The knowledge of the water fluxes and groundwater contributions to plant water use under fluctuating groundwater levels is crucial for the sustainable utilization of groundwater resources and the preservation of healthy ecosystems in semi-arid regions, such as the desert oasis croplands in the Mu Us Sandy Land (MUSL), Northwest China. A 3-year field trial was conducted in an oasis cropland within MUSL, involving five treatments with surface irrigation depths ranging from 5 mm to 60 mm per application and 5 to 11 irrigation applications per year. The major physical characteristics of the soils at the site were ascertained through laboratory measurements. Hydrogen and oxygen isotopes (deuterium and oxygen 18) in different water sources for the treatment with an irrigation depth of 45 mm per application (I-45) in the maize field were analyzed from in situ samples, and root water uptake (RWU) was further identified using the IsoSource model. A numerical model based on HYDRUS-1D for soil water fluxes was calibrated and validated using observation data to estimate the actual evapotranspiration (ETa) and water productivity (WP) under various irrigation treatments. The isotope analysis indicated that maize water uptake in the I-45 treatment incorporated both groundwater and soil water during the growing stage. The large negative d-excess values in groundwater suggested a secondary evaporation effect of recharge water due to irrigation return flow. According to simulations with Hydrus-1D, groundwater use contributed 9 % to 37 % of the maize ETa for the I-45 treatment between 2014 and 2016 when the depth to the water table varied between 0.81 m and 1.13 m. With an optimal irrigation depth of 37 mm per application, the maximum yield and WP can be achieved in oasis cropland. Even in dry years, with a 16 % probability of rainfall occurrence, maize production was not affected due to the capillary rise of groundwater and the pumping of groundwater for irrigation. These results offer potential scientific insights for agricultural water management of spring maize under irrigation in desert oasis farmlands of the MUSL and other regions with similar soil texture, climatic conditions, and cropping practices.