In the energy balance of groundwater flow a hydraulic equivalent of Joule's law is used:
where:
*  = loss of hydraulic energy () due to friction of flow in -direction per unit of time (m/day), comparable to 
*  = flow velocity in -direction (m/day), comparable to 
*  = hydraulic conductivity of the soil (m/day), the hydraulic conductivity is inversely proportional to the hydraulic resistance which compares to