Table 4.
An overview of methods to determine soil moisture content for point and area scales.
| Scale | Methods | Example | Description |
|---|---|---|---|
| Point/local | Gravimetric | Oven-drying | Standard method, destructive sampling |
| Nuclear | Neutron scattering | Fast neutrons emitted from a radioactive source are slowed down by hydrogen atoms in the soil | |
| Gamma attenuation | The scattering and absorption of gamma rays is related to the density of matter in their path | ||
| Nuclear magnetic resonance | Soil water is subjected to both a static and an oscillating magnetic field at right angles to each other | ||
| Electro-magnetic | Resistive sensor | Soil resistivity depends on the soil electrical properties and moisture | |
| Capacitive sensor | Using the dielectric constant by measuring capacitance between two electrodes implanted in the soil | ||
| Time-domain-reflectometer | Propagation of electromagnetic signals. Velocity and attenuation depend on soil properties: water content and electrical conductivity | ||
| Frequency domain | An oscillator detects changes in soil dielectric properties linked to variations in soil water content | ||
| Tensiometric | Soil matrix tension | Measures the soil matrix potential (capillary tension) | |
| Hydrometric | Thermal inertia | Relationship between moisture in porous materials and the relative humidity. Since thermal inertia of a porous medium depends on moisture, soil surface temperature is indicative | |
| Heat dissipation | Heat pulse | Rising or cooling of temperature in a porous block is measured after a heat pulse | |
| Feel and Appearance | Manual | Soil moisture interpretation chart based on texture classification and manual squeezing of soil samples | |
| Optical | Polarized light | The presence of moisture at a surface of reflection tends to cause polarization in the reflected beam | |
| Fibre optic sensors | Light attenuation in the unclad fiber embedded in the soil varies with the soil water amount in contact with the fiber because of its effect on the refractive index | ||
| Near-infrared | Molecular absorption of water in the surface layers | ||
| 1D hydrologic models | WAVE, SWAP | Based on solving the 1-D Richards equation with knowledge on atmospheric upper and soil bottom boundary conditions | |
| Spatial/regional | Remote sensing | VIS, NIR, SWIR | Reflected electromagnetic energy from the soil surface |
| TIR emittance | Emitted EM energy in the thermal spectral band from the soil surface | ||
| Microwave emission RADAR | Emitted microwave EM energy from the soil surface Attenuation/backscattering of microwave energy as an indication of moisture content of porous media | ||
| Catchment models | SWAT, MIKE-SHE | Solving the 3D Richards equation knowing atmospheric upper and soil bottom boundary conditions |