Infiltration is the process by which water on the ground surface soaks into the soil, moving precipitation into the ground where it can recharge groundwater. In AP Environmental Science, it's a core step of the hydrologic cycle (Topic 1.7) and the counterpart to surface runoff.
Infiltration is what happens when rain hits the ground and sinks in instead of flowing away. Water moves from the surface down into the pore spaces of soil, where it can be taken up by plant roots, held as soil moisture, or keep moving downward to recharge groundwater. It's one of the main pathways in the hydrologic cycle, the sun-powered movement of water between sources and sinks (EK ERT-1.G.1).
Here's the intuitive way to think about it: every drop of precipitation faces a fork in the road. It either infiltrates into the soil or becomes runoff across the surface. What decides the split? Soil composition (sandy soil with big pores absorbs fast, clay absorbs slowly), vegetation cover (roots and leaf litter slow water down and give it time to soak in), how saturated the soil already is, and land use (pavement and compacted soil block infiltration almost entirely). That infiltration-versus-runoff tradeoff drives a huge number of AP Enviro scenarios, from deforestation to urban flooding to groundwater depletion.
Infiltration lives in Unit 1 (The Living World: Ecosystems), Topic 1.7, under learning objective 1.7.A, which asks you to explain the steps and reservoir interactions in the hydrologic cycle. Groundwater is one of the smaller but critical water reservoirs (EK ERT-1.G.2), and infiltration is the only way water gets into it naturally. Cut off infiltration and you cut off groundwater recharge. That's why this term keeps reappearing far beyond Unit 1. Urbanization questions, deforestation questions, and water-resource questions all hinge on whether water can soak into the ground or gets forced across the surface as runoff.
Runoff (Unit 1)
Runoff is infiltration's opposite. Water that can't soak in flows across the surface instead, carrying sediment and pollutants with it. When an exam question says a surface got paved or compacted, translate it instantly to less infiltration, more runoff.
Groundwater Recharge (Unit 1)
Recharge is what infiltration accomplishes. Infiltrated water that keeps moving downward refills aquifers. If a city pumps groundwater faster than infiltration can replace it, recharge is the phase of the cycle that gets disrupted, which is exactly how exam questions frame groundwater depletion.
Permeability (Unit 4)
Permeability is the soil property that sets the infiltration rate. Sandy, porous soils are highly permeable and absorb water quickly, while clay-heavy soils are nearly waterproof. When you study soil properties in Unit 4, you're learning the 'why' behind infiltration rates from Unit 1.
Soil Erosion (Unit 4)
Low infiltration feeds erosion. When water can't sink in, it runs over the surface with enough force to strip topsoil. Deforestation scenarios chain these together: lost vegetation, less infiltration, more runoff, more erosion. That cause-and-effect chain is FRQ gold.
Infiltration shows up most often in multiple-choice scenarios about the hydrologic cycle and human disruptions to it. Expect stems that trace water's pathway after precipitation in a forested ecosystem (precipitation, then infiltration, then groundwater), or that ask what happens to a watershed after deforestation (infiltration drops, runoff rises). Another common setup is a coastal city pumping groundwater faster than natural recharge, where you need to identify infiltration and recharge as the disrupted phase. On FRQs, infiltration is your go-to mechanism word when explaining why pavement causes flooding, why wetlands or vegetation reduce runoff, or why aquifers deplete. Don't just name it. Explain the direction of change: what factor increased or decreased infiltration, and what happened to runoff or groundwater as a result.
Infiltration and runoff are the two fates of precipitation, and they trade off directly. Infiltration is water sinking INTO the soil; runoff is water flowing ACROSS the surface because it couldn't sink in. If a question changes the landscape (paving, deforestation, soil compaction), one goes up and the other goes down. Saying 'deforestation increases infiltration' is one of the most common ways to lose easy points.
Infiltration is the step of the hydrologic cycle where surface water soaks into the soil, and it's the natural pathway that recharges groundwater.
Infiltration and runoff are inversely related, so anything that decreases infiltration (pavement, deforestation, compacted or saturated soil) increases surface runoff.
Infiltration rate depends on soil permeability, vegetation cover, existing soil moisture, and land use.
Vegetation boosts infiltration by slowing water down with roots and leaf litter, which is why deforested watersheds flood more and recharge less.
When groundwater is pumped faster than infiltration can recharge it, the result is groundwater depletion, a scenario AP Enviro tests repeatedly.
This term supports learning objective 1.7.A, explaining the steps and reservoir interactions in the hydrologic cycle.
Infiltration is the process where water on the ground surface enters the soil. It's a key step in the hydrologic cycle (Topic 1.7) because it moves precipitation into the ground, where it can recharge groundwater reservoirs.
Infiltration is water sinking into the soil; runoff is water flowing across the surface when it can't or doesn't sink in. They're inversely related, so paving a surface or removing vegetation decreases infiltration and increases runoff.
Decrease. Removing vegetation eliminates the roots and leaf litter that slow water down and let it soak in, so deforested watersheds show less infiltration, more runoff, more erosion, and reduced groundwater recharge.
Not exactly. Infiltration is water entering the soil at the surface, while groundwater recharge is infiltrated water continuing downward until it actually refills an aquifer. Recharge depends on infiltration happening first, which is why blocking infiltration with pavement also blocks recharge.
Soil composition and permeability (sandy soils absorb fast, clay slowly), vegetation cover, how saturated the soil already is, and land use. Impervious surfaces like roads and parking lots can drop infiltration to nearly zero.