In hydrologic terms, which term represents water that infiltrates from the surface into the soil?

• A. Precipitation
• B. Evapotranspiration
• C. Infiltration
• D. Runoff

Answer: (C)

Water infiltrating from the surface into the soil is infiltration. This term describes the entry of water from rainfall or surface water into the soil rather than letting it run off or evaporate. Precipitation is the broad process of water arriving at the land from the atmosphere, which includes rain, snow, and other forms; it isn’t itself the act of entering the soil. Evapotranspiration is the loss of water to the atmosphere from soil and plants, not the entry of water into soil. Runoff is water that moves over the land surface to streams and rivers when the soil can’t absorb it quickly enough. Infiltration depends on factors such as soil texture and structure, soil moisture at the time of arrival, surface cover, and land management; if rainfall is slower than the soil’s capacity to absorb, infiltration proceeds and water may percolate down to recharge groundwater, rather than becoming surface runoff. Percolation is the downward movement after infiltration, reaching deeper into the soil, whereas infiltration is the initial entry into the soil.