Water Reservoir Management: Droughts, Evaporation, Falling Levels, Sedimentation, Leaks, and High Withdrawal Rates are Serious Issues

Water Storage Reservoir Challenges

     A new report by the American Geophysical Union shows that water storage reservoirs in the U.S. are facing some serious issues. Longer and more intense periods of low storage levels have become more common. This is not just happening in the dry Western areas but in wet areas as well. Droughts lower water availability. Hot weather increases evaporation. Sediment buildup makes reservoir storage space smaller, and high withdrawal rates mean lower water levels. Sediment can also clog pipes and damage pumps. Utilities need to invest more in sediment removal and infrastructure upgrades. Most of the reservoirs in the U.S. were built between 1930 and 1970 and some of their issues are due to the aging infrastructure. Dams provide water for power generation, flood control, water supply, and irrigation

     The study basically concludes that U.S. water storage reservoirs “are experiencing longer, more severe, and more variable periods of low storage than several decades ago,” Of course, the problems are most severe in the western and central states. The report is expected to help with forecasting and managing storage reservoirs.

     Combined with related problems such as low river levels and over-pumped groundwater the low reservoir levels are concerning. Phys.org writes:

 

“Drought, water withdrawals, and sediment buildup behind dams determine how much water can be stored in a reservoir. Each of those factors has been changing, in many cases pushing reservoirs away from the conditions under which they were designed to operate.”

 

Caelan Simeone, a hydrologist at the U.S. Geological Survey's Oregon Water Science Center who led the study and colleagues analyzed water levels in 250 large reservoirs from 1981 to 2020, looking for changes in baseline, maximum, and minimum water levels. The study excluded reservoirs in the Northeast region. The surprising results showed that reservoirs are not filling to the levels they once were. Some of the key findings are given below (CONUS=conterminous U.S.):

 

·        Low-storage periods are longer, more severe, and more variable in over-year storage reservoirs and in the western and central CONUS

 

·        Longer periods of low storage for some regions in recent years suggests decreased reservoir reliability in a changing hydroclimate

 

·        Maximum annual storage is also declining across CONUS, furthered by storage losses from sedimentation

 

     Reservoirs are typically used to lessen the impacts of droughts by providing stored water in times of increased need. However, the droughts are also leading to lower levels due to both increased evaporation from heat waves and increased withdrawals. Sediment buildup reduces the total storable volumes in the reservoirs. Increasing reliability and decreasing vulnerability are the goals of water storage. Those goals have been compromised in recent years, according to the paper, which defines them as follows:

“Reliability, which reflects how often a system deviates from desired operations, is defined as the fraction of time operational reservoir storage targets are met, and vulnerability, which reflects the relative significance and consequences of these deviations, is defined as the maximum departure from storage targets when an anomaly occurs. Both can be used to evaluate how water management systems, and specifically reservoirs, have adjusted to multi-decade trends in water supply and demand (Jones & Hammond, 2020).”

     For the study reservoirs were classified as within-year, over-year, or highly variable. The highly variable classification was mainly for storage reservoirs used as flood control.

     Conclusions from the study include the following description of the approach and what was found:

“We developed a generalized approach for exploring low-storage anomalies in reservoirs as an avenue to understand drought manifestation. We found low-storage anomalies in reservoirs are longer, more severe, and more variable in more arid western and central CONUS reservoirs, and for reservoirs with primarily over-year storage. Low-storage anomaly duration and severity have increased for reservoirs in central, southcentral, southwestern, and coastal northwestern CONUS, suggesting the specifications to which these reservoirs were built may not support a changing hydroclimate and sedimentation. This will potentially make these reservoirs less reliable and more vulnerable. Maximum annual storage is declining across most of CONUS, possibly due to sedimentation reducing available storage.”

     A 2022 study in Nature explored ResOpsUS, a data set on U.S. reservoirs and dams. The paper noted that there are more than 52,000 dams in the U.S. They explain the data set as follows:

“There are over 52,000 dams in the contiguous US ranging from 0.5 to 243 meters high that collectively hold 600,000 million cubic meters of water. These structures have dramatically affected the river dynamics of every major watershed in the country. While there are national datasets that document dam attributes, there is no national dataset of reservoir operations. Here we present a dataset of historical reservoir infows, outfows and changes in storage for 679 major reservoirs across the US, called ResOpsUS.”

     Graphics from the paper and dataset are given below.

     Evaporation is also a factor in water loss from reservoirs due to increased heat exacerbated by climate change. The graph below from the EPA shows that evaporation has been increasing pretty drastically since the 1970s.

