This document defines mathematical models and describes their components and characteristics. It provides Darcy's law, which shows that flow rate in porous media is a function of flow area, elevation, fluid pressure, and a proportionality constant. Darcy's law can be expressed in different forms depending on flow conditions. It also gives the one-dimensional flow equation, defining variables like flow rate, flow area, hydraulic conductivity, flow path length, hydraulic head, and change in head over the path length.