What does a change in water storage mean in a catchment water-balance equation?

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In the context of hydrologic sciences, the water-balance equation is an application of the conservation of mass to a specific area, such as a catchment. The equation is expressed as:

$S = P - Q - E - G$

In this formula, $S$ represents the change in water storage within the catchment over a defined time period.1

This term accounts for the net difference between the water entering the area and the water leaving it. When $S$ is positive, it indicates that the total volume of water stored within the catchment—such as in soil moisture, groundwater, or surface water bodies—has increased during that interval. Conversely, a negative value for $S$ signifies a depletion of stored water within the system. Evaluating this change is essential for understanding how catchments respond to different climatic conditions and for quantifying the movement of water across the landscape.1