In the early 1970s, the National Park Service made a bold decision about fires in the remote Illilouette Creek (Illilouette) and Sugarloaf Creek (Sugarloaf) Basins in Yosemite and Sequoia/Kings Canyon National Park.  They reversed 100 years of fire suppression, allowing naturally occurring fires to burn without intervention. 

Fifty years later, these "managed wildfire" basins - particularly Illilouette - are different to the rest of the Sierra Nevada.  Fire area and severity has increased since the 1970s in the Sierra Nevada - but fires in these basins remain unchaged, frequent, and small.  Their outlines make a "jigsaw" pattern, where the last fire forms a boundary stopping the spread of the next one.  The landscape is diverse, ranging from dead snags, dense regenerating pine trees, and, excitingly for water scientists, places where new wetlands are forming from dry forest.   

I wanted to understand how the changes to the fire regime had, over 50 years, changed the vegetation in these watersheds, and what it meant for water resources.  I wanted to know if the basins were wetter, and if this would alter future fires in the Sierra Nevada.  I wanted to equip students in the outdoor education facility NatureBridge to make their own measurements to help answer these questions.  The questions are hard to answer - no one in the 1970s collected information about how water was changing as the fires returned.  So, I needed to combine remote sensing, air photos, field studies, laboratory experiments and numerical models in a piece of hydrological detective work.   

Luckily, aerial photographs of Illilouett and Sugarloaf were taken right at the start of the managed wildfire period - smoke from the first fire that would burn freely in Sugarloaf is even visible at the edge of one of the images! The types of vegetation visible in these early air photos (and later ones too) could be worked out using object-oriented classification, a kind of image analysis that I could test in 2012, when air photos were taken and detailed vegetation mapping was done in Illilouette.  The interpretation of the images was good, and our project produced a series of maps showing how the forest cover changed as the managed wildfire period went on.  In Illilouette, forest cover had dropped by 1/4, from about 80% to about 60% forest cover. Sugarloaf, though, had barely changed at all.   

To understand how water was changing, we took thousands of soil moisture measurements in Illilouette and Sugarloaf.  Wetness of the soil was closely related to vegetation cover, letting us use our vegetation maps to estimate changes in soil water.  The results showed especially large increases in wet soil in the middle of Illilouette.  We provided students at NatureBridge with their own soil moisture measurement equipment and developed teaching modules to use the equipment in outdoor classrooms.   We installed weather stations and soil moisture monitoring in areas where meadows, forests and shrublands abutted.  Snowpack melted much faster under forest canopies, but persisted in the meadows, which also stayed much wetter through summer.  These data and flow observations in Illilouette Creek helped us train a hydrologic model.  We ran the model using our vegetation maps, and compared the results to when we ran it with 1970 forest cover.  The results show that Illilouette now produces about 50 mm/yr more streamflow than it did when fire-suppressed.  We re-ran all the modeling for future climate scenarios, and found the same streamflow increases due to managed wildfire.   We measured fuel moisture and found that the wetter soils in Illilouette shorten the fire season and change fire risk.    

More fires in Illilouette and Sugarloaf could also increase the negative outcomes of wildfire for water - erosion and flooding.  But, our modeling and flow data showed no increase in flooding.  Lab experiments showed that the water repellent soils that contribute to flooding and erosion after fires were destroyed by freezing. The cold, wet winters in Illilouette and Sugarloaf likely reduce erosion risks.   

Finally, we wanted to explain why Sugarloaf forest cover hadn't changed while Illilouette forest cover had.  We trained and tested a simple model on all the data we'd collected.  The model showed that the lower fire frequency in Sugarloaf is likely explains the lack of change in that basin.  

Many forested locations aren't suitable for managed wildfire: they-re too close to roads, settlements, or infrastructure. But for places where we can use managed wildfire, this study tells a success story.  Thanks to managed wildfire, Illilouette is more diverse, safer from fire hazards and drought, and provides more water.  Sugarloaf shows us that this experience can't be repeated unless enough fires occur on the landscape.  This project tells us that careful and judicious use of managed wildfire should be on the policy agenda of those seeking to improve resilience in Sierra Nevada forests. 

Last Modified: 10/18/2021
Modified by: Sally Thompson