Aerial view of desert body of water with 2 house boats

Editor’s Note: We look forward to any opportunity to highlight women who are outstanding examples of climate scientists and globally recognized as leaders in their areas of expertise.  Kathy Jacobs is the Director of the Center for Climate Adaptation Science and Solutions (CCASS) at the University of Arizona. We invite you to learn more about Kathy and her accomplishments in this short film from Landmark Stories: One Nation Under Science

Water is our most precious resource. Water is life and as global citizens, we need to educate ourselves about the future of water regardless of our geographical location. Kathy’s article explores issues through the lens of the Tucson region but has global application. Thank you to Kathy for sharing this article with our community and thank you for taking this opportunity to learn more about water.

‘That’s the effect of living backwards,’ the Queen said kindly: ‘it always makes one a little giddy at first’  ‘Living backwards!’ Alice repeated in great astonishment. ‘I never heard of such a thing!’ 

‘— but there’s one great advantage in it, that one’s memory works both ways.’  From Through the Looking Glass, Chapter 5: Wool and Water

The tricky part about adapting to climate change is trying to understand what exactly we are adapting to.  Most water managers and other decision-makers would appreciate accurate predictions of the future, and many feel uncomfortable making decisions about climate adaptation given the wide range of possible future conditions. Yet the alternative, which is to do nothing, is almost never the prudent option.  Thankfully, we do have a general understanding of the trends that are already visible, and can use these to guide our choices. 

If, like the White Queen in Through the Looking Glass, we lived backwards, all of our decisions would be much easier to make.  But since we, like Alice, can’t do that, we need to use the best scientific information that is available, along with a good dose of common sense, to anticipate future risks and opportunities.  Looking into the future, what can and can’t we learn from the past? And how can understanding current trends, as well as the basic physics of the earth system, help decision-makers make the best decisions?  And how well do our current water supply and past decisions protect us from future impacts?

Until about a decade ago, water managers assumed that the climate since the last ice age has been essentially stationary.  Though there was plenty of variability, it was thought to be variability around a stable average climate condition.  What we are now experiencing is a new paradigm: variability around accelerating trends of changing climate.  The earth’s atmosphere is warming because greenhouse gases (carbon dioxide and methane are the largest culprits) are trapping more of the sun’s energy than would otherwise have occurred, and human activity is now widely understood by scientists to be the largest driver of increasing global temperatures.

This means that we can no longer use past water supply conditions as a looking glass to the future, but it doesn’t mean that there are no lessons to be learned from history.  Looking into the past using tree rings and other sources of data, we can see that there have been floods and droughts that far exceed what has been experienced since people started recording river flows.  That helps us understand how serious the impacts of these events can be even in the absence of climate change, and the information about mega-droughts and floods in the past is sobering.

Climate Change and Water

So what are the trends that we are already observing about the relationship between water and climate change?  Virtually all observations of temperature on the globe show increases over the last 50 years, and all recent national and international science assessments have concluded that it is virtually certain that global temperatures will continue to increase over the next 50 years, if not the coming century. 

Simple physical principles tell us higher temperatures will result in more evaporation and less runoff because warmer air holds more moisture and evaporates more water from land and water bodies.  In addition, transpiration (use of water by plants) increases when it is warmer.  So, in the absence of changes in precipitation, we will face a drier future, and both natural vegetation and agricultural crops will need more water to survive.  But for Southern Arizona, most climate models also anticipate at least some reduction in total precipitation.  This “double-whammy” in our region is why climate expert Jonathan Overpeck has called Arizona “ground-zero” for climate impacts.

We are already see drying of the land surface and reductions, on average, in river flows due to higher temperatures. Ironically, increased flooding is another likely result of warming.  How do we know this?  The same physical principle that causes warm air to hold more moisture means that when it does rain, it rains more intensely (more water per unit of time). And this is what scientists have been observing – the intensity of precipitation has been increasing to some degree regionally and quite substantially across the entire northern tier of the US and Canada. More intense precipitation, under the right conditions, can lead to flooding – so increases in both floods and droughts are anticipated as part of climate change impacts in many regions of the US.

