For simulating the exact physical processes of water movement through a watershed's soil layers, what type of hydrologic model (empirical, conceptual, or physically-based) is most suitable?

For simulating the exact physical processes of water movement through a watershed's soil layers, a physically-based hydrologic model is most suitable. A hydrologic model is a simplified representation of a real-world hydrologic system, used to simulate water movement and storage processes. A watershed is an area of land where all water drains to a common outlet. Water movement through soil layers encompasses processes like infiltration, where water enters the soil surface; percolation, its downward movement through the soil profile; and subsurface flow, its lateral movement within the soil layers. For these "exact physical processes," physically-based models are designed to represent these mechanisms by solving fundamental physical laws and equations that govern fluid dynamics and transport in porous media. These models explicitly incorporate principles such as the conservation of mass and momentum. For water movement through soil layers, they utilize established equations like Darcy's Law for saturated flow, which describes flow in fully water-filled pores, and Richards' Equation for unsaturated flow, which describes water movement in partially saturated soil, accounting for changes in soil moisture content and pressure. They require detailed spatial data on soil hydraulic properties, such as hydraulic conductivity and porosity, and discretize the watershed into small computational units. This detailed spatial and temporal resolution allows them to capture the heterogeneity of soil properties and the complex pathways of water movement accurately, providing a mechanistic understanding of how water interacts with the soil matrix at a fundamental level. In contrast, an empirical hydrologic model relies on observed relationships and statistical correlations between inputs and outputs, without explicitly representing the physical mechanisms of water movement. For example, it might use a simple equation to relate rainfall to runoff without simulating infiltration physics. A conceptual hydrologic model represents hydrologic processes using simplified, interconnected conceptual storages and transfer functions. While it offers a more abstract representation of processes like soil moisture storage, it does not solve the fundamental physical equations that govern water flow through the soil layers in detail; instead, it uses simplified rules and parameters. Therefore, neither empirical nor conceptual models are capable of simulating the "exact physical processes" at the detailed, mechanistic level that physically-based models achieve by directly applying the laws of physics.