An evapotranspiration (ET) model for sparsely vegetated canopies under partial root-zone irrigation (PRI) was developed and tested using measurements from a vineyard in the arid region of northwest China. This model (PRI-ET) was mainly based on the Shuttleworth–Wallace (S-W) model and took into consideration the differences in soil water content between furrow-irrigation ditch and ridge under the PRI scheme and variable canopy shading over the surface. Estimates of ET and its components (plant transpiration and soil evaporation) by the PRI-ET and S-W models were compared to ET, plant transpiration and soil evaporation measured by Bowen ratio–energy balance, heat-pulse sap flow sensor and micro-lysimeter, respectively. The PRI-ET model can estimate the vineyard ET and its components more accurately than the S-W model, indicating that the PRI-ET model is suitable for estimating vineyard ET under PRI.