Forecasting water future for
Phoenix Metropolitan Area using
Water Sim, an on-line program
Authors: Lance Ward, Mitchell Little, Sean Hayes, Ethan Vickers,
Tiffany Cogan, Jessica Laurino, Jessie Giacomelli, Dustin C.
Sandberg, Alyssa Ralls, Jessica Kashian, Clayton Kammerer, Jessica
Woody, and Andrew Axelband
A class paper by the students enrolled in UNVR 195a-002, Water
Resources in the Tucson Basin, The University of Arizona, spring
2008. Instructor: James J. Riley
May 1, 2008
2
TITLE: Forecasting water future for Phoenix Metropolitan Area
using Water Sim, an on-line program
AUTHORS: UNVR 195a-002 Water Resources Colloquium, spring
2008, The University of Arizona
ABSTRACT: The spring 2008 Water Resources in the Tucson Basin
First Year Colloquium at the University of Arizona took a diversion
to examine the future water resources outlook for Maricopa County.
We used the on-line program/model developed by the Decision
Center for a Desert City at Arizona State University called, WaterSim.
The program enables users to explore the impact of changes in inputs
on the output water source and use for the next 25 years. The
program displays inputs and outputs in easily read charts. Before
using the program, the class became familiar with the water situation
in Arizona by reading the,” Layperson’s Guide to Arizona Water”.
The class was divided into groups paralleling the inputs to the
program. Each section defined pessimistic and optimistic future
scenarios both within the range of possibility. The pessimistic
scenario resulted in cessation of Central Arizona Project, CAP, water
by 2012; a high rate of groundwater use; and the imposition of much
lower than current levels of GPCD levels by 2010. The more
optimistic scenario resulted in a balanced use of Salt River Project,
CAP, and groundwater for the entire period though 2030; much less
mining of groundwater; and the imposition of less stringent GPCD
levels in 2020. The population of Maricopa County significantly
affected the fluctuation of water usage and water levels as seen
through the optimistic and pessimistic scenarios. Sensitivity analysis
indicated that the outputs were sensitive to the changes in the flow of
the Colorado River and the Salt and the Verde rivers (to a lesser
extent). Surprisingly, the response to climate change was
insignificant. However, in the present model it only impacts the flow
of the Salt and the Verde rivers, which was forecasted to be generally
low.
3
INTRODUCTION:
General Design and Rationale for the Class
The Arizona State University Decision Center for a Desert City has
developed an on-line model for exploring future scenarios for the
water situation in metropolitan Phoenix. The URL of the site is:
http://watersim.asu.edu/ In a previous class the students tried
different input alternatives for the model. (see:
http://cals.arizona.edu/swes/tucwater1/ It was found that some of
the selections were unrealistic as the students were insufficiently
prepared in regard to the history and general projections on water
use in Arizona. Therefore, this class was designed to better prepare
the students by utilizing a new publication; namely: “Layperson’s
Guide to Arizona Water”, prepared by the Water Education
Foundation and The University of Arizona Water Resources Research
Center. It is available on-line:
http://ag.arizona.edu/AZWATER/WRRC_Events_News/LPG/Laypers
on%27s_Guide_to_Arizona_Water.pdf
MATERIALS AND METHODS:
Two class periods were devoted to discussing the above publication.
Then, the class was divided into working groups, whose name and
assignment was related to the primary inputs of the Water Sim
model. They group names were (their principal assignment is given
in parentheses):
1. Colorado River (variation in flow over the last century,
highlighting periods of low and high flow)
2. Salt and Verde River (variation in flow over the last century,
highlighting periods of low and high flow)
3. Climate Change in Arizona (What variations have been observed
and what are the general forecasts due to global warming?)
4. Population Trends in Metropolitan Phoenix (document trends
over the last 100 years)
5. Agriculture in Maricopa County (trends in agriculture acreage
from 1900 to present. What is responsible for changes?)
4
Student groups were given two weeks to prepare their reports. These
reports are included in this paper in the following section.
Reports were presented in class and suggestions for improvement
were given. Based on this input, the reports were revised prior to
their inclusion in the class report.
