Taking the average total outflow per year (6,947,951.965 af) to account for changes in management in both wet and dry years, we can calculate how far an expanded Shasta Dam would last us in a hypothetical “no inflows” scenario.
Problem & Background
Located 10 miles north of Redding, the Shasta Dam is a linchpin of California’s water supply. Built by the Roosevelt administration from 1935 to 1945, it holds back a vast reservoir, that when full, contains enough water for the yearly needs of 20 million Californians.¹
Much of the water from Shasta is used to irrigate Central Valley farms, but some also goes to Bay Area cities via the Santa Clara Valley Water District and the East Bay Municipal Utility District.
California’s recent drought has placed unprecedented demands on our freshwater resources, renewing enthusiasm for surface water infrastructure investments such as raising dams to capture more water in wet years.²
Using historical data of Shasta Dam, our group wanted to estimate how successful the Bureau of Reclamation’s expansion of Shasta Dam would be.
Our key questions are as follows:
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In the context of the 2007 to 2009 drought and the 2011 to 2015 drought, how frequently did Shasta Dam meet capacity?
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Given a proposed dam expansion, would we expect a dam to fill to capacity in the near future?
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How much total water flowed out of Lake Shasta per year, in the past 10 years, when there were several drought years? Did actual outflows change based on management and policy decisions in drought years?
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Given a proposed dam expansion and anticipated future outflows, how much longer could an expanded dam provide water?
Our belief is that an investment to increase the capacity of Lake Shasta may be justified if the reservoir is filling up on a regular basis. Furthermore, how effective a large, full dam is depends on estimating how many extra years of water it provides in dry years.
Using Lake Shasta’s past water use to forecast what a future dam expansion may mean
Andrew Tom, Jason Leung, Judy Chen & Bobby Miyashiro
Analysis & Findings
Figure 1: Lake Shasta Water Storage and Outflow from 1994 to 2016⁴
Using reservoir data provided by the California Data Exchange Center (CDEC)³, we retrieved data for the storage (acre feet), outflow (cubic feet per second), and elevation (feet) of Lake Shasta over the period of 12/28/1994 to 11/21/2016 for our initial exploration.
First, we visualized the storage and outflow data together (Fig. 1). We found the data, particularly for outflow, to be inconsistent before 2006.
Figure 2 reveals that Lake Shasta reached capacity, or neared capacity, seven times in the last ten years. The longest gap of storage not reaching capacity was about five years.
Key Question 2: Given a proposed dam expansion, would we expect a dam to fill to capacity in the near future?
Table 1: Total Acre-feet Outflows by Year of Shasta Dam from 2005-2015
Figure 3: Lake Shasta Total Outflows (cfs) Per Year from 2006 to 2016⁶
To generate this table, we first averaged the outflow data per year to calculate the average cubic feet per second. Then we multiplied the average cubic feet per second by a constant, 723.968, to convert into total outflow of water in acre feet per year.
The average acre feet outflow per year over this ten year period is 6,947,951.965 acre feet. Despite experiencing two dry periods of increased severity, average outflows were increased ever so slightly.
We visualized these total outflows by year in Figure 3 below.
Understanding the frequency of wet years is important because it gives temporal context to the environmental constraints on engineering any dam expansion. Dam expansions should be engineered, not just to capture additional inflows, but also to optimize water availability over California dry periods
Recommendations For Future Study
Given all of the storage and outflow data, we were able to analyze how often the reservoir fills and how often the reservoir is flowing water out at its maximum levels. An important point that we need to look at for future study is inflows. Without an inflow assumption, no model for the amount of extra time an increased reservoir capacity would last us can be made. Inflow is a consideration we overlooked for our conservative estimates, given that with expected increases in water demand, it would only make sense to expand the dam and increase capacity if it made more water available for a significant time.
Another limitation of our study is that our calculations assumed average outflow per year. In reality, outflows change and vary per day, throughout the year depending on the time. This calculation for how much water can actually be stored should be adjusted to account for low-inflow years for a more accurate estimator of water availability and security.
Temperature change may also affect ET rates. An overlooked consideration of our analyses was the benefits of cooler water temperatures as a result of increasing the capacity of the water body. In further study, this could be examined to better inform cost-benefit analyses.
