Reviewer: Reinsch, S1
Measurement unit: m3 m-3; Measurement scale: plot; Equipment costs: €€; Running cost: €; Installation effort: moderate; Maintenance effort: moderate; Knowledge need: moderate; Measurement mode: data logger
Soil moisture is the amount of water in the soil (Robinson et al., 2008; Vereecken et al., 2008). It provides the biological moisture pool for microbial activity and plant transpiration supporting terrestrial life. Soil moisture is a key variable in hydrological, agricultural, and climate- and global-change research. Soil moisture dynamics are likely to respond in different ways to climate change, depending on whether it leads to drought, warming, or excess rainfall (Seneviratne et al., 2010). This will directly affect the biologically available moisture pool and oxygen levels in the case of wet soils. Moreover, because soil moisture controls microbial activity, carbon and nutrient cycling will be affected, as will greenhouse gas fluxes of CO2, CH4, and N2O.
Research is now emerging that suggests extreme events such as drought can lead to unforeseen feedbacks in moisture dynamics and potential soil moisture state shifts through structural alteration (Robinson et al., 2016) or the development of water repellency (Goebel et al., 2011). We are only just beginning to identify the occurrence and importance of such phenomena. At continental scales we now understand the important role of soil moisture for the energy balance and how soil moisture deficits contribute to increasing the magnitude of heatwaves (Seneviratne et al., 2006). This is why efforts to create community databases, such as the international soil moisture network (Dorigo et al., 2013) and the global database on soil infiltration data (Rahmati et al. 2018), are important.
3.1.1 What and how to measure?
Soil moisture sensors
The most commonly used methods for soil moisture determination are dielectric sensors with time domain reflectometry (TDR; Ferré & Topp, 2002; Robinson et al., 2003). A range of sensors are available and compared in Blonquist et al. (2005a). Given the different performance characteristics arising from measurement frequency, the new generation of digital time domain transmission (TDT; Blonquist et al., 2005b) or TDR sensors are less susceptible to errors experienced by lower frequency capacitance and impedance-based sensors. Most sensors require low power and are affordable with their own data loggers. The signal processing is generally performed on a microprocessor in the head of the sensor, so for TDR, cable length is not a major limitation. The digital TDR and TDT family of sensors has well-characterised sampling volumes, larger than other low-frequency sensors. Moreover, because they work in the GHz frequency range they are the least susceptible to electrical loss due to solution electrical conductivity. The addition of a temperature sensor in the head of most sensors today gives another important measurement parameter for understanding soil physical behaviour. The preferred location of soil moisture sensors is discussed in protocol 1.5. Meteorological measurements.
Other methods of soil moisture measurement
The standard method for soil moisture determination is taking a volumetric core and oven drying. Although important for calibration, the destructive manual nature of the method renders it unsuitable for most climate-change and long-term monitoring experiments. A range of techniques are described in Evett et al. (2008).
Where to start
Choosing a sensor can be challenging: price is not always a good guide. Sensors determine water content from permittivity (Robinson et al., 2003). See the paper by Blonquist et al. (2005a) to compare performance and electrode length characteristics of a number of commonly used sensors. The new generation of digital sensors offers low-cost robust measurements (Blonquist et al., 2005a). For monitoring you will then need to decide if you want to measure vertically to determine moisture storage, or horizontally to obtain a definitive depth. Sensors are often installed horizontally by digging a small trench and then inserting them into the soil layer of interest (see Evett et al., 2008 for more information on installation).
3.1.2 Special cases, emerging issues, and challenges
Horizontal installation at multiple depths is helpful in determining the water balance and convenient for comparison with models such as HYDRUS-1D (Šimůnek et al., 2016). Calibration from permittivity to soil moisture content can be done using a standard calibration for mineral soils (e.g. Topp et al., 1980) and many sensors now incorporate something similar. For clay soils and organic soils more specific calibration may be required. Placing three sensors in the soil is ideal: for example, one near the soil surface at around 5 cm, one at the depth of maximum rooting density, and one below the roots (see Figure 1.5.1 in protocol 1.5. Meteorological measurements).
Theory, significance, and large datasets
D’Odorico & Porporato, (2004), Jung et al. (2010), Rodriguez‐Iturbe (2000), Seneviratne et al. (2010), Taylor et al. (2012)
More on methods and existing protocols
Bittelli (2011), Jones et al. (2002, 2005), Or et al. (2011), Topp et al. (1980), Vaz et al. (2013)
Bittelli, M. (2011). Measuring soil water content: A review. HortTechnology, 21(3), 293-300.
Blonquist, J. M., Jones, S. B., & Robinson, D. A. (2005a). Standardizing characterization of electromagnetic water content sensors. Vadose Zone Journal, 4(4), 1059-1069.
Blonquist, J. M., Jones, S. B., & Robinson, D. A. (2005b). A time domain transmission sensor with TDR performance characteristics. Journal of Hydrology, 314(1), 235-245.
D’Odorico, P., & Porporato, A. (2004). Preferential states in soil moisture and climate dynamics. Proceedings of the National Academy of Sciences USA, 101(24), 8848-8851.
