Question

this can drop during a long drought
erosion
tributary
water table
water cycle

Explanation:

The water table is the upper surface of the zone of saturation (where soil or rock is fully saturated with water). During a long drought, less water infiltrates the ground to recharge the zone of saturation, so the water table drops. Erosion is the wearing away of land, a tributary is a stream feeding a larger stream, and the water cycle is the movement of water on Earth—none of these directly "drop" in the context of a drought like the water table does.

Answer:

water table