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Evapotranspiration (ET) dominates water fluxes in arid and semiarid ecosystems. The partitioning of ET into transpiration (T) and evaporation (E) has drawn increasing attention, particularly with respect to quantifying the control of shrub cover on water budget studies. The main objective of this study was to simulate the effects of shrub coverage spatial patterns on water partitioning into T and E and its relation to the simulation cell sizes. Correlation lengths of spatially distributed shrub cover in fine resolution grid cells were used to distinguish spatial patterns (i.e. patchiness) of shrub distributions. We then aggregated the fine resolution grid cells into larger cells of various hierarchies, calculated the potential T of each large cell as a portion of potential ET in the aggregated cell, and simulated the partitioning of actual ET into T and E. Results illustrated that simulated T increases with aggregated cell size, while E rate and total E do not change significantly (i.e. T/ET increases with aggregated cell size). The spatial distribution of shrub pattern influences the partitioning of ET: T increases with decreasing correlation length of shrub spatial pattern. Significant differences in T/ET are linked to the different correlation lengths, as a result of the highly non-linear relationship between potential T and % shrub cover, which indirectly take into account subsurface lateral flows in the root zone. Copyright (c) 2014 John Wiley & Sons, Ltd.