How Darcy-scale daemons lead theory developments for soil-water dynamics astray

Theory in soil physics is tightly bound to integral lab observations of dynamics of soil water content and matric potential. In addition, the perceptual model of (linear) filter flow water movement is deeply embedded in measurement procedures and projections of soil water dynamics. Such Darcy-scale principles have been found to mismatch with observations and application requirements at the landscape-scale including the effect of boundary conditions. While this discrepancy is often attributed to soil heterogeneity and the requirement for effective parameterisation, we seek to discuss that assumptions about scalability of lab-measured soil hydraulic properties taken out of the capillary context of soils potentially render our "physics" ill-posed.

In this PICO we will present a series of analyses of soil water state dynamics from lab, plot and hillslope scales. We will show how scaling coincides with a change in boundary conditions and hydraulic gradients, which can fundamentally alter the inferred properties in similar soils at different locations. However, these effects are largely ignored when generalising soil-water constitutive theory and pedotransfer functions.

We propose a scale- and information aware evaluation concept for pedotransfer function derivation and application. Given the many theoretical obstacles in scaling non-linear three-phase characteristics in porous media, we argue that reducing the scale-gap between the level of derivation and application of soil physical characteristics is more promising. A smart, standardised and repeatable field experiment at the pedon scale could be a first step towards a more physically consistent reference of macroscale soil functioning.