Drought, climate change, and California water management

Coulterville DWRClimate change will stress all aspects of our water management system, particularly the Delta; however, multiple strategies do exist to minimize the effects, says Dr. Ted Grantham

The ongoing and extensive drought has had an effect on all aspects of California’s natural environment and agricultural resources, but it may be a sign of things to come.  Most climate change research predicts that  with global warming, there will be more variability in California’s climate, with more intense storms, longer dry periods, and less snowpack.  How will these changes affect water management in California?

In this presentation from the UC Division of Ag and Natural Resources’ Insights: Drought and Water series, Dr. Ted Grantham, Cooperative Extension Specialist in climate and water, University of California’s Agriculture and Natural Resources Division builds upon Dr. Patak’s presentation on the effects climate change on agriculture, with a presentation focusing on the effects of drought and climate change on California’s water management system and particularly the Delta.

The main message that I want you to take home today is that the current drought offers a glimpse into our future in California,” said Dr. Grantham.  “We can expect that just as the drought has affected much of our system, that climate change will stress all aspects of our water management system, particularly in the Delta.  However, multiple strategies do exist to minimize the effects of climate change and drought.”


In 2016, Northern California received above average precipitation; however, much of the state has actually received less than average rainfall in this winter and is still in the condition of extreme drought, so it’s important to keep in mind that drought is not over, he said.

UCANR Grantham_Page_08Droughts are a natural occurring feature in California; we’ve had a lot of droughts in the past and we can expect to have more in the future, so what is it about this drought that is really unique?” Dr. Grantham presented a slide from a recent paper that explored drought patterns in the state of California.  The chart shows the Palmer Drought Index from 1880 to present day; he explained that the Palmer Drought Index is a measurement of drought severity that combines information on precipitation and temperatures to give a measure of moisture in the environment.

When that Palmer Drought index falls below negative 1, we consider that to be a drought in the state, so all those dots that you see in the lower portion of this graph represent droughts that we’ve had in history,” he said.  “What you see is that from the start of this period of record which is 1896, we’ve had about 14 severe droughts from 1896 to 1994, and we’ve had six drought since about 1995.  We look at the percentage of time that we’ve experienced droughts, and in the earlier record, we only had droughts about 14% of the time, whereas in this past 20 years, we’ve had droughts about 30% of the time.  There seems to be an increase in the frequency of drought in California.  So what’s going on here?

UCANR Grantham_Page_09“When we look at historic precipitation and temperature patterns, again these dots represent our drought years, we see that the precipitation patterns have been highly variable over the past century or so, but there’s no apparent trend in our statewide precipitation,” Dr. Grantham said. “However, when we look at our temperature graph here on the right, we see a significant increase in statewide temperatures associated with climate change.  What we see is that during years of droughts, we tend to have higher temperatures, so in the future, it seems to be saying that we can expect to see a higher frequency of drought because of this general increase in temperatures statewide.”

UCANR Grantham_Page_10Another paper published in 2015 projected future increased in drought and flooding, he said.  “So these general patterns of increase in climate extremes are predicted by our current climate models, and the numbers suggest that we expect to see an increase of about 50% in the number of severe drought and flood events in the state by the end of this century.”

So what does this mean for California water?  “Some of the direct impacts of these trends are associated with less snow and earlier snowmelt, prolonged lower flows in our rivers and streams, higher water temperatures and sea level rise,” he said.  “I’m going to talk about what this actually means for our water supply system, but before that, I want to provide a little bit of context about California’s climate.”

UCANR Grantham_Page_12California has strong variability in its precipitation patterns; there is strong seasonal precipitation, he said.  “The graph here in the upper left shows our precipitation patterns for the San Francisco Bay Area, and what we see (and this is fairly true for most of the state) is that we receive about 90% of our rainfall and precipitation between November and March or April, followed by a prolonged dry season,” he said.  “We also have extraordinary interannual variability, and the lower map shows the interannual variability in rainfall across the U.S. and we can see that California really is an outlier.  Those darker colors of greens and blues represent much higher variability and interannual rainfall, so much greater frequency of wet and dry years compared to much more stable conditions in the eastern US.”

UCANR Grantham_Page_13Not only is there strong temporal variability in the way that the rain falls within years and across years, there is also strong spatial variation in where the rainfall falls on the state.  Dr. Grantham noted that the map shows runoff patterns:  “This is the water that’s actually coming off the landscape that can be used to fill our reservoirs and flow in our rivers.  Most of the runoff that’s generated in the state comes from the areas that are in blue.  Those darker and lighter blues represent about 40% of California’s land area but generate about 80% of our total runoff.”

The light and darker oranges also represent about 40% of the landscape but they only generate about 1% of our state’s annual runoff, so there’s a large difference in where and how much runoff is generated across the state,” he said, noting that the southern Central Valley is shown in dark orange, meaning one of the state’s most important agricultural producing regions in the state that naturally produces very little rainfall locally.

