Using Lidar to Measure Reservoir Storage

Elevation-Area-Capacity Relationships of LakePowell in 2018 and Estimated Loss of Storage Capacity Since 1963

Scientific Investigations Report 2022-5017
Water Resources Mission Area
Prepared in cooperation with the Bureau of Reclamation

For a while now, we’ve been waiting for LIDAR, light detection and ranging technology, to bring us the elusive self-driving car. Unfortunately, it always seem to be 5 years out. However, I read with amazement about a team of scientists from South America and the United Kingdom who used helicopter-mounted lidar to peer below the rainforest foliage and get a view of the remains of structures below the trees discovering villages that are hundreds of years old and had been swallowed by the jungle.

Now, scientists are finding many other uses for Lidar. The U.S. Geological Survey (USGS) , in cooperation with the Bureau of Reclamation (Reclamation), surveyed Lake Powell between fall 2017 and spring 2018 topographic light detection and ranging (lidar) data (land elevation) and multibeam bathymetry (bed elevation of a water body to calculate the capacity of Lake Powell, the second largest reservoir in the nation.

Lake Powell is located on the Colorado River across the Utah– Arizona border and was created in 1963 by the completion of the Glen Canyon Dam. Nearly 200 miles of the Colorado River was flooded upstream from the dam creating the reservoir/lake. In the United States only Lake Mead, which is approximately 300 miles downstream on the Colorado River is larger.

Though the instrumental record of the Upper Colorado River Basin is robust, with daily stream gage monitoring going back decades, only two studies have estimated the Lake Powell storage capacity. The original, pre-Glen Canyon Dam elevation-area-capacity tables (Bureau of Reclamation, 1963) that were calculated from contour maps and a reservoir-wide, range-line bathymetric survey that was completed 25 years post-impoundment in 1986 (Ferrari, 1988). Both studies utilized the best-available technology at the time but lacked the precision of current surveying methods.

Lake Powell has continuously trapped sediment from the sediment-laden Colorado and San Juan Rivers at the river deltas, diminishing the storage capacity at the highest elevations of the reservoir. During the most recent survey of Lake Powell, USGS scientists used high-resolution multibeam bathymetry and lidar to create the equivalent of an underwater topographic map of the reservoir. The data were then combined to create a topobathymetric digital elevation model (TBDEM), a continuous representation of submerged bathymetry and subaerial topography.

Just as the land above the water has its highs and lows, so too does the land beneath the water’s surface. Those features are known as bathymetry. In a reservoir the build-up of sediment slowly over time reduces the capacity of to the reservoir – how much water it can hold.

The lidar topographic data were acquired during a 2-day airborne survey on April 2 and April 3, 2018, and completed by The Atlantic Group, LLC they found that the total storage capacity of Lake Powell is now 25,160,000 acre-feet. This is a decrease of 1,833,000 acre-feet or 6.79% of storage capacity from 1963 to 2018. The average annual loss in storage capacity was approximately 33,270 acre-feet per year between 1963 and 2018.

Locally, the Occoquan Reservoir in an urbanized area has suffered a 15% loss of capacity associated with accelerated siltation over a shorter period of time.

GRACE Watches Building Water Crisis

Earlier this month Dr. Jay Famiglietti, a professor of Earth System Science at the University of California, Irvine, and Director of the UC Center for Hydrologic Modeling (UCCHM) and Matt Rodell, now Chief of the Hydrological Sciences Laboratory at NASA’s Goddard Space Flight Center have published an new paper in Science entitled, “Water in the Balance.” The scientists draw conclusions and trend from the ten years of data that has come from the Gravity Recovery and Climate Experiment (GRACE) and Global Land Data Assimilation System (GLDAS) to quantify groundwater depletion. A group of researchers at the University of California, Irvine, the University of Texas, and the Hydrological Sciences Branch at NASA GSFC have worked in partnership to apply GRACE and GLDAS to various real world groundwater monitoring.

GRACE data has provided a global picture of water storage trends for over a decade and could be an invaluable tool for understanding water resource availability. The GRACE mission is able to monitor monthly water storage changes within river basins and aquifers that are 77,000 square miles or larger. While this area may be too large for community water management, it can be used on the regional and national scale, and to aide international policy discussions. This information could someday be used to develop a unifying principal of cross border water resource allocation. Now, though, the first use has been to study the trends on groundwater in various regions during this period.

Stressed aquifers are in yellow, orange and red

Dr. Famiglietti points out that groundwater represents almost half of all drinking water worldwide, though a lesser proportion of irrigation water. In the United States groundwater is an important natural resource, representing about 30% of all consumptive water use especially in those parts of the country that don't have ample surface-water sources, such as the arid West and in times of drought. Groundwater is a renewable resource, but not in the way that sun light is. Groundwater recharges at various rates from precipitation. Changes in rainfall patterns and the actions of man can impact the recharge rate of groundwater. Increasing the amount of impermeable area by paving or building and other changes to land cover can reduce groundwater recharge. The climate of the planet has continually changed over the millennia and some groundwater aquifers are legacies of an earlier climate and are not being recharged.

