Lens (hydrology)

In hydrology, a lens, also called freshwater lens or Ghyben-Herzberg lens, is a convex-shaped layer of fresh groundwater that floats above the denser saltwater and is usually found on small coral or limestone islands and atolls. This aquifer of fresh water is recharged through precipitation that infiltrates the top layer of soil and percolates downward until it reaches the saturated zone. The recharge rate of the lens can be summarized by the following equation:

$${\displaystyle R=p-ET}$$

Where ${\displaystyle R}$ is the recharge rate in meters, ${\displaystyle p}$ is precipitation (m), and ${\displaystyle ET}$ is evapotranspiration (m) of water. With higher amounts of recharge, the hydraulic head is increased, and a thick freshwater lens is maintained through the dry season. Lower rates of precipitation or higher rates of interception and evapotranspiration will decrease the hydraulic head, resulting in a thin lens.^[1]