Abstract: Water consumption of vegetation is the core of SPAC (Soil-Plant-Atmosphere Continuum) research; it's also the precondition on the research of water requirements by natural vegetation. This paper simulated the water consumption of riparian Populus euphratica forests, the representative natural vegetation in the lower reaches of Tarim River, in a relatively small time scale based on the SHAW(Simultaneous Heat and Water) model. Conventional weather elements, vegetation parameter and hydrothermal data of soil profile were adopted as initial inputs of the model. The results show that there is a deviation between the simulated value and observed value of water consumption by Populus euphratica forests. To further improve the appropriateness of the SHAW model application in arid regions, we established a modified SHAW model by introducing the groundwater level factor GSI (Groundwater-Soil water Interaction). We comparatively analyzed the water consumption of Populus euphratica by the two models of SHAW and GSI-SHAW. The results show that the correlation coefficients between simulated and observed water consumption are 0.8533 and 0.9075 by SHAW model and GSI-SHAW model, respectively. The relative errors of simulated values by SHAW model and GSI-SHAW model are respectively 21.4% and 16.9% against observed water consumption rates. It can be seen that the simulated values of the modified GSI-SHAW model are more approximate to the measured values. We conclude that the integration of GSI model further improves the simulation accuracy of SHAW model, providing a new method and scientific proof for the calculation of water consumption by natural vegetations in arid areas.