Then, the class was asked to go over the Water Sim Tutorial
thoroughly prior to class and then each group was to identify two
scenarios in their areas, one more optimistic and one less so, but both
within the realm of possibility using Scenario Builder in Water Sim.
These reports follow the initial reports in the following section of this
report.
Next, prior to integrating the inputs from the initial groups, the
groups were assigned a new topic. Three of the topics were from
Water Sim Inputs; namely:
1) Water Policy
2) Water Shortage Policy (with fixed residential water use)
3) Water Shortage Policy (with fixed total water use)
The other two groups began to work on integrating the input
scenarios from all the groups into two comprehensive scenarios
composed of the inputs from the initial groups using Water Sim’s
Scenario Builder.
The integrated report includes the Outputs of Water Sim based on the
scenarios developed by the class. It is included in part two of the
results section in Water Sim’s Scenario Builder format.
The class as a whole then reviewed the outputs and contributed to
composing the Discussion and Conclusions part of the paper. Finally,
the class drafted the Abstract for their paper.
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RESULTS:
Student Reports on Observed/Projected Variations of Inputs to
Water Sim which affect Phoenix Municipal Water Use
Colorado River
Lance Ward and Mitchell Little
Background Report
The Colorado River is a major source of water for the region of central Arizona.
Seven states as well as Mexico share the water of the Colorado River and each regions
share is based on the Colorado River Compact, which is the federal guideline for use and
division of the Colorado River. The river basin has been divided into an upper basin and
a lower basin. The upper basin includes the river portions located in Wyoming, Utah,
Colorado, and New Mexico. The upper basin also includes the portion in Arizona that
drains into the river above Lee Ferry. The lower basin includes the river regions in the
remainder of Arizona, Nevada, and California. The flow of the Colorado River fluctuates
in regards to the conditions of the climate, seasons, and water releases from dams. The
Colorado River is a historical landmine that shaped the future of the West. The River
determined the settlement patterns of various communities in the 19th and 20th centuries
and the settlers began plumbing to support the economies of mining and agriculture.
During periods of shortage in managing the Colorado River, water was stored in Lake
Powell and Lake Mead. This was a plan put together by all the seven states that share the
River Basin. There have been decades of battles between the seven states that share the
Colorado River. There is currently a shortage-sharing agreement between Arizona and
Nevada which lead to a decreasing amount of water flow to Mexico.
6
References
http://www.desertusa.com/colorado/intro/du_introcr.html
http://en.wikipedia.org/wiki/Colorado_river
Water Sim Input Scenarios
Scenario 1: Pessimistic
Year: 1940
Runoff: 60%
Start: Now
End: 25 years
Scenario 2: Optimistic
Year: 1970
Runoff: 90%
Start: Now
End: 25 years
The year 1940 was selected for the input scenario because during this time, the
Colorado River water flow progressively decreased. With respect to this rather dry timeperiod, we altered the runoff to 60 percent to help demonstrate the effects of a low water
runoff. We chose to have this scenario to begin now and end in 25 years to exemplify the
decreasing water flow over the years, if in fact this dry time-period was to continue. This
scenario shows a drastic decrease to the Colorado River water flow as well as its total
storage. From this, we can assume that if this dry stretch of time did continue, then the
river would be much less beneficial to the communities that rely so heavily upon it.
The year 1970 was chosen for Scenario 2 due to the Colorado River’s continual
growth over the following years which led to its highest level of acre-feet of water which
happened to be about 24 million acre-feet in 1984. The water runoff input was increased
to 90 percent to help exaggerate the river flow and its progressive increase. Thus, this
progressive increase could be a result of cloud seeding which would help reach a higher
river runoff. We chose this scenario to begin now and end in 25 years to help express
what would happen if this actual scenario were to occur. The end result leaves the
7
Colorado River with a more realistic and attainable river flow and river storage, which
will be able to support the communities that the river supplies.
Scenario 1
Scenario 2
8
Salt and Verde River
Sean Hayes and Ethan Vickers
Background Report
The Salt River was first used for irrigation purposes in 1867. At this point in
time, the flow of the river was tremendous, and at often times, too great. However, many
water scientists believe that it wasn’t necessarily the amount of the water that was the
problem, but the differentiation of level in flow. On multiple occasions there have been
reports of the change in flow from a small stream to extreme floods in just a short period
of time. Therefore, the farmers around this area could be extremely wiped out due to
droughts. Since the irrigation at this time led miles away from the original source, when
the water levels were too low, water would not even reach the distant farms. There was
an immense amount of need for storage facilities and other systems to fix these problems.