What does all of this mean for water in the Tucson Region?  Recent work by Udall and Overpeck (2017) spells out what higher temperatures mean for our primary source of renewable supplies – the Colorado River.  Reductions in total flows could reach 20% by midcentury, and even higher by 2100 – even if average precipitation increases in the headwaters area.  Snowpack volume is also decreasing at lower elevations, and less water stored in snowpack affects runoff timing (when the peak flows occur) and total volume.  In addition to the impacts on the Colorado, this has serious implications for riparian and aquatic ecosystems in its tributaries and in other rivers and streams across the Southwest. 

In Arizona and Sonora, where riparian systems are already seriously threatened by water diversions and groundwater pumping, it is likely that heat rather than changes in precipitation will drive the largest impacts for natural systems. Increasing water temperature and water quality problems are also anticipated, presenting more challenges for biodiversity.

What are we going to do for water in 100 years?

Given all of this grim news, we might be tempted to jump back through the looking glass and avoid knowing what the White Queen knows.  But in reality, those who are in the Tucson basin have already made many investments to ensure future water supplies, so there’s also much positive news to report.  Because generations of politicians and water managers in Arizona have understood that increasing use of over-drafted groundwater in our desert environment seriously threatens our economy, Arizona adopted the 1980 Groundwater Management Act. Arizona (and our Federal Partners) completed the $4 billion Central Arizona Project (CAP) in the 1990s bringing renewable Colorado River water supplies through Phoenix to Tucson. 

We in the Tucson region have a more secure water supply due to investments in the largest municipal CAP allocation in the state, in multiple underground storage facilities, in a reclaimed water system, and in conservation.  The question we have to ask ourselves, and perhaps the Queen, is have we done enough to get us through the next 100 years?

Implications of climate change for the Central Arizona Project (CAP).

The majority of the Tucson basin’s water use is now supplied via the CAP, and the Arizona Department of Water Resources has declared its “Tucson Active Management Area” to now be at “safe yield.”   This means that the average amount of groundwater withdrawn from the aquifer is now less than the amount being recharged naturally and through artificial aquifer recharge projects (mostly through percolation basins in the Avra Valley and agricultural exchanges).  This is quite a remarkable feat given that Tucson’s metro area population has roughly doubled in the almost 40 years since the Groundwater Management Act.  The area was previously one of the largest urban areas in the US 100% dependent on groundwater. 

Our biggest concern now is no longer groundwater depletion but the supply in the Colorado River system. The major reservoirs on the Colorado are currently at or near their lowest levels since they were filled in the 1950s.  There is a more than 50% probability of a shortage in the near term, which poses serious implications for Arizona and the CAP.  We have already crossed the first shortage threshold in the Drought Contingency Plan, triggering reductions in delivery to Arizona.  The risk of Lake Mead reaching the even more critical low level of 1,025 feet is significant; using the most recent  data from the last 28 years of falling lake levels as the new normal, the probability is 50% by 2026.

Shortages on the Colorado River are a major issue for us because Arizona has the lowest priority CAP allocation among its neighbor states, and the Tucson area’s water supply is the most dependent on CAP water.  That said, with the current priority system, it is unlikely that Tucson’s municipal allotment will be curtailed any time soon because municipal supplies are cut last. The story for agricultural users, many of whom have been using excess CAP water, is very different, since excess water and agricultural supplies will be cut first.

Clearly, increasing temperatures and population, and decreasing flows in the Colorado, is not a good combination.  Though we have neither a magic mirror nor a crystal ball, a century from now there will almost definitely be more people and less agriculture in Arizona.  Urban land use already is less water consumptive than agriculture, and it would also be very surprising if we don’t achieve even more conservation than we have today thanks to  continued improvements in technology. Housing and land use patterns are currently changing in directions that reduce water use. And it is logical to expect that water will continue to become more expensive.  All these factors could help stem the historical trend in ever greater demands for water.  

Adaptation options for those connected to the CAP and effluent and municipal delivery systems are dramatically easier to achieve than options for those who are dependent entirely on groundwater.  This is a very significant challenge for the region: the “have’s” and “have-not’s” in Southern Arizona are rather clearly demarcated by the service area of the CAP and the boundaries of the Tucson Active Management Area.  For the rest of Southern Arizona and northern Sonora, there are no significant protections from groundwater over-pumping, and no access to imported sources of renewable supplies. 

