Geographic Perspectives on Earth System Science

Geography 210

Assignment #5: Water use in the Colorado River

Due on Canvas by Friday April 18 at 11:59 PM

Purpose

Water is the most important resource for sustaining life on Earth and across the cosmos (as far as we know). Where we find water, we find life. Dependable availability of this precious resource then becomes paramount to sustaining and growing populations. As some areas face increasing water shortages, understanding the pressures on this resource becomes critical. In this assignment we will explore water levels along the Colorado River over the last ~100 years and explore what changes have occurred that are leading to water shortages today. This assignment uses stream gauge data, demographic data, and climate change projections to help you see a comprehensive picture of the changes and pressures facing the Colorado River.

Objectives

1.   To obtain and use geographic data to explore and understand physical patterns.

2.   To grasp the difference in use vs availability of resources.

3.   To become familiar with drivers of water consumption change.

4.   To perform. basic analysis of scientific data.

Your deliverables to be uploaded to Canvas

1.   A document (Word or PDF) including thorough, original answers to all questions using

complete sentences and correct grammar. Some questions require more than a one-sentence response, either a couple sentences or more. Some questions also require you to plot data from Excel. Include your plots in your writeup (copy and paste them).

2.   Your Excel file from which you’ve performed all analyses and created your plots.

Submissions not including the Excel file in which analysis was performed will be penalized 50%.

Both are required by the due date for you to receive a grade. Also, if you are including any outside information, you are required to include a list of references at the end and in-text citations.

Please reach out if you have questions about this.

Background on the Colorado River and its water

The Colorado River is a vital source of water for the American Southwest, in terms of drinking water, irrigation, and hydropower. Water from the Colorado River irrigates 15% of the US agricultural lands and those lands produce 90% of winter vegetables for the US market. The river is a vital resource that supports life, livelihoods, and industry across the Southwest.

The Colorado River provides water for seven states, multiple Native American tribes, and two countries (Figure 1). The allocation of the river’s water to these various entities is governed by a set of legal and administrative frameworks, including compacts, treaties, and decrees collectively known as the “Law of the River.” Together, these establish how much water each entity is entitled to use from the river annually. In particular, the Colorado River Compact of 1922 divides the   river’s flow between Upper Colorado River Basin states (Wyoming, Utah,   Colorado, and New Mexico) and Lower Colorado River Basin states (Nevada, Arizona, and California), specifying allocation amounts in MAF (Million Acre-Feet). MAF is a unit of water volume commonly used in the US to measure large quantities of water, such as the annual flow of rivers or volumes of water within reservoirs. Simply, one  acre-foot is one foot of water across one acre of area, equivalent to 325,851 gallons.

Figure 1: Colorado River watershed and users (via https://www.coloradoriverdistrict.org/map-gallery/)

Under the Colorado River Basin compact of 1922, access to 7.5 MAF was given to the upper basin states and 7.5 MAF to the lower basin states. In addition, 1 MAF was allocated for Mexico. It’s important to note that this allocation of 16 MAF per year total was based on river flow estimates at the time of writing (1922), which have since proven to be overly optimistic compared to the river’s actual  average flow.

Part 1: Colorado River allocations

1.   The 1922 Colorado River Compact is the founding agreement that defines water

allotments for each of 7 states. In the A5 Excel workbook, the ‘CRC Allocations’ tab contains the allocations for each state. Use that data to answer the following questions. The data values are given in terms of Acre Feet or the amount of water required to cover  an acre of land with a foot of water.

a.   According to the data, what is the allocation for the upper basin, lower basin, and total of both? (3 points)

b.   Create a pie chart with an appropriate title, labels, and include the percent values for each state on the pie chart. (5 points)

Part 2: Colorado River flows

2.   The Colorado River flows through 7 states with its headwaters in the namesake Colorado.

Look at the USGS stream gauge data for the Colorado River in Glenwood Springs, CO for the last year. https://waterdata.usgs.gov/monitoring-

location/09085100/#period=P365D&dataTypeId=continuous-00060-

0&showMedian=false

a.   What is the current discharge rate (cubic ft/s)? (2 points)

b.  What is the maximum rate over the last month? (2 points)

c.   What is the maximum rate over the last year and when does it occur? (2 points)

d.  Thinking about the shape and timing of this peak flow, do you think this is driven by rain events or by mountain snow melt? Justify your answer. (3 points)

