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Department of Environmental Studies and Sciences

Iris Stewart-Frey

Iris Stewart-Frey


Office: Varsi Hall 219
Phone: 1-408-551-7186

Ph.D., Stanford University

Teaching and Research Vision

My teaching vision is to engage and inspire every student in my class to become an active learner. I believe that if students have a solid foundation in the basics, a keen interest, and knowledge on how they learn best, they are well equipped to learn in the classroom and well beyond. I strive to kindle this interest, provide foundational knowledge in Environmental Studies and Sciences, and have students explore their learning preferences through a variety of teaching strategies, such as discussions, research projects, student presentations, workshops, fieldwork, case studies, and traditional lectures. In my classes we explore the geological, atmospheric, water, and soil processes and their interactions with society, pondering how plate tectonics is relevant for cell phones,  why more than half of the world?s population today does not have access to the quality of water systems available to the citizens of Rome 2,000 years ago, and how in 200 years we have managed to lose half of the topsoil of the Great Plains, which took hundreds or thousands of years to form. I also teach Geographical Information Systems (GIS), an approach to mapping and spatial analysis, which is used in many academic and professional fields. In the class, students conduct a research project using GIS. Several longer term student projects, such as vegetative mapping at the Blue Oak Ranch and Reserve and an investigation into food access and environmental justice issues in Santa Clara County have evolved from this classwork.
My research interests encompass environmental issues that affect the water cycle and water supply. I have worked on both shallow groundwater pollution issues and on surface water processes that are related to climate variability and climate change.
Leaching of pesticides to the water table is a common occurrence in intensively farmed agricultural regions. I have developed a type transfer function model (TTF) to assess pesticide leaching to groundwater at the regional scale. I have then used this model with in a GIS (Geographical Information Systems) framework to estimate Atrazine (a common herbicide) concentrations that would result at the water table from routine farming operations in the San Joaquin Valley in California (Stewart and Loague, 2003; 2004).

In the arid regions of western North America, nearly all precipitation arrives in the Winter and is stored as mountain snowpack. Hence, the spring snowmelt runoff is the main component for the region?s water supply, delivered to urban and agricultural regions through the summer drought period. I analyzed changes in the timing of the spring snowmelt runoff across western North America that have resulted from warmer air temperatures and precipitation changes to date (Stewart et al., 2005). This work documented that streamflow timing for snowmelt dominated streams has changed toward an earlier spring over the past several decades and for an area much larger than previously recognized (Fig. 1). These changes could have serious implications for the water supply of the dry southwestern regions and beyond. For another study, I used the relationships between past streamflow and climate to assess the impact of a warming climate on streamflow timing under a business-as-usual climate change scenario for the 21st century (Stewart et al., 2005). In follow-up work, I and collaborators investigated if the 2000 ? 2010 decade, which to date had been recorded as the warmest on record, had seen an acceleration of streamflow timing shifts using trend analysis and a statistical model (Fritze et al., 2011). This work found that although an acceleration of streamflow timing shifts is not clearly indicated, a set of highly vulnerable basins has experienced runoff regime shifts such that previously snowmelt dominated watersheds are now mainly rain dominated (Fig. 2).


Fig 1: Throughout the mountainous West, spring runoff timing has shifted towards earlier in the year over the past several decades.



Fig. 2: Changes in snowmelt domination categories. Orange symbols indicate regime shifts towards rain-dominated regimes.


Stream temperature, and associated water quality parameters play a critical role in the health of aquatic ecosystems. Warmer air temperatures and changes in the hydrologic cycle expected from global change are likely to affect stream temperature and general water quality, but these processes and interconnections have only recently been explored. Information on how stream temperature and water quality within mountain streams may change in the future could aid in the management of these watersheds. My collaborators and I are assessing how water flow and quality in mountain streams throughout California and the West are likely to be affected by projected climatic changes. This work is funded by a current research grant from the USEPA on which I am the PI. For our simulations, we are using output from an ensemble of Global Circulation Models to drive the SWAT (Soil and Water Assessment Tool) hydrologic model. Stream temperatures are affected by many factors, such as the heat exchange at the air/water interface, and the temperature of the contributing hydrologic component to a stream reach. Commonly, simulation models neglect the contribution of hydrologic processes to the water temperature in a stream. To better capture likely future changes, we developed a new model for assessing stream temperature that is based on individual hydrologic components (Fig. 3 and Ficklin et al. 2012a, WRR). Simulations showed that by the end of the century, hydrologic contributions to mountainous subbasins in California are likely to experience a shift towards earlier in the year. Spring and Summer flows are likely to decrease substantially (Fig. 4), and the ratio of snowmelt to rain runoff is expected to decrease (Ficklin et al., 2012b, JAWRA). A related study for the Mono Lake Basin, a sensitive and unique California ecosystem, indicated decreased water availability and a more frequent occurrence of drought conditions by the end of the century (Ficklin et al., 2012c, Climatic Change). In addition, stream temperature simulations project increases in Spring and Summer stream temperatures by1 to 6 ÂșC (Fig. 5) with concurrent decreases in dissolved oxygen. Our results suggest that large changes in water quantity and quality are likely under future climates. For the most vulnerable basins, dissolved oxygen levels are likely to fall below levels necessary to maintain aquatic life (Ficklin et al, 2012d, submitted to WRR).


