What If? Lab
What if a drought lasted for years?
Sometimes it does not rain for a long, long time. What if the dry time lasted for years? Let's imagine it with Olly and find out how people save water.
A drought is a long stretch with much less rain or snow than normal, long enough that water runs short. Droughts can last a few weeks, a few months or several years. Imagine a land where the dry years keep coming, and follow its rivers, the ways people save water, and what it takes for a drought to end.
Drought.gov defines drought as a deficiency of precipitation over an extended period, usually a season or more, resulting in a water shortage. This thought experiment follows an imagined region through a multi-year drought: how streams, reservoirs and wells respond with different delays, how demand can be cut, and why one storm rarely ends it.
The big question
Sometimes it does not rain for a long time. Plants get thirsty and the ground gets dry.
What if the dry time lasted for years and years? Let's imagine it together.
A drought is a long dry spell: much less rain or snow than normal, for long enough that water starts to run short. Drought.gov says a drought can last from just a few weeks or months to several years.
So what if a drought lasted for years? In this thought experiment you can follow three paths: rivers and lakes that shrink, the ways people save water, and how a long drought finally ends.
Drought.gov gives a common definition: a deficiency of precipitation over an extended period of time, usually a season or more, resulting in a water shortage. It also notes that researchers in the early 1980s counted more than 150 published definitions, reflecting differences in regions, needs and approaches. USGS adds that a farmer, a meteorologist, a water manager and a hydrologist may each mean something different by the word.
This thought experiment imagines a region where the deficit continues year after year. It follows three threads: how streams, reservoirs and groundwater respond; how water use can be reduced; and what it takes for a drought to end.
Explore the idea
Make a prediction
When rain stops for a long time, which shows it first?
When a drought begins, which of these usually shows it first?
According to USGS, where does the first evidence of a drought usually appear?
- Rainfall records
- Water levels in wells
- Lake and reservoir levels
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Established fact
In a long drought, rivers, lakes and wells get lower.
When rain stays below normal for months or years, rivers carry less water, lakes and reservoirs drop and wells must reach deeper (USGS).
Sustained precipitation deficits reduce soil moisture, streamflow, reservoir storage and groundwater levels, with lags from weeks to more than a year (USGS, drought.gov).
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An idea scientists discuss
The dry land in this story is pretend. We imagine what might happen.
The land in this experiment is imagined. What might happen to it is an idea built from what scientists have seen in real long droughts.
The imagined region combines effects that USGS and drought.gov document separately; its outcomes might differ greatly from any real place.
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Established fact
A drought is a long time with much less rain than usual.
A drought is a long stretch, usually a season or more, with much less rain or snow than normal, so water runs short.
Drought is generally defined as a deficiency of precipitation over an extended period, usually a season or more, resulting in a water shortage (drought.gov).
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Established fact
Most dry times are short. Some last for years.
A drought of a few weeks or months is called short-term. One that lasts more than six months is usually called long-term.
Drought.gov treats precipitation deficits of a few weeks or months as short-term drought, and those lasting more than six months as typically long-term.
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Established fact
Rivers and lakes get lower when the rain stays away for a long time.
When rain is below normal for weeks, months or years, rivers carry less water, lakes and reservoirs drop, and the water in wells lies deeper.
Sustained below-normal rainfall reduces streamflow, lowers lakes and reservoirs and increases the depth to water in wells; well levels may not reflect the shortage for a year or more (USGS).
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Established fact
A very big lake in the United States, Lake Mead, got very low in 2022.
In July 2022, Lake Mead, the largest reservoir in the United States, was only 27 percent full, its lowest level since 1937, when it was still being filled for the first time.
On July 18, 2022, Lake Mead's surface stood 1,041.30 feet above sea level at Hoover Dam, 27 percent of capacity and the lowest since April 1937; at the end of July 2000 it had been at 1,199.97 feet (NASA Earth Observatory, using U.S. Bureau of Reclamation data).
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Established fact
Low, warm water can be less healthy for fish.
In a drought, lower and warmer water can grow more algae and hold less oxygen, which is hard on fish and other water life.