Between the time when irrigation first started on the Salt River and 1902, there
were many plans to solve the irrigation and flooding problems. There were various
diversion dams, canals, and laterals created in order to make farming and other water uses
more feasible. These dams and canals were not only built by the surrounding people, but
by private companies. These companies were looking to divert the water to their specific
location so that they could fulfill their business needs. In due time, it would be realized
that these dams were not constructed properly and that there was a shortage in supply. A
reservoir was the solution to this problem.
The problems that we are encountering today are that the surface water levels are
running dangerously low. This could create a major shortage of water and electricity in
the Phoenix area. Also, this shortage could affect other cities that are getting their
supplies form Phoenix. The annual runoff of the Salt and Verde Rivers has been
9
decreasing rapidly ever since 1985. As the population of Phoenix keeps increasing, the
water demand is going to do the same, and unless there is more development of storage
facilities, then there could be catastrophic problems.
The Salt River Project includes a service area of about 240,000 acres spanning
portions of Maricopa, Gila, and Pinal Counties in central Arizona. The land within the
project is furnished by a full irrigation and home water supply from the Salt and Verde
Rivers and from 250 groundwater wells; about 26,500 acres are furnished by
supplemental irrigation water. Six storage dams, two of which were constructed by the
Bureau of Reclamation, control the rivers flow.
The total storage capacity of the Salt River reservoirs is more than 2.4 million
acre-feet. The combined storage capacity of the two reservoirs on the Verde River is
317,715 acre-feet. The Salt River Project helps in power generations through five
hydroelectric plants; three steam plants, two with separate combustion-turbine
installations; and a combined-cycle plant.
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Water Sim Input Scenarios
Scenario 1: Pessimistic
Year: 1950
Runoff: 30%
Start: 2015
End: 25 years
Scenario 2: Optimistic
Year: 1970
Runoff: 150%
Start: 2020
End: 20 years
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Climate Change in Arizona
Tiffany Cogan, Jessica Laurino, and Jesse Giacomelli
Background Report
Climate is one of the fundamental drivers of the water cycle. It determines how
much water the population of Arizona needs and how much water is available. Today,
Arizona is considered in a drought, and a major concern is how global warming will
affect climate change and specifically, Arizona’s water supply.
Arizona’s water delivery system is deteriorating under the effects of rising
temperatures. The timing of weather has a huge effect on the quantity of water in supply.
Due to shorter winters, a significant amount of snow melt is lost in the mild Winters.
This difference has already reduced snow melt runoff by about 75%. This is a serious
issue considering that runoff provides Arizona with as much as 75% of its surface water.
Milder weather could also change winter snow to more rainfall. Instead of storing the
water efficiently, rain sinks into the soil, evaporates, or causes flooding (McKinnon).
The majority of this climate change stems from human activity; mainly greenhouse gases
caused by automotive pollution and power plants. These greenhouse gases have caused
changes in snowpack, streamflow, and temperature. Not only will this make water levels
decrease but also have effects on how the water is stored and released. In fifty years,
Arizona residents can expect a 1-2 degree temperature increase in the spring and fall as
well as a 2.5 degree increase in winter and summer. Additionally, Arizona is currently in
a dry cycle, which typically lasts for 20 to 25 years. But due to global warming, it is
expected that the dry cycle will continue and perhaps intensify. All of these are
important natural changes that will happen if global warming causes Arizona’s
temperature to continue rising for the next three decades.
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Works Cited
ASU Water Sim Model. http://watersim.asu.edu/.
McKinnon, Shaun, Sources of Water for West at Risk: Forests, farmers, cities depend
upon mountain-snow runoff. 25 November 2007. The Arizona Republic.
Water Sim Input Scenarios
Scenario 1: Pessimistic
Most Pessimistic
Scenario 2: Optimistic
Average
For scenario 1 we chose the most pessimistic point of view for climate change.
