A Regime-Based Framework for Understanding the Dominant Mechanisms of Flow in Heterogeneous Agricultural Soils

Soil moisture dynamics in unsaturated soils of agricultural fields are driven by the complex interplay of hydraulic heterogeneity, root water uptake (RWU), and atmospheric forcing; however, the specific conditions under which each process becomes dominant are not yet fully understood. We propose a regime-based framework to identify the dominant flow mechanisms in an agricultural field, using the wheat cropping season as a primary case study. For this, 3-D Richard’s equation simulations were performed for silty loam soil, considering crop-specific data and atmospheric conditions from an experimental site at IIT Kanpur. Simulations comparing homogeneous and heterogeneous soil profiles (spatial heterogeneity in van Genuchten parameters, K_s  and α) reveal that near-surface layers (10–25 cm) exhibit higher early-season variability and faster post-irrigation responses than deeper layers (50 cm), with heterogeneity effects diminishing over depth and time. Sensitivity profiles based on the ratio of RWU to vertical flux divergence indicate stronger near surface control, with values close to one during irrigation periods and declining with depth, reflecting reduced influence of vertical flux divergence in deeper soil. Variance-based dominance diagnostics, evaluated against variability in both  and , reveal a distinct transition in governing processes. As the ratio of soil moisture variance to heterogeneity variance decreases from approximately 0.30 near the surface during early growth to below 0.15 at greater depths and later stages, the system shifts from a heterogeneity-dominated behavior to an RWU-dominated regime. Collectively, these diagnostics categorize the subsurface into four distinct regimes: heterogeneity-dominated, RWU-dominated, atmospheric demand-dominated, and transition zones. This classification provides a physically interpretable framework for analyzing process dominance and refining model selection in structured agricultural soils.