What is water scarcity called that occurs due to increasing numbers of people relying on fixed levels of runoff? 

A)  desiccation 
B)  drought 
C)  aridity 
D)  water stress

Water Scarcity

Water scarcity is a situation in which the available water supply is insufficient to meet the demands of water use within a region.

Runoff

The portion of rainfall, melted snow, or irrigation water that flows across the ground's surface and is not absorbed into the soil, often reaching rivers, lakes, and oceans.

Increasing Numbers

A term indicating a situation where the quantity or amount of a certain entity is growing over time.