We chose this outlook because it shows the worst possible climate change for Maricopa
County, and due to the current drought facing Arizona, this is the logical scenario for the
future. By 2030, the Salt and Verde River will be at approximately 1 million acre-feet of
water but only storing approximately half of that. The maximum capacity for these rivers
is roughly 2.5 million acre-feet, which is significantly above our estimated stored water.
For our second scenario, we are expecting a less than optimistic climate change
scenario. This “average scenario” is chosen because it seems the most realistic of the
three. Between the current drought facing Arizona, which lasts approximately 20 years,
and the expected drought due to climate change, an optimistic scenario is unrealistic.
When considering the average scenario, the expected runoff is about 2 million acre-feet
while storage is half below capacity. Throughout the thirty estimated years, there are also
13
peaks in water storage dispersed throughout the years which will help sustain the water
level.
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Population Trends in Metropolitan Phoenix
Dustin C. Sandberg, Alyssa Ralls, and Jessica Kashian
Background Report
History behind Population Trends in Metropolitan Phoenix
Metropolitan Phoenix, which consists of Phoenix, Mesa, Scottsdale, Tempe, and
Glendale, contains the largest populated statistical area within Arizona. Over the past
few hundred years, Metropolitan Phoenix, which used to consist of only one or more
counties, has rapidly become more urbanized and increased in population amounts. The
original Phoenix metropolitan area, “consisted only of Maricopa County (which has more
than 9,000 square miles) until the results of the 1990 census were tabulated, when Pinal
County (which has more than 5,000 square miles) was added to the metro area”
(Development of Metropolitan Phoenix: Historical, Current and Future Trends). The
inclusion of this county, although it is a rural county, increased Metropolitan Phoenix’s
population amount.
Throughout the years, the population of Metropolitan Phoenix has fluctuated in
different amounts. From 1990 to 2000, the increase in population was 1,013,396, which
was a 45% increase in population (United States Census Bureau). As of July 1, 2006,
Metropolitan Phoenix’s population is, according to the United States Census, an
estimated 4,039,182 (United States Census Bureau).
The inclusion of Pinal Country is not the only reason why Metropolitan Phoenix
has increased in population amounts. Over the years, Phoenix has turned into a highly
urbanized area. “Urbanized areas are defined by the U.S. Bureau of the Census, with the
boundaries updated every 10 years based on decennial census data” (Development of
Metropolitan Phoenix: Historical, Current and Future Trends). The Phoenix urbanized
area now includes Pinal County and the Apache Junction area (Development of
Metropolitan Phoenix: Historical, Current, and Future Trends). Since the urbanized areas
of Metropolitan Phoenix increased, the population increased as well.
As Arizona was starting to become populated in early 1900, “The population of
Maricopa County in 1900 was only 20,000” (Development of Metropolitan Phoenix:
Historical, Current and Future Trends). Population growth accelerated between 1910 and
1920, but then began to stabilize through 1940. In these areas, there were 120,000
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residents in 1940, and in small towns, the populations were no more than
7,000(Development of Metropolitan Phoenix: Historical, Current and Future Trends).
The Growth Factor Affecting Metropolitan Phoenix
Most new development utilizes low water use systems and landscaping. Thus, the
average household water use of a new house is less than the average household water use
of existing houses more than 10 years old. Commercial and industrial uses are also more
efficient than past uses. The result is that in the city of Phoenix, the rate at which total
water consumption is increasing is lower than our growth rate. To illustrate, the city's
population has grown by 77 percent since 1980, though the water use has grown by only
35 percent. The regional growth rate is 3.9 percent per year outside Phoenix’s city limits.
The city’s strategy is growth management, not zero growth. Stopping growth would
negatively impact the well being of the city’s and the region. It would cause dislocations
in employment throughout the region, and significantly dampen commercial activity,
manufacturing, and service industries.
Through long-range water resource planning efforts, the city continually strives to
ensure that the demands of this growth can be met far into the future. These efforts
involve both management of our existing supplies and acquisition of new supplies. As
the drought depends, this becomes especially critical in protecting our local economy.
