The primary factor that makes precipitation and runoff possible is gravity. Gravity pulls water from clouds in the form of precipitation (like rain, snow, sleet, or hail) back to the Earth's surface. Once it reaches the ground, gravity also drives the movement of water as runoff, guiding it toward rivers, lakes, and oceans.
While other factors like radiant energy (which helps in the evaporation process), vapor (water vapor in the atmosphere), and groundwater are involved in the broader water cycle, gravity is the key force that enables the actual processes of precipitation and runoff.