) involves the transport of soil water through plants and its subsequent loss by evaporation through their stomata. T
together with evaporation (E
) from soil (Es
) and wet vegetation canopy (interception Ei
) forms evapotranspiration (ET). ET is the driving component of earth surface-plant-atmosphere water fluxes [1
can occur simultaneously and are both controlled by solar radiation, temperature, wind velocity, and vapor pressure gradient. These factors determine the amount of energy available for vaporization and the removal of water vapor from the evaporating surface. In addition, T
can be both limited by a lack of available soil moisture and are influenced by plant characteristics and plant density.
It is important to distinguish between E
fluxes because they respond differently to, e.g., air temperature [2
], interception [3
], and soil moisture [4
], and since T
is directly related to plant production [5
]. Information about E
fluxes is vital, e.g., to assess groundwater recharge and storage under changing climate, land use, and vegetation management practices. Both terms serve as input to models on, e.g., groundwater flow, weather forecasts, and climate change projections.
are difficult to measure accurately [6
]. A comparison of 31 studies in which E
, and ET were measured individually, revealed that the sum of E
comprised 34%–162% of the measured ET [6
]. This large span indicates measurement inaccuracies with the used methods for all three terms. These inaccuracies are due to the accuracy of the method and measurement devices as such and to human-induced errors (installation, maintenance, operation) [7
]. Moreover, interactions between vegetation and its abiotic and biotic environment are very complex, with great variations spatially and temporally. These complexities are reflected in ET measurements, yielding in a mismatch in the theoretical and measured sum of ET. In natural, undisturbed systems, it is virtually impossible to measure all factors that could affect E
, e.g., a plant’s access to soil water and nutrients and stress factors, such as toxic heavy metals. This also implies that measurements at one location may not be representative for a larger area, which poses serious limits to the application potential of the collected field data.
The complexity of measuring T
can be reduced by experimentally controlling one or more factors which affect E
. Typically, this is done by means of container experiments which can be conducted outdoors (with rain exclusion), in climate chambers, or in greenhouses. Depending on the research interest, for example soil moisture or CO2
concentration is controlled (e.g., [9
]). Different sap-flow measurement methods are available for measuring T
]. When the soil surface is covered to minimize E
, whole-plant T
can be measured directly by gravimetry. This can be done by placing the plant containers onto weighing devices (e.g., [12
]) by using a hanging scale [14
], or with a pallet truck scale [16
]. The containers are manually watered to compensate for T
losses. Due to practical constraints, the container size is usually limited and, with that, the size of the plant or vegetation under study. Logically, only a few studies measured woody plants taller than 50 cm [13
]. Therefore, a cost effective system is needed which facilitates the measurement of T
in a less time- and labor-consuming manner, thereby also enabling the measurement of larger woody plants.
To this end we designed a measurement device which measures whole-plant T directly and which can be applied for a wider range of container (and, therefore, plant) sizes. Measurements of T can be automated in this system.
This technical note presents the working principle and the design of the measurement device. We will describe how its accuracy was tested both in the laboratory and in the field under optimal water conditions and how T measured by the device compares to gravimetric measurements of T in an outdoor experiment.
4.1. Accuracy of the Transpiration Measurements for Different Refill Volumes
The laboratory test showed a positive relative error of the water meter, which decreased exponentially with an increasing amount of water abstracted from the water supply device. The relative error of the water meter varies to different extents per abstraction moment and volume, with smaller variation for a higher abstraction volume. That means that individual refill moments are not suitable to determine the actual water consumption, especially when refill volumes are low. Rather, measurements done after several refill moments will be more reliable. Our data indicate that 5–10 abstractions are sufficient to obtain a mean close to the mean of a larger number of abstraction records. Ultimately, the dimensions of both container and water supply device, as well as the water use by the vegetation determine the time resolution of the measurements.
Since larger refill volumes yield lower relative errors, we decided to use refill volumes of at least 1500 mL, which comprise an average measurement error of 4.49%. Expressed in mm, this amount equals an accuracy of 0.02 mm for a transpiring surface of 3 m².
During the setup of the field experiment, the accuracy of the transpiration measurements was tested again since the field setup differed in one main aspect from the laboratory tests: several devices were refilled at once instead of just one. The test under field conditions, where nine connected devices were refilled simultaneously for an abstraction volume of 1435 mL yielded mean measurement errors ranging from −7.1% to 4.2%. These observations differed substantially from the laboratory test, as both over- and underestimations were measured. We can exclude that this is caused by an inherent difference in accuracy between the water meters, since the device tested in the laboratory (Figure 4
and Figure 5
) and device no. 9 in Figure 6
was the same device. Rather, the reason for the different errors measured under field conditions is related to the rate at which the devices are filled. One device at a time is refilled at a rate of about 384 L/h. This value falls between the transitional and permanent flow rate of the meter used where an error of ±2% is expected [20
]. The more devices are refilled at the same time, the lower the flow rate per device. Flow rates lower than the minimum flow rate of 31 L/h increase measurement errors. Furthermore, when the valve opens, the devices at the end of the row are at first slowly refilled (with no detection by the water meter) until all previous devices in line are fully filled, resulting in an underestimation of the measured refill volumes. The duration and, therefore, volume of this undetected slow refill depends on the volumes that are refilled in the previous devices in line. Under field conditions, this will differ per refill moment. Additionally, the valve was open for a maximum of five minutes at a time, possibly resulting in an additional error due to inflows shorter than what the meters were designed for [21
]. Moreover, fluctuations in the water pressure in the feeding system depending on, e.g., water abstractions from other users influence the behavior of the water meter. Longer and larger flows, therefore, decrease the relative measurement error. To obtain consistent correctable errors we, therefore, recommend to refill the devices individually.
Errors measured with our device are low compared to other methods for transpiration measurements which are not based on weighing [7
]. Additional errors possibly introduced by the user are limited: reading the water meter is simple and the device requires little maintenance for proper functioning. However, adjusting the number and timing of the refill moments requires careful observations of the systems under study.
4.2. Field Tests: Tree Water Use Measured by Two Methods
Whole-tree transpiration of two individuals of the same species (Sorbus aucuparia) and of a similar size was measured with two methods. In general, both trees showed the same pattern of water use over the course of the measurement period from May until November 2018. However, an 8% higher total (over the entire measurement period) transpiration was measured for the tree connected to the water supply device than the one on the weighing balance. The higher total transpiration can be explained by a larger total leaf area of that tree: its approximate maximum transpiring leaf area was 10.7% larger than that of the tree on the weighing balance. The shift in the transpiration peak and decline of the device-tree compared to the balance-tree could have been caused by earlier bud break and leaf development, as well as earlier initiation of leaf senescence by the tree connected to the device.
Comparing the water use data obtained from the water supply device and the weighing balance shows that both follow the pattern of the reference ET. This indicates that water used by the tree was largely determined by meteorological conditions and that soil water was not limiting. This means that the water supply device adequately replenished the consumed soil water.
4.3. Potential Applications
The water supply device can in principle be used for experiments where the position of water table needs to be controlled and where plant water consumption needs to be measured in an easy and reliable way. The system is most suitable for moist to wet soil conditions and measurements are most accurate for periods that contain multiple refill moments.
Here we used the water supply device for single trees. In principle it can be employed for single or multiple plants of all vegetation types. This makes the device suitable for, e.g., diversity experiments, such as measuring the water use of different species combinations [15
], as well as agricultural studies, e.g., measuring effects of the water table on crop growth and yield.
Next to measuring transpiration under non-limiting water conditions, the device can also be used to let the container periodically fall dry by manipulating the refills with the valve to measure for example plant physiological responses to water shortages.
When setting up the device, a balance has to be found between the relative position of the water table and the (expected) rooting depth of the vegetation under study to avoid a too high or a too low soil water content in the rooting zone. To determine whether a soil is suitable and which distance between the water table and the soil surface should be chosen, we recommend insight into the water retention curve and the hydraulic conductivity of the soil and the expected transpiration fluxes. If necessary, the water table can also be adjusted when the device is already connected to the container by placing it higher than the plant container.
The advantages of the device are:
Made from easily accessible, durable and relatively inexpensive materials (approx. 80 € material per device);
The device does not require any electronics, thereby enabling an easy installation and set up;
Installation both in- and outdoors, as long as rain is excluded from the plant container and temperatures are above 0 °C;
Measurements of transpiration also in large and/or heavy containers where weighing becomes cumbersome or expensive;
Fast and easy data acquisition; and
The device can be modified to enable fully automated data acquisition by installing electronic devices such as level sensors or water meters which can be read remotely.