Dorigo, W. A., Xaver, A., Vreugdenhil, M., Gruber, A., Hegyiová, A., Sanchis-Dufau, A. D., … Drusch, M. (2013). Global automated quality control of in situ soil moisture data from the International Soil Moisture Network. Vadose Zone Journal, 12(3), vzj2012.0097.
Evett, S.R., Heng, L.K., Moutonnet, P., & Nguyen, M.L. (2008). Field Estimation of Soil Water Content: A Practical Guide to Methods, Instrumentation, and Sensor Technology. 131 pp. IAEA-TCS-30. International Atomic Energy Agency, Vienna, Austria. ISSN 1018–5518. Available at http://www-pub.iaea.org/mtcd/publications/PubDetails.asp?pubId=7801
Ferré, P. A., & Topp, G. C. (2002). Time domain reflectometry. Methods of Soil Analysis, 4, 434-446.
Goebel, M. O., Bachmann, J., Reichstein, M., Janssens, I. A., & Guggenberger, G. (2011). Soil water repellency and its implications for organic matter decomposition – is there a link to extreme climatic events? Global Change Biology, 17(8), 2640-2656.
Jones, S. B., Wraith, J. M., & Or, D. (2002). Time domain reflectometry measurement principles and applications. Hydrological Processes, 16(1), 141-153.
Jones, S. B., Blonquist, J. M., Robinson, D. A., Rasmussen, V. P., & Or, D. (2005). Standardizing characterization of electromagnetic water content sensors. Vadose Zone Journal, 4(4), 1048-1058.
Jung, M., Reichstein, M., Ciais, P., Seneviratne, S. I., Sheffield, J., Goulden, M. L., … Dolman, A. J. (2010). Recent decline in the global land evapotranspiration trend due to limited moisture supply. Nature, 467(7318), 951-954.
Or, D., Wraith, J. M., Robinson, D. A., & Jones, S. B. (2011). Soil water content and water potential relationships. In P. M. Huang, Y. Li, & M. E. Sumner (Eds.), Handbook of Soil Sciences: Properties and Processes (2nd ed., pp. 4.1-4.28 ). Boca Raton: CRC Press.
Rahmati, M., Weihermüller, L., Vanderborght, J., Pachepsky, Y. A., Mao, L., Sadeghi, S. H., … Vereecken, H. (2018). Development and analysis of soil water infiltration global database. Earth System Science Data, 10, 1237-1263.
Robinson, D. A., Jones, S. B., Wraith, J. M., Or, D., & Friedman, S. P. (2003). A review of advances in dielectric and electrical conductivity measurement in soils using time domain reflectometry. Vadose Zone Journal, 2(4), 444-475.
Robinson, D. A., Campbell, C. S., Hopmans, J. W., Hornbuckle, B. K., Jones, S. B., Knight, R., … Wendroth, O. (2008). Soil moisture measurement for ecological and hydrological watershed-scale observatories: A review. Vadose Zone Journal, 7(1), 358-389.
Robinson, D. A., Jones, S. B., Lebron, I., Reinsch, S., Domínguez, M. T., Smith, A. R., … Emmett, B. A. (2016). Experimental evidence for drought induced alternative stable states of soil moisture. Scientific Reports, 6, 20018.
Rodriguez‐Iturbe, I. (2000). Ecohydrology: A hydrologic perspective of climate‐soil‐vegetation dynamics. Water Resources Research, 36(1), 3-9.
Seneviratne, S. I., Lüthi, D., Litschi, M., & Schär, C. (2006). Land–atmosphere coupling and climate change in Europe. Nature, 443(7108), 205-209.
Seneviratne, S. I., Corti, T., Davin, E. L., Hirschi, M., Jaeger, E. B., Lehner, I., … Teuling, A. J. (2010). Investigating soil moisture–climate interactions in a changing climate: A review. Earth-Science Reviews, 99(3), 125-161.
Šimůnek, J., van Genuchten, M. T., & Šejna, M. (2016). Recent developments and applications of the HYDRUS computer software packages. Vadose Zone Journal, 15(7), vzj2016.04.0033.
Taylor, C. M., de Jeu, R. A., Guichard, F., Harris, P. P., & Dorigo, W. A. (2012). Afternoon rain more likely over drier soils. Nature, 489(7416), 423-426.
Topp, G. C., Davis, J. L., & Annan, A. P. (1980). Electromagnetic determination of soil water content: Measurements in coaxial transmission lines. Water Resources Research, 16(3), 574-582.
Vaz, C. M. P., Jones, S., Meding, M., & Tuller, M. (2013). Evaluation of standard calibration functions for eight electromagnetic soil moisture sensors. Vadose Zone Journal, 12, vzj2012.0160.
Vereecken, H., Huisman, J. A., Bogena, H., Vanderborght, J., Vrugt, J. A., & Hopmans, J. W. (2008). On the value of soil moisture measurements in vadose zone hydrology: A review. Water Resources Research, 44(4), W00D06.
Authors: Robinson DA1, Jones SB2
Reviewer: Reinsch, S1
1 Centre for Ecology & Hydrology, Environment Centre Wales, Bangor, UK
2Department of Plants, Soils and Climate, Utah State University, Logan, USA