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Lecture 3a - Water on Earth
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From the course by University of Arizona
Biosphere 2 Science for the Future of Our Planet
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The Science of Water Availability
Where is Earth’s water? In Module 3, you will learn about the global water cycle and discover how much water exists, where it exists, and how it moves around the environment. Learn how Biosphere 2 and Critical Zone research are used to study the water cycle. With Dr. Peter Troch, Director of Science at Biosphere 2, UA Hydrology.
Meet the Instructors
Kevin E. Bonine, PhDDirector of Education and Outreach
Biosphere 2
Hi, my name is Peter Troch.
I'm a professor in
the Department of Hydrology and Atmospheric Sciences at the University of Arizona,
and I'm also Biosphere 2 Science Director.
My lecture series deals with the science of water availability.
And in this first lecture,
we will address the question of how much water is there on Earth.
When we look back from space to planet Earth,
what we see is a lot of water,
that's why we call it the water planet.
There's water in the oceans,
water in the atmosphere,
water on the poles,
in terms of ice caps et cetera.
Most of that water is actually not available for human consumption though.
Let's first see how much water exactly is available on earth.
All water has a total volume of more than one billion cubic kilometers,
that's enormous amount of water.
But if you would be able to isolate all that water in its own water planet,
strip it from earth,
and bring it back into a sphere,
that sphere would only have a diameter of about 700 kilometers.
When we project that water planet on top of the dry earth,
it shows that basically in perspective,
the amount of water with respect to the size of our planet is relatively small.
The, all water in our planet would occupies a sphere with a 860 miles diameter.
That's all water, but not all water is fresh water.
Only 2.5% of all water in the planet is fresh water.
Of that 2.5% about 70% of that fresh is locked up in glaciers,
permanent snow pack and permafrost.
The other 30% is stored in aquifers,
and if we pump aquifers,
we basically mine water because of recharging of aquifers takes a lot of time.
So that means that leaves only 1% of all fresh water as surface water,
that means water that flows through
our rivers and is available in lakes and ponds and therefore,
readily available for human consumption.
That fresh water at the surface takes part in the hydrological cycle.
Water evaporates from the oceans,
condenses into the atmosphere,
precipitates over land in the form of snow or rain,
and runs off to our rivers and lakes and returns back to the ocean.
That cycle is called the hydrological cycle.
Some of the water will infiltrate and be available for plants
to take up through their roots and transpire back into the atmosphere,
and some of that water will percolate to deeper aquifers and
return to the ocean over long time periods.
So, of all the freshwater,
99% in the hydrological cycle is
surface water and only one percent returns to the ocean as groundwater.
The hydrological cycle is coupled to the carbon cycle on Earth.
And understanding the carbon cycle on earth is important to
understand the effects of increasing carbon dioxide and global warming.
The way the hydrological cycle and the carbon cycle
are coupled is through transpiration by plants.
When plants take up carbon through photosynthesis,
they lose water through their stomata,
and that process is called transpiration.
When the carbon that is taken up by plants is stored in
their biomass above ground and below ground,
and below ground microbes decompose
the biomass and return carbon back into the atmosphere through respiration.
Similar processes occur also in the ocean where
phytoplankton takes up carbon through similar processes as photosynthesis.
Those two cycles are powered by the sun.
The Earth receives a certain amount of energy from the sun,
on top of the atmosphere that amount is about 340 Watt per square metre.
About 30% of that is reflected back into space.
So, the surface of the Earth has the reflectivity of about point three,
which means 30% of the incoming solar radiation is reflected back to space.
The rest is used to warm up our planet,
and at a certain temperature of the planet,
it emits thermal radiation and that's about 240 Watt per square meter.
So, that energy balance of incoming and outgoing
radiation at the planetary level is balanced,
but of course any changes that we make in our atmosphere to capture more heat
or in the reflectivity of
our planet in terms of more cloudiness will change that balance,
and the amount of carbon dioxide increase in
our atmosphere has resulted in increasing temperature of the earth,
which leads to global warming.
Returning back to the question of how much water is available.
As hydrologists, we make
calculations about water that is potentially available from a catchment.
This slide introduces or defines what we mean by catchment.
A catchment is the land that drains water to a certain point along a river.
So when we look back to our landscape and we are interested
in the discharge running through this river at this point,
then the green line indicates the land surface that
will drain water to that particular point along the river.
The area of the catchment will define define
how much water can potentially be captured at that point when it rains.
We can use simple calculations to compute how much water would be
available at that particular point along the river
for a given storm size and a given catchment area.
So, let's assume that we receive a storm of 10 millimetres,
and that our catchment area is 10 square kilometers.
You are familiar with the fact that we report
rainfall in millimeters or in inches by listening to the weather forecast.
Now, if you pour one liter of water on the surface of one square meter,
you will fill up a depth of one millimetre or a thousands of a cubic meter.
So given that fact,
we now know that 10 millimeter would be 10 litres per square metre,
and that 10 square kilometers is 10 times thousand,
times a thousand square meters,
multiplying those two numbers will gives us the amount of
water that potentially will flow through the river.
In our example, that would be a 100,000 cubic metres,
enough water to fill up 200 swimming pools of 25 metre long,
that's a lot of water that is available.
Now, not all rainfall will be available as discharge in a river.
Some of that rainfall will infiltrate into the soil as we mentioned earlier and
be available for plants to taken up by their roots,
and be transpire back into the atmosphere,
so that water will not be available in the river.
Other rainwater can be stored in the soil,
or in ponds and lakes and then from those storages,
it can evaporate directly into the atmosphere.
So, that water will also not be available in the river.
So, we can now make a simple balance between what we'll receive in terms of rainfall P,
what we lose in terms of evaporation E,
and what we lose in terms of transpiration T,
and the rest will be available for river discharge.
The Deltas S indicates the change in storage of water in
the catchment and you can think of that as
the level of lakes or ponds or the wetness of the soil.
So that simple water balance can now be used to make simple computations
about how much water we can expect from
a certain catchment area in a certain river during rainfall.
That amount will strongly depend on the type of climate where we live.
We can classify climates as humid versus arid.
From a hydrological perspective, in humid catchments,
there's a lot of rainfall and
potentially less energy available for evaporating water and so,
the rest term assuming there are no changes in storage,
is the discharge, which it can potentially be large in humid catchments.
So, that's why we expect in humid areas to see river flow throughout most of the year.
On the other hand in areas catchments,
there is less rainfall,
most of that will evaporate because of high energy
availability and only a small fraction will flow through the rivers.
That's why we expect in arid catchments such as Tucson Arizona,
not to have water available year round.
We can classify catchments as a function of the climate, and when we do so,
we can use a aridity index,
which is the ratio of the amount of
energy available for evaporating water versus the amount of rainfall.
So, in arid catchments that ratio will be large and larger than one,
and in humid catchment that ratio will be less than one.
And when we collect data from different catchments,
what we notice is that the evaporation fraction
is basically the ratio of amount of evapotranspiration over rainfall,
follows a specific pattern,
which we call the Budyko curve.
With that Budyko curve,
we are now able to predict or estimate,
given a certain aridity index of a catchment,
how much water will evaporate
proportional to the amount of rainfall that catchment receives.
Alternatively, since ET plus P,
minus ET, minus Q, is zero,
we can also express that as a function of a runoff coefficient.
So, with that, we can estimate the amount of water availability of a certain catchment.
So that concludes the first lecture,
The Science of Water Availability.
In next lecture, we will address the question of
how much water we actually use. I'll see you next time.
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