Background on Soil Moisture Modeling

This chapter provides the soil moisture modeling foundation needed for closed-loop irrigation scheduling. It first introduces the agro-hydrological system, the vadose zone, common measures of soil moisture, and soil–water potential concepts. It then presents Darcy's law and derives Richards’ equation for a one-dimensional vertical soil profile. Key soil hydraulic properties required to solve Richards’ equation are discussed, including the soil–water retention curve and the hydraulic conductivity. The chapter also outlines a numerical technique based on the finite difference method for solving Richards’ equation and presents the boundary conditions commonly imposed during the solution process. Root water uptake is then introduced as a sink term in Richards’ equation. Finally, the chapter presents a generic form of Richards’ equation and explains how it can be tailored to the various domains that arise in agricultural settings.