What If? Lab
What if it rained every day?
Pitter-patter! What if rain fell every single day? The ground would get soggy and rivers would fill up. Let's imagine it with Olly.
Rain is wonderful: it waters plants and fills rivers. But what if it rained every day? Soil can only soak up so much, so more water would run into rivers. Some rainforests and mountain slopes get rain very often, and life there is built for it. Explore what daily rain would change.
Infiltration falls as soil saturates, so persistent rain converts more precipitation into runoff and raises streams. Rainforests receive 2,000–10,000 mm a year, yet even the Amazon has a dry season, when NASA notes the sunnier forest puts out extra leaves. This thought experiment imposes daily rain everywhere.
The big question
Rain helps flowers grow. It fills up puddles to splash in!
But what if it rained every single day? What would happen?
Rain is one of the most important things in nature. It waters plants, fills lakes and rivers, and soaks into the ground to refill the water underground. Most places get a mix of rainy days and dry days.
So what would happen if it rained every single day, everywhere? Would we have more of everything good, or would something go wrong? This thought experiment follows three paths: soggy soil, rising rivers and places on Earth that get rain very often.
Precipitation partitions into infiltration, runoff and evapotranspiration, and the split depends strongly on how wet the ground already is. Rain on most days would shift that balance.
This lab examines the hydrology of persistent rain using USGS explanations, compares it with the wettest biome, tropical rainforest, using NASA data, and labels consequences of daily rain everywhere as ideas.
Explore the idea
Make a prediction
The ground is already soaking wet. More rain falls. Where does most of it go?
If the soil is already soaked from yesterday's rain, where does most of today's rain go?
On saturated soil, additional rainfall mostly becomes…
- It runs off into streams
- It all soaks in
- It disappears
-
Established fact
Soaked soil can't take much more rain.
Soil already soaked from earlier rain can't absorb much more, so more rain runs off into streams.
Saturated soil absorbs little more; additional rainfall becomes surface runoff (USGS).
-
An idea scientists discuss
Rain every day is pretend. We imagine it carefully.
It won't really rain every day everywhere. What would happen is an idea built from how rain, soil and rivers work.
Every consequence here is reasoned from hydrology and observed rainforests, not observed under daily global rain.
-
Established fact
Some rain soaks into the ground.
Part of the rain that falls soaks into the ground. How much depends on the rain, the soil and the land.
Infiltration depends on precipitation amount and intensity, soil type, saturation, land cover and slope (USGS).
-
Established fact
Wet soil is like a full sponge.
Soil already soaked from earlier rain can't absorb much more, like a wet sponge, so more rain runs off.
Like a wet sponge, soil saturated from previous rainfall can't absorb much more; more rainfall becomes surface runoff (USGS).
-
Established fact
Clay soil soaks up water slowly.
Clay soils soak up less water, and more slowly, than sandy soils.
Clays absorb less water at a slower rate than sandy soils, producing more overland runoff (USGS).
-
An idea scientists discuss
The ground might stay muddy all the time.
With rain every day, the soil might almost never dry, and some places might turn into marshy ground.
Persistent saturation could favor wetland conditions in poorly drained areas; this is reasoning, not a model result.
-
Established fact
Roads and parking lots send rain straight to drains.
Hard surfaces like roads and parking lots don't let rain soak in. They send it quickly into drains and streams.
Impervious surfaces act as a 'fast lane' for rainfall into storm drains and streams; vegetation slows runoff (USGS).
-
Established fact
A rain garden helps rain soak into the ground.
A rain garden is a low patch of garden that collects rain from a roof or driveway and lets it soak into the ground.
Rain gardens are planted depressions that collect runoff from roofs, driveways or streets and let it infiltrate, reducing runoff (EPA).
-
Established fact
Always stay out of floodwater.
Weather experts remind everyone never to walk or drive into floodwater. Turn around and find another way.
NWS advises never driving around barriers on flooded roads and not walking into flood waters; people underestimate the force of moving water.
-
Established fact
Rainforests get the most rain of all.
