Why does groundwater help sustain streamflow between storms?

In the context of hydrologic sciences, groundwater is essential for maintaining streamflow during periods without precipitation because it provides the base flow component of streams.1

When a stream or lake intersects the water table, it communicates directly with the groundwater body. If the water table is higher than the level of the stream, a pressure head develops that causes groundwater to flow into the stream channel. This steady, continuous discharge from groundwater flow ensures that water remains in the channel even when there is no immediate input from storm-related surface runoff or subsurface storm flow.1

While surface runoff and subsurface storm flow are responsible for the "flashy" or rapid responses of streamflow to individual precipitation events, groundwater acts as a reliable, long-term source of water that sustains the stream between these events.1