Fig 3: The new stream temperature model calculates stream temperature as a function of the hydrologic inputs and surface air temperature within a subbasin.



Fig. 4: Under the A2 emissions scenario, Spring and Summer flow in Sierra Nevada (CA) mountain streams are likely to decline substantially. The simulations were performed using the SWAT hydrologic model driven by output from an ensemble of Global Circulation Models.
Fig 5: Median projected changes in Summer stream temperature for the Sierra Nevada under the A2 emissions scenario.
Currently, my colleagues and I are investigating future changes in the likelihood of extreme hydrologic events in the Sierra Nevada, the physical processes and characteristics of watersheds with are particularly sensitive to climate change, the likely impact of projected changes on aquatic indicator species. We are also expanding the assessment of future climate impacts on water flow and quality to the Colorado basin, the Pacific Northwest, and other mountain regions. In collaboration with ESS colleagues, I am using GIS to study issues of food access and environmental justice in Santa Clara County.
Currently working with me on the EPA funded study is postdoctoral researcher Darren Ficklin. Darren received his Ph.D. in hydrogeology from UC Davis and is an expert SWAT modeler. 

ENVS 20: The Water Wars of California

ENVS 23: Soil, Water, and Air

ENVS 115: GIS in Environmental Science

ENVS 116: Introduction to GIS

Ficklin DL, Stewart IT, Maurer EP, 2012. Projections of 21st century Sierra Nevada local hydrologic flow components using an ensemble of General Circulation Models. Jour. Am. Water Res. Assoc. (JAWRA), in press. 
Ficklin DL, Stewart IT, Maurer EP, 2012. Effects of projected climate change on the hydrology in the Mono Lake Basin, California. Climatic Change, in press.
Ficklin DL, Luo Y, Stewart IT, Maurer EP, 2012. Development and application of a hydroclimatological stream temperature model within SWAT. Water Resour. Res. Vol 48, W01511, doi:10.1029/2011WR011256.
Fritze H, Stewart IT, Pebesma EJ, 2011. Shifts in Western North American snowmelt runoff regimes for the recent warm decades. J. Hydromet. doi: 10.1175/2011JHM1360.1
Stewart IT, 2009. Changes in snowpack and snowmelt runoff for key mountain regions. Hydrologic Processes 23(1):78-94.
Maurer EP, Stewart IT, Bonfils C, Duffy PB, Cayan DR, 2007. Detection, attribution, and sensitivity of trends towards earlier streamflow in the Sierra Nevada, J. Geophys. Res., 112, D11118, doi:10.1029/2006JD008088.
Stewart, I.T., D.R. Cayan, and M.D. Dettinger. 2005. Changes towards earlier streamflow timing across western North America. Journal of Climate 18: 1136-1155.
Stewart, I.T., and K. Loague. 2004. Assessing groundwater vulnerability in the San Joaquin Valley (CA) with the type transfer function model. Journal of Environmental Quality 33:1487-1498.
Stewart, I.T., D.R. Cayan, and M.D. Dettinger. 2004. Changes in snowmelt runoff timing in western North America under a ?Business as Usual? climate change scenario. Climatic Change 62:217-232.
Stewart, I.T., and K. Loague. 2003. Development of type transfer functions for regional-scale non-point-source groundwater vulnerability assessments. Water Resources Research 39(12):1359.
Curriculum vitae
Curriculum vitae

In the News

May 17, 2021

Iris Stewart-Frey lead a study on the allocation of water from the Tuolumne River during extreme droughts.