Modeling the impact of stone content on the shape of water retention curve

Heterogeneous soils with stone fractions are very common in non-agricultural areas. The characterization of their hydraulic properties is important but face technical challenges. Therefore, the retention of the stony fraction (larger than 2 mm) is often considered as null. However, when soil stone content is large (>15%), even a slight change of water with suction in the stone fraction will affect the shape of the bulk soil retention curve.

In this study, we analyzed the retention data, between pF 0 and pF 4.2, of more than 2400 aggregates extracted from 48 soil horizons in forests down to 2-m depth. For each horizon, at each suction level, we characterized water content and stone content of at least 8 replicates of aggregates. We propose a novel methodology to extract and separate the hydraulic properties of the stony and of the fine fractions from these data. It proved to be efficient beyond 15% of stone content.

In general, the change of volumetric water content between pF 2 and pF 4.2 was below 5% for stones but for some of them, it could reach up to 15%.  In addition, we could propose a general expression of the bulk retention curve that explicitly contains the fraction of stones. It is observed that the shape of the bulk retention curve (mono or bimodal) evolves with stone content for a given horizon.