3.   Look at the A5 Excel file and use the ‘Colorado River Flows’ tab to answer the following questions. The data values are given in terms of Acre Feet.

a.   When the Colorado River Compact was signed (1922), what was the average flow over the preceding decade (1911-1920)? (2 points)

b.   Given this amount, does an allocation of 16 MAF seem appropriate? (2 points)

c.   What was the average flow over the following decade after the signing of the Compact (1923-1932)? (2 points)

d.   Given this amount, does an allocation of 16 MAF seem appropriate? (2 points)

e.   What was the average flow over the following two decades after the signing of the Compact (1923-1942)? (2 points)

f.    Given this amount, does an allocation of 16 MAF seem appropriate? (2 points)

g.   What was the average flow since the signing of the Compact (1923-2024)? (2 points)

h.   Given this amount, does an allocation of 16 MAF seem appropriate? (2 points)

i.    Since 2000 (2000-2024), what was the average stream flow for the Colorado River? (2 points)

j.    Plot the total annual stream flow of the Colorado River from 1922 – 2024 as a line graph. Add an appropriate title and format the axes to make it legible. Select the  data within the graph, right click on the data and select “add trendline”. Within the “Format Trendline” Options, click on Add Equation. Insert that graph into this document. (6 points)

k.   The trendline and equation provide a brief analytical description of the dataset.

The slope (seen visually and described numerically as the first term in the equation) indicates the amount the dataset (in acre feet) is increasing or decreasing each year. Is this dataset increasing or decreasing over time? (2 points)

l.    Based on the data you just analyzed, what does this difference between

allocations and actual flows mean for current and future water use? (at least 2 sentence answer in your own words) (3 points)

Part 3: Population change data

During this time of changing decreasing water availability, the area has seen increasing water demand, driven in large part by increasing population in the Southwest. No where is this more apparent than in Arizona.

4.   In the data sheet, go to the ‘AZ Pop’ tab.

a.   What was the population of AZ in 1922? (2 points)

b.  What was the population of AZ in 2024? (2 points)

c.   By how much has the population of Arizona increased in the last 100 years (2024 vs 1925)?  (3 points)

d.   Create a graph of Arizona population over the last 100 years. Create an

appropriate title with useful labels for each axis and insert that graph here. (4 points)

Part 4: Central Arizona Project (CAP)

To service this growing population the Central Arizona Project was devised to direct water from the Colorado River to supply water to the growing population centers in Arizona.

Read through the followings story map to understand more about the CAP

https://storymaps.arcgis.com/stories/b7b28dd4c36a413e8d533ba540f998cb

This description of the CAP highlights the achievements but fails to address the fundamental underlying challenge of water accessibility.

5.   In the A5 Excel datasheet, use the ‘CAP’ tab to complete the following questions.

a.   Between 2000 and 2023 what was the average amount of water drawn through the CAP? (2 points)

b.   Comparing this amount with the total amount of Colorado River flows during this  same time period, what proportion of the Colorado River is being diverted through the CAP? (3 points)

c.   Compared to Arizona’s original allocation, how much does the CAP increase Arizona’s water take from the Colorado River? (3 points)

Part 5: Future conditions

6.   Let’s use the IPCC’s Interactive Atlas for the following questions: IPCC Interactive Atlas

Select variable “Total Precipitation” and select the medium term (2041-2060) time period and SSP 5-8.5.

Click on the Western area of the US (Regions: Western North America).

a.   Using the Table Summary view, what is the Median (%) change in 2050 for this region? (3 points)

b.  Applying that % change to the average flow for the Colorado River from 2000-

2024, what is the expected water flow for the Colorado river in 2050? (3 points)

c.   Given what you know about the current water shortages in the West, what do you expect will happen to the Colorado River by mid-century? (4 points)