P R O J E CT G U T S
W AT E R R E S O U R CE S
Water Pumping Model
in StarLogo TNG
Introduction
The Hydrologic cycle
The hydrologic cycle begins with the evaporation of water from the surface of
the oceans and the land. Moist air is lifted as the land heats up during the day,
the moist air cools and condenses into water vapor and forms clouds. Moisture is
transported around the globe as clouds until it returns to the surface as
precipitation.
Once the water reaches the ground, one of two processes may occur; 1) some
of the water may evaporate back into the atmosphere or 2) the water may fall
as rain or snow, penetrate the surface of the earth, and become groundwater.
Groundwater either seeps back into the oceans, rivers, and streams, or is
released back into the atmosphere through transpiration (the evaporation of
water into the atmosphere from the leaves and stems of plants). The balance of
water that remains on the earth's surface is runoff, which empties into lakes,
rivers and streams and is carried back to the oceans, where the cycle begins
again.
Figure 1: Zones of an Aquifer (image credit: http://library.thinkquest.org/04oct/01590/humans/sources.html)
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INTRODUCTION
Fresh water is a limited resource. The amount of fresh water supply provided by
the hydrological cycle does not increase. Water everywhere on the planet is an
integral part of the global hydrologic cycle.
Aquifers:
An aquifer is a body of saturated rock and
silt through which water can easily move.
Aquifers are both permeable and porous.
There are three different types of aquifers:
confined, unconfined, and perched. A
confined aquifer has at least one aquitard,
or clay layer, at its top and, if it is stacked
with others, an aquitard at its base. Multiple
confined aquifers can be stacked on top of
one another and separated by aquitards. A
unconfined aquifer has a aquitard only at
its base. The top of an unconfined aquifer is
the water table.
Figure 2: A simple aquifer (cut away view)
http:wikipedia.org/aquifer
Aquifers have Recharge zones where water enters an aquifer through rain,
snowmelt, river and resevoir leakage, and Discharge zones where water leaves
an aquifer such as near springs, wetlands, and also from wells. Aquifers cqn be
part of a larger system; their boundaries can be gradations into other aquifers.
Aquifers are filled with moving water and the amount of water in storage in an
aquifer can vary from season to season and year to year. Ground water flow
through an aquifer can be fast or slow depending on the permeability. Water
will eventually discharge or leave an aquifer and must be replaced by new
water to replenish or recharge the aquifer. Thus, every aquifer has a recharge
zone or zones and a discharge zone or zones.
Aquifers are usually good options for water supplies because in natural systems,
the water is filtered through hundreds—sometimes thousands—of feet of dirt and
sediment before it enters the aquifer and is usable. Water often cannot be
pumped out of the ground until it reaches the aquifers because it is not
concentrated enough. However, when water reaches an aquifer, it saturates
the sediment and can be pumped out.
INTRODUCTION
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Wells:
A well is a hole drilled into the ground to penetrate an aquifer. Wells are drilled
into aquifers to harvest water. Normally well water must be pumped to the
surface. If water is pumped from a well faster than it is replenished, the water
table is lowered and the well may go dry. When water is pumped from a well,
the water table is generally lowered into a cone of depression at the well.
Groundwater normally flows down the slope of the water table towards the well.
The drilling of millions of irrigation wells has pushed water withdrawals beyond
recharge rates. The failure to limit pumping has resulted in water tables that are
now falling. Most of the world's aquifers are replenishable, meaning that when
they are depleted, the maximum rate of pumping will be automatically
reduced to the rate of recharge. Others such as fossil aquifers, however, are not
replenishable.
Impact on Food production:
Farmers who lose their irrigation water must either farm different crops or use
alternative techniques such as dry-land farming (i.e. depend on rainfall) or
import water (which is often energy intensive and expensive). In the desert
climate of the southwestern United States, the loss of irrigation water would
mean the loss of agriculture. In the United States, three leading grain-producing
states (Texas, Oklahoma and Kansas), have underground water table levels that
have dropped by more than 30 meters (100 feet) over the past 60 years. As a
result, wells have gone dry on thousands of farms in the southern Great Plains.