Planned future management of the dam should also be considered because if water consumption behaviors change, the amount of water the dam may need to hold may change. Similar analyses to the one we conducted can be replicated for all surface water infrastructure for California, with potential for process-flow optimization across regions.
Even with all of this considered, this is not a decision that a small group of us should or can make alone. A proposal such as this needs more extensive research into all of the habitats and people affected. This is meant to provide some context into proposals, like increasing dam capacity.
Key Question 1: In the context of the 2007 to 2009 drought and the 2011 to 2015 drought, how frequently did Shasta Dam meet capacity?
Figure 2: Lake Shasta Water Storage and Outflow from 2006 to 2016⁵
To keep consistent with our data source and provide the most relevant analysis of recent data, we focused our time frame to 2006 to 2016.(Fig. 2). This time interval captured California’s most recent droughts, and provided the most complete datasets provided by the CDEC.
Given that the periods of 2007 to 2009 and 2011 to 2016 were drought periods, the graphs imply that there has been potential, within the last ten years, to fill a larger dam. Because both outflows and storage levels are at their maximum capacity, we made the assumption that outflows are at their maximum to prevent overflow of the dam during this time. Given more information on anticipated El Niño and megadrought frequency and their impact on inflows, we could make a better estimate of whether or not the proposed dam increase under-engineers or over-engineers Lake Shasta.
Key Question 3: How much total water flowed out of Lake Shasta per year, in the past 10 years, when there were several drought years? Did actual outflows change based on management and policy decisions in drought years?
Using the outflow data provided by the CDEC, we summed the outflow data on an annual basis to determine total outflows for Shasta Dam from 2005 to 2015. The sum totals are presented in Table 1.
Key Question 4: Given a proposed dam expansion and anticipated future outflows, how much longer could an expanded dam provide water?
We visualized water elevation data from Shasta Lake from 1994 to 2016 (Fig. 4.). Without precise knowledge of the topography of Lake Shasta, we would need to correlate elevation and storage to be able to translate proposed height increases in the dam to storage in acre-feet.
The total outflows per year from Shasta Dam shows a very slight increasing trend over the last ten years. To approximate for future outflows, we found it reasonable to use the average total outflow per year (6,947,951.965 af) to account for changes in management in both wet and dry years.
Figure 4: Lake Shasta Water Elevation from 1994 to 2016⁷
To our relief, water elevation followed the pattern of storage closely enough for our purposes. We operated on the assumption that the rate of storage increase was equivalent to the rate of elevation increase in the dam. A proposed increase to the height of the dam would be directly proportional to some unknown storage quantity. Luckily, the U.S. Bureau of Reclamation provided the storage quantities given different increases in dam heights (Table 2).
Table 2: Raising Specifications of Shasta Dam⁸
About Storage AF
We’re team Storage AF, inspired by the field name of our dataset, and internet memes. We’re a team of four friends, having benefit from undergraduate student experiences at West Valley College, De Anza, SJSU, and UC Davis.
Our objective going into this project was to examine one problem really well. We also sought to apply our fledgling data analysis and inquiry to a new project, and learn all that we could in the meantime.
Ultimately, we created a website, this website, and hosted our visualizations using plot.ly as a proof of concept for how data information can be better shared, and interacted with. Since our submission, we’ve checked our work and course-corrected for a more conclusive finding. Our exercise hopes to highlight the power of citizen science, and the intelligent design of data visualization and communication.
Table 3: Water Availability Estimates for Shasta Dam and its Proposed Expansions
If you have any further questions about this project or would like to reach out to Storage AF, feel free to fill out the contact form or reach out to us individually via email.
Andrew Tom: andrewtom.careers@gmail.com
Jason Leung: jasonleungsj@gmail.com
Judy Chen: judy.chen.sj@gmail.com
Bobby Miyashiro: Rmiyashiro14@gmail.com
Contact Us
In summary, we operated on a critical assumption that reservoir storage would last us for several years (as demonstrated in Figure 2). This calculation reveals that in a hypothetical “no inflows” scenario, a full reservoir would be depleted very quickly.