Understanding Wildfires and Hydrological Cycles through Evapotranspiration (ET) in the Foothills of Southern California

Introduction

The increasing number of wildfires in California has caused loss of life and property. With more research on the causes and prevention of wildfires, we start to realize the complicity of wildfires not only concerning their origins but also their effects on different aspects of both natural and urban environments. Recent studies have focused on the relationship between climate change and fire frequencies, particularly on how the rising temperature contributes to fires in the region. In the context of a multi-year drought beginning in 2011, scientists in southern California need to conduct more studies on the relationship between wildfire disturbances and hydrological cycles because water plays a vital role in determining the susceptibility of an ecosystem to fire and in the post-fire resilience. This blog post 1) provides an overview of the existing studies on fire and hydrological cycles around the world, 2) describes how various ecological communities of the Southern California foothills respond to wildfire disturbances and changes in hydrological cycles, and 3) recommends future policies on wildfire prevention and management in the foothills.

Evapotranspiration: Significance & Application

Evapotranspiration, often referred to as ET, is the process of transporting moisture from the earth into the atmosphere. Evapotranspiration is the combination of the evaporation of water in the ground the transpiration of water from the ground through plants (Figure 1). Evapotranspiration is important because it is a crucial aspect of the hydrological cycle on the earth, and it accounts for about 10-15% of the water in the atmosphere. Factors that influence evapotranspiration include temperature, relative humidity, wind, solar radiation, etc.

Measurements of evapotranspiration are widely used in agriculture to provide guidelines for irrigation systems. ET is a good indicator of how much water has entered and left the plants. An ET-based irrigation system helps reduce water cost and increase agricultural productivity. Ecologists also use ET as a measurement to study the health of a forest ecosystem since it is a crucial aspect of the forest’s hydrological cycle.

Figure 1: Hydrological Cycle on Earth Surface, Highlighting the Components of Evapotranspiration

Existing Case Studies

Many studies have been conducted in Canada regarding the conditions of evapotranspiration in disturbed forests. A study conducted by Kettridge et al. (2006) indicates that subcanopy evapotranspiration critically determines peatland carbon stock vulnerability to wildfire. Besides, it affects water availability between different landscapes. Bond-Lamberty et al. (2009) concluded that wildfire is a significant driving factor in changes in hydrological processes in the Canadian boreal forest. These changes influence both the growth of the forest and the regional climate.

Recently, scientists have also conducted similar studies in western United States. Poon and Kinoshita (2018) studied the spatial and temporal trends of evapotranspiration after the 2011 Las Conchas Fire in New Mexico. They concluded that post-fire annual ET has decreased from pre-fire ET. Furthermore, they observed a shift of vegetation from mostly conifer to grassland, which also contributed to the decrease in the post-fire annual ET. Such observations suggest that when wildfire disturbances cause vegetation conversion, they alter the hydrologic cycle in the area as well. In the Sierra Nevada, Roche et al. (2018) studied the effects of fuel-load treatments, specifically thinning, on evapotranspiration. As the region has been facing water shortage, this study concluded that ET