Unsaturated hydraulic conductivity in dual-porosity soils: Percolation theory

Abstract

Modeling fluid flow in dual-porosity media with bi-modal pore size distributions has practical applications to understanding transport in agricultural soils. Dual-porosity soils are typically formed of two domains: (1) structure and (2) texture. The former mainly incorporates macropores, while the latter contains micropores. In 2013, Hunt and his coworkers proposed a bi-fractal capillary pressure curve model and applied concepts from critical path analysis (CPA) to estimate unsaturated hydraulic conductivity, K(Sw), in dual-porosity soils. By comparing with eight experiments, they found K(Sw) underestimations at intermediate and low water saturations. In this study, we revisit their capillary pressure curve model and demonstrate that the modified bi-fractal model results in more accurate K(Sw) estimations, particularly at the dry end. For this purpose, we use two datasets collected under two different cultivation conditions: (i) conventional tillage (CT) and (ii) no-tillage (NT). Each dataset consists of 13 soil samples for which capillary pressure curve and unsaturated hydraulic conductivity were determined over a wide range of water saturation. We estimate the unsaturated hydraulic conductivity from the measured capillary pressure curve using the CPA approach in combination with: (1) the original capillary pressure curve model, and (2) the modified power-law capillary pressure curve model proposed in this study, both adapted for dual-porosity media. Comparing the theoretical estimations with the experimental measurements shows that CPA in combination with the modified capillary pressure curve model resulted in more accurate K(Sw) estimations than CPA in combination with the original capillary pressure curve model. estimations. Results also show that CPA estimates K(Sw) for the CT samples more accurately than that for the NT samples. We demonstrate that the precise estimation of K(Sw) via CPA requires the accurate characterization of the capillary pressure curve and correct determination of the crossover point separating the structure domain from the texture one.