During severe drought, communities and water managers must often make difficult decisions about how scarce water resources will be used. How much water can be used to satisfy immediate water needs but minimize adverse impacts on future supplies? Can groundwater pumping be increased to augment depleted surface water supplies? If so, what are the long-term impacts of groundwater pumping? These are just a few of the questions that decision makers face during times of drought.
USGS science provides accurate, trusted, hydrologic data and scientific analysis to help decision makers who must address complex issues and competing interests in times of drought. USGS California Water Science Center scientists work with California communities through the Cooperative Water Program to help:
Changes in world climate are influencing water resources by reducing snowpack and causing snow to melt earlier in the spring. About 70% of California's water supply is stored in the snowpack and the State depends it for water supplies throughout the year as it melts slowly over the course of the summer, keeping reservoirs full. Climate change also impacts landscapes, vegetation, animals, and agriculture by causing longer dry seasons, more frequent extreme storms, fewer chilling hours, and higher snowlines.
Because of these challenges, land and and resource managers need scientific information in order to plan for current and future water availability. Scientists at the USGS have developed software that can be used to simulate and evaluate hydrologic parameters such as natural and human water movement, the combined use of groundwater and surface water, and impacts to water availability at regional, watershed, and landscape scales, as caused by changes in temperature and precipitation. Descriptions of two of these tools follow.
The One-Water Hydrologic Flow Model (MF-OWHM) is a MODFLOW-based integrated hydrologic flow model (IHM). MF-OWHM is designed for the analysis of a broad range of conjunctive-use issues. Conjunctive use is the combined use of groundwater and surface water. MF-OWHM allows the simulation, analysis, and management of human and natural water movement within a physically-based supply-and-demand framework.
Diagram showing the types of interdependencies within MF-OWHM and the related constraints on the supply and demand components (modified from Schmid and Hanson, 2009).
The Basin Characterization Model (BCM) is a recently developed tool that translates fine-scale maps of climate trends and projections into hydrologic consequences, to permit evaluation of the impacts to water availability at regional, watershed, and landscape scales, as caused by changes in temperature and precipitation.
Maps of hydrologic output variables for (a) average recharge (net infiltration below the root zone) for water years 1981-2010 and (b) change in average April 1st snow water equivalent (SWE) between 1951-1980 and 1981-2010 calculated by the Basin Characterization Model for the California hydrologic region.
Provisional Simulated Unimpaired Mean Daily Streamflow in the Russian River and Upper Eel River Basins, California, under Historical (1910-2013) and Projected Future (2001-2099) Climates: U.S. Geological Survey Data Release
The Precipitation-Runoff Modeling System (PRMS) was developed to evaluate the impacts of various combinations of precipitation, climate, and land use on streamflow, sediment yields, and general basin hydrology. Basin response to normal and extreme rainfall and snowmelt can be simulated to evaluate changes in water-balance relationships, flow regimes, flood peaks and volumes, soil-water relationships, sediment yields, and groundwater recharge.
Map of the Merced River Basin PRMS study area.
California's vast reservoir system, fed by annual snow- and rainfall, plays an important part in providing water to the State's human and wildlife population. There are almost 1,300 reservoirs throughout the State, but only approximately 200 of them are considered storage reservoirs, and many of the larger ones are critical components of the Central Valley and State Water Project facilities. Storage reservoirs capture winter precipitation for use in California's dry summer months. In addition to engineered reservoir storage, California also depends on water "stored" in the statewide snowpack to significantly augment the State's water supply as it melts slowly over the course of the summer.
The Central Valley and State Water Projects are a complex system of reserviors, lakes, dams, pumping plants and conveyance structures. Source: California Department of Water Resources
Of the State's almost 200 storage reservoirs, about a dozen major reservoirs hold about half of the water stored in California's reservoirs (Dettinger and Anderson, 2015). Each of these major reservoirs is multipurpose, and operated to meet environmental mandates, including providing flows and cold water storage for anadromous fish. Nine of these 12 major reservoirs either impound storage in, or provide water supply to, the Sacramento River and San Joaquin River watersheds. These reservoirs include Shasta, Oroville, Trinity, New Melones, Don Pedro, Exchequer, Pine Flat, Folsom, and Millerton. The other three major reservoirs—San Luis, Castaic, and Perris—are off-stream storage reservoirs built to help optimize the operations of either the Federal Central Valley Project, or the California State Water Project. Many of the smaller reservoirs are operated primarily for hydropower production, and supply millions of megawatts of water-powered electricity to the State's population. These reservoirs—large and small—all play an important role in keeping the California community thriving. Despite having a cumulative storage capacity of almost 45 million cubic acre-feet, these reservoirs currently hold much less than their full capacity for a variety of reasons.