And what about future water supplies for our unique natural environment?

It is hard to imagine ways to adequately prepare for the impacts of climate change and drought on riparian habitat and vegetation in general.  It is one thing to protect the urban system from the implications of severe drought, and quite another to protect the desert and mountain ecosystems that we all depend on for our own quality of life and that support the intricate network of biological assets of the region.

The implications of climate change for river and stream flows and riparian habitat have received very little attention, but may be the most visible and irreversible impact of climate change in our region.  The Queen would likely not be impressed by our failure to anticipate the multiple consequences of failing to protect what riparian and aquatic systems we have left.

Limitations of our regulatory systems

If we were living backwards, we would likely already know that the Groundwater Management Act, the Recharge and Recovery Act, and the associated Assured Water Supply rules (that require a 100 year supply of renewable water for new subdivisions) were giant steps forward in protecting the developed areas from the impacts of climate change—even though climate change was not a motivator for those investments.  Living in a desert, we already recognize water scarcity and benefit from the foresight of prior governors, businesses, farmers and members of Congress who recognized the value of water.  We are an example for others, but we still face dramatic challenges… and our environmental assets are at the top of the list.

With 20-20 hindsight, we can now see that the provisions of the Groundwater Management Act were totally inadequate to protect water supplies for our natural environment.  The Act focused on safe yield and on water for human use, not streamflow or environmental benefits.  Arizona has no overarching regulation that protects environmental assets. Other than the Santa Cruz Active Management Area (the Santa Cruz River basin north of Nogales and south of Amado), where protecting the flows of the river from pumping is a goal, no provisions explicitly focus on preserving river flows or habitat. Furthermore, the provisions of the Groundwater Management Act are largely limited to the Active Management Areas, which cover less than a quarter of Arizona’s lands.

What is the landscape of the future going to look like?

Climate change impacts on ecosystems in our region are already substantial.  We have seen very large wildfires destroy significant forested portions of the sky islands.  The regional drought that has gripped our region since roughly 2002 has had a big impact on the potential to support ranching in the area.  Perhaps of greatest concern is the impact on the few flowing streams that still exist in our region.  A large percentage of biodiversity in our region is dependent on riparian areas that are drying up.  This past year, Sabino Creek stopped flowing for the first time in recorded history.  Over the past several decades, the cottonwood canopy in the Tanque Verde area has diminished considerably.  The San Pedro’s plight has received national and international attention. 

These issues are not unique to our region, and the implications are not limited to biodiversity. 

What we also know – beyond broad, general statements of concern, is that water-related impacts of climate change will vary by basin for a number of reasons.  This  means that the risk to water supplies and environmental assets varies dramatically from one basin to the next, and therefore the adaptation options that need to be considered will not be a “one size fits all” solution.

What we don’t know

Obviously, it would be great to have perfect knowledge of what is at risk and what the best adaptation options might be.  But there are still many gaps in our knowledge of existing and future trends.  For example, it would be nice to know:

  •  What are the implications of changes in seasonality of runoff on groundwater recharge rates?
  •   Will there be increased recharge during large flood events, and might that be enough to offset the overall losses in recharge that come from increased drought and heat?                
  •   What changes might there be in availability of alternative supplies (e.g., municipal effluent)?  Might we go to direct potable reuse of effluent, which would reduce its availability for landscaping and riparian flows?
  •   What might the effect of changes in energy supplies and costs be?  Will a transformation to renewable energy or new cooling technologies change the demand and use of water?
  •   Will surface water shortages result in more groundwater pumping at a time when energy costs are increasing?
  •   How will global economic trends, such as shifts in agricultural production or economic downturns, affect water demand and supply?

The Lower Santa Cruz Basin Study

 To address some of these gaps in our knowledge, a regional effort is underway to understand the implications of climate change for water supply and demand.  The project is led by the US Bureau of Reclamation, in partnership with Pima County, the City of Tucson, the Arizona Department of Water Resources, and the Central Arizona Project, along with numerous other water utilities, agricultural, tribal and mining interests.  The University of Arizona, through its Center for Climate Adaptation and Solutions, is also a project partner, providing technical and scientific support. 

