Average layer soil moisture is the depth-averaged amount of water present in a specific soil layer beneath the surface. Soil moisture content can be measured by Gravimetric Soil Moisture (GSM). It is the mass of water compared to the mass of solid materials per unit volume of soil. Soil moisture can also be expressed in Volumetric Soil Moisture (VSM) which is the volume of water per unit volume of soil. As water is of a known density, the mass of water per unit volume of soil (g/cm^3) can be easily determined.