Like many of the largest urban areas in the United States, the whole of the Phoenix
region has become greater than its parts. Over the past 50 years, the Phoenix area has
become a major urban region facing a wide range of issues critical to its future. With its
rapid growth, the fastest in the country over the last 10 years, it has transcended the
traditional definition of a region with clearly defined boundaries. Issues of growth,
economy, open space and sustainability are now regional issues over which no single
local governmental agency has authority.
Sources:
Rex, Tom R. Development of Metropolitan Phoenix: Historical, Current and Future
Trends. August 2000.
<http://wpcarey.asu.edu/seidman/ccpr/PDFs/development.pdf>
16
United States Census Bureau. International Data Base. February 12, 2008.
<http://www.census.gov/population/cen2000/phc-t3/tab05.txt>
United States Census Bureau. Arizona. February 12, 2008.
<http://quickfacts.census.gov/qfd/states/04000.html>
United States Census Bureau. Phoenix City, Arizona Statistics and Demographics.
February 12, 2008. <http://phoenix.areaconnect.com/statistics.htm>
Understanding Growth of the Phoenix Area. February 12, 2008.
<http://alliance.la.asu.edu/lessonpackets/phoenixgrowth/studentpacket/phoenixgr
owthstudent.html>
Water Sim Input Scenarios
Scenario 1: Pessimistic
Double Projected Growth
Scenario 2: Optimistic
As Projected
17
The graphs presented above reveal a large amount of information on the projected
growth of future populations. These projections are presented under the DES population
prediction, which stands for the Department of Economic Security. Therefore, the
fluctuation in population and the intended DES population are in accordance with the
department’s information.
This projection is set in the year 2050, because a reasonable and realistic time line
could be set to this future time that would produce desired information. For Scenario 1,
we are predicting a more pessimistic situation in the population growth. As assigned, we
intended to include a more negative outcome to predict the course of future water
situations. Yet, due to the limited choice that fell between predicted projected growth and
the double projected growth there was no in-between choice. Therefore, we could only
use the exaggerated sample from the options, although we do not believe that this is a
realistic outcome. This unrealistic projection would include the highest population for
Maricopa County by 2050. In Scenario 2 we have selected a more realistic idea of the
population growth as predicted by DES. We see in scenario 1, that water usage will
increase substantially in the residential areas and in turn increase agricultural and
commercial use. Also in scenario 1, we see that the groundwater supply is substantially
in deficit, limiting the amount of groundwater that could be supplied. In the second
scenario we also see a dip into the negative million-acre feet but by not as large a margin.
In the Gallons per Capita per Day figure, we do not see much of difference between
scenario 1 and scenario 2.
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Agriculture in Maricopa County
Clayton Kammerer, Jessica Woody, and Andrew Axelband
Background Report
Arizona as a whole is notorious for over usage of our precious and scarce water
supplies, and it is commonly recognized that Maricopa County in particular is where
much of the water is directed. Specifically, agriculture in Maricopa County accounts for
a very large percentage of the water used. Farms now utilize about 55% of Maricopa’s
water, which was closer to 85% around 1980. Farmers also have been very innovative
and zealous about using current technology in order to decrease the use of water. Some
farmers even use a technique of leveling off fields with lasers in order to reduce runoffs.
But all in all, with lettuce, cotton, alfalfa, and citrus all being of such importance in the
agriculture industry, Maricopa County still is to blame for much of the water use, even
with the efficiency farmers have recently displayed in response to water scarcity.
Agriculture in Maricopa has seen an increasing decline in acreage in the last few
decades for its farms and orchards. In 1982 there were over 1,400,000 acres of farm land,
and only ten years later that number was reduced to half. There are various reasons
causing this. Agricultural land is disappearing in part due to the encroachment of the
urbanized area. Residential and commercial buildings take priority over agricultural
zones. Agriculture has also been decreasing partly due to changes in the nature of
farming. About 6,000 acres of agricultural land - an area the size of the town of Paradise
Valley - permanently goes out of production each year. The development of nonagricultural uses of the land, as well as advances in farming technology have been
significant at reducing the amount of acres used for agriculture. As an example, as hybrid
crops are improved, they require less subsistence from the soil, and therefore require less
19
space and can be planted closer together. A farmer will be able to produce a higher yield
of crops in a smaller acreage, so the greater amount of acreage is no longer needed as it
becomes unnecessary and no longer cost effective within certain boundaries of crop
yield.
