Evaluation of Sap Flow Sensors to Measure the Transpiration Rate of Plants during Canopy Wetting and Drying

Accurate measurement of transpiration is needed to quantify the different components of evaporation losses during sprinkler irrigation. Among the different methods for direct measurement of transpiration rate, measurement of sap flow on the basis of heat flow has been widely used. To evaluate the applicability of this method for use in field experiments involving sprinkler irrigation, stem flow measurements on small pot plants were compared with transpiration values estimated from successive mass measurements using pre-calibrated mini-lysimeters. Experiments were conducted in the glass house at USQ during the period August to October 2010. From this study it was found that during steady conditions, the sap flow sensors were able to measure the transpiration rate of plants in reasonable agreement (11% overestimation) with the lysimeter measurements. During canopy wetting (sprinkler irrigation) the decline in sap flow rate was less than and lagged the decline in transpiration rate due to the storage and buffering capacity of the stem. Sap flow sensors at different heights on the plant responded at different rates for the same reason. The study showed that the sap flow sensors do not give an accurate measurement of transpiration during canopy wetting and drying but will still aid interpretation of measurements of total evaporation during sprinkler irrigation.