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Direct and continuous measurements of water use by range grasses are needed by both range scientists and land managers. This study tested a heat balance sap flow gauge on individual culms of the tallgrass prairie species big bluestem (Andropogon gerardii Vitman) and indiangrass [Sorghastrum nutans (L.) Nash]. Gauge performance was evaluated on potted plants in the laboratory, greenhouse, and field by comparing sap flow to gravimetric measurements of transpiration. In the laboratory, gauge-measured water loss was consistently within +/- 10% of gravimetric measurements for both species at flow rates less than or equal to 4 g hour-1. The first-order time constant of the gauge was calculated to be < 20 seconds. In the greenhouse, sap flow estimates were consistently below gravimetric water loss and negative flows were often computed because of suspected errors in the radial heat flux component. Laboratory data showed that despite the gauge being surrounded with insulation, errors in the heat balance could occur because of external air temperature changes. In the field, environmental alterations in the stem energy balance affected the accuracy of gauges placed outside a plant canopy, but accurate measurements did occur when the plants were placed within a plant canopy. Heat transfer analysis indicated that foam insulation should be 20 to 25 mm thick to minimize the effect of the environment on gauge performance. The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020