Monthly totals of water stored in (dark blue) 12 major reservoirs and (light blue) 148 other, mostly smaller reservoirs, stacked on top of each other, and (green bars) estimated statewide-total of water stored in April 1 snowpacks each year, January 1970 through April 2015. Source: Dettinger and Anderson, 2015
Snowpack plays an important role in keeping California's reservoirs full. Winter and spring snowpack typically melt gradually throughout the year, flowing into and refilling reservoirs. Snowpack accounts for the bulk of California's water source and storage, as early spring snowpack "contains about 70% as much water, on average, as the long-term average combination of the major and 'other' reservoirs" (Dettinger and Anderson, 2015).
Recently, Michael Dettinger, a U.S. Geological Survey Research Hydrologist, collaborated with Michael Anderson, California State climatologist, to investigate California's reservoirs' long-term water storage. They looked as far back as 1970 to determine the reservoir's involvement in California's water landscape, specifically studying how annual snowpack has historically impacted the water supply within the 12 major reservoirs and 148 of the smaller reservoirs. They then analyzed the recorded snow water content to determine the total water in the snowpack for every April 1 since 1970. By examining the correlation between these two sets of data, Dettinger and Anderson concluded the 12 major reservoirs have, historically, been operated aggressively to alleviate the impacts of drought and flood. They determined that the inter- and intra-seasonal managed fluctuations of the 148 smaller reservoirs were not as significantly pronounced.
Dettinger and Anderson also used this understanding of the historical relationship between snowpack and reservoir usage to investigate how the current California drought affected reservoir storage. In April 2015, the California Department of Water Resources measured the snow water content as essentially zero. Because the April snow water content helps recharge surface reservoir storage during the spring and summer months, a snow water deficit results in storage reservoirs—depleted throughout the year—to go without crucial refilling. Dettinger and Anderson determined that reservoir replenishment in winter 2015 was only about 9% of normal. Thus in 2015, California's major reservoirs—which are important tools to manage water supply through drought conditions—did not receive the snowpack runoff necessary to refill them after three years of drought. The authors state, "The current challenge to statewide water managers is less the lack of water in the reservoirs and much more the lack of water in snowpack that normally would be expected to melt soon and replenish our reservoirs." Unfortunately, due to the expected consequences of climate change, the lack of snow storage experienced in the current drought could become more the norm than the exception in years to come.
Rainfall is another essential water source. It replenishes water in soils, groundwater, streams, lakes, and reservoirs alike. During droughts, dry soils absorb and store a substantial amount of water in the watersheds above reservoirs, preventing rainfall from flowing into and refilling reservoirs.
USGS scientists Alan Flint and Lorraine Flint recently analyzed the current drought conditions for two of California watersheds—the Feather and Tuolumne. The Feather and Tuolumne watersheds flow into the Oroville and Don Pedro Reservoirs, respectively. They examined the dry soil in the watersheds, seeking to estimate the rainfall needed to recover them from the drought. Although intense rain could cause enough runoff to be generated without the soil moisture deficit being corrected, Flint and Flint determined that significant rainfall would first be needed to fill the current soil moisture deficit. While intense rainfall will generate runoff into reservoirs, much of the precipitation would go to amend the soil moisture deficit before becoming the greatly-needed inflow to the reservoirs. Flint and Flint predict that—when considering the soil moisture deficit and the volume of the reservoirs—large storms would be needed to alleviate storage deficit conditions in Oroville and Don Pedro Reservoirs caused by the drought. They suggest the Oroville Reservoir would need a storm totaling 14.8 inches to fill completely, while the Don Pedro Reservoir would need a storm totaling 15.9 inches to fill. These values are an optimistic estimate as they assume all the rainfall happens at once, with no drainage or evapotranspiration. These storms are sizable when considering the average annual precipitation of the 8-Station Index (i.e. eight monitoring stations in the Northern Sierra area) is 50 inches, and of the 5-Station Index (i.e. five stations in the San Joaquin area) is 40.8 inches.
Climatic water deficit (CWD) accumulated through water-year 2015 as a percent of the baseline CWD (average year for 1951-1980). Source: Updated from Flint and others, 2013.
