Run-off, Infiltration and Groundwater

Introduction

When rain falls on a hillside, where does it go? Some of it runs straight over the surface and into the nearest stream. Some soaks into the soil and is taken up by plant roots. Some sinks deeper still, into the rocks beneath, where it may stay for years or even thousands of years before it reappears in a spring or a well. These journeys over and through the land link precipitation back to rivers and the sea, and they decide whether a heavy storm causes a flood or quietly recharges underground water stores.

Core explanation

Three main routes. Once precipitation reaches the ground, the water that is not evaporated straight back into the air follows one of three broad routes:

- Run-off (surface run-off) — water flows over the surface, collecting in rills, streams and rivers, and eventually reaches a lake or the sea. - Infiltration — water soaks down through the surface into the soil. Some is held in the soil and used by plants; some moves sideways through the soil towards rivers (called throughflow). - Percolation to groundwater — water keeps moving downwards through gaps in the rock until it reaches a zone where every pore is full. This saturated zone holds groundwater , and its upper surface is the water table .

Porous and permeable rocks. A rock is porous if it has tiny spaces (pores) between its grains, and permeable if water can pass through it, either through connected pores or through cracks. Sandstone and chalk are porous and permeable, so they make good aquifers — underground stores of water that can be pumped out through wells. Granite and clay let very little water through, so rain falling on them tends to run off.

What controls the balance? The share of rain that runs off rather than soaking in depends on several factors:

Factor More run-off when… More infiltration when… --- --- --- Rock and soil impermeable (clay, granite) permeable (sand, chalk) Slope steep gentle or flat Ground already wet? saturated or frozen dry Rainfall very heavy and sudden light and steady Surface cover concrete, tarmac, roofs forest, grassland

Groundwater moves slowly. Water in rivers may travel several kilometres an hour, but groundwater typically moves only centimetres to metres per day. It eventually returns to the surface at springs , seeps into river beds (keeping rivers flowing in dry weather) or flows out under the sea.

Step-by-step reasoning

To predict what happens to rain on a piece of land:

1. Ask whether the surface lets water in (soil, grass) or blocks it (concrete, frozen ground). 2. Check the rock beneath: permeable rock allows percolation; impermeable rock forces water sideways. 3. Consider the slope: steep ground speeds run-off. 4. Consider the rainfall: an intense downpour can exceed the rate at which soil absorbs water, so the excess runs off.

Visual explanation

Imagine a cross-section through a hill. Arrows of rain fall on the top. Some arrows skim down the surface into a river at the bottom. Others point down into a brown soil layer, then into a pale sandstone layer shaded blue below a dotted line — the water table. At the foot of the hill, where the water table meets the surface, water trickles out as a spring.

Real-world analogy

Pouring water onto a sponge on a tilted tray is like rain on a hillside. A dry sponge soaks up the water (infiltration). Once the sponge is full, extra water runs off the tray (run-off). A plastic chopping board in place of the sponge sends almost all the water straight off.

Real-world example

Towns flood more easily than countryside because roofs, roads and car parks are impermeable. Rain cannot infiltrate, so it rushes into drains and rivers within minutes. Many cities now use permeable paving, green roofs and grassy "rain gardens" so that more water soaks in and the peak flow in rivers is reduced.

Why?

Why do many rivers keep flowing during weeks without rain? Because groundwater slowly seeps out of aquifers into the river bed. This steady supply, called baseflow, comes from rain that infiltrated weeks or months earlier and has been moving slowly through the rock ever since.

Common misconception

"Groundwater is found in huge underground lakes." Most groundwater is not in open caverns; it fills the tiny pores and cracks of solid rock, rather like water held in a wet sponge. Large underground lakes exist only in a few limestone cave systems.

Worked example

Question: A field of dry grassland and a car park of the same area each receive 10 mm of rain in one hour. Which sends more water into the nearby stream, and how quickly?

Reasoning: Tarmac is impermeable, so almost all the rain on the car park runs off into drains and reaches the stream within minutes. Dry grassland lets most of the water infiltrate, and plants intercept some of it, so only a small fraction runs off, and it arrives more slowly.

Answer: The car park sends far more water into the stream, and much more quickly.

Quick check

1. What name is given to the upper surface of the saturated zone underground? Answer: The water table.

Exam focus

Be ready to define run-off, infiltration and groundwater, and to explain how urbanisation, deforestation or saturated ground increases run-off and flood risk. Use the words porous, permeable and impermeable correctly when describing rocks.

Advanced insight

Water moving underground dissolves minerals from the rocks it passes through. Groundwater that has spent a long time in limestone or chalk therefore contains more dissolved calcium ions than surface water — one of the main reasons why some regions have hard tap water. Some deep aquifers hold "fossil" water that fell as rain more than ten thousand years ago; pumping it faster than it is replaced lowers the water table permanently.

Summary

Rain that reaches the ground runs off over the surface, infiltrates into soil, or percolates down into rock to become groundwater below the water table. Porous, permeable rocks such as sandstone and chalk form aquifers. Steep slopes, impermeable surfaces, saturated ground and heavy rain increase run-off; vegetation and permeable rock increase infiltration. Groundwater moves slowly and feeds springs and rivers.

Practice questions

1. Explain the difference between run-off and infiltration. Answer: Run-off is water flowing over the ground surface into streams; infiltration is water soaking down into the soil. 2. Give two reasons why building a new housing estate can increase flooding downstream. Answer: Roofs and roads are impermeable, so less water infiltrates; drains carry water rapidly to rivers, so it arrives in a short, high peak. 3. Why is sandstone a better aquifer than granite? Answer: Sandstone is porous and permeable, so it can store water and let it flow; granite has very few connected pores. 4. Why does heavy rain after a long drought sometimes cause flash floods? Answer: Very dry, baked soil can have a hard surface that absorbs water slowly, and intense rain arrives faster than the soil can absorb it, so much of it runs off.