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During the night, plant water loss can occur either through the roots, as hydraulic redistribution (HR), or through the leaves via the stoma, as nocturnal transpiration (E-n), which was methodologically difficult to separate from stem refilling (R-e). While HR and E-n have been reported across a range of species, ecosystem, and climate zone, there is little understanding on the interactions between E-n and/or R-e and HR. As water movement at night occurs via gradients of water potential, it is expected that during periods of high atmospheric vapor pressure deficit (VPD), water loss via E-n will override water loss via HR. To test this hypothesis, sap flow in stems and roots of Populus euphratica Oliv. trees, growing in a riparian zone in a hyperarid climate, was measured once in a year. Nocturnal stem sap flow was separated into E-n and R-e using the forecasted refilling method. Substantial nocturnal sap flow (38% of 24-hr flux on average) was observed and positively correlated with VPD; however, the strength of the correlation was lower (R-2=.55) than diurnal sap flow (E-d) (R-2=.72), suggesting that nocturnal stem sap flow was attributed to both water loss through the canopy and replenishment of water in stem tissues. Partitioning of nocturnal sap flow shows that R-e constituted approximately 80%, and E-n similar to 20%, of nocturnal sap flow. The amount of root sap flow attributed to redistribution was negatively related to E-d (R-2=.69) and the amount of acropetally sap flow in stems, R-e (R-2=.41) and E-n (R-2=.14). It was suggested that the magnitude of HR is more strongly depressed by R-e that was recharge to the water loss via E-d than by E-n. It was consistent with whole-tree water balance theory, that the nighttime upward sap flow to xylem, stem refilling and transpiration, may depress hydraulic redistribution of roots.