Rainforests get the most rain of all the biomes: 2,000 to 10,000 millimeters a year.
Rainforests receive the most rain of any biome, typically 2,000–10,000 mm a year; tropical rainforest is hot and moist with rain all year (NASA).
-
Established fact
The Amazon gets more sunshine in its drier time.
The Amazon rainforest has a dry season, mostly in June, July and August. Then the clouds clear and the plants put out extra leaves.
During the Amazon's dry season, mostly June to August, the cloud blanket clears and plants put out extra leaves and photosynthesize more (NASA Earth Observatory, 2009).
-
Established fact
Mountains help make lots of rain in Hawaiʻi.
In Hawaiʻi, steady trade winds and mountains work together to make clouds and rain, so rainfall changes a lot over short distances.
Mountainous terrain, persistent trade winds and a stable layer near 7,000 ft produce uplift, cloud and sharp rainfall contrasts (Rainfall Atlas of Hawaiʻi).
-
An idea scientists discuss
Too much rain might mean too little sunshine for plants.
If it rained every day, plants might get less sunshine, and some might grow more slowly.
Persistent cloud and rain could reduce light-limited growth; responses would differ among ecosystems.
In our world
When rain falls, some soaks into the ground.
Some runs over the ground into streams and rivers. Plants drink some too!
When rain falls, part of it soaks into the ground. The U.S. Geological Survey calls this infiltration. Some of that water stays near the surface where plant roots drink it, and some sinks deeper to refill the water underground. The rest runs over the land into streams and rivers.
How much soaks in depends on the rain, the soil and the land. Sandy soil soaks up water faster than clay. Plants slow the water down so more can sink in, while roads and parking lots send it rushing into drains.
USGS lists the main controls on infiltration: precipitation amount and intensity, soil characteristics (clays absorb less, more slowly, than sands), soil saturation, land cover and slope. Vegetation slows runoff and allows more infiltration; impervious surfaces act as a 'fast lane' to storm drains.
Infiltrated water sustains plants and recharges aquifers, and groundwater seeping into stream beds provides base flow, which keeps streams running between rains.
Rain, rain, every day
In our pretend world, it rains every day. No dry days at all!
The ground gets wetter and wetter. Puddles stay and stay.
In our pretend world, rain falls every day, everywhere. At first the ground soaks it up. But soil is like a sponge, and a sponge can only hold so much.
Follow the paths in the lab: soggy soil, rivers rising, and places on Earth where it already rains very often. Each card says whether it's a fact or an idea. Nobody can make it rain every day everywhere, so the consequences are careful guesses.
Impose daily precipitation everywhere. With little time to drain between events, soils trend toward saturation, infiltration capacity drops and a larger fraction of each day's rain becomes surface runoff.
The scenarios follow soils, streams and ecosystems already adapted to frequent rain. The hydrological relations are from USGS; outcomes such as how rivers or forests would respond are reasoned ideas.
Why soil gets soggy
Soil is like a sponge. A dry sponge soaks up lots of water.
A wet sponge can't hold any more. So the extra water runs away!
The USGS compares soil to a sponge. A dry sponge soaks up water easily. But once it's full, any more water just runs off. Soil that's already soaked from earlier rain can't absorb much more, so more of the new rain flows over the surface into streams.
So after several rainy days in a row, more and more of the rain can end up in rivers. In a world where it rained every day, the soil might almost never get the chance to dry out. That's an idea.
Infiltration capacity declines as soil water content rises; USGS puts it plainly: like a wet sponge, soil already saturated from previous rainfall can't absorb much more, so more rainfall becomes surface runoff.
Persistent rain therefore raises the runoff fraction of each successive event. Under daily rain everywhere, soils might stay near saturation, with runoff and stream levels elevated: a reasoned idea, since real outcomes would depend on rain intensity, soil type, slope and evaporation.
Places that get lots of rain
Rainforests get rain in every month. The plants there love it!
But even the Amazon rainforest has a drier time. Then the sun shines more, and the plants put out new leaves!
Tropical rainforests are the rainiest places of all. NASA says they get 2,000 to 10,000 millimeters of rain a year, and it rains all year long. Some mountain slopes, like those in Hawaiʻi, also get a lot of rain, because steady winds push moist air up the mountains to make clouds.
But here's a surprise: even the Amazon has a dry season, mostly in June, July and August. NASA says that's when the clouds clear, sunshine reaches the trees, and the plants thrive and put out extra leaves.
NASA's biome data give rainforests 2,000–10,000 mm a year; Manaus, Brazil, averages 2,323 mm, from about 310 mm in April to 68 mm in August (1970–2000). The Rainfall Atlas of Hawaiʻi attributes the islands' extreme rainfall contrasts to persistent trade winds lifted by mountainous terrain.
NASA Earth Observatory notes that the Amazon's dry season, mostly June to August, is when the cloud blanket clears; with more sunlight the plants put out extra leaves and photosynthesize more. Even the wettest forests rely on drier, sunnier spells, which an every-day-rain world would remove.
Amazing facts
-
Rain that soaks in can keep streams flowing on dry days.
Water that soaks into the ground slowly seeps into streams, so many streams keep flowing even when it hasn't rained for a long time.
Groundwater seeping into stream beds supplies base flow, sustaining streams between rain events (USGS).
-
Rainforest leaves rot fast on the wet ground.
On the warm, wet rainforest floor, fallen leaves rot quickly, like a compost pile, and give their nutrients back to the soil.
Rainforest leaf litter decomposes rapidly in warm, wet conditions, returning nutrients to the soil; microorganisms thrive there (NASA).
-
Rainforest clouds can come from the trees!
On some sunny Amazon afternoons, little 'popcorn' clouds form from water vapor that the trees give off.
NASA imaged uniform afternoon 'popcorn' clouds over the Amazon in August 2009, likely formed from water vapor released by plants through transpiration.
Words to know
- infiltration
- rain soaking into the ground
- runoff
- rain that flows over the land into streams and rivers
- saturated
- so full of water that it can't hold any more
- rain garden
- a low garden that collects rain and lets it soak into the ground
- dry season
- the part of the year when much less rain falls
Discovery challenge
-
What happens to rain on soaking-wet soil?
What happens to rain that falls on soil that's already soaked?
Rain on saturated soil mostly becomes…
- It runs off into streams
- It all soaks in
- It turns into snow
Show the answer
It runs off into streams
It runs off. The soil sponge is full!
Most runs off, because full soil can't absorb much more.
…surface runoff (USGS).
-
Which soil soaks up water faster?
Which kind of soil soaks up water faster?
Which soil typically has the higher infiltration rate?
- Sandy soil
- Clay soil
- They are the same
Show the answer
Sandy soil
Sandy soil soaks it up faster.
Sandy soil; clay soaks up less, more slowly.
Sand; clays absorb less, more slowly (USGS).
-
When does the Amazon rainforest get the most sunshine?
When do the Amazon's plants put out extra leaves in the sunshine?
NASA reports that Amazon plants put out extra leaves and photosynthesize more during…
- In its drier season
- In its wettest months
- At night only
Show the answer
In its drier season
In its drier, sunnier time!
In the dry season, mostly June to August, when clouds clear.
…the June–August dry season, when the cloud blanket clears.
Experiments to try
- Watch What If? Lab What if there were no clouds? Clouds shade the ground by day, send heat back down at night and are where rain comes from. Imagine a sky with no clouds at all, and follow what might happen to hot days, cold nights, rain and even the colors of the sky.
- Watch What If? Lab What if a river changed its course? A winding river wears away the outside of each bend and piles sand on the inside, so slowly, year by year, it moves. Sometimes it cuts off a loop and leaves a lake behind. Imagine a big river changing its course: what would happen to the old loops, and to the towns and bridges beside it?
- Watch What If? Lab What if a rainforest had little rain? Rainforests are tall, green forests that get a great deal of rain. In 2005 and 2010, big droughts hit the Amazon rainforest, and satellites watched what happened to its trees. Use those real events to imagine a rainforest with little rain: drier leaves, hungry animals and a forest that might change.
Explore next
- Watch What If? Lab What if there were no clouds? Clouds shade the ground by day, send heat back down at night and are where rain comes from. Imagine a sky with no clouds at all, and follow what might happen to hot days, cold nights, rain and even the colors of the sky.
- Watch What If? Lab What if a river changed its course? A winding river wears away the outside of each bend and piles sand on the inside, so slowly, year by year, it moves. Sometimes it cuts off a loop and leaves a lake behind. Imagine a big river changing its course: what would happen to the old loops, and to the towns and bridges beside it?
- Watch What If? Lab What if a rainforest had little rain? Rainforests are tall, green forests that get a great deal of rain. In 2005 and 2010, big droughts hit the Amazon rainforest, and satellites watched what happened to its trees. Use those real events to imagine a rainforest with little rain: drier leaves, hungry animals and a forest that might change.
- Watch Geography The water cycle around the world Earth's water is always on the move. The Sun's heat lifts water from oceans, lakes and plants into the air as invisible vapor. It cools into clouds, winds carry it around the world, and it falls as rain or snow. Then rivers and groundwater carry it back to the sea. Some places get far more rain than others because of how air rises and sinks around the planet.
- Watch Geography Monsoons A monsoon is a seasonal change in the direction of a region's strongest winds. In South Asia, the summer monsoon blows warm, wet air in from the Indian Ocean and brings months of heavy rain, while the winter monsoon blows dry air from the land. About three-quarters of India's yearly rain falls in the monsoon season, so farms, wells, rivers and power stations all depend on it.
- Watch Try it What If? Lab What if Earth had no atmosphere? Earth's air works like a jacket: it keeps heat in and carries it around. Take the air away in our model of a sandy patch of ground on the equator, and the coldest part of the night drops from about 13 °C to about 2 °C.
- Watch Try it What If? Lab What if there were no greenhouse gases? Greenhouse gases such as carbon dioxide and water vapor hold some of Earth's heat in, like a blanket. Take them all away in our energy-balance model and Earth's average temperature falls from about 15 °C to about −19 °C, well below freezing.
- Watch Try it What If? Lab What if there were more greenhouse gases? Adding greenhouse gases makes it harder for Earth's heat to escape, so the planet warms until it balances again. In our model a small change in how freely heat escapes warms Earth by about 2.6 °C, and the oceans make the warming take years.
For parents & teachers
What children will learn
- Some rain soaks into the ground (infiltration) and some runs off into streams.
- Soil that is already saturated can't absorb much more rain.
- Plants, soil type and hard surfaces change how much rain soaks in.
- Even very rainy places, like rainforests, have drier seasons that matter for life.
Recommended ages: 4 to 17 · About 15 minutes
Talk about it
- Why might rivers rise more after a week of rain than after one rainy day?
- Where does rain go when it falls on your school's playground or parking lot?
- What could your town do to help rain soak into the ground?
Try at home or in class
- Pour a cup of water slowly onto a dry sponge, then onto a soaked sponge, over a tray. Which one lets water run off?
- Put dry sand in one cup and clay or packed soil in another, each with holes in the bottom. Pour the same water into each and time how fast it drains.
Key vocabulary: infiltration, runoff, saturated, rain garden, dry season
Where these facts come from
- Infiltration and the Water Cycle (Water Science School) , U.S. Geological Survey
- Biome: Rainforest , NASA Science (Mission: Biomes)
- Afternoon Clouds over the Amazon Rainforest (2009) , NASA Earth Observatory
- Rainfall Atlas of Hawaiʻi , University of Hawaiʻi at Mānoa, Department of Geography
- Soak Up the Rain: Rain Gardens , U.S. Environmental Protection Agency
- Turn Around Don't Drown , National Weather Service
Checked, not yet human-reviewed. Last updated October 26, 2026. Spotted a mistake? Tell us.