In hydrology, what do we refer to the space filled with water in the saturated zone?

The term used to describe the space that is filled with water in the saturated zone is "groundwater." This area exists beneath the Earth's surface where all the pore spaces in the soil or rock are completely filled with water, creating a substantial reservoir of water that can move through geological formations.

Groundwater plays a crucial role in hydrology, as it contributes to stream flow, supplies water for wells, and sustains ecosystems. The saturated zone is below the water table, and here, every pore in the geological material contains water, differentiating it from the unsaturated zone above, where pores may contain both air and water.

While other terms also relate to water and the subsurface environment, they do not specifically describe the water-filled space in the saturated zone. For instance, "pore space" refers to the voids in soil or rock, "capillary fringe" denotes the area above the water table where water is held in small pores due to surface tension, and "aquifer" describes a rock or sediment layer that can store and transmit significant amounts of groundwater, but it does not directly denote the water itself. Thus, the designation of "groundwater" is the most accurate for describing the water-filled portion of the saturated zone.