Measuring the Bathtub Ring: Calculating Reservoir Surface Area Changes in the Colorado River Basin

Graphics and Data Analysis By: Leanne Abraham

On August 16th, 2021 the US Bureau of Reclamation declared an unprecedented water shortage for the Colorado River, a vital artery that winds through seven states in the western United States over 1,450 miles. From its inception in the Rocky Mountains to its drainage in the Gulf of California, the river generates electricity at multiple dams, supports the region’s extensive agricultural industry, and provides an essential water supply to roughly 40 million people. However, the current megadrought—a decades-long period of extreme dryness—has reduced the river’s system storage to 40% capacity and lowered its reservoirs to historic levels, prompting drastic government intervention. 

Quantifying the effects of drought is a critical step in monitoring their severity and providing the data necessary for regional authorities to act upon. Several indices exist for measuring hydrological drought, but a deficit of Earth-observation data has restricted their effectiveness. Planet’s remote sensing satellites can aid in calculating the surface area of inland water bodies, including reservoirs, at an unprecedented spatial and temporal scale, contributing to drought quantification that is repeatable globally and at scale.

Planet detected the surface area of four reservoirs in the Upper Colorado River Basin. The two-week median extents in Lake Powell in Warm Creek Bay from May 2021 to August 2021 are mapped above, as well as the maximum two-week extent for 2019. Our data shows a significant decrease in reservoir surface area both year-over-year and at two week intervals during the summer of 2021. © 2021, Planet Labs Inc. All Rights Reserved.

Historical Context

Lake Powell, the second largest human-made reservoir in the United States, serves as a fulcrum for water distribution between the Colorado River’s upper (Colorado, New Mexico, Utah, and Wyoming) and lower (Arizona, California, and Nevada) basins. It’s the ‘bank account’ of the four upper states to ensure that they meet their required deliveries to the lower three, in times of drought-related stress. When its water reserves are threatened, so too are the network of reservoirs, and the people and industries that depend on them.

Decades of exacerbating consequences due to climate change have ripened the conditions for severe drought. Stored water resources are depleting rapidly as drier soils swallow water runoff before they enter streambeds and warmer temperatures evaporate snowpack storage. As a result, reservoirs throughout the western United States are drying up and their water levels are reaching historical lows, visually marked by ‘bathtub rings’ of recently exposed bands of rock along its edges. Lake Powell is expected to receive just 30%, or 3.2 million acre-feet, of its natural inflow in 2021. On July 24th, Lake Powell dropped to a record low pool elevation of 3,554.9 feet, and the Bureau of Reclamation projects even further declines.

In response, the bureau announced that in August they would begin to divert water from three reservoirs—Blue Mesa, Navajo Lake, and Flaming Gorge—to Lake Powell in order to ensure that it retains an adequate supply. By December, Blue Mesa is expected to lower by 8 feet, Navajo Lake by 2 feet, and Flaming Gorge by 4 feet. Together the releases will deliver a total of 181 thousand acre-feet of water to Lake Powell and raise its surface by 3 feet. The primary incentive for the diversion is to keep its level safely above its critical threshold of 3,525 feet. Below that point, water is unable to pass through the penstocks of the Glen Canyon Dam, which generates billions of kilowatt-hours of hydroelectric power. 

How Can Planet Help

Creating a means of measuring changes in the surface area of reservoirs at sub-monthly intervals could help local authorities manage water resources and better our understanding of hydrological systems. Current measurements, however, are impeded by limited observation data and model uncertainties. High-resolution, high-cadence satellite imagery is uniquely positioned to fulfill this demand and complement the existing indices based on drought-impacted reservoirs. 

In order to track how the bureau’s diversions are impacting water supply in the four reservoirs, we incorporated public data sources such as the Global Reservoir and Dam Database (GRanD), in addition to our own daily PlanetScope 3m imagery. We then used a semi-supervised learning approach on the Google Cloud Platform to detect water and calculate the two-week median surface area of the four reservoirs. This classification was performed on each waterbody from March through August using data from 2019, 2020, and 2021.

Flaming Gorge Reservoir, Wyoming and Utah

The first planned release from the Flaming Gorge Reservoir began on July 15, 2021 and is scheduled to continue through October with a planned release of 125 thousand acre-feet (kaf) of water. Despite relatively constant elevation levels up until this point, our surface area measurements for Flaming Gorge reflect a significant decrease in area, exposing land in the upper portion of the reservoir as the drought continues. 

Graphs are not scaled to zero.

Blue Mesa Reservoir, Colorado

Releases from Blue Mesa are scheduled to begin in August 2021 and continue through October, with a planned release of 36 kaf.

Graphs are not scaled to zero.

Navajo Lake, New Mexico

A 20 kaf planned release from Navajo Lake is scheduled for November and December 2021.

Graphs are not scaled to zero.

Lake Powell, Utah and Arizona

Since March, Lake Powell has had two periods of significant loss. The reservoir lost water more rapidly in the second decrease during the warm summer months, indicated by a steeper slope in elevation level. As water is diverted into Lake Powell over the following months we expect to see a relative increase in both elevation and surface area.

Graphs are not scaled to zero.

Our surface area calculations strongly correlate with the reservoirs’ changing elevation, aligning with the reported decrease in volume across all four water bodies during the late spring and summer months. By quantifying the surface area we are able to both visualize and add a much needed data point towards measuring the effects of drought and evaporation on inland bodies of water. 

Planet’s near-daily and high-resolution satellite imaging allows us to survey, analyze, and visualize changing reservoir levels across the Earth. This database provides another critical measurement tool for researchers, water managers, and local communities to use in monitoring vital water resources and the environmental variables threatening their supply.

Additional Data Sources

Reservoir Elevation Data Source: Bureau of Reclamation

Planet maximum reservoir extents clipped using the National Hydrology Dataset