that some geographers would say is the most  important to understand all life on Earth:  

the map of the world's precipitation. 

We can think of precipitation as the final  flourish in the hydrological cycle that  

circulates water molecules between the  four Earth systems. It's all the rain,  

snow, sleet, hail or any liquid or solid  that falls from clouds in the atmosphere. 

And we need water for so many parts  of our lives: agriculture, industry,  

transportation, recreation, and not least  for all the flora and fauna that live here. 

Water is the universal solvent, which  means it can dissolve more substances  

than any other liquid and we almost  never find completely pure water.  

That's pretty important, because as  water moves through the water cycle,  

it transports both vital nutrients and  harmful pollutants across spaces and places.

So using a map of precipitation helps us  track water on Earth and reveals potential  

consequences of differing access to water.  Like if we compare our precipitation map with the  

map of population distribution, we can understand  a simple but powerful pattern of human geography:   

where there is water, there are people. 

But it gets a little more complicated than  that, because where there are people and  

limited resources, there's often conflict  and bigger geographical questions at stake. 

I'm Alizé Carrère and this  is Crash Course Geography.

[INTRO]

We started our journey into physical  geography by looking at the big,  

big picture to reveal the geographic  patterns and processes that create  

Earth's environments and  support all living things. 

And we've learned that the spheroid shape,  

rotation, revolution, and tilt of the Earth  cause insolation, air temperature, pressure,  

and wind to form worldwide patterns  that strongly depend on latitude.

Ultimately, precipitation comes  from clouds in the atmosphere,  

which are complex structures that  change based on many of those patterns.  

So precipitation varies a lot between different  places, especially different latitudes. 

Looking at our map, there are  areas with a lot of precipitation,  

like the island we call Borneo. The air here near  the equator is hotter and has a higher dew point,  

the temperature when the air is saturated with  water vapor and condensation is imminent. 

But areas like what we call the Svalbard  islands in the Arctic Ocean get very  

little precipitation because the air  at those latitudes is cold and dry. 

A different kind of precipitation variability  can happen within a place that spans similar  

latitudes. Like there's a region that makes  up a large chunk of the continental interior  

of the US and Canada, often called  the Great Plains or the Prairies. 

Because the Great Plains sit  deep within the interior,  

far from oceans, a phenomenon  called the continental effect causes  

huge temperature fluctuations with  scorching summers and frigid winters. 

In addition, the Rocky Mountains, which are west  of the Great Plains, form a barrier to the warm,  

moist winds blowing in from the Pacific.  Basically, as the winds hit the side of the  

mountains, the air is forced to rise.  As it rises, the air expands and cools  

enough that the water vapor molecules can  condense to form clouds and precipitation. 

  The resulting rain or snow (or fog or whatever!)  

is called orographic precipitation, which got its  name from “oros” -- a Greek word for mountain.

Then, as the air descends the  other side of the mountains,  

it gets warmer as the air molecules are  compressed together, and any leftover water  

droplets evaporate. So we say the side of the  mountains not facing the winds -- like where  

the Great Plains are -- is in a warm, dry area  called the rain shadow of the Rocky Mountains. 

Orographic precipitation patterns  can be found on mountains worldwide.  

In Argentina, the Patagonia desert lies  in the rain shadow of the Andes Mountains,  

while the Trans-Himalayan region of Tibet  and Central Asia lies in the rain shadow  

of the Himalaya Mountains. The Great Plains straddle the  

98th meridian. So there's precipitation  sometimes... it's just unpredictable. 

The result is a steppe or semi-arid  climate which is too dry to support forest,  

but too moist to be a desert., 

The dryness can be linked to some combination of  the continental effect, the rain shadow location,  

and subtropical high pressure systems in  the atmosphere. The unpredictability comes  

from local conditions and the constantly flowing  atmospheric and ocean circulation. So, basically,  

rainfall amounts can change dramatically from  one year or season or month to the next. 

A year with lots of rain could be followed by  several years of below average precipitation.  

So drought can be a major, recurring  problem for people, animals, and plants.

In fact, at one point the Great Plains  was called the Great American Desert,  

even though it's technically a stretch  of grasslands between forests to the  

east and deserts to the west. By  thinking about the physical space,  

we learn more about how our perception  of the perceived space has changed. 

It wasn't until the inventions  of barbed wire, the steel plow,  

well-drilling techniques, and  the railroad solved the region's  

unique spatial problems that it became  a place where European people settled. 

Over long periods of time, grasslands produce  excellent soils, making them extremely productive  

farmlands -- but also prone to severe soil  erosion from overfarming and overgrazing. 

So the combination of mass settling and  farming, unpredictable precipitation,  

and high temperatures led to  devastation in this physical space.  

One of the most significant droughts in the last  century was the Dust Bowl, which ravaged the  

Great Plains for nearly a decade from 1930 until  the fall of 1939, when the rains finally came. 

So now our idea of the Great Plains as a place  is forever tied to hardship and even lack of  

opportunity, not just precipitation patterns  like the rainfall effect. All because of the  

physical geography of the space layered with the  human geography of our lived experiences there. 

On the other side of the Rockies  and east of the Sierra Nevadas,  

there are also vast stretches of semi-arid  regions, with some true deserts in the southwest.  

