Soil-water characteristic curve
The soil-water characteristic curve, moisture characteristic curve or water retention curve is the relationship between the volumetric water content, θ, and matric suction, ψ. Since matric suction is equal to ua-uw, this can be replaced with soil suction, since the pore air pressure, ua, is commonly taken as equal to the atmospheric pressure (i.e. ua = 0).
Due to the hysteretic effect of water filling the pores of soil as well as draining from the pores, different wetting and drying curves may be distinguished, as seen in figure 1 as the adsorption curve and desorption curve. When the matric suction, or soil water potential, is equal to zero, the degree of saturation of the soil is close to 100% and water is held in the soil primarily by capillary forces. As the volumetric water content of the soil reduces, the water binds together creating suction (lower potentials) within the soil. In a sandy soil the reduction in the volumetric water content will occur at much higher potentials than clayey soils. The amount of water a soil can retain is directly related to the porosity.
The shape of soil-water characteristic curves can be characterized by several models, one of the original models is known as the van Genuchten model and a more recent model is a modified version by Fredlund & Xing (1994).
There are multiple procedures and methods that can be used to determine the soil-water characteristic curve of a soil. Some of the following methods are not directly used to determine the soil-water characteristic curve but can be easily modified to do so.
Filter Paper Method
- Rifat Bulut (2001), Total and Matric Suction Measurements With the Filter Paper Method (inc "Soil Suction Measurements by Filter Paper" manuscript) (Download PDF).
- Head, K. H. (2006). Manual of Soil Laboratory Testing - Soil Classification and Compaction Tests (Third ed. Vol. 1): Whittles Publishing, pg 110-113.
- ASTM D5298 - Standard Test Method for Measurement of Soil Potential (Suction) Using Filter Paper.
Instantaneous Profile Method
- Watson, K.K.. (1966). "An instantaneous profile method for determining the hydraulic conductivity of unsaturated porous materials". Water Resources Research 2 (4): 709–715. https://dx.doi.org/10.1029%2FWR002i004p00709
Non-Linear Least-Squares Method
- Chou, T.K. (2016). "A free GUI application for solving the van Genuchten parameters using non-linear least-squares minimization and curve-fitting". www.cmcsjc.com. January: 1–5.
- Lourenço S., et al. (2007), Determination of the Soil Water Retention Curve with Tensiometers (Download PDF).
- University of Connecticut, Measurement of Soil Water Characteristic Curve – Sensor Pairing and Laboratory Methods (Download PDF).
- D.G. Fredlund and A. Xing (1994). Equations for the soil-water characteristic curve. Canadian Geotechnical Journal, 31(3): 521-532, 1994. (Download PDF)
- van Genuchten, M.Th. (1980). "A closed-form equation for predicting the hydraulic conductivity of unsaturated soils". Soil Science Society of America Journal 44 (5): 892–898, https://dx.doi.org/10.2136%2Fsssaj1980.03615995004400050002x. (Download PDF)