Soil Moisture Curve
Soil Water Retention Curve (Soil Moisture Characteristic Curve) · Also known as: Water Retention Curve, pF Curve, Characteristic Curve, SWRC
The soil moisture curve (or soil water retention curve, SWRC) describes the relationship between soil water content and soil matric potential (water tension). It characterizes how tightly water is bound in pores of different sizes: large pores drain at low tensions (wet soils), while smaller pores retain water at high tensions (dry soils). Quantifying this relationship is essential for water balance modeling, unsaturated flow prediction, and assessing plant-available water.
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When to use it
Use soil moisture curves when: (1) you develop water balance or crop growth models requiring unsaturated flow; (2) you assess soil water availability across soil types and depths; (3) you design irrigation systems and need to know soil water depletion rates; (4) you model contaminant transport in vadose zone. Essential input for field-scale hydrological and agronomic models. Less critical if you only measure soil moisture in situ with sensors (TDR/capacitance probes).
Strengths & limitations
- Mechanistically relates soil pore structure to water retention, bridging capillary physics and field hydrology
- Enables prediction of unsaturated hydraulic conductivity, which is difficult to measure directly
- Standardized models (van Genuchten) allow easy sharing and scaling of soil properties across regions
- Pedotransfer functions (PTFs) enable estimation from texture and bulk density alone, reducing measurement cost
- Captures nonlinearity: water release is steepest near saturation and saturation, matching plant root uptake dynamics
- Laboratory measurement is time-consuming and expensive; tensions above 1500 kPa are difficult to maintain accurately
- Hysteresis: the curve differs depending on whether soil is wetting or drying; most studies report only the drying curve
- PTFs trained on temperate soils may perform poorly in tropical or highly organic soils
- Cracks and macropores (common in clay soils) bypass the capillary matrix, invalidating assumptions
- Water retention changes over time as soil structure degrades, organic matter decomposes, or compaction increases
Frequently asked
What is the difference between matric potential and water content?
Water content (θ, m³/m³ or %) is how much water occupies the soil pore space. Matric potential (ψ, kPa or bars) is the attraction (negative pressure) that soil exerts on water, reflecting pore-size distribution. The SWRC relates them: as you increase tension (more negative ψ), water drains from large pores and content (θ) decreases. You need the curve to convert between the two.
How do I estimate available water without measuring the SWRC?
Use pedotransfer functions (PTFs): published equations that predict SWRC parameters from easily measured soil properties (sand%, silt%, clay%, bulk density, organic matter). Databases like the Rosetta Model (USDA-ARS) provide PTFs trained on thousands of samples. Input your texture and bulk density; output is θsat, θfc, θwp, and the SWRC. Accuracy is ±5-10% volumetric water content.
What is field capacity and why is it important?
Field capacity is the water content remaining 24-48 hours after irrigation or rain, when large pores have drained but fine pores still hold water. It's usually defined as pF 2.0 (~20 kPa or 0.2 bar). It represents the upper bound of plant-available water. The lower bound is wilting point (pF 4.2, ~1500 kPa), the water content at which plant roots can no longer extract water. AWC = θfc - θwp.
Why does the SWRC matter for irrigation scheduling?
Because the rate at which water moves through the soil and becomes available to roots depends on θ and the conductivity K(θ). A soil at high water content drains fast (K is high); at low content, drainage is slow. The SWRC and K(θ) tell you how fast the soil 'refills' by capillary rise after irrigation, and thus how much water is in the root zone at any time.
How does soil texture affect the shape of the SWRC?
Clay soils have steep, S-shaped curves: they retain a lot of water over a narrow range of tension (small pores dominate), then release little more even at high tension. Sandy soils have gentler curves: they lose water quickly at low tension (large pores drain first) and retain little at high tension. Loams are intermediate. Knowing texture lets PTFs predict the curve shape without measurement.
Sources
- Gardner, W. R. (1956). Representation of soil aggregate-size distribution by a logarithmic-normal distribution. Soil Science Society of America Journal, 20(2), 151-153. DOI: 10.2136/sssaj1956.03615995002000020003x ↗
- Brooks, R. H., & Corey, A. T. (1964). Hydraulic properties of porous media. Hydrology Papers No. 3, Colorado State University, Fort Collins. link ↗
- van Genuchten, M. T. (1980). A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Science Society of America Journal, 44(5), 892-898. DOI: 10.2136/sssaj1980.03615995004400050002x ↗
How to cite this page
ScholarGate. (2026, June 3). Soil Water Retention Curve (Soil Moisture Characteristic Curve). ScholarGate. https://scholargate.app/en/agronomy/soil-moisture-curve
Which method?
Set this method beside its closest kin and read them side by side — the library lays the books on the table; the choice is yours.
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