The population explosion of Maricopa County has increased the need for
residential homes, and the agricultural acreage is the first to decline as the urban sprawl
increases. As of July 2006 the population of Maricopa County was estimated at over
3,768,000, making it the 4th largest county in population in the United States. It is also
one of the fastest growing, which explains the rapid decline in agricultural land as
acreage is quickly being gobbled up for urban and municipal uses.
References
McKinnon, S. (2005). Farms Swallowing Most of Arizona’s Water.
http://www.azcentral.com/specials/special26/articles/0103conserve-main03.html
ASU, WaterSim Model. http://watersim.asu.edu/
Arizona Department of Agriculture. http://www.azda.gov/Main/maricopa.htm
Maricopa Association of Governments.
http://www.mag.maricopa.gov/archive/vv2025/Presentations/Seg6/SEGMENT61/index.htm
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Water Sim Input Scenarios
Scenario 1: Pessimistic
Agriculture phased out by 2040
Scenario 2: Optimistic
Agriculture phased out by 2060
In a worst case scenario, the agricultural land in Maricopa County could
be phased out by the year 2040. If the population continues to rapidly grow and the
drought becomes more severe, the importance of water will become ever more essential.
Water in municipal and commercial usage tends to have a priority over agricultural
usage, and thus the amount of agricultural land is declining. Since agriculture requires
enormous amounts of water, it will become more expensive to continue the development
of agriculture and it may become nearly impossible with the lack of water. The growing
population is expected to cause the water crisis to become more severe, so it is very
possible that the agriculture land will phase out even faster than predicted, as the severity
increases.
Because agriculture in Maricopa county accounts for such a large portion of
Arizona’s agriculture, not to mention the nation’s agriculture (especially lettuce, alfalfa,
cotton, and pecans), it is likely that it will take quite some time to phase out. Until there
is an alternative to using the land in Maricopa County as prime agriculture production,
much of Maricopa’s water will also be directed toward irrigation for its purposes. It
seems unlikely that agriculture will be transitioning out of the picture anytime in the near
future due to the abundance of agricultural productivity.
It is certainly not a linear problem, as more and more agriculture has been phased
out year after year, making it probable that Maricopa County could see a near total
depletion of its farms and orchards in a very short period of time, as soon as 2040. In a
pessimistic scenario, laws may come into place that limit the usage of water to only the
21
most critical needs, and agriculture will have less priority. This would hurt the
agricultural economy even further. In an optimistic scenario agriculture would survive as
long as 2060. This is realistic if the current trends continue on their current path. It is
improbable that the agriculture in Maricopa County would last longer than this, but it is
possible that it could be phased out even sooner. Inevitably, agriculture will be phased
out to an extent; it just depends on the current trends and the trends of the future that
dictate exactly when.
2040 Model (pessimistic):
2060 Model (optimistic):
The above graphs depict the amount of agricultural land in the model's study area over
the course of the simulation.
22
Water Policy & Water Shortage Policies for Phoenix
Water Sim Input Scenarios
Water Shortage Policy
Scenario 1: Pessimistic
Fixed Residential GPCD
Start Implementation: 2010
Scenario 2: Optimistic
Fixed Residential GPCD *
Start Implementation: 2020
* We had selected other options but found we could not impose fixed total or residential
rates if we used them. So we decided to just use the options shown.
Water Shortage Policy (Residential) GPCD
Scenario 1: Pessimistic
75 GPCD
Scenario 2: Optimistic
175 GPCD
When looking at the most pessimistic scenario, we found that the amount of water that
would be saved would only equal out the amount of water that is being recharged into the
groundwater. That means that continuing the most pessimistic scenario; we would never
refill the groundwater to what it originally was. We chose the inputs for the scenario
based on the average of 170 gallons per capita per day. We reduced that by about 100
gallons to try and save as much as we could which results in a figure of 75 gpcd.
For the optimistic scenario, we chose to give about 175 gallons per capita per day.
This showed a steady increase in the depletion of the groundwater supply.
23
Water Shortage Policy (Total)
Scenario 1: Pessimistic
175 GPCD
Scenario 2: Optimistic
275 GPCD
These were selected with the idea of conserving water by constraining total water use in
the Phoenix Metro area. Note that these levels mean that only 100 GPCD is available for
use on purposes other than residential consumption.
