how to convert gravimetric soil water content to volumetric soil water content

The amount of water or moisture in soil can be measured as either gravimetric soil water content (GWC) or volumetric soil water content (VWC).

But what is GWC and VWC? And how do you convert GWC to/from VWC?

The easiest way to think of GWC and VWC is that GWC is related to the mass of water and soil, whereas VWC is related to the volume of water and soil.

GWC is the mass of water per mass of dry soil in a given sample. To determine GWC, collect soil sample from the field, weigh that soil then dry it in the oven at 105°C for at least 3 days (or, more precisely, weigh the soil periodically until there is no more loss in weight which indicates that all of the water has been dried from the sample). The equation for GWC (θg) is:

θg = (M_wet – M_dry) / M_dry     (Equation 1)

where M_wet is the weight of the soil sample from the field and M_dry is the weight of the dry soil sample.

VWC is the volume of water per volume of soil. To determine VWC, you will need to measure M_wet and M_dry as described above for GWC. You will also need to know the bulk density of the soil sample (ρ_soil). In this case, it is not enough just to collect a handful of soil sample from the field. It is better practice to collect a sample of soil in a known volume, such as a metal cylinder. The ρ_soil is the ratio of M_dry to the volume of your sampler. (A You Tube video at the bottom of this article outlines how to collect soil samples to measure soil bulk density.) The equation for VWC (θv) is then given by:

θv = θg *  ρ_soil     (Equation 2)

Soil scientists also incorporate the density of water (ρ_water) into θv as:

θv = θg *  (ρ_soil / ρ_water)     (Equation 3)

However, the value of ρ_water is close to 1 and is typically ignored when calculating θv.

In summary, Equation 1 is used to calculate gravimetric soil water content and Equation 2 is used to calculate volumetric soil water content.

 

how to measure soil bulk density