One concept scientists use to understand soil water deficit is the climatic water deficit (CWD). This indicator is calculated as potential evapotransipiration (the amount of water used by plants), minus actual evapotranspiration, and reflects the loss of water in the soil, as well as the soil-water deficit accumulated throughout the year as a result of landscape vegetation demands. Water-year 2015 accumulated between 125% and 400% greater CWD than baseline over about 90% of the state. In the Central Valley that has deep soils, and for the Modoc Plateau that had July precipitation the CWD was normal for water-year 2015. High CWD indicates high landscape stress, which in California has lead to massive forest die-off and extreme wildfire conditions in 2015, as well as very poor forage conditions on rangelands.
The studies conducted by Dettinger and Anderson, and Flint and Flint, are telling of the impact both snow and rain have upon our vital California Reservoirs. Without a doubt, California needs precipitation to help alleviate drought conditions, but studies such as these can help managers plan for new water storage options, and to optimally manage current reservoirs so that Californians can be better prepared for future drought.
During this unprecedented drought, the rising temperatures in many of California's rivers have become potentially lethal to anadromous fish, steel head and other fish species. Even in rivers controlled by reservoirs, where operators have traditionally been able to help control river temperature by timed releases, the combination of low flows, reduced cold-water pools in reservoirs and high air temperatures has resulted in elevated river temperatures.
Low water levels can be seen on the American River from Watt Ave. in Sacramento on January 16, 2014. From California Department of Water Resources archive.
"Stream temperature has long been recognized as an important water-quality parameter. Temperature plays a key role in the health of a stream's aquatic life, both in the water column and in the benthic habitat of streambed sediments. Many fish are sensitive to temperature. For example, anadromous salmon require specific temperature ranges to successfully develop, migrate, and spawn [see Halupka and others, 2000]. Metabolic rates, oxygen requirements and availability, predation patterns, and susceptibility of organisms to contaminants are but a few of the many environmental responses regulated by temperature." 1
In order to understand how river temperatures are being affected by drought and climate change, careful collection and analysis of temperature data is key. USGS stream gages, already established throughout California and the nation to monitor river flows, are ideally located to also monitor temperature in California's rivers. Of the approximately 500 USGS steam gages in California, 151 monitor temperature continuously, 132 collect data that data in real-time.
This graph shows temperature data collected at a stream gage on the American River. Temperatures above the orange bar are increasingly dangerous to steelhead egg incubation and parr-smolt transformation. Temperatures in excess of the red bar are increasingly dangerous to migrating juvenile steelhead. 2
This graph shows temperature data collected at a stream gage on the American River. Temperatures above the orange bar are increasingly dangerous to steelhead egg incubation and parr-smolt transformation. Temperatures in excess of the red bar are increasingly dangerous to migrating juvenile steelhead.
In June 2014, assisted by such temperature monitoring data, the California Department of Fish and Wildlife forecasted that summer water temperatures would exceed 78°F in the American River-too warm for young trout and salmon to survive. To save fish, CDFW decided to evacuate the American River and Nimbus Hatcheries that raise rainbow trout, salmon and steelhead (respectively) from eggs through release size. This was the first time all stocks of fish, totaling 430,000 fingerlings, were evacuated and released six months ahead of the normal February release time. Scientists will know in Summer 2017 if this action was successful.
In another drought-related incident, high Sacramento River temperatures in 2014 resulted in loss of the winter and fall run salmon in the upper river when water released from Lake Shasta was too warm to sustain incubating salmon. To prevent further losses of the federally protected endangered specie, the U.S. Fish and Wildlife Service, in cooperation with California Department of Fish and Wildlife, decided to truck juvenile salmon from Coleman National Fish Hatchery near Redding, to Rio Vista in the Bay-Delta during this drought. Trucking the fish downstream shortens their trip and avoids the lethal high-temperature river waters, but reduces the likelyhood that fish will return upstream in the Sacramento to spawn. However, officials believe that the transported salmon will have a better chance of surviving the journey to the ocean and that more will be able to return upstream to mate. Additionally, the Shasta Temperature Management Plan was created in 2015 to conserve cold water storage in Lake Shasta. The plan, developed by the U.S. Bureau of Reclamation in coordination with the NMFS, USFWS, CDFW, the California Department of Water Resources and the State Water Resources Control Board, makes changes in the Federal Central Valley Project and State Water Project operations in order not to lose temperature control in September, as happened in 2014.
