S3 Water cycling
Editors: Reinsch S & Robinson D
Within ecosystems, the exchange of water and energy between the soil, plants, and the atmosphere is often termed the soil–plant–atmosphere continuum (SPAC). Understanding how SPAC works is a major research challenge, of interest to climate-change research and wider research on environmental change. We need to know what enters the system via precipitation and how this water is partitioned between runoff, interception, and infiltration. Of the water that infiltrates soil we need to know whether it drains to groundwater, is evaporated, or is transpired by plants.
Studies in this area have uncovered a range of processes that enable ecosystems to thrive, often under challenging environmental conditions. Active research is underway regarding processes such as hydraulic redistribution (Ryel et al., 2002), preferential soil water flux (Clothier et al., 2008), alternative soil moisture states (Robinson et al., 2016), plant-induced wetting patterns (Franz et al., 2011), and the development and persistence of soil water repellency and how it alters infiltration (Doerr et al., 2000). This research is important because soils and plants provide feedbacks to climate. We need to understand these feedbacks to ensure that ecosystem management strategies, interventions, or policy regulations do not exacerbate environmental pressures (Robinson et al., 2019).
In this chapter, we focus on general soil measurements for site characterisation and soil physical measurements that are relevant to understanding SPAC. The major focus is on soil hydraulic measurements, which include soil moisture, hydraulic conductivity, water retention, and water potential. The methods included are a starting point for determining parameters that link to, or are used in, modelling SPAC. In addition, we include some measurements used to track the progress of water through the plant to the atmosphere (for other protocols relevant to the SPAC also see 5.8 Psychrometry for water potential measurements, 5.9 Pressure-volume curve – TLP, ε, Ψo, 5.10 Maximum leaf hydraulic conductance, 5.13 Stable isotopes of water for inferring plant function and 5.16 Leaf hydraulic vulnerability to dehydration).
The purpose of this chapter is not to provide a fully comprehensive review of methods, but to identify some critical ones that will get a research team started with measurements for their work in climate-change studies. The methods may also be applied to other types of studies investigating global-change drivers (e.g. nutrients, land-use change). The extensive references are intended to provide the reader with links to a wider literature selection.
Clothier, B. E., Green, S. R., & Deurer, M. (2008). Preferential flow and transport in soil: progress and prognosis. European Journal of Soil Science, 59(1), 2-13.
Doerr, S. H., Shakesby, R. A., & Walsh, R. (2000). Soil water repellency: its causes, characteristics and hydro-geomorphological significance. Earth-Science Reviews, 51(1), 33-65.
Franz, T. E., King, E. G., Caylor, K. K., & Robinson, D. A. (2011). Coupling vegetation organization patterns to soil resource heterogeneity in a central Kenyan dryland using geophysical imagery. Water Resources Research, 47(7), W07531.
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.
Robinson, D. A., Hopmans, J. W., Filipovic, V., van der Ploeg, M., Lebron, I., Jones, S. B., … Tuller, M. (2019). Global environmental changes impact soil hydraulic functions through biophysical feedbacks. Global Change Biology. https://doi.org/10.1111/gcb.14626
Ryel, R., Caldwell, M., Yoder, C., Or, D., & Leffler, A. (2002). Hydraulic redistribution in a stand of Artemisia tridentata: evaluation of benefits to transpiration assessed with a simulation model. Oecologia, 130(2), 173-184.