Location: Watershed Physical Processes Research
Title: Soil water potential and matric potentialAuthor
Submitted to: Protocols.io
Publication Type: Research Notes Publication Acceptance Date: 9/20/2024 Publication Date: 9/20/2024 Citation: O'Reilly, A.M., Baffaut, C. 2024. Soil water potential and matric potential. Protocols.io. https://dx.doi.org/10.17504/protocols.io.8epv5rzb4g1b/v1. DOI: https://doi.org/10.17504/protocols.io.8epv5rzb4g1b/v1 Interpretive Summary: This protocol is part of a larger set published at protocols.io for the LTAR Common Experiment. This protocol outlines how to measure soil water potential and matric potential. These measurements are important to agriculture because they are required in order to calculate the speed and direction of water movement in soil, which affects the growth of crops and the influence of crop production on the surrounding environment. The goal is to provide repeatable guidelines to achieve consistent data collection, instrument maintenance, data processing, and quality control for obtaining these data at cropland sites across the U.S. Technical Abstract: The "potential" of subsurface water measures the energy associated with position or internal conditions, thus representing the potential energy of a parcel of water. Given the typically small flow velocity of subsurface water, its kinetic energy commonly can be assumed negligible; thus, the "total potential" represents the "total energy" status of the water parcel. Potential energy due to position depends on the location of the water parcel within the Earth’s gravitational field, and potential energy due to internal conditions depends on the temperature and solute concentration of the water parcel. Because all systems equilibrate by moving from higher to lower energy states, a difference in potential energy from one point to another drives water moving between those points. The absolute value of the potential energy of a water parcel at any given point is irrelevant to its movement. Because the change in energy drives water movement, a reference state is necessary. Thus, potential is the work per unit quantity (mass, volume, or weight), or "specific potential energy", required to transport an infinitesimal parcel of water reversibly and isothermally to a point of interest from a point representing a reference state for a pool of water defined by the following criteria: (1) pure water, (2) free water (not bound, i.e., adsorbed to solid phase), (3) air phase at atmospheric pressure, and (4) located at an established vertical datum (arbitrary reference elevation). Total potential is used to define the subsurface flow of water under saturated and unsaturated conditions. In the vadose zone, total potential is commonly called "soil water potential" and obtained by summing the gravitational, pressure, and osmotic potentials generated by the respective force fields acting on water. "Matric potential" represents the specific potential energy attributable to capillary and adsorptive forces, i.e., the total effect resulting from the affinity of water to the whole soil matrix (pores and particle surfaces). |