Long-term tempertaure data is one of the foundations to understanding the quality of water in California's and our nation's waterways. Baseline and continuous river temperature information is needed so that resource managers can understand how ecosystems fare under normal climatic conditions, and so that they can make agile and informed decisions to better steward those resources as conditions change.
1USGS Circular 1260, Introduction, page 1, Heat as a tool for studying the movement of ground water near streams
22009 NOAA National Marine Fisheries Service (NMFS) Biological Opinion for the Long-term Operations of the Central Valley Project and the State Water Project, pg 285: "Temperatures of 52°F or lower are best for steelhead egg incubation. However temperatures less than 56°F are considered suitable." Yellow bar on graph is 56 °F. pg 288-89, "Steelhead in the American River exhibit symptoms of thermal stress ... at temperatures over 65°F", which is the Orange bar on the graph.
USGS California Water Science Center researchers are monitoring the San Joaquin Valley area of the Central Valley for land subsidence, a compaction of the aquifer and subsequent sinking of the land surface, associated with increased pumpage of groundwater during the current drought. The Central Valley is of concern due to it's contributions to the Nation's agricultural supply, and the State's water supply infrastructure that runs through the Valley.
Groundwater is essential to the water supply in the Central Valley. On average, groundwater provides about 50% of water supply in the Central Valley. The City of Fresno's main water supply, for example, comes from groundwater. About 20% of the Nation's total groundwater pumping is from the Central Valley.
Graph showing surface water deliveries and cumulative storage changes simulated by the Central Valley Hydrologic Model (CVHM). One cubic kilometer is about 811,000 acre-feet. Since the majority of the surface water delivery system has been in place, the CVHM simulates that on average about 40% of the water supply of the Central Valley has come from groundwater (ranging from about 30% during wet years to 70% during dry years). Over time, the extra pumping has stressed the aquifer, which for decades has had an overall loss in storage. The Central Valley has been depleted by about 1.85 km3 per year on average since 1960 (Faunt et al. 2009), and has been depleted about twice this rate during the current drought.
The hydrology of the present-day Central Valley is driven by surface-water deliveries and associated groundwater pumpage, which in turn reflect the spatial and temporal variability in climate, water availability, and land use. Since the early 1990s, the availability of surface water has decreased because of operational changes of the federal Central Valley Project and the California State Water Project. Although irrigation has become more efficient, since 2000, land use in the Central Valley has trended toward the planting of permanent crops (vineyards and orchards), replacing non-permanent land uses. During the recent droughts of 2007-2010 and 2012-present, groundwater pumping has increased from the combined effects of the drought and these changes. The increased pumping has re-initiated subsidence. In order to document historical subsidence and monitor continued changes, the USGS has gathered and interpreted data from a variety of source.
The land surface can decline when groundwater levels drop as a result of groundwater being withdrawn faster than it is replenished. In order to show the extent of this, scientists interpreted data from satellites to map land subsidence from 2008 to 2010. Due to satellite communication loss, recent drought-related data were unavailable. New satellite data have become available this year. To fill the gap, the U.S. Geological Survey used other methods to monitor subsidence in California and is analyzing that data now to determine recent drought-related subsidence.
Subsidence is reducing the capacity of water conveyances and other infrastructure that transport floodwater and deliver water to agriculture, cities, industries and wildlife refuges across California. Systems such as the Delta-Mendota Canal, California Aqueduct, Eastside Bypass, and various local canals are at risk for damage such as reduced freeboard (the distance between the water surface and infrastructure that crosses it, such as bridges), damaged panels of lined canals, erosion in unlined canals, and damaged wells. Land subsidence can also exacerbate flooding and damage pipelines, roads, and railways.
ALOS interferogram showing large subsidence feature affecting Checks 15-21, January 2008-January 2010, San Joaquin Valley, California. Graphs showing subsidence computed from repeat leveling surveys along Highway 152 for 1972-2004 and along the Delta-Mendota Canal for 1935-1996, subsidence computed from GPS surveys at selected check stations for 1997-2001, and contours showing subsidence measured using PS InSAR during March 9, 2006-May 22, 2008, San Joaquin Valley, California. The above is preliminary data and is subject to revision.
