Improving the Scientific Understanding of the Paradox of Irrigation Efficiency: An Integrated Modeling Approach to Assessing Basin-Scale Irrigation Efficiency

Irrigation is vital for the sustainability of water-limited agricultural areas. However, higher irrigation efficiency (IE) rarely reduces irrigation water consumption, a paradox well noted in the real world. Resolving this paradox requires rigorous assessment of basin-scale IE, which remains rare. This study holistically analyzed basin-scale IE using the Zhangye Basin (ZB), a typical agricultural area in arid northwestern China, as the testbed. A new basin-scale IE index was proposed that factors in irrigation return flow and groundwater's direct contributions to crop evapotranspiration. A novel approach was also developed to calculate this new index based on integrated ecohydrological modeling. The major study findings for the ZB include the following. First, return flow accounts for approximately 13% of irrigation water, which creates a difference of 0.1 between the new basin-scale IE index and a traditional field-scale index. Second, although basin-scale IE has notable interannual fluctuation, its multiyear average shows stability, which may be dependent on the basin's physical characteristics. Third, basin-scale IE reflects large spatial heterogeneity and is influenced by the intensity of surface water-groundwater interaction. Finally, under the projected future climate, return flow in the ZB will be enhanced by 3.2% per decade, and basin-scale IE shows an increasing trend. Overall, rigorous evaluation of basin-scale IE is critical to making heterogeneous policies and developing adaptive management strategies under the changing climate. The integrated modeling approach developed in this study provides a methodological foundation for overcoming misunderstandings about the IE paradox and benefits the reform of the current IE policy agenda.