Does our concept of hydraulic head exist outside of ideal water conditions?

Answer 1

Hydraulic head, is a measure of two components: Elevation head and pressure head. Elevation head can be represented by potential energy while pressure head is the height of a liquid column above or below a datum which corresponds to a particular pressure value. The existence of the pressure head, calculated by pressure divided by the density of water times gravity is non-applicable to water of different chemistries such as brine or salt water. This is because the presence of NaCl increases the density of the water (1.202 g/mL) vs 1.027 g/mL in pure water. What this means is that the concept of hydraulic head, when applied to real world scenarios is typically null or used as an approximation because of this density difference. This is especially true in the case of saltwater intrusion, where there's a boundary between freshwater and salt water. And so, boundaries and approximations must be drawn when considering using Darcy's equation to define an aquifer or saturated zone of various chemistries.