Soil water retention curve inflection point: Insight into soil structure from percolation theory

Quantifying soil structure has been a long-standing challenge in soil physics. Among the proposed indices and parameters, slope at the inflection point of soil water retention curve has been widely used. In this short communication, we provide theoretical insights and show that under full saturation conditions, the pore-throat radius at the inflection point (r(inf)) is equivalent to the critical pore-throat radius within percolation theory. The inflection point, in fact, corresponds to a critical saturation (critical fraction of pore space) at which a sample-spanning cluster forms and a medium starts percolating. We discuss that r(inf) is theoretically linked to saturated hydraulic conductivity (K-sat), in a power-law form within the critical path analysis framework. Using 59 soil samples from the GRIZZLY database, we show that the K-sat is correlated to the r(inf), although there exists scatter in the data. Interestingly, the experimental exponent 2.219 found from the K-sat-r(inf) data is less than 5% greater than the estimated theoretical value 2.111 determined from the average fractal dimension of the measured soil water retention curves.

Automated summary

This study proposes a method for quantifying soil structure and verifies its feasibility through theoretical models and experimental data. The study finds that there is a correlation between the pore-throat radius at the inflection point of the soil water retention curve and the saturated hydraulic conductivity. The experimental results are very close to the theoretical values, indicating that this method can be used to evaluate the saturated hydraulic conductivity of soils.