USDA LTAR common experiment measurement: saturated hydraulic conductivity

Authors: Schreiner-Mcgraw, Adam; Baffaut, Claire

Submitted to: Protocols.io
Publication Type: Research Notes
Publication Acceptance Date: 9/24/2024
Publication Date: 9/24/2024
Citation: Schreiner-Mcgraw, A.P., Baffaut, C. 2024. USDA LTAR common experiment measurement: saturated hydraulic conductivity. Protocols.io. https://dx.doi.org/10.17504/protocols.io.eq2lywz1qvx9/v1
DOI: https://doi.org/10.17504/protocols.io.eq2lywz1qvx9/v1

Interpretive Summary: The saturated hydraulic conductivity represents the speed at which a fluid can move through a porous medium, and it is a fundamental parameter that governs water flow through soil or rock. It is difficult, however, to measure this parameter in field conditions. This is because soil is heterogeneous in space and because it is difficult to make measurements without disturbing the soil. In this protocol, I provide guidance to researchers working with the Long-Term Agroecosystem Research (LTAR) network common experiment on best practices for measuring saturated hydraulic conductivity. This guidance can be used across the LTAR network so that measurements of saturated hydraulic conductivity can be standardized and comparable across locations.

Technical Abstract: The saturated hydraulic conductivity (Ksat) represents the speed at which a fluid can move through a porous medium, and it is a fundamental parameter that governs water flow through soil or rock. Darcy’s law governs one-dimensional flow through a porous medium: Q = Ksat * (dh/dx), where Q is the flux of water [m/s], Ksat is the saturated hydraulic conductivity [m/s], and dh/dx is the gradient in the hydraulic head [m/m]. More complicated forms of Darcy’s equation apply to representing three-dimensional flow in a saturated medium. Additionally, flow in an unsaturated medium, as is often the case in soil, is represented through equations such as Richards’ equation, based on Darcy’s law. Usually, the saturated hydraulic conductivity is the most important parameter governing water flow in the saturated zone. The basic idea when measuring saturated hydraulic conductivity is to saturate the medium (i.e., the soil) and measure how quickly water moves through it. This procedure can be quite simple. For example, the lab setup that formed the basis of Darcy’s experiments in the 1850s was homogenous sand placed in a tube with a known gradient in the hydraulic head so that Ksat could be calculated from the equation above. The setup becomes more complicated in the field, where the soil is heterogeneous; we are typically interested in a bulk estimate of Ksat in a soil that is nonuniform and may contain soil cracks or holes from roots or worms. It is also difficult to completely saturate a soil column in the field and control the applied hydraulic gradient. The techniques presented here help account for these challenges.