The application of the widely used Penrnan-Monteith evapotranspiration equation to hybrid poplar trees is impossible without a valid surface resistance. The increase in applications of drip-irrigated hybrid poplar trees for wood chip stock and veneer production, as well as bioremediation, constitutes a need for estimating the evapotranspiration of these trees. To the author's knowledge, there are no published estimates of surface resistance for poplar trees. Six years of weekly soil moisture content for drip-irrigated, hybrid poplar trees were used in a water balance to compute evapotranspiration. The weekly data were adjusted with reference evapotranspiration data to compute a daily evapotranspiration. Only data that represent fully leaved hybrid poplars are used in this study and the data were screened for the effects of drainage. Additional parameters applied in this study include solar radiation, temperature, wind speed and relative humidity taken at a nearby AGRIMET Weather Station. The results of this study indicate that surface resistance values cannot be described as a function of meteorological data within the constraints of the current experiment design. The graph of poplar evapotranspiration versus surface resistance shows that for a given evapotranspiration there can be multiple rs values. This scatter is the influence of parameters other than rs within the Penman-Monteith model. The use of an instrument to directly measure the surface resistance is recommended in further studies. / Graduation date: 2000