Why are ground-based precipitation estimates unreliable?
Reliable ground-based precipitation measurements are difficult to obtain because most of the world is covered by water and many countries don’t have precise rain measuring equipment (i.e., rain gauges and radar). Precipitation is also difficult to measure because precipitation systems can be somewhat random and evolve very rapidly. During a storm, precipitation amounts can vary greatly over a very small area and over a short time span. As a result, these systems may fall more- or less-intensely at the location of the ground-based equipment- or it may miss the equipment entirely, introducing error to precipitation estimates.
Fun Fact: Did you know if you gathered all the rain gauges in the world in one place, they would cover an area the size of approximately two basketball courts, or 18,800 square feet (1,740 square meters)? In contrast, satellite observations from space can provide global coverage.