What is the term for the upper surface of saturated soil or rock?

The upper surface of saturated soil or rock is referred to as the water table. This term specifically describes the boundary between the saturated materials below, where all the pore spaces are filled with water, and the unsaturated materials above, where some pore spaces contain air. The water table can fluctuate based on factors such as precipitation, evaporation, and groundwater extraction, and it plays a critical role in the hydrological cycle.

The other terms do not accurately describe this boundary. The term "groundwater level" can sometimes be confused with the water table, but it is less specific and may refer to the level at which groundwater is found in a well or other excavation. "Surface runoff" pertains to water that flows over the ground surface, typically during or after precipitation events, and does not involve saturated soil or rock. The term "water reservoir" usually refers to a natural or artificial storage area for water, used for various purposes such as irrigation, drinking water supply, or recreation, but it does not describe the concept of the upper boundary of saturated soil or rock.