Soil Water Retention Quiz: Hysteresis, Curves, and Water Holding Capacity

Hysteresis in soil water retention means that the soil water content at a given matric potential is path-dependent. A soil that is being dried retains more water at any given matric potential than the same soil being rewetted from a drier state to the same potential. This means a single soil water retention curve cannot describe water content at all matric potentials, and separate drying and wetting curves are needed to fully characterize soil water behavior.

The primary drainage curve is the reference drying curve of the soil water retention function, beginning from full saturation where all pores are water-filled and matric potential is approximately zero. As matric potential decreases and suction increases, water progressively drains from pores in order of decreasing size. This drainage path typically retains more water at any given potential than the corresponding wetting path because of the mechanisms creating hysteresis.

The ink-bottle effect occurs because most soil pores have narrow constrictions connecting them to adjacent pores or to the soil surface. During drainage, air must first penetrate the narrowest pore throat, requiring a suction corresponding to that throat radius. During wetting, water fills the wide pore body at a lower suction than needed to drain through the throat. Because pores drain at a higher suction than they fill, a hysteresis loop develops where the drying curve lies above the wetting curve at the same matric potential.

Contact angle hysteresis arises because the angle at which a water-air interface meets a solid surface differs depending on whether the interface is advancing over a dry surface or receding from a wet surface. Advancing contact angles are larger than receding angles because rough surfaces, chemical heterogeneity, and adsorbed organic matter create energy barriers that impede water spreading. Larger contact angles correspond to larger capillary pressures needed to wet a pore, contributing to the systematically higher water contents on drying curves compared to wetting curves at the same matric potential.

When a soil that is drying reverses direction and begins wetting before reaching the primary wetting boundary, it follows a scanning curve that lies between the two boundary curves. Similarly, a soil rewetting that reverses to drying follows a drying scanning curve. These scanning curves connect smoothly to the boundary curves when the process reaches the boundary. In detailed soil water transport modeling, accurately representing scanning curves requires hysteresis models that capture the complete family of possible scanning paths within the hysteresis loop.

When a soil water transport model uses a single water retention function, it cannot correctly predict water content during both wetting and drying sequences. During drying, the actual soil follows the higher drying curve, retaining more water than the single curve predicts. During wetting, the actual soil follows the lower wetting curve, retaining less water than predicted. These systematic errors in water content translate to errors in calculated matric potential gradients and unsaturated hydraulic conductivity, accumulating over multiple drying and wetting cycles and producing inaccurate predictions of water redistribution and plant water availability.

Hysteresis arises from multiple physical mechanisms. The ink-bottle effect creates different filling and emptying pressures for the same pore geometry. Contact angle hysteresis produces different capillary pressures for wetting versus drying at the same surface. Air entrapment during rapid wetting traps air in large pores, reducing water content on the wetting curve below what complete saturation would produce. Instantaneous equilibration is a simplifying assumption in static retention measurements but not a mechanism of hysteresis itself.

In irrigated systems, soil water sensors measure volumetric water content, but converting this to matric potential for plant water stress assessment requires knowing which retention curve applies. A soil on the drying curve after irrigation drainage holds more water at the same matric potential than during the wetting phase. Interpreting sensor readings without accounting for hysteresis can cause errors in irrigation timing, either delaying irrigation unnecessarily or irrigating too frequently. Precision irrigation management increasingly uses hysteretic models to improve scheduling accuracy.

The magnitude of hysteresis is related to the ratio between pore body radius and pore throat radius in the soil pore network. Sandy soils have well-sorted relatively large pores but with significant bottleneck effects at pore throats, creating large contrasts between filling and emptying pressures. Clay soils have more uniformly small pores where body-to-throat ratios are smaller. The hysteresis loop, measured as the horizontal distance between drainage and wetting curves at the same water content, is typically wider in coarser soils reflecting larger ink-bottle contributions to hysteresis.

The domain or independent domain model treats soil pore space as an assembly of elementary pore domains that fill and empty independently at specific matric potentials. Each domain is characterized by a threshold for drying and a lower threshold for wetting, creating the ink-bottle characteristic for that pore. The overall water retention curve is constructed by integrating over the distribution of all domain thresholds, weighted by their frequency. The model successfully reproduces hysteresis loops and scanning curves with relatively few parameters fitted to measured boundary curves.

During rapid wetting, water advancement traps air in large pores because water enters through multiple pathways and can seal off air-filled regions before they connect to the soil surface. This entrapped air, sometimes called occluded air, reduces the saturated water content achievable during wetting. The maximum water content on the primary wetting curve is therefore typically less than the water content at zero matric potential on the primary drainage curve, creating a further discontinuity between the two boundary curves in addition to the hysteresis loop.

Multiple approaches exist for incorporating hysteresis into soil water models. Separate parameterizations for drying and wetting boundary curves using van Genuchten or Brooks-Corey equations are widely applied. Switching between these curves based on flow direction provides a simplified representation. More complete implementation tracks soil history and uses scanning curve models to interpolate between boundaries when flow direction reverses. Using a single average curve ignores hysteresis entirely and does not account for wetting or drying history, which is precisely the approach producing errors that hysteresis modeling aims to correct.

Soil water retention measurements require equilibrating samples at defined matric potentials and measuring resulting water content. Pressure plate extractors apply gas pressure to expel water at defined suctions for the drying curve. Hanging water column systems apply negative pressure for low-suction measurements. For wetting curves, dried samples are placed on tension tables or subjected to controlled rewetting while equilibrium water content is measured at each tension. Careful step-by-step equilibration is essential because kinetic effects can cause apparent rather than true hysteresis if equilibration times are insufficient.

Following rainfall, the soil near the surface wets rapidly and follows the wetting branch of the retention function, while deeper soil layers may still be drying from a previous cycle on the drainage branch. As the storm ends and evapotranspiration resumes, the surface layer transitions back toward drying. At any moment the soil profile includes zones following different branches of the hysteresis loop, and an accurate simulation requires tracking the wetting-drying history of each layer rather than assuming a single unique relationship between water content and matric potential throughout the profile.

Water repellency from hydrophobic organic coatings on mineral surfaces dramatically amplifies contact angle hysteresis. When initially dry hydrophobic surfaces are wetted, the advancing contact angle can approach 90 degrees or greater, requiring high capillary pressures and very negative matric potentials before wetting can proceed. Once wet, receding contact angles are much smaller because the surface energy state changes as organic molecules reorient. This produces extreme hysteresis where the wetting curve shifts to much more negative potentials, and very dry conditions develop before rewetting proceeds in water-repellent soils even when matric potential is relatively high.