Where Does Rain Go? Runoff, Infiltration, and the Water Cycle

In August 2017, Hurricane Harvey dropped more than 60 inches (152 cm) of rain on Houston, Texas in just four days — the most rainfall ever recorded from a single storm in U.S. history. The water had nowhere to go. Houston's concrete streets, parking lots, and rooftops could not absorb it. About 30,000 people had to be rescued from their flooded homes. This disaster shows exactly why understanding runoff and infiltration can save lives.

By the end of this lesson you will be able to: • Define runoff and infiltration and explain the difference between them • List three factors that control how fast water soaks into the ground • Explain why cities flood more easily than forests • Describe two ways communities reduce runoff damage

When rain falls on land, gravity determines what happens to every drop: Path 1 — Infiltration: The water soaks into the soil and moves downward. This water may reach underground aquifers (like giant underground sponges) that supply wells and springs. It also feeds plant roots. This is the helpful path — it recharges groundwater and slows flooding. Path 2 — Runoff: The water flows across the surface, downhill, collecting in streams, rivers, and eventually the ocean. Runoff carries whatever is on the surface with it — dirt, fertilizer, pet waste, oil from roads — polluting waterways along the way. Every landscape produces some combination of both paths. The big question is: what controls the ratio?

Three main factors decide the infiltration rate: 1. Soil type: • Sandy soil has large particles with big gaps — water infiltrates fast, up to 5 cm per hour or more. • Clay soil has tiny flat particles packed tight — water soaks in slowly, sometimes less than 0.1 cm per hour. Clay swells when wet, sealing itself further. • Loam soil (a mix of sand, silt, and clay) infiltrates at a moderate rate and holds water well, making it best for farming and gardens. 2. Compaction: When soil is packed hard by vehicles, construction, or heavy foot traffic, the air pockets collapse and water cannot get through. Urban soils under parks are often heavily compacted even if they look like natural grass. 3. Vegetation cover: Plant roots create channels in soil and plants intercept rain, slowing it down. A square meter of old-growth forest floor can absorb more than 14 cm of rain per hour. A square meter of compacted bare soil might absorb less than 0.5 cm per hour.

An impervious surface is any material water cannot pass through: concrete, asphalt, rooftops, compacted gravel. A natural forested watershed converts only about 10% of rainfall into runoff. A highly urbanized watershed can convert 55% or more into runoff — more than five times as much water rushing into streams all at once. This causes two major problems: 1. Flooding: Streams and storm drains receive a massive surge of water much faster than they can handle. 2. Pollution: Runoff picks up motor oil, heavy metals, fertilizers, and bacteria from roads and carries them directly into rivers and bays without any filtering. Hydrologists use a number called the runoff coefficient (C) — between 0 and 1 — to represent what fraction of rainfall becomes runoff. A dense city center might have C = 0.85; a meadow might have C = 0.10.

Cities around the world are redesigning themselves to let water infiltrate instead of run off: Rain gardens: shallow depressions planted with native plants that tolerate both wet and dry conditions. A 3-meter-wide rain garden can capture runoff from a 30-meter-long driveway. Pervious pavement: special paving materials have gaps that allow water to trickle through. A pervious concrete parking lot can absorb 3–5 gallons of water per minute per square foot. Green roofs: rooftops covered with soil and plants absorb the first half-inch or so of each rainstorm before any water runs off the building. Urban trees: each large tree intercepts 1,000–4,000 gallons of rainfall per year before it reaches the ground. After a 1992 flood, Chicago planted 75,000 trees as part of its green infrastructure response.