Like in the Great Plains, precipitation is  rather unevenly distributed, so the Colorado  

River is actually the region's largest water  source, dubbed the "lifeline of the southwest." 

Actually ninety percent of the surface water in  

the Colorado River comes from  snow in the Rocky Mountains,  

which melts, flows down a network of smaller  tributary streams, and reaches the main river. 

The challenges of relying on water  from one source like a river,  

instead of from widespread rainfall,  involve both who needs the water and  

where they are relative to the source.  Managing water resources is a spatial problem. 

For example, the biggest water-users are farms,  factories, and towns, but they aren't all on  

the riverbanks, so they have to find ways to  transport the water they need.And those that  

live near the upstream parts of the Colorado  River can use a ton of water if unregulated,  

leaving less for anyone who lives downstream. 

And the Colorado River is well, a river. So  as it winds from its source to its mouth,  

it's unaware of any political boundaries, like  those from counties, Native American tribal land,  

or even international boundaries. But the  humans that have made this semi-arid region  

their home generally use all these political  boundaries for decision making about water.

Understanding why precipitation is unevenly  distributed, how drought can change seasonally,  

and how people use water are key parts of  geography. And the intersection between  

these physical geography processes and  human geography decision-making can be  

the source of a lot of tension -- especially  when it comes to environmental policies.

For example, Native Americans have used  the Colorado River's water and managed  

its resources for thousands of years.  But the modern legal doctrine that governs  

water rights in the West -- which goes back  to the Gold Rush of the 1840s and 50s in some  

places -- is the “prior appropriation doctrine.”  This doctrine allocates rights based on who  

started using the water first...except  traditional Native American claims. 

Then in 1922 the seven states  of the Colorado River Basin  

drew up the Colorado River Compact  on how to divide the waters, because  

seasonal precipitation alone wouldn't provide  enough water for everyone that lived there. 

But they overestimated the flow of the  river and didn't account for how the  

amount of water varies year-to-year. So  each state was allocated more water than  

actually exists -- a problem that's led  to intense legal battles between states. 

Like California is a downstream  user but also a very powerful state,  

and for decades was using more water  than it was allocated. So in 2003,  

after threats that their water would be cut  off, California agreed to reduce its use of  

Colorado River water over the next 14 years  to allow the upstream states their share. 

The other big problem was that the river's  natural flow had to be physically moved to  

suit certain human wants and needs,  which made things even less fair. 

Like two huge dams -- the Hoover dam on the  Arizona-Nevada border and the Glen Canyon dam  

in Arizona --  were built in the 1930s and 60s  to store and hold back water in reservoirs. 

Lake Mead, the reservoir for the Hoover dam,  supplies water to 25 million people in California,  

Arizona, and Nevada, and generates  hydroelectricity for the region.  

But all that water still has to be  divided between different needs. 

And since the 1950s, explosive  urban growth in upstream states,  

like the growth of cities like Phoenix  and Las Vegas, meant skyrocketing demand  

for water there -- and, as a consequence, less  water for farmers and other rural communities.

Basically, early miscalculations and  mismanagement has created a water  

crisis that affects 40 million people  and 5.5 million acres of farmland.

As of 2021, privatizing water rights is being  proposed as a new solution. Private investors  

would buy water rights, and cities, states,  and individual farms could buy water from them,  

even across state lines. This way the market  would decide whether water was more valuable  

serving urban or rural populations, redefining  the century-old rules for sharing the river. 

So yes, this would turn the river water into  a commodity that could be bought and sold.  

Private investors would redraw the map of water  distribution in the West and make a profit.

But while water management enters a new phase  with big players from Wall Street staking a claim,  

Native American tribes are still working to obtain  

their water rights that they  were finally awarded in 1908.

There is no substitute for water. Today we  mainly focused on one region with one kind  

of precipitation pattern, but we still saw how  studying precipitation opens up lots of deeper  

questions in geography, from what makes a region  habitable to political struggles over resources. 

Some of the most serious geopolitical  issues in the Middle East, North Africa,  

South Asia, and the Western US  relate to control over water.

Like water shortages affect public health,  reduce agricultural productivity, and  

damage ecological systems on which we depend.   So, really, that map of the Earth's precipitation,  

and where and how much water falls from  the sky, is the foundation of a bunch  

of big geographical questions: who should  control water? Is water a basic human right?  

And how have humans altered the environment  to get the water we need -- and at what cost?  

As geographers, we'll keep looking for new  answers in the stories and patterns of the Earth.

Like next time when we'll look at cyclonic  systems that bring dramatic weather and spatial  

implications affecting human activities like  where we build our homes and choose to live.

Many maps and borders represent modern  geopolitical divisions that have often  

been decided without the consultation, permission,  or recognition of the land's original inhabitants.  

Many geographical place names also don't reflect  the Indigenous or Aboriginal peoples languages.  

So we at Crash Course want to acknowledge these  peoples' traditional and ongoing relationship  

with that land and all the physical  and human geographical elements of it.

We encourage you to learn about  the history of the place you call  

home through resources like native-land.ca  and by engaging with your local Indigenous  

and Aboriginal nations through the  websites and resources they provide.

Thanks for watching this episode of Crash  Course Geography which is filmed at the  

Team Sandoval Pierce Studio and was made with  the help of all these nice people. If you want  

to help keep all Crash Course free for everyone,  forever, you can join our community on Patreon.