How does groundwater help sustain streamflow between storms?

Groundwater plays a critical role in sustaining streamflow during periods without precipitation by providing the base flow component of streams.1

When a stream channel intersects the water table, groundwater and surface water communicate directly. 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 consistent discharge from underground sources ensures that many streams continue to flow even when there is no immediate input from storm-related surface runoff or subsurface storm flow.1

On a global basis, groundwater contributes approximately 30 percent of the water found in total river runoff. While surface runoff from storms is often responsible for the "flashy" or rapid responses in streamflow, groundwater acts as a reliable, steady supply that maintains the perennial nature of many river systems between these precipitation events.1