UCANR Grantham_Page_14In order to cope with the extreme seasonal and interannual variation, the state has built a massive system of water storage and conveyance infrastructure to move water around the state.  What you see here in the boxes are various large dams, they are roughly sized by capacity.  There are federal, state, and local projects that manage a network of dams, aqueducts and canals.

So through this water storage and distribution system, we’re basically able to move a drop of water that falls in the Trinity Alps in Northern California down to the tap of a resident in San Diego; it’s really extraordinary system of water storage and distribution.”

UCANR Grantham_Page_15So what are these trends in these climate and hydrology mean for this water storage and distribution system?  “First, one of the most pronounced effects of climate change and drought is on our snowmelt in the Sierra Nevada mountains, which is the major mountain system running on the eastern side of the state,” he said.  “These graphs show trends and projections into the future of various climate parameters.  What you see first is that we don’t expect to see a significant change in precipitation … when we look at cold season precipitation, basically precipitation that is falling as rain or snow, for this period of record, we don’t see a significant trend, starting from 1916 looking into the future.”

However when we look to the right of cold season temperatures, we see a significant increase in those temperatures consistent with statewide patterns,” he said.  “The warmer cold season temperatures mean that less precipitation will be falling as snow, and so that panel on your lower left shows the decrease in the percentage of precipitation that is falling as snow in these mountain regions.  It also means a general shortening of the snow season, and so the last day of the snow season is expected to become earlier and earlier.

UCANR Grantham_Page_16Less snow means less water supply for the state,” Dr. Grantham said, presenting snowpack projections under different climate scenarios for the period of 2070-2099 under two different climate scenarios.  “We can expect to see somewhere between a 20 to 40% decrease in the amount of water available as snowpack which supplies our system.


These changes in the snow runoff and water availability have implications for the entire water management system, but I’m going to focus specifically on the San Francisco Bay Delta, which is generally considered California’s water management hub,” he said.

UCANR Grantham_Page_19The Bay Delta is located at the confluence of the two largest rivers in the state of California, the Sacramento and San Joaquin rivers, where they meet and flow into the San Francisco Bay which is where the saltwater from the ocean mixes with freshwater to create seasonally variable saline conditions.

The reason the Delta is so important from a water management perspective is that we have several major diversion pumps in the southern part of the Delta that take freshwater from the Sacramento and San Joaquin rivers and deliver that to the south for agricultural and municipal water needs,” he said.  “These water exports are really critical to water users in Southern California.  They deliver on average over 9 million acre-feet of water from these water management projects, which support nearly 4 million acres of irrigated agriculture and contribute to the drinking water supplies for about 25 million people in Southern California.”

UCANR Grantham_Page_22The Delta also is an ecosystem and supports an extraordinary diversity of fish and wildlife that have been severely affected over the past century of land use and water use in the Delta region, and what we’re actually seeing is the collapse of several estuarine and migratory fish species in this region, including the Delta smelt, the longfin smelt, striped bass, and winter run Chinook that pass through the Delta on their migration to their spawning areas in rivers in Northern California.

Dr. Grantham acknowledged there are several factors that have contributed to the decline of these freshwater fishes.  “But one important contributing factor appears to be the relationship and the dependence of these freshwater fishes on Delta outflows, and these are data that show that basically the abundance of longfin smelt are highly correlated with the amount of outflow from the Delta.

The graph on the right shows the patterns and volume of water coming out of the Delta from the 1930s to the present time; it also shows the amount of water that’s being diverted or withdrawn upstream of the Delta, and the amount of water that’s been exported from the water projects in the Delta.

There’s been a high variation in the amount of outflows and a general increasing volume of water has been withdrawn upstream of the Delta,” said Dr. Grantham.  “Starting about the 1960s, we’ve seen a slight growth in the volume of water exported from the Delta by those major projects.  What’s important to note here is that when that purple line falls below those bars, it basically means that more than half of the water that would naturally flow to the Delta is being used upstream or being exported.”

In the past 40 years, there have been about 15 above normal and wet years in terms of precipitation, but the ecosystem itself has experienced only 7,” he continued.  “There’s actually been only one naturally occurring critically dry year in the past 40 years, but the ecosystem has experienced 19 because of these upstream water uses and exports.  What this means is essentially is the Delta, as an ecosystem, has experienced has experienced a prolonged artificial drought.

UCANR Grantham_Page_25The way the Delta as California’s water management hub works is in order to get freshwater from these rivers into these pumps to export to the south, a sufficient level of outflow must be maintained to prevent the saltwater from the Bay from coming into the Delta.  “In other words, we must maintain a barrier of flow coming out of the Delta in order to prevent the water that we want to send south from being contaminated by salts, so this is a really important constraint that this management system has to deal with,” Dr. Grantham said.

