Institute of Botany, University of Basel, Switzerland
Department of Earth and Planetary Sciences, Weizmann Institute of Science, Israel
Transpirable water content is the amount of soil water (in kg or mm) available for plant water-use based on local and species-specific parameters of soil and plant hydraulics. This is an important parameter in the interpretation of tree water-use and calculation of forest water balance.
Knowledge of the amount of water available for plant water-use is fundamental to many plant eco-physiology studies. In such studies soil water content (SWC) is often measured and interpreted in relation to plant water-use patterns, but with very limited meaning. This is because: (i) plant water transport follows changes in water potential and not water content; and (ii) The ability of plants to withdraw water from soil is species-specific.
Due to water retention in soil, the transpirable soil water content (tSWC) is always smaller than the measured SWC. This depends on both soil and plant hydraulics. Calculating the amount of water available for plant transpiration can be useful to interpret plant hydraulic measurements. For example, tree sap flow patterns are not fully understood from parallel SWC time-series. tSWC were shown to better explain sap flow dynamics (Klein et al. 2014). A second advantage is the ability to partition the water sources for transpiration, in terms of the different soil layers. In addition, in a mixed vegetation ecosystem, co-occurring species may have very different tSWC for the same SWC values.
What information do you need in order to calculate transpirable soil water content?
- Soil water potential, either measured in situ or calculated based on soil water content and soil water retention curve(s). Soil water potential values are time-dependent and soil layer-specific.
- Information about the species-specific water potential threshold of root water uptake. This depends on xylem and stomatal physiology.
By applying the plant threshold value (ttSWC) on the layer-specific soil water retention curve, one would obtain the transpirable water amount for any soil layer. For example, a root uptake threshold of -1.5 MPa applied on a retention curve of a clay-rich soil layer can obtain a ttSWC of 17% v/v for that layer, meaning that anything below a SWC value of 17% is physiologically considered zero. The tSWC is hence: tSWC = SWC – ttSWC. To get a number in mm water (as for rainfall), tSWC (in % v/v) must be multiplied by the layer width (in mm), minus any stone fraction.
To perform the calculation, soil water content data series are required, together with additional information on soil water retention and the plant root water uptake threshold. To obtain these data, you would need the following equipment:
- Soil corer for taking soil samples. Samples should be of undisturbed soil.
- Soil water content probes (or alternatively, an oven for drying soil samples for SWC determination).
- Soil water potential probes. A variety of methods exist, nevertheless with many limitations. Among these are: (1) resistance block: inexpensive but unreliable, measures at water potentials (wp) of -0.1 to -0.7 MPa, i.e. not in dry soil; (2) psychrometer: good for dry soil (-0.2 > wp > -10.0 MPa) but expensive, temperature sensitive and unsuitable to field; (3) Tensiometer: reliable but not suitable for plant studies due to low range (0.0 > wp > -0.09 MPa). A new type of tensiometer claims to measure at a suitable range (0.0 > wp > -1.50 MPa).
- Alternatively, soil water potential can be estimated from SWC (measured by ii), and soil water retention curves. To this purpose you would need to use the soil samples in (i) and determine experimentally the changes in soil water potential as function of changes in SWC, e.g. using the capillary head method.
- Alternatively, soil water retention curves are estimated using measured values of SWC (ii) and a pedo-transfer equation such as the van-Genuchten-Mualem model. To this purpose you would need to use the soil samples in (i) and determine the soil particle size distribution, e.g. using the density method.
- Scholander pressure chamber to measure plant water potential, and a porometer (or gas exchange system) to define the root uptake water potential threshold. Plant water potential is often measured at the leaf or shoot level and not at the root. Therefore the root uptake water potential threshold can be estimated from the stomatal closure water potential threshold assuming a water potential gradient of ca. 0.8 MPa between leaves and roots. Alternatively, it can be assumed that the root uptake water potential threshold is equivalent with the leaf water potential at 50% stomatal closure. In tall trees, an additional decrease of -0.1 MPa should be considered for every 10 m height increment.
- Optionally, if a complete water balance is desired, a root survey must be performed in order to characterize the soil layers where roots are prevalent.
Units, terms, definitions
The product of this protocol is transpirable soil water content (tSWC, in % v/v), which is calculated for any specific soil layer by:
Eq. (1) tSWC = SWC – ttSWC.
Where SWC is the measured soil water content (in % v/v) and ttSWC is the species-specific root water uptake threshold (in % v/v). To get a number in mm water (as for rainfall), tSWC (in % v/v) must be multiplied by the layer width (in mm), minus any stone fraction
The procedure is exemplified using a case study of a semi-arid pine forest.
Step 1. Obtain the SWC data series (Fig. 1). In this case four discrete layers were measured. The lower panel shows soil water potential dynamics based on the same dataset and soil water retention curves (Fig. 2).
Step 2. Calculate the ttSWC by applying the species-specific root water uptake threshold on the soil layer-specific retention curves (Fig. 2). In this case the ttSWC values are 13.2, 15.2, 17.0, and 17.6 % v/v for 0-10, 10-20, 20-40, and 40-60 cm depth layers, respectively.
Step 3. Build the tSWC graph by subtracting the ttSWC (Fig. 2) values from the measured SWC values (Fig. 1). Figure 3 below shows the tSWC graph (in mm) together with a graph of sap flow dynamics of trees growing on site (bottom panel). Tree water-use can be interpreted by tSWC dynamics.
Notes and trouble shooting tips
Plant water potential is often measured at the leaf or shoot level and not at the root. Therefore the root uptake water potential threshold can be estimated from the stomatal closure water potential threshold. It can be assumed that the root uptake water potential threshold is equivalent with the leaf water potential at 50% stomatal closure. A list of water potential at 50% stomatal closure values for 70 woody species is available at the supporting material in Klein (2014).
Links to resources and suppliers
Pedo-transfer software can be found here:
Klein, T., Rotenberg, E., Cohen-Hilaleh, E., Raz-Yaseef, N., Tatarinov, F., Preisler, Y., Ogee, J., Cohen, S., Yakir, D. (2014) Quantifying transpirable soil water and its relations to tree water use dynamics in a water-limited pine forest. Ecohydrology 7: 409-419.
Health, safety & hazardous waste disposal considerations
Measurements of water potential at the plant level must follow safety guidelines due to the high pressures involved.
Search terms and classification
This protocol has been used in a semi-arid forest dominated by Pinus halepensis (Pinaceae). Further applications are currently being performed in a temperate mixed forest in Europe. The method is general and can be applied at any site, with various soil types and plant species.