The amount of water needed for agriculture usage has been declining significantly
over recent years, but this policy would suggest that agriculture usage would be reduced
even more severely. In the pessimistic scenario agriculture water usage could only be as
high as 40% of the total, which is far lower than the current percentage of 55% around
2005.
24
Summary of Student Output Scenarios on water use in Phoenix
from 2006 till 2030
Outputs
Pessimistic
This graph depicts the
amount of water
available from various
sources based on the
simulation.
This graph depicts the
amount of water
calculated to be
demanded by various
uses, subject to the
availability of water
from the sources.
This graph depicts the
calculated accumulated
change in groundwater
since the starting year
of the model simulation
due to recharging and
pumping.
Optimistic
25
This graph depicts the
calculated commercial
and residential water
usage in gallons per
person per day after
taking into account
water supply, water
demand, and water
policy.
Maricopa County Water Sources
(written by the original Colorado River Group)
After reviewing the two scenarios, the graphs illustrate the drastic differences
between the two outputs. The first scenario shows the extreme increase in acre feet of
groundwater as well as the decrease in the Salt River Project and how the Central
Arizona Project perished. The second scenario exemplifies a steady and balanced output
with no severe changes. The Salt River Project, Central Arizona Project, and the
Groundwater remain stable throughout the given time period.
Maricopa County Water Uses
(written by the original Salt and Verde Rivers Group)
When comparing the graphs, the data from the pessimistic graphs shows an increase in
the use of groundwater reserves. There is a usage of the full amount of residential water
and a reduction in the agricultural usage. The optimistic graph shows a steady plateau of
rising in the residential and an increase in the usage of water for commercial use. There
is a decrease of groundwater use and an overall recharge.
26
This shows that after a few years, there will be a steady increase of water usage
for the residential and a decrease in the usage of the water for the agriculture. After a few
years, the remaining water that was put aside for the residential is used for the recharge
and commercial water use.
Change in Groundwater Supply
(written by original Climate Change in AZ Group)
The two scenarios for the outputs of groundwater both show opposing outcomes.
The most pessimistic scenario shows that by 2030 there will be a large increase due to the
dependency on groundwater. This is very unrealistic due to the understanding that only a
little bit more groundwater is going to have to be utilized since there will be a decrease in
surface water. 50 million-acre feet is an extreme exaggeration of how much groundwater
will need to be used. However, the output for scenario two looks more realistic. By
2030, the graph shows that if everything follows the projected outcome then there will be
about 20 million-acre feet of groundwater used. This seems realistic due to the fact that
in the upcoming years most of our water will be coming from groundwater but still with
some surface water.
Most Pessimistic
Average
27
Gallons per Capita per Day
(written by original Agriculture in AZ Group)
In the pessimistic scenario (Scenario 1), the gallons per capita per day dropped to
a mere total of 175, which is most certainly grim. In this case, the commercial use
appears to actually be greater than the residential use past the year 2010. Residential use
would be at an all time low of only 75 gallons per day, climbing down from over 200
before 2010. This scenario would be unrealistic in meeting the demands of the residential
areas, while probably not having a severe affect on commercial use.
In the second scenario, the more optimistic situation, the residential water use
would be about where it is now for some time, with a slight decrease around the year
2020. Residential gallons per capita per day would not get far below 200 gallons, and
once again the commercial use would remain fairly steady in comparison with past
decades. There would be much more water reserved for the residential use than
commercial, much like as has been in the past. The scenario would be much more
desirable than the first.
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DISCUSSION:
(written by the class, in class and recorded by the original Population
Trends in AZ Group)
We compared the change of the outputs for the pessimistic scenarios by replacing the
pessimistic scenarios with the optimistic scenarios, one input at a time; a form of
sensitivity analysis.
We found that the change in the flow of the Colorado River made a big difference in
available water resources for Maricopa County. There was more Central Arizona Project,
CAP, water available throughout the length of the forecast. In the pessimistic scenarios
the CAP was completely phased out as a water source after a few years. This change also
resulted in an increase in groundwater use.