     A May 2024 paper in the American Geophysical Union’s Geophysical Letters shows that globally, there has been a trend toward reservoirs not filling up to desired levels. This often results in decreased hydropower output which can affect local and regional electricity availability. The study utilized satellite altimetry from CryoSat-2 to observe 525 reservoirs around the world. Some of the key findings are:

·        Water levels of reservoirs in the southern hemisphere show a declining trend

·        About 93% of the 525 studied reservoirs have not been fully filled up at least once in the past 12 years

·        Less developed economies need to develop informed reservoir operation rules to cope with climate change

     The study looked at reservoir levels from 2011-2022 and noted that the Southern Hemisphere in particular during this period has shown dropping reservoir levels. Their data for the Northern Hemisphere showed overall increasing levels in Eastern North America and Europe but some decreasing levels in drier areas in the North American West and Central regions. The study compared data from the Standardized Precipitation Evapotranspiration Index (SPEI), a common drought metric. It also examined the effect of the El Nino Southern Oscillation (ENSO) on these reservoirs, which was found to be considerable.

“The strong correlation between water level and SPEI indicates that climate plays a decisive role in regulating the dynamics of reservoirs, and reservoirs have varying degrees of susceptibility or a certain latency in response to different intensities or durations of meteorological droughts. Further analysis reveals that much attention should be paid to the impact and associated extent of ENSO on reservoirs.”

     Another 2024 study in Nature Water considered the effects of upwind moisture on water security.  The authors concluded that upwind moisture supply is often overlooked when considering water security. They noted that globally, around 40% of continental precipitation originates in evaporation from land. In some regions in Eurasia, South America, and Africa this amount increases to 80-90%. Thus, upwind changes in the amount and timing of evaporation entering the atmosphere are very important in determining risks to water availability. Some of the differences between considering and not considering upwind moisture supply are shown below.

 

References:

US Reservoirs Are Failing, And Utility Companies Are Thirsty For Bill-Payers’ Dollars to Fix It. Joe Lysikatos. Eco Hugo. August 31, 2024. US Reservoirs Are Failing, And Utility Companies Are Thirsty For Bill-Payers’ Dollars to Fix It (msn.com)

US water reservoirs are shrinking and becoming less reliable, new study finds. American Geophysical Union. Phys.org. August 22, 2024. US water reservoirs are shrinking and becoming less reliable, new study finds (phys.org)

Declining Reservoir Reliability and Increasing Reservoir Vulnerability: Long-Term Observations Reveal Longer and More Severe Periods of Low Reservoir Storage for Major United States Reservoirs. Caelan E. Simeone, John C. Hammond, Stacey A. Archfield, Dan Broman, Laura E. Condon, Hisham Eldardiry, Carolyn G. Olson, Jen C. Steyaert. American Geophysical Union. Geophysical Research Letters. Vol. 51, Issue 16. August 22, 2024. Declining Reservoir Reliability and Increasing Reservoir Vulnerability: Long‐Term Observations Reveal Longer and More Severe Periods of Low Reservoir Storage for Major United States Reservoirs - Simeone - 2024 - Geophysical Research Letters - Wiley Online Library

ResOpsUS, a dataset of historical reservoir operations in the contiguous United States. Jennie C. Steyaert, Laura E. Condon, Sean W. D. Turner & Nathalie Voisin. Nature. Scientific Data (2022) 9:34. ResOpsUS, a dataset of historical reservoir operations in the contiguous United States (nature.com)

Reservoir Filling Up Problems in a Changing Climate: Insights From CryoSat-2 Altimetry. Zhiwei Wang, Liguang Jiang, Karina Nielsen, Lei Wang. Geophysical Research Letters. May 13, 2024. Reservoir Filling Up Problems in a Changing Climate: Insights From CryoSat‐2 Altimetry - Wang - 2024 - Geophysical Research Letters - Wiley Online Library

The risk of global water scarcity is greater when accounting for the origin of rain, study shows. Science X staff. Phys.org. September 2, 2024. The risk of global water scarcity is greater when accounting for the origin of rain, study shows (msn.com)

Best Practices to Consider When Evaluating Water Conservation and Efficiency as an Alternative for Water Supply Expansion. EPA-810-B-16-005. December 2016. Best Practices to Consider When Evaluating Water Conservation and Efficiency as an Alternative for Water Supply Expansion (epa.gov)

Upwind moisture supply increases risk to water security. José Posada-Marín, Juan Salazar, Maria Cristina Rulli, Lan Wang-Erlandsson & Fernando Jaramillo. Nature Water (2024), September 2, 2024. Upwind moisture supply increases risk to water security | Nature Water