Technical note: A device to directly measure transpiration from vegetation grown in containers

Information about evaporation and transpiration fluxes is vital for water budgets, modeling of water flows and climate, as well as for assessing the hydrological impact of land management practices. Under natural conditions, these fluxes are difficult to measure accurately, which results in large measurement inaccuracies. These inaccuracies can be reduced in controlled experiments. We present a device that is especially useful for transpiration studies conducted in large and/or heavy containers where weighing becomes too cumbersome or expensive. With our device we set a water table and control soil moisture of potted small trees by periodically replenishing soil water consumed by the tree, thereby measuring the inflow volume, which represent whole-tree transpiration. The device is made of inexpensive, easily available and durable materials and can be used for in- and outdoor experiments. Data acquisition is fast and easy. The mean measurement error of the device is 4.5% (±3.2% SD) for refill (i.e., transpiration) volumes of 1.5 L or larger. For a transpiring surface of 3 m², this amount is equal to an accuracy of 0.02 mm. Validation on field data showed that transpiration measured by the device is comparable to transpiration measured by gravimetric changes.