Degree of Saturation: Soil Moisture Ratio
The degree of saturation (Sr) represents the ratio of water-filled pore volume to total pore volume in a soil sample. Expressed as a percentage or fraction, it is calculated as Sr = Vw / Vv, where Vw is water volume and Vv is void volume. Alternatively, Sr = θ / n, where θ is volumetric water content and n is porosity. Saturation informs soil strength, hydraulic conductivity, and aeration status. This article defines Sr, recounts measurement history, explores conceptual frameworks, and showcases applications across engineering and agriculture.
Combine this primer with the porosity explainer to derive pore volumes, and consult the greywater irrigation sizing calculator when applying saturation targets in reuse systems.
Definition and Measurement
Relationships among water content variables
Degree of saturation links volumetric water content θ, gravimetric water content w, porosity n, and bulk density ρb. The conversion θ = w ρb / ρw permits calculation of Sr from oven-dry sample measurements. Maintaining consistent units (cubic metres or cubic centimetres) avoids errors.
Laboratory and field determination
Laboratory methods involve saturating and draining undisturbed cores, then determining moisture content and porosity. Field sensors such as time-domain reflectometry probes and neutron gauges provide θ, which is combined with survey-based porosity to compute Sr in situ.
Saturation curves
Soil-water characteristic curves relate Sr to matric potential. These curves inform irrigation scheduling and geotechnical analyses by showing how soils transition from saturated to residual moisture content under suction.
Historical Development
Foundations in soil physics
Early twentieth-century soil scientists such as Edgar Buckingham formalised the relationships among porosity, water content, and suction, establishing degree of saturation as a key variable in unsaturated flow theory.
Geotechnical engineering adoption
Karl Terzaghi and successors integrated Sr into effective stress concepts, linking saturation to shear strength and consolidation behaviour. Standard compaction tests (Proctor, Modified Proctor) use saturation lines to evaluate moisture-density relationships.
Modern sensing and modelling
Contemporary remote sensing missions and land surface models estimate Sr over large areas using microwave observations and data assimilation, informing drought monitoring and flood forecasting.
Applications and Importance
Geotechnical stability
Engineers evaluate Sr when assessing slope stability, bearing capacity, and earth dam performance. Maintaining saturation below critical thresholds prevents loss of shear strength. Saturation data feed into finite element models for seepage analysis.
Agricultural water management
Farmers track Sr to ensure root zones retain adequate moisture without causing hypoxia. Combining saturation data with the rain garden sizing calculator supports design of on-farm infiltration basins and buffer strips.
Stormwater and reuse systems
Designers of green infrastructure and greywater systems target saturation ranges that balance infiltration with storage. The greywater irrigation sizing calculator helps align discharge volumes with soil capacity.