Equation of Continuity

Darcy's law furnishes three equations of motion for four unknowns (qx, qy, qz, and <f>). A fourth equation notes that the flow phenomenon must satisfy the fundamental physical principle of conservation of mass. When an elementary block of soil is filled with water, as shown in Figure 9.2.2, no mass can be gained or lost regardless of the pattern of flow.

The conservation principal requires that the sum of the three quantities (the mass flow) is zero, hence when divided by Ax • Ay • Az

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