Hydrologic Equation: Meaning, Formula, and Uses      The hydrologic equation is one of the most important ideas in hydrology. It gives a simple way to understand how water moves through a drainage basin, watershed, or larger hydrologic system. In plain language, it tells us that water entering a system must either leave the system or be stored inside it.      Hydrology studies the occurrence, distribution, movement, and chemical properties of water on Earth, and the hydrologic equation helps scientists and engineers quantify that movement. This is why it is often treated as the foundation of water balance studies. What Is the Hydrologic Equation?      The hydrologic equation is a statement of the law of mass conservation. It means water is not created or destroyed inside a closed system; it only changes form, moves in, moves out, or gets stored.      In the provided resource, the equation is written as: Inflow = Outflow ± Chang...