For more information see Chapter 4, "Emerging Water Shortages," in Plan B 3.0:
Mobilizing to Save Civilization. www.earth-policy.org/Books/PB3/pb3book.pdf
Figure 3. A diagram of a water table, an unconfined aquifer, a confined aquifer, wells, and the level to
which water would rise by the force of hydraulic pressure in a confined aquifer.
(http://www.waterwatchalliance.us/ch3.html)
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INTRODUCTION
The Base Model:
The base model for this unit is an “idea” model that simulates a part of the
hydrological cycle in which water falls as rain, seeps into an aquifer, and is
pumped out by one pump. We will walk through each part of the model, run
experiments to understand the model, then give tips on how to alter model to
reflect different conditions.
In the base model, called pump04rain.sltng, “setup” initializes the program.
Within the setup procedure, 1000 turtles are created to represent water
molecules, their color is set to blue, and they execute a turtle procedure called
“fill”.
INTRODUCTION
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In the fill procedure, the goal is to have
each turtle agent find its random starting
position in the brown part of the
background. The setxy block is used to
randomly select an x and y position and
the if/then block is used to determine if the
selected position meets the requirement
by being the location of a brown patch.
The fill procedure uses a technique called “recursion” to find a position for the
water molecule. Recursion is when a procedure calls itself. In this case, the fill
procedure calls itself if the patch color where the water molecule is positioned is
not brown. In effect, it works like this: first time through setxy puts the water
molecule at (20, 34) the color at that patch is cyan (not brown) so the
procedure calls itself. The next time through, setxy puts the same water
molecule at (-34, 29), the color of the patch there is cyan so the procedure calls
itself again. The third time through setxy puts the same water molecule at -38, 23 and this time, the color of the patch there is brown and the procedure exits
with the water molecule positioned on a brown patch. Note that each
molecule goes through a similar process to find a starting position on a brown
patch.
In the “forever” procedure that runs the
simulation, there are two procedures called
each time through the “forever loop”. The first
is called “gravity” that simulates the pull of
gravity on water molecules, and the second is
called “pump” that simulates groundwater
being pumped out of an aquifer.
In the gravity procedure, each turtle agent as water molecule moves
downward by a simulated force of gravity. The basic concept is that if the
water molecule is in the ground, it moves slowly downward unless impeded by
another water molecule whereas if the water molecule is in the air, it moves
rapidly towards earth as a raindrop.
The first if/then statement in the gravity procedure checks to see if the water
molecule is positioned at the top of the screen. If so, it is made visible. Next, the
water molecule is given a heading that is mostly downward but has some
probability of facing off slightly to the left or right. Afterwards, the patch color of
the patch ahead of the agent is checked.
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INTRODUCTION
If it is brown, then the water
molecule moves forward one step
and tests if that space is already
occupied by another water
molecule. If so, then the water
molecule must step back one
step to avoid overlapping with
another water molecule. The smell
block is used to determine if
another water molecule is already
there. If the patch color of the
patch ahead is not brown, then
we check to see if it is cyan,
representing sky. If so, the water
molecule drops straight down at a
rapid rate.
The pump procedure simulates
water being pumped at a well.
The procedure simply tests to see if
the patch color directly ahead of
the water molecule is yellow
(indicating the wellhead) and if so,
the water molecule is hidden and
repositioned at a location at the
top of the screen where it will
“become rain”.
Using the model:
1) simply run the model by first clicking the “setup” button, then clicking the
“forever” button. What do you observe?
2) Can you identify an emergent pattern that develops?
3)
4)
5)
6)
How would you use this model to test a hypothesis?
What is your hypothesis? (in the form of a question)
How would you test the hypothesis?
What would you change? How?