So, the velocity is related to potential through Darcy's law. he found out that the velocity is proportional to the potential gradient. And this case has a hydraulic conductivity, which we are talking, which is quite non-linearly with the potential. And so if you substitute in the continuity potential this expression for velocity we get this expression which is called the equation for isothermal flow. Now, what we have done for the experiment is quite easy if we measure variable, which is volumetric moisture content.
And therefore we define another variable which is a volumetric moisture content, that is shows volume of water upon total volume.