The Sacramento-San Joaquin River Delta (Delta) is part of the largest estuary on the West Coast. It is a vitally important ecosystem and the hub of California's water supply system, providing water for more than 25 million Californians, and for millions of acres of farmland in the San Joaquin Valley. The Delta is a transition zone where saltwater from the Pacific Ocean and San Francisco Bay meets freshwater from the Sacramento and San Joaquin Rivers. Its network of waterways and numerous islands provide habitat for hundreds of aquatic and terrestrial species, including the threatened Delta smelt and winter–, fall– and spring-run Chinook salmon. The Delta is home to over 500,000 Californians and is scattered with historic towns and approximately 500,000 acres of agriculture, and supports many services, including transportation, utilities, and recreation. The Delta's many uses have resulted in numerous conflicts over its limited resources.
As in all estuaries, salt water brought in on tidal flows cause daily fluctuations in Delta salinity. However, during dry years, when freshwater rivers flowing into the Delta are low, tidal movements exert a greater influence over Delta waters, resulting in saltwater being pushed farther upstream. This salinity moves upstream into Delta waterways on flood tides, affecting the quality of the water for human use, as well as for a number of environmentally sensitive species, such as delta smelt and Chinook salmon. Freshwater flows were so low in late January, 2014 that the USGS flow network recorded low upstream flows unprecedented at the Freeport stream monitoring gage.
Location of hydrodynamics flow stations in the Delta (Source: US Geological Survey California Water Science Center Estuarine Hydrodynamics Program). Click image to enlarge.
The U.S. Geological Survey (USGS) installed the first gage to measure the flow of water in the Sacramento River in the late 1800s. Today a network of 35 hydro-acoustic meters measures flow throughout the waterways, sloughs and islands that comprise California's Sacramento-San Joaquin River Delta. With the data provided by this flow station network — updated every 15 minutes and available from the National Water Information System (NWIS) — state and federal water managers make critical daily decisions about how much freshwater can be pumped for human use, at which locations in or out of the Delta, and when. Fish and wildlife scientists, working with water managers, also use this information to protect fish species affected by pumping and loss of habitat. The data can also be helpful in determining the success or failure of efforts to restore ecosystem processes in what has been called the "most managed" watershed in the country.
Water operations in the Delta are complex, controversial, and often pit human needs against those of estuary and species health. These operations include diversions of freshwater from the Sacramento River at the Delta Cross Channel; diversions at the pumping stations in the southern Delta (Federal Central Valley Project and the California State Water Project) that withdraw water for human consumption, and industrial and irrigated agricultural uses south of the Delta; and the need to maintain fresh water in the Delta for agriculture. While restrictions on water deliveries have been designed to protect Delta fish species, they also reduce the flexibility to meet statewide water supply needs. The addition of drought conditions and the associated water quality issues are added challenges to those tasked with balancing the many needs and uses of water from the Bay-Delta estuary.
Buoy and Instrument package at Liberty Island Superstation. (Source: US Geological Survey California Water Science Center Biogeochemistry Program)
The dynamic and complex nature of the Bay Delta system makes it difficult to understand the transport and fate of contaminants. CAWSC scientists are working to add technology to existing flow stations that can measure a broad suite of physical, optical, particle, and water quality indicators and report that data in real–time. This multi–analytical approach allows for better integration of environmental factors and more powerful diagnostic tools. It allows scientists and water managers to continuously characterize parameters related to nutrient uptake, phytoplankton community structure, zooplankton and fish foraging efficiency at the same time as more common water quality information (e.g., dissolved oxygen, nutrients, and pH) in the context of the existing flow and turbidity monitoring network. Co-locating these sensors can provide better information about ecosystem and drinking water quality fluxes.
Delta smelt (Source: U.S. Fish and Wildlife Service)
Populations of several native fish species, including Chinook salmon, Central Valley steelhead and delta smelt, declined dramatically over the last two decades. Biological opinions under the state and federal Endangered Species acts rendered to protect these species have been the principal drivers of water supply curtailments in recent years. However, several key gaps remain in the knowledge of life history, habitat preferences, and the factors limiting abundance. The CAWSC Hydrodynamics Program has been conducting multi-disciplinary, collaborative research studies to inform decision makers about strategies to reduce the mortality of these species due to Delta water operations.
CAWSC has maintained long-term monitoring programs, and a broad program of multi-disciplinary research in the Bay-Delta and its watersheds. These studies help us understand the extent of changes to the system related to drought, and also help decision-makers use science to help mitigate adverse effects of drought.The current and future studies in the Bay-Delta are part of USGS efforts to understand and help inform sound management of the estuary's resources to help meet the dual goals of the Delta Plan: