Abstract: The transpiration variation in space and time in a young poplar plantation(Populus deltoides Bartr.cv.‘Lux’ex．I-69/55)was studied in 1983—1984 for the purpose of inquiring into the relationship between transpiration water-consumption and growth in young poplar plantation so as to provide an evidenee for rational irrigation．The research has revealed that the transpiration of abaxial surface is higher than that of adaxial;the transpiration rates of different position on identical leaf surface are Similar；there is a fluetuaring movement in transpiration rate along an ordinal leayes of a branch;the transpiration is higher in low layer of crown，medium in middle layer and lower in the upper layer;the daily transpiration process(sunny day)forms a curve with a peak at about 10 am．；the seasonal fluctuation of transpiration is as follow：transpiration is higher in early spring，then goes down，rises in the autumn and goes down again by the end of growing season．A formula for estimation of crown transpiration water-consumption has been determined according to the analysis of the factors influencing transpiration in space and time and meteorological factors，and the transpiration water-consumption of crown was estimated by use of the formula．