In arid and semi-arid inland regions such as northwestern China, landscapes typically exhibit a zonal pattern from oasis to oasis-desert ecotone and then to desert, influenced by proximity to inland rivers that recharge groundwater and regulate vegetation distribution. The oasis-desert ecotone, as a transitional and ecologically fragile zone, plays a critical role in resisting desert encroachment and maintaining regional ecological stability. However, its structure and function are highly sensitive to groundwater dynamics, particularly phreatic water depth. The study employed a combination of theoretical derivation and field investigation for validation. A hydro-ecological conceptual framework was first developed by dividing the vadose zone into two functionally distinct layers: the phreatic influence zone and the root action zone. The interactions between these two zones are analyzed to elucidate the mechanisms governing vertical soil water movement in arid environments. Based on this framework, a phreatic water distribution curve is derived, and the thickness of the phreatic influence zone is quantitatively estimated. Furthermore, theoretical expressions are proposed to determine the critical groundwater burial depths demarcating the inner boundary (between oasis and ecotone) and the outer boundary (between ecotone and desert) of the transition zone. Taking the Luocheng Irrigation District in the Heihe River Basin as a case study, the thickness of the phreatic influence zone is estimated as 1.29 m, the critical groundwater depths are estimated at approximately 4 similar to 6 m for the inner boundary and 8-13 m for the outer boundary. The findings offer theoretical support for ecological zoning, oasis restoration, and sustainable land management in arid inland river basins. Future work should expand comparative studies across diverse climatic regions and promote large-scale application of these theoretical results in ecological restoration practices.