What is the upper boundary of the saturated groundwater zone known as?

The upper boundary of the saturated groundwater zone is known as the water table. The water table represents the level at which the soil or rock is saturated with water. Below this level, all the pore spaces are filled with water, whereas above it, the soil or rock may contain air-filled pores. Understanding the position of the water table is crucial for various applications in environmental health, hydrology, and resource management, as it influences groundwater availability and is an important factor in assessing potential contamination risks and groundwater recharge rates.

In contrast, the other terms provided do not accurately describe this specific boundary. For instance, "water shelf" does not refer to a recognized concept in hydrology. "Groundwater limit" is a vague term that isn't widely used to denote the boundary of the saturated zone, while "surface line" does not have a standard definition in this context related to groundwater. Consequently, the water table is the correct and widely accepted terminology for identifying the upper limit of saturated groundwater.