The flow of the Salt and the Verde Rivers provided more water via the Salt River Project
to Maricopa County when the optimistic scenario inputs replaced the pessimistic values
for these rivers. Ground water use was decreased as well.
Replacing only the pessimistic climate forecast with the optimistic one, surprisingly
hardly made any change in the outputs.
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When the optimistic population projection replaced the pessimistic forecast there was less
use of the Salt and the Verde rivers water as there would be fewer people in the county.
Also, there was reduced groundwater use.
When Agriculture was extended by replacing the pessimistic projection with the more
optimistic one, there was an increase in total water use and groundwater consumption.
We decided to see what the outputs would look like if the pessimistic scenario forecasts
were replaced with what the class thought would be the most likely future situation as
shown in the table below:
Most likely Input Scenarios
Colorado River
Salt and Verde Rivers
Climate Change
Population Growth
Agriculture
Water Shortage Policy
Year: 1970
Runoff: 80%
Start: Now
End: 25 Years
Year: 1950
Runoff: 50%
Start: 2015
End: 25 Years
Most Pessimistic
Double Projected Growth
Phased out by 2040
Fixed Residential GPCD
Start Implementation: 2020
Residential GPCD: 100
Total GPCD: 225
The impact of the most likely inputs, above, are compared with the Pessimistic Scenario
results in the charts below. The most likely outputs are on the right and the pessimistic
outputs are on the left . You will note that the there is little water use available from the
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Salt River Project in either scenario, but there is a bit more in the most likely scenario.
There is more CAP water used throughout the latter. The use of groundwater is about the
same over the forecast period, but the annual changes differ.
The change in the date of limitation of Gallons per Capita per Day,GPCD, affects the
residential water use with a step change taking place in 2010 for the pessimistic scenario
and in 2020 in the most likely scenario. In the latter the total water use is higher as the
date for limitation is delayed compared to the pessimistic scenario.
There is slightly less use of groundwater in the most likely scenario. The GPCD changes
abruptly when the GPCD constraints are applied which is 10 years later in the most likely
scenario compared to the pessimistic scenario This results in higher water use over the
longer unconstrained period.
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This graph depicts the amount of water available from various sources based on the
simulation.
Pessimistic Scenarios
Most Likely Scenarios
This graph depicts the amount of water calculated to be demanded by various uses,
subject to the availability of water from the sources.
32
This graph depicts the calculated accumulated change in groundwater since the starting
year of the model simulation due to recharging and pumping.
This graph depicts the calculated commercial and residential water usage in gallons per
person per day after taking into account water supply, water demand, and water policy.
33
CONCLUSIONS:
(Written by the class, in class and recorded by the original Population
Trends in AZ Group)
In this study, pessimistic, optimistic and most likely forecasts were made of water inputs
to Maricopa county and via the WaterSim program the resulting outputs were obtained.
The water shortage policy, population growth rates, and the use of the Colorado River
water had the greatest affect on the outputs, which means that politicians in the future
have the opportunity to control the water future for Maricopa County. Animals and plants
which are dependent upon water will decrease as shortages are experienced. Surpisingly,
in these scenarios, climate change had little affect on the pattern of water availability and
use. In the WaterSim program, climate change only affects the Salt and the Verde Rivers,
and since they were already constrained by the selected inputs the amplitude of climate
change had little affect.
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REFERENCES:
1. Water Sim, an ASU on-line program: http://watersim.asu.edu/
2. Website for fall courses on Water Resources:
http://cals.arizona.edu/swes/tucwater1/
3. Layperson’s Guide to Arizona Water, prepared by the Water
Education Foundation and The University of Arizona Water
Resources Research Center.
http://ag.arizona.edu/AZWATER/WRRC_Events_News/LPG/Laypers
on%27s_Guide_to_Arizona_Water.pdf
AFTERWORD: This class nominally focuses on water resources in
the Tucson Basin, as indicated by the class name. However, this
semester we departed from that theme to take advantage of the
Water Sim model for the Phoenix municipality. At present there is
no equivalent model for the Tucson area, but hopefully there will
be in the future. It was felt that an understanding of the future of
water in the Phoenix area would give the students a solid
background on which to better understand the situation in Tucson
even though the two are not equivalent.
JJRiley