Representing the effect of (de)compaction on soil hydraulic properties using segmental constitutive laws

Soil (de)compaction is widespread and has a large impact on soil constitutive relationships including hydraulic and gas-exchange properties. Surveys estimate that about a third of EU soils are severely degraded by compaction, and lab experiments show that the effect of compaction on soil hydraulic conductivity can be as large as the differences between textural classes. Nevertheless, the effect of (de)compaction on soil properties remains absent or only provisionally represented in present-day soil-crop, ecosystem, and land-surface models. That is despite the formulation of soil structure evolution models for key land management practices, biotic factors, and wet-dry or freeze-thaw cycles. The slow maturing and uptake of these models results from observational limitations and from difficulties with upscaling and with relating their outputs to soil properties of interest. Here we address the latter by extending established models of soil hydraulic properties to represent a dynamically evolving soil structure parametrized by a discrete pore-size distribution. The extension decomposes a given water retention curve into smooth algebraic segments corresponding to predefined pore size classes. The segments are then individually scaled to represent the changing pore volumes and summed to obtain the new retention and conductivity functions. We validate this approach using lab-based compaction experiments and demonstrate its use at site scale by leveraging the soil hydrology scheme of the ORCHIDEE land surface model. Finally, we discuss new applications that our approach enables, with a focus on representing the interaction between ecosystem engineers, soil structure, and soil water.