To recharge groundwater, it must rain and the soil must be able to absorb the water. When you withdraw the groundwater from fine-grained compressible sediments and do not replace it, the land subsides. In the pursuit of wealth the ground water in the incredibly fertile Central Valley was pumped to such an extent that the ground subsided more than 75 feet in some places. The area was identified by the research efforts of Joseph Poland as the location of maximum subsidence in the United States due to groundwater mining. Once the land subsides, it loses its water holding capacity and will never recover as an aquifer. Groundwater mining in the Central Valley was believed to have slowed in the past few decades, but it continues as documented by the recent data from Drs. Famiglietti and Rodell’s work and the continual falling of the groundwater level.

Though, ten years of data may not be adequate to determine accurate changes in water availability and groundwater recharge. Using GRACE data, Drs. Famiglietti and Rodell identified what appear to be in this 10 year window water ‘hotspots’ in the United States, and these include the important food producing regions in California’s Central Valley, and the southern High Plains; large areas of the southeastern U. S. that has been plagued by persistent drought, including Houston, Texas, Alabama, and portions of the Mid-Atlantic region. Based on the data since 2003, the wetter, northern half of the U.S. has become wetter, while the drier, southern half has become generally drier.

Dr. Rodell hopes to have the next generation of satellites able to monitor groundwater changes on a weekly basis and to be able to monitor groundwater and river basins that are closer to 1,000 square miles in area. In addition, to have the resources to interpret the date in a more timely fashion so that communities can use it to manage water resources in real time. Our water resources are the urgent need. Water is life. We must develop sustainable water, economic and agricultural policies to ensure the certainty and security of our food supply and water supply. If the water use is not sustainable, then ultimately we are not sustainable on a much shorter scale than climate change.

Water Rights, Water Use and Sustainable Life

The Felicity Barringer reported for the NY Times yesterday that due to the increasing drought in California farmers have been pumping more groundwater to irrigate their crops, lowering the level of the groundwater. As a result the state has begun to try and collect data on groundwater supplies with an eye to regulation. When the level of groundwater falls it is an indication that water use is unsustainable. California, my former home, is a semi-arid state rich with sun shine and a long growing season, but all crops need to be irrigated. There is a huge demand for water for farming and for people, and limited water resources to supply it. California seems to go through drought cycles, but the long term forecasts by the climate change model builders is that water resources available to the state will decrease as the Sierra snow pack decreases.

According to the US EPA: "Ground water is an important natural resource. More than 50 percent of the people in the United States, including almost everyone who lives in rural areas, use ground water for drinking and other household uses. Ground water is also used in some way by about 75 percent of cities and by many factories. The largest use of ground water is to irrigate crops. We can run out of ground water if more water is discharged than recharged. For example, during periods of dry weather, recharge to the aquifers decreases. If too much ground water is pumped during these times, the water table may fall and wells may go dry.” Wells going dry is what some Californians are worried about. Compliance with the request for water monitoring in California has been limited. Regulation of the water use, charging for private well pumping is something the farmers and rural residents fear.

Americans do not appreciate the true value of clean, potable, on demand water. We take for granted its unlimited availability and overlook the interconnected nature of our water supplies. There are those who think only in the short term, those who do not appreciate the experience of the past, and those who think the past is a perfect predictor of the future. Like 15% of all Americans my water is supplied from a well on my land. All my water comes from groundwater. One of the selection criteria for this house was the water and its quality. Many of the decisions I make on how I live are about protecting my water and the watershed and river in the woods behind my yard. Who owns the water and has rights to it is an interesting question. Ideally, all property owners would see themselves as the custodians of the land resources that they are. We need to think beyond carbon footprint, to understand that all the resources of the earth are limited in some way. Air and water are critical elements of life. Water rights are tied not only to the land but to life.

Septic Systems and Our Water Resources

It is widely accepted, but not documented that improperly managed septic systems contribute to major water quality problems. The US EPA states in the “Volunteer National Guidelines for Management of Onsite and Clustered Treatment Systems” that improper design, construction, installation, operation and/or maintenance are the source of these onsite waste treatment failures. EPA hopes to better determine the extent of the relationship as documentation becomes available.

In the “1996 Report to Congress on the National Water Quality Inventory” the second most frequently cited contaminated source for water was improperly constructed and poorly maintained septic systems causing nutrient and microbial contamination to groundwater. In that survey 500 communities were noted to have had public health problems caused by failed septic systems. In 2003 EPA reported that 168,000 viral and 34,000 bacterial illnesses occur each year from drinking water contaminated by waterborne pathogens from fecal contamination. Proper maintenance of septic systems (both traditional and alternative) is essential for protection of public health and local water resources. In 1996 more than 25% of existing homes and 33% of new developments were served by septic systems. The EPA estimated that by 1999 over 30% of the households were served by onsite septic systems, and that number has probably crept up with the building boom that took place in 2000-2006. More than half of the existing onsite systems are over 30 years old and 10% of these older systems back up into homes or yards each year. Reportedly, the homeowner was unaware that there was a problem with their system until it backed up. This problem will only be made worse by the increasing number of alternative systems that require more maintenance. Long before global warming impacts the earth’s populations; lack of clean reliable potable water will. Our water resources need to be protected.

