This lecture delves into the concept of water balance, which asserts that inflows to a water system equal its outflows plus any changes in storage over a given time period. It applies to various hydrological domains like soil columns, drainage basins, or urban areas, and is crucial for understanding how water is distributed and managed. The water balance equation, reflecting conservation of mass, helps in managing water resources and predicting shortages by accounting for precipitation, evaporation, runoff, and storage. The lecture also differentiates between Water Resource Systems (WRS) and Water Use Systems (WUS), highlighting their roles and challenges in maintaining water balance. Proper management of these systems is essential to address issues like declining water storage and ensuring efficient water use.