Being an important desert riparian forest in the lower reaches of the Heihe River Basin,Populus euphratica Oliv.forest functions as a natural barrier in maintaining and preserving the stability of local oases.Accordingly,accurately estimating the water use of P.euphratica is important for vegetation protection and water resource allocation.To date,little data are available for evaluating the hysteretic effects between sap flow and environmental variables,and for estimating the water use of desert riparian forest.In this study,we measured the sap flow velocity(V_s)of P.euphratica using the heat ratio method during the growing season of 2012.Based on the response of V_s to solar radiation(R_s)and vapor pressure deficit(VPD),we estimated the hourly Vsand daily V_s using the multivariable linear regression and a modified Jarvis-Stewart(JS)model,respectively.Hysteretic response of V_s to environmental variables was then evaluated using a sap flow model.We found the thresholds of V_s responses to R_s and VPD at both hourly and daily scales during the growing season,and successfully estimated the seasonal variations of hourly V_s and daily V_s using the JS model.At an hourly scale,the maximum V_s occurred earlier than the maximum VPD by approximately 0.5 h but later than the maximum R_s by approximate 1.0 h.At a daily scale,the maximum V_s lagged behind the maximum VPD by approximately 2.5 h while occurred earlier than the maximum R_s by approximately 2 h.However,hysteretic response of V_swas weakened when R_s and VPD were measured together using the JS model at both hourly and daily scales.Consequently,short-term and intensive field campaigns,where V_s and environmental variables can be measured,may be used to estimate short-run sap flow and stand transpiration using only two environmental variables. Being an important desert riparian forest in the lower reaches of the Heihe River Basin, Populus euphratica Oliv.forest functions as a natural barrier in maintaining and preserving the stability of local oases. Accredited, accurately estimating the water use of P.euphratica is important for vegetation protection and water resource allocation. To date, little data are available for evaluating the hysteretic effects between sap flow and environmental variables, and for estimating the water use of desert riparian forest. In this study, we measured the sap flow velocity (V_s) of P.euphratica using the heat ratio method during the growing season of 2012. Based on the response of V_s to solar radiation (R_s) and vapor pressure deficit (VPD), we estimated the hourly Vsand daily V_s using the multivariable linear regression and a modified Jarvis-Stewart (JS) model, respectively. Hysteretic response of V_s to environmental variables was then evaluated using a sap flow model. We found the thresholds of V_s respons es to R_s and VPD at both hourly and daily scales during the growing season, and successfully estimated the seasonal variations of hourly V_s and daily V_s using the JS model. At an hourly scale, the maximum V_s occurred earlier than the maximum VPD by approximately 0.5 h but later than the maximum R_s by approximate 1.0 h.At a daily scale, the maximum V_s lagged behind the maximum VPD by about 2.5 h while showing an earlier than the maximum R_s by approximately 2 h.However, hysteretic response of V_swas weakened when R_s and VPD were measured together using the JS model at both hourly and daily scales. Conclusion, short-term and intensive field campaigns, where V_s and environmental variables can be measured, may be used to estimate short-run sap flow and stand transpiration using only two environmental variables.