My libertarian streak would love to believe that homeowners would care for their septic systems appropriately to avoid the system backing up in the future, contamination of the groundwater (which may be the source of the local drinking water), and future septic system repair bills of tens of thousands of dollars to remediate and replace a system. Unfortunately, many homeowners are unaware of how septic systems work and what is necessary to maintain them. In addition, people do not seem to be able take appropriate responsibility for their systems. One method to deal with this problem is to eliminate all but the most basic systems in the most geologically favorable locations (reduce percolation rate tolerances and design the systems as conservatively as possible). The other method is to regulate, control and track. Establish system performance and monitoring and maintenance requirements, establish a tracking system and operating permits for compliance monitoring, and establish fee system and fines to fund and enforce the program. As a society we collect taxes, we license, register, and inspect cars; how different would it be to license, register and inspect/maintain a septic system. After all, unlike cars, septic systems stay put and should be easy to track.

Septic Systems and the Ecologically Sustainable life

Your carbon footprint is not the only measure of the sustainability of your lifestyle. An ecologically sustainable life is in natural balance and respectful of humanity's dependence on the Earth's natural ecological cycles. Preserving precious water resources, clean air and open land are necessary to maintaining the earth’s ecological cycles. One of the steps that a large portion of us can take is to understand and maintain our septic systems. It is estimated by various sources that 25-35% of all US homes use septic systems.
There are many different types of septic system designs. The most common type used for single family homes consists of a septic tank and leach field. A septic tank can be an anaerobic (without air) tank or an aerobic tank (with air). The anaerobic system is a single chamber tank that receives the toilet and drain waste from the house and allows the solids to settle down to the bottom of the tank where the anaerobic bacteria that live in the tank digest the organic materials while the effluent (water around all that stuff) flows out to the leach field to be purified by passing through soil until it reaches the groundwater. Scum consisting of oil and grease floats on top of the water layer and can be pulled into the leach field limiting its effectiveness.
The septic tank effluent water is either pumped or allowed to flow to a leach field or other soil absorption system, where it percolates into the soil, which provides final treatment by removing harmful bacteria, viruses, and nutrients. Suitable soil is necessary for successful waste water treatment. The “percolation rate” is the rate at which water moves through soil. The acceptable rates are between one minute and one hour per inch of soil. Take either more or less time for the water to pass through your soil and the natural soil is unsuitable for treatment of the waste water. If the water moves too slowly through the soil the leach field will flood with contaminated, foul smelling water or the water will back up into the house. If the water moves too quickly thought the soil the water will be untreated and contaminate nearby ground or surface water.
An aerobic system consists of a multi chamber tank or several tanks. After separation of solids in the first tank waste is forced through a filter into a second chamber or tank where air is pumped in to enhance aerobic bacteria which decomposes the organic material. The waste then flows into a third chamber or settling chamber which collects the bacteria and passes the liquid on to the leach field or drip field. Aerobic systems can remove more than 90% of the organic material and suspended solids within the tanks themselves, but require much more maintenance. (These systems are like the British sports cars of the septic world.) The biological load delivered to the leach field or other absorption system is much reduced and would allow (if permitted under regulation) the successful treatment of septic waste where soils are rocky, impermeable or groundwater is particularly shallow.
Indoor water use in the typical single-family home is between 50-70 gallons per person per day. Septic systems are sized by bedrooms, which is an estimate of the number of people living in a home. However, even if the number of people living within your home is appropriate for the size of the septic system, you can still overload the system. Use too much water in a short period of time and the system will be overwhelmed. Each time the system is overwhelmed untreated sewage will leave the tank and begin to clog the leach field. A leaky toilet alone can add as much as 200 gallons of waste water to the system each day. The less water used the less water enters the septic system, and reduces the risk of failure. If the amount of waste water entering the system is more than the system can handle, the waste water containing raw sewage eventually backs up into the house or yard and creates a health hazard. By the time you can smell or see a problem, however, the damage to the leach field might already be done. Replacement of a leach field can run to the tens of thousands of dollars. So caring for your septic system not only cares for the earth but also cares for your wallet. By limiting your water use and spreading out peak demands on the system you can reduce the amount of waste water your system must treat. When you have your system inspected and pumped as needed, you reduce the chance of system failure.
The US EPA’s Homeowner’s Guide to Septic Systems is a terrific basic guide to caring for and maintaining your septic system. Follow the Dos and Don’ts and your septic system may last forever. Remember though, what goes into your septic system goes into the earth. Rethink the products you use to clean your house. Paint, solvents, gasoline, insecticides and poisons should never go down your drain. Every chemical you pour down your drain is buried in your yard. In a multitude of ways your yard is part of the earth’s yard.