Precipitation and runoff are primarily influenced by several factors, but the key elements among the options you provided are:
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Radiant Energy: This is crucial as it drives the water cycle through processes like evaporation, which adds moisture to the atmosphere.
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Vapor: Water vapor is essential for precipitation; it condenses in the atmosphere to form clouds and ultimately falls back to the earth as precipitation.
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Gravity: Gravity plays a vital role in runoff. Once precipitation falls to the ground, gravity helps to move that water across the land surface and into rivers, lakes, and oceans.
Groundwater is important in the broader hydrological cycle but is more related to the storage and movement of water underground rather than the immediate processes of precipitation and surface runoff.
In summary, the most significant factors in making precipitation and runoff possible among your options are vapor and gravity, with radiant energy also playing a crucial role in the cycle.