, as well as return flows. The results reveal low farm-level irrigation efficiency (30–50%), high non-beneficial water losses (such as soil evaporation), and varying water withdrawal trends across the riparian countries. Turkey demonstrates improved farm-level efficiency (≥40%), whereas Syria exhibits unsustainable groundwater use. The overall basin efficiency (65–77%) reflects a high level of water reuse; however, actual water consumption still increased by 80% in Turkey, 94.1% in Syria, and 25% in Iraq, largely due to the expansion of irrigated areas and increased irrigation intensity. This study emphasizes the importance of improving water productivity and promoting cooperative water management to mitigate transboundary tensions. This study presents three main contributions: (a) a scalable and transferable framework for evaluating agricultural water policies in transboundary basins; (b) the quantitative distinction between beneficial and non-beneficial water consumption across all riparian countries of the Tigris-Euphrates basin; and (c) evidence that return flows significantly buffer upstream water withdrawals, thereby explaining the irrigation efficiency paradox in this basin.
Delavar et al. (Mon,) studied this question.