3. Law of water flow in soil

In the remainder of this article, we'll focus solely on free water, also known as gravity water.

Soil is defined by saporosity:

$n=\frac{\text{ volume des vides }}{\text{ volume total }}$

assumed constant over time (in fact, to be completely accurate, we need to take the effective porosity, defined as the ratio of the volume of voids in which free water can circulate to the total volume). The soil is assumed to be saturated throughout the area affected by the flow. It is also assumed to be homogeneous and, unless otherwise stated, isotropic.