 Water-filled pore space

Water-filled pore space is the fraction of a soil sample's pore volume occupied by water. If $V_w$ is water volume and $V_p$ is total pore volume, the fraction is $V_w/V_p$; multiplying by one hundred expresses it as a percentage. The denominator is the pore volume, whereas volumetric soil water content uses the entire sample volume.^[ref]

Let total sample volume be $V$, porosity be $\epsilon =V_p/V$, and volumetric soil water content be $\theta =V_w/V$. ThenFor matching dry-solid mass $M_s$, soil bulk density is ${\rho }_b=M_s/V$ and soil-particle density is ${\rho }_s=M_s/V_s$. Under an additive solid-and-pore volume model,These identities require compatible volume definitions; shrinkage or swelling can change the measured sample geometry.

Consider two hypothetical samples with $\theta =0.30$ and ${\rho }_s=2.65\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$. For ${\rho }_b=1.325\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$, porosity is $0.50$ and water-filled pore space is $0.60$. For ${\rho }_b=1.59\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$, the corresponding values are $0.40$ and $0.75$. Equal water per total volume fills different fractions of the available pores. This compares two stipulated samples, rather than assuming a compaction process preserves every other quantity.

The 1984 study related water-filled porosity to carbon dioxide production, oxygen uptake and nitrous oxide production. Its reported maximum aerobic activity near sixty percent applies to the studied conditions, rather than a universal threshold.^[ref] Pore occupancy describes geometry; pore connectivity and biological demand also matter when interpreting gas exchange.