As I mentioned earlier, there are also these fish species in the Delta that have been experiencing severe population declines that also require and benefit from outflows from the Delta,” he said.  “However, one of the misconceptions that I want to address is this idea that these fish species are the primary constraint on exports to Southern California.  This is an issue that has been very politicized in our state.”

UCANR Grantham_Page_27Dr. Grantham then presented a chart showing how much water exports are actually being constrained by environmental versus other constraints in the Delta ecosystem.  “We looked at the data from the last three or four years, and looked on a day by day basis at what seems to be constraining exports from the Delta, and what we see is that for about 75% of the time, the constraints on Delta exports are actually related to the need to maintain the salinity barrier where the Delta flows into the Bay.  Less than 20% of the time are Delta exports actually restricted by regulations to protect fish species such as salmon and Delta smelt.”

UCANR Grantham_Page_28While salinity is an important constraint in the system, it’s particularly disturbing to look at projected sea level rise, said Dr. Grantham, presenting a map showing the Delta with an estimated 3 feet of sea level rise.  “Clearly, most of the Delta would be at least partially inundated by these rising sea levels which will make it likely difficult if not impossible to continue managing that salinity barrier where it currently is today.  This suggests that our ability to export water from the Delta is going to be severely even further restricted under future climate change scenarios and this is likely to affect agriculture that’s already suffering from other stressors.”

UCANR Grantham_Page_29He presented a map developed by the California Energy Commission that shows the agricultural regions with the greatest vulnerability, noting that many of the vulnerable regions occur south of the Delta.  “Given that water supplies are likely to become even more limiting than they are today, we really need to focus on these vulnerable areas and take steps to build the resilience of these communities.”


So in summary, the frequency of drought appears to be increasing in California.  Less snow and earlier runoff means less water supplies for people and the environment.  The collapse of several native and estuarine and migratory species will likely lead to tighter restrictions on Delta water management operations, but really salinity and other water quality requirements are currently the greatest constraint on Delta exports and are likely to become more so.”


UCANR Grantham_Page_31We’ve actually seen some really promising movement to adapt California’s water management system to new and changing conditions,” Dr. Grantham said.  “Even before the drought, we started to see a decline in both urban and agricultural water use, despite significant increases in ag productivity and population growth, so this is very promising.  And there is certainly room for improvement, but we should not forget the progress that we’ve made in improving water use efficiency, managing flood and drought risk and protecting the environment.”

Meeting our varied demands for water use in California will continue to be a challenge, particularly under warmer, and variable climate future, and difficult decisions and tradeoffs will continue to need to be made.  But there are several promising strategies that exist and are being explored right now that we should be aware of.”

Groundwater management:  California has made significant progress in managing its groundwater resources, the importance of which cannot be overstated, he said.  “If managed sustainably, the ability to access and use groundwater particularly in times of drought gives us tremendous flexibility to minimize impacts and sustain productivity, particularly in the ag sector.  In general, the water supply benefits of sustainable groundwater management far exceed those that could be achieved through raising or expanding our surface water system.  It’s too early to know the outcome of the 2014 Sustainable Groundwater Management Act, but it’s certainly a step in the right direction.”

Environmental management:  “One of the lessons we’ve learned from our current drought is that we’re largely unprepared to deal with its impacts to the environment.  Many environmental protections were rolled back, and several ad hoc measures were put into place to save endangered species.  The state would greatly benefit from a more strategic and deliberate approach to ecosystem management.”

Multi-objective management: Tradeoffs among the multiple management objectives will continue to be necessary; there’s also tremendous potential for managing our environment for multiple benefits, and to really improve the resilience of both ecosystems and society.  Often the restoration and careful management of ecosystems can provide direct benefits to people, including recreation, water quality benefits, flood protection, and in some cases, even water supply benefits.”

Dr. Grantham said that such win-win approaches are really being put into practice now, such as the intentional flooding of agricultural fields on a temporary basis to provide habitat for migratory birds and fish as well as to provide recharge benefits to groundwater; levees being set back to restore the natural functions of floodplains and reduce flood risks downstream; and improvements in water use efficiency that are freeing up new supplies that can be dedicated to the environment to sustain endangered species and retain recreational and fisheries uses.

Overall we’re seeing a breakdown of our water management silos among water supply management, flood control management, and environmental management to explore these multi-benefit approaches that are really critical I think to creating a more resilient water management system overall.”


I’ve shown that our current drought really offers a glimpse into California’s water management future, and the steps we take today to deal with this drought will really benefit us as we deal with a warmer and more variable hydrology,” said Dr. Grantham.  “Climate change can stress all aspects of our water management system, particularly the San Francisco Bay Delta; however multiple strategies exist to really reduce these impacts that we can expect to see and overall maintain our water supply reliability and conditions of the environment.”


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