Drought.gov notes that decreased water levels, warmer temperatures and runoff can lead to algal growth, lower dissolved oxygen and increased turbidity, posing risks to people and aquatic life.
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An idea scientists discuss
In the pretend land, rivers might get smaller each dry year.
In the pretend land, rivers and reservoirs might keep dropping each dry year, and wells might show it last of all.
The imagined region might see streams and reservoirs fall first and wells later; how far each might drop would depend on its storage, its groundwater and how much water people draw.
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Established fact
A short shower uses less water than a full bath.
A shower uses about 10 to 25 gallons of water, but a bath can use up to 70 gallons.
WaterSense gives 10 to 25 gallons for a shower and up to 70 gallons for a bath, and estimates about 400 gallons a day for a U.S. family of four.
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Established fact
Turning off the tap while you brush saves water.
Turning off the tap while you brush your teeth, morning and night, can save as much as 4 to 8 gallons of water.
Turning off the tap while brushing in the morning and at bedtime can save as much as 4 to 8 gallons, more than 200 gallons a month (WaterSense).
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Established fact
Some gardens get water in small drips, right at the roots.
Microirrigation, such as drip watering, gives water slowly and straight to plants' roots, where it is needed most.
Microirrigation applies water slowly and directly to the root zone; WaterSense also advises against watering in the middle of the day, when the sun evaporates much of it.
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An idea scientists discuss
If everyone in the pretend town saves water, it might last longer.
If the people in the pretend land saved water at home, on farms and in gardens, their rivers and wells might last longer through the dry years.
Combined savings in homes, landscapes and farms might stretch the imagined region's stored water; how much it might help would depend on how large each use is there.
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Established fact
Gentle, soaking rain is the best help for dry land.
Soaking rains help most, because the water sinks into the soil and refills the groundwater that feeds streams. A quick shower or a heavy storm helps much less.
Light showers mostly evaporate or are used by plants, and intense thunderstorms mostly run off; soaking rains infiltrate and recharge groundwater, which sustains vegetation and streamflow between rains (USGS).
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Established fact
Scientists make a drought map every week.
The U.S. Drought Monitor map comes out every Thursday. It shows where it is abnormally dry and four levels of drought, from moderate to exceptional.
The U.S. Drought Monitor, published weekly since 1999, uses D0 (abnormally dry) and D1 to D4 (moderate, severe, extreme, exceptional); it notes that an area in drought for a while usually needs more than one or two rains to end it.
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Established fact
Sometimes one big, rainy storm in the right place can end a drought.
USGS says a single tropical storm, at the right place and time and with the right amount of rain, can break a drought.
USGS notes that one tropical storm at the right place and time, with the right rainfall, can break a drought, but its benefits depend on rain falling upstream of reservoirs, and intense rain can also cause runoff and flooding.
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An idea scientists discuss
In the pretend land, rivers might fill first and wells might fill last.
When the rains return to the pretend land, the soil and rivers might recover first, and the wells might take the longest to come back.
In the imagined region, recovery might also come at different speeds: soils and streams first and groundwater last, possibly long after the rains return.
In our world
In most places, rain and snow come again and again. Rivers fill up. Plants drink from the wet soil.
Some years are a little drier. Then the rain comes back.
Rain and snow fall unevenly. USGS notes that the deserts of the American Southwest get less than 3 inches of precipitation in an average year, while Atlanta gets about 50 inches. Each place has its own normal, so a drought is measured against what is usual there.
The amount changes from year to year, but over many years the average stays fairly steady. Drought.gov calls drought a normal climate pattern that has happened again and again through history, sometimes short and mild, sometimes long and severe.
USGS points out that precipitation at any site varies from year to year but averages out fairly steadily over a period of years, and that normals differ hugely: under 3 inches a year in Southwestern deserts versus about 50 inches in Atlanta. Drought is therefore judged against local normal conditions.
Drought.gov separates short-term drought, lasting a few weeks or months, from long-term drought, which typically lasts more than six months. Wet spells can interrupt a drought, and dry spells can interrupt wet periods. Drought.gov reports that between 1895 and 2010, on average around 14 percent of the United States was in severe to extreme drought in any given year.
Imagine years without enough rain
Now imagine no big rains for years. In our pretend land, streams get smaller. Lakes go down. Fields get dry.
People, plants and animals must share less water.
Now imagine a pretend region where the rain stays low year after year. USGS describes what happens when rain is below normal for months or years: streams and rivers carry less water, lakes and reservoirs drop, and the water in wells sits deeper underground.
In the pretend region, grass and crops might dry out first, and later even trees might lose branches. People might pump more groundwater to make up for it. These are ideas, but each one is something drought.gov describes in real droughts.
In the imagined region, the deficit might appear in the order USGS describes: first in rainfall records, soon after in soil moisture, then in streams and reservoirs after several weeks or months, and in well levels a year or more after the drought begins.
Drought.gov describes how the damage builds as a drought goes on: shallow-rooted grasses and crops such as corn and wheat suffer first; the branches of woody plants may die back; crops and forage can fail completely; and groundwater pumping often increases, which can deplete aquifers and, near coasts, let seawater into them. Which of these the imagined region might face would depend on its soils, plants and water supply.
Why the land dries out
Sun and wind dry the ground. Plants also let water go into the air from their leaves.
When little rain falls, the ground loses water faster than it gets it back.
Rain is only half the story. Some water goes back into the air when it evaporates from lakes and damp soil, and plants pull water from the soil and let it out through their leaves. Scientists call these together evapotranspiration.
USGS explains that sunshine, heat, wind and dry air speed up evapotranspiration, so soils dry out. In cool, cloudy weather, a little less rain than normal might not cause a drought at all. That is why a hot, windy dry spell can be much worse than a cool one.
Drought develops from the balance between supply (precipitation) and loss. USGS describes evapotranspiration, the sum of evaporation from water and soil and transpiration from plants, as depending on sunlight, humidity, temperature and wind. When it is high, soils lose moisture quickly; during cool, cloudy weather it may be small enough to offset below-normal precipitation, so a drought may be milder or may not develop.
Drought.gov notes that flash droughts develop quickly when low rainfall combines with unusually high temperatures, winds or incoming radiation. Long droughts usually creep instead: drought.gov says drought is often described as a 'creeping phenomenon', and its beginning and end are hard to mark.
Long droughts for real
Real droughts have lasted for years. A huge lake called Lake Mead got very low.
Scientists make a new drought map every week. Families help by saving water at home.
Long droughts are real. USGS notes that in the 1930s most of the United States was much drier than normal; in Missouri the dry spell lasted from 1930 to 1941. In July 2022, NASA reported that Lake Mead, the largest reservoir in the United States, was only 27 percent full, its lowest level since 1937, when it was still being filled for the first time.
Scientists keep watch. Every Thursday the U.S. Drought Monitor publishes a map showing where drought is and how bad it is. Families help too: EPA's WaterSense program says turning off the tap while you brush your teeth, morning and night, can save as much as 4 to 8 gallons of water.
A 2017 study by NOAA scientist Richard Heim, summarized by NOAA NCEI and drought.gov, identified 13 major U.S. drought episodes between 1900 and 2014. The three longest ran from July 1928 to May 1942, July 1949 to September 1957, and June 1998 to December 2014; each covered 60 percent or more of the contiguous United States at its peak and lasted 99 months or longer. Tree rings indicate that droughts like those of the 1930s have occurred occasionally over the last 2,000 years.
Monitoring makes such events visible. The U.S. Drought Monitor, produced since 1999 by the National Drought Mitigation Center, NOAA and USDA, blends precipitation, streamflow, reservoir, soil-moisture and vegetation data with input from more than 450 experts into a weekly map running from D0 (abnormally dry) to D4 (exceptional drought). It looks back at recent conditions; it is not a forecast.
Amazing facts
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Scientists have written lots of different ways to say what a drought is.
Drought is so hard to pin down that, in the early 1980s, researchers found more than 150 published definitions of it.
Researchers in the early 1980s counted more than 150 published definitions of drought, reflecting differences in regions, needs and approaches (drought.gov).
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Tree rings can show dry years from long, long ago.
Tree rings show that droughts like the ones of the 1930s have happened now and then over the last 2,000 years.
Tree-ring archives indicate that agricultural droughts like those of the 1930s Dust Bowl era have occurred occasionally over the last 2,000 years (drought.gov).
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When Lake Mead got low, pale rings showed on its shores.
When Lake Mead dropped, satellites saw a pale 'bathtub ring' of mineral-coated shore that used to be under water.
Landsat images of Lake Mead in 2021 and 2022 show light-colored, mineralized shoreline that had been under water when the reservoir was fuller, often called a 'bathtub ring' (NASA Earth Observatory).
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For a farmer, even two weeks with no rain can be hard on crops.
USGS says that to a farmer, even two weeks without rain can stress many crops at certain times in their growth.
What counts as drought depends on perspective: USGS notes that even two weeks without rainfall can stress many crops during certain stages of growth.
Words to know
- drought
- a long period with much less rain or snow than normal, so that water runs short
- evapotranspiration
- water returning to the air by evaporating from soil and water and by passing out through plants' leaves
- reservoir
- a lake, often made by a dam, where water is stored for people to use
- groundwater
- water stored underground in the spaces between grains of soil and rock
- soaking rain
- steady rain that sinks into the ground instead of running off
Discovery challenge
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Which can take a year or more to show a drought?
Which of these can take a year or more to show that a drought has started?
According to USGS, which of these may not reflect a rainfall shortage for a year or more?
- Water levels in wells
- Rainfall records
- Soil moisture
Show the answer
Water levels in wells
Wells are often the last to show a drought.
Rainfall records show a drought first and soil dries soon after, but wells can take a year or more.
Well levels may lag the rainfall deficit by a year or more; rainfall records and soil moisture respond first (USGS).
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Why does one rain shower not end a long drought?
Why doesn't one rain shower end a long drought?
Why does a single light shower give only short-term relief in a drought?
- Most of its water dries up or is used by plants before it soaks deep
- Rain cannot fall during a drought
- Rain makes rivers smaller
Show the answer
Most of its water dries up or is used by plants before it soaks deep
The water dries up or plants drink it before it soaks deep down.
A quick shower mostly evaporates or is used by plants. Many soaking rains are needed to refill the ground.
In the growing season most of a light shower evaporates or is taken up by plants, so little reaches groundwater (USGS).
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Which uses less water?
Which uses less water, according to EPA's WaterSense program?
WaterSense compares a shower (10 to 25 gallons) with a bath. Which uses less?
- A shower
- A full bath
- They use the same
Show the answer
A shower
A shower uses less water than a full bath.
A shower uses about 10 to 25 gallons; a bath can use up to 70.
A shower uses 10 to 25 gallons; a bath up to 70 (WaterSense).
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What is the U.S. Drought Monitor?
What is the U.S. Drought Monitor?
Which description fits the U.S. Drought Monitor?
- A weekly map of where drought is and how bad it is
- A machine that makes rain
- A forecast of next year's weather
Show the answer
A weekly map of where drought is and how bad it is
It is a map that shows where it is dry. A new one comes out every week.
It is a map published every Thursday that shows where drought is and how bad it is.
A weekly map, from D0 to D4, that looks back at recent conditions; it is not a forecast.
Experiments to try
- Watch What If? Lab What if a town used up its groundwater? Below the ground, water fills the tiny gaps between grains of rock and sand. In the United States, this groundwater is the drinking water for about half of all people. Rain and snow refill it, but often slowly. Imagine a town pumping faster than nature refills it, and follow what might change and how people help.
- Watch What If? Lab What if a river flooded every year? For thousands of years, the Nile in Egypt flooded every year, spreading fresh mud across its banks. That mud made the land rich for farming, in the middle of a desert. Imagine a river that flooded every year: you could plan for it, farm on it, and build banks called levees to control it. Many of the earliest towns grew up beside rivers like this.
- Watch What If? Lab What if cold wind blew over a warm lake? In late fall and winter, the Great Lakes are often much warmer than the freezing air that often blows in from Canada. As cold wind crosses the open water, it picks up warmth and moisture, rises and builds narrow bands of clouds that can drop 2 to 3 inches of snow an hour, while a town a mile or two away stays sunny. This thought experiment follows the air across the lake.