The Lower Santa Cruz Basin Study (LSCBS) is a technical assessment of supply and demand imbalances in the Tucson Active Management Area through 2060; it will not produce specific recommendations for action, but will establish a foundation for future risk management action by local entities.  It will evaluate the costs and benefits of adaptation options to enhance water security for water users and the environment.

The LSCBS is the first study supported by either the state or the federal government to look at climate impacts on the surface and groundwater supply in the Tucson region.  By using the latest version of a regional climate model developed by Chris Castro and Hsin-I Chang at the University of Arizona, it will generate more accurate and relevant insight for this particular region than currently exists.  In addition, there is significant effort to study the impacts of climate change on riparian areas and other environmental assets, and to evaluate the effectiveness of adaptation options that achieve environmental protection.  

What adaptation options are there?

Now that we have at least a partial view through the looking glass – what are the responses we can consider?  The good news is that we do have a multitude of options, and many are practices with which we are already familiar. They include integrated water management – which really means thinking about all water supplies as part of a portfolio of sources and matching quality to use.  Recycling of municipal effluent, and matching the quality of that water with specific uses such as landscape irrigation, is a good example.  Although Tucson is already a leader in conservation, even more can be accomplished without significant impacts on our quality of life. 

Another option that is already in place but may be expanded is using the groundwater aquifer to store surplus water during wet years in order to ‘bank’ it for later use.  Understanding where it is most beneficial to store water, and where it is best to pump it, can be a powerful tool for protecting environmental assets. 

Stormwater capture and storage, also known as rainwater harvesting, has received substantial attention and should be part of a package of strategies.  But it is challenging for a number of reasons.  First, it only works when it rains; higher temperatures and more extended droughts reduce its effectiveness.  Second, if we have more intense precipitation, flood control is more challenging – even though it means more water is available on a temporary basis.  Third, there are health issues if retention ponds or water harvesting systems are not managed to control mosquitoes and other disease vectors.

An adaptation option that is receiving significant attention across the US, including in Pima County, is the use of green infrastructure.  For instance natural greenways can be developed for flood control as opposed to cement-lined channels.  Such projects provide co-benefits, including environmental enhancement and recreation, while also reducing flood risk.


In contrast to the White Queen, whose memory goes in both directions, we have to use our own imperfect understanding of future conditions to manage climate-related risks.  The good news is that we have a very firm water management foundation from which to begin this adventure.  The not-so-good news is that we have many challenges ahead, and though we have a broad solution set, it is not clear whether our current and future decision makers will be as motivated, congenial and innovative as their predecessors in overcoming those challenges.  Investing in the future, in light of all of this complexity can “make one a little giddy at first.” But nothing is more reassuring than looking back through the looking glass, and appreciating the way Arizonans came together in the past to solve the grand water challenges of their day.

Kathy Jacobs

Kathy Jacobs is a professor of Environmental Science at the University of Arizona and Director of the Center for Climate Adaptation Science and Solutions (CCASS), focused on building adaptation and assessment capacity at multiple scales. CCASS is a component of the Arizona Institutes for Resilience, and builds capacity to accelerate adaptation and on-the-ground solutions to climate issues.  From 2010 – 2013, Jacobs worked in the Office of Science and Technology Policy in the White House. She was director of the Third National Climate Assessment, and the lead advisor on water science, policy, and adaptation. From 2006-2009 Jacobs was Executive Director of the Arizona Water Institute, a consortium of Arizona’s three universities focused on water sustainability. She worked 23 years for the Arizona Department of Water Resources, including 15 as the director of the Tucson Active Management Area.  She was engaged in multiple aspects of implementing Arizona’s Groundwater Management Act, including development of water conservation programs and the Assured Water Supply Rules.  Jacobs earned her M.L.A. in environmental planning from Berkeley.

Please note, this article was originally published in Sonorenis, Arizona-Sonora Desert Museum, in 2018