- 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 Try it What If? Lab What if a habitat lost its water supply? Water makes plants grow, and plants feed animals. In our model, a dry spell cuts the plant food from 50 tonnes a year to 20, so the land can feed far fewer deer. Watch the herd shrink to fit, then find out how drought affects real wildlife and how people help.
Explore next
- Watch What If? Lab What if a town used up its groundwater? Below the ground, water fills the tiny gaps between grains of rock and sand. In the United States, this groundwater is the drinking water for about half of all people. Rain and snow refill it, but often slowly. Imagine a town pumping faster than nature refills it, and follow what might change and how people help.
- Watch What If? Lab What if a river flooded every year? For thousands of years, the Nile in Egypt flooded every year, spreading fresh mud across its banks. That mud made the land rich for farming, in the middle of a desert. Imagine a river that flooded every year: you could plan for it, farm on it, and build banks called levees to control it. Many of the earliest towns grew up beside rivers like this.
- Watch What If? Lab What if cold wind blew over a warm lake? In late fall and winter, the Great Lakes are often much warmer than the freezing air that often blows in from Canada. As cold wind crosses the open water, it picks up warmth and moisture, rises and builds narrow bands of clouds that can drop 2 to 3 inches of snow an hour, while a town a mile or two away stays sunny. This thought experiment follows the air across the lake.
- 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.
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Watch
Natural wonder
Salar de Uyuni
Salar de Uyuni is a giant salt flat high in the Andes of Bolivia. It covers 10,582 square kilometers and was left behind when ancient lakes dried up. NASA calls it the largest salt flat in the world.
- Watch Try it What If? Lab What if a habitat lost its water supply? Water makes plants grow, and plants feed animals. In our model, a dry spell cuts the plant food from 50 tonnes a year to 20, so the land can feed far fewer deer. Watch the herd shrink to fit, then find out how drought affects real wildlife and how people help.
- 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.
For parents & teachers
What children will learn
- A drought is a long period of below-normal precipitation that leads to a water shortage, judged against what is normal for a place.
- Drought shows up in rainfall records first, then in soil, streams and reservoirs, and in groundwater last.
- Heat, wind and dry air speed up evapotranspiration and make drought worse.
- Saving water stretches supplies, and ending a long drought takes many soaking rains.
Recommended ages: 9 to 17 · About 15 minutes
Talk about it
- Why might the same amount of rain be a drought in one place and a normal year in another?
- Why do wells take so much longer than soil to show a drought?
- Which ways of saving water could your family try this week, and how could you measure the difference?
Try at home or in class
- Try EPA WaterSense's tuna-can test with a grown-up: set a few empty cans on the lawn while the sprinkler runs and time how long it takes to collect half an inch of water.
- Time your shower with a clock and try to keep it under five minutes, as WaterSense suggests.
- Fill two identical cups with the same amount of water, put one in a sunny window and one in a shady spot, and mark the levels each day to compare how fast they evaporate.
Key vocabulary: drought, evapotranspiration, reservoir, groundwater, soaking rain
Where these facts come from
- Drought Basics , National Integrated Drought Information System (Drought.gov)
- Drought Timescales: Short- vs. Long-Term Drought , NOAA National Integrated Drought Information System (drought.gov)
- Historical Drought , NOAA National Integrated Drought Information System (drought.gov)
- A Historical Perspective on Drought , NOAA National Centers for Environmental Information
- Monitoring Drought , NOAA National Integrated Drought Information System (drought.gov)
- Water Utilities , NOAA National Integrated Drought Information System (drought.gov)
- Droughts: Things to Know , USGS Water Science School
- What is the USDM , U.S. Drought Monitor, National Drought Mitigation Center, University of Nebraska-Lincoln
- Lake Mead Keeps Dropping , NASA Earth Observatory
- WaterSense Kids , U.S. Environmental Protection Agency, WaterSense
- Watering Tips , U.S. Environmental Protection Agency, WaterSense
Checked, not yet human-reviewed. Last updated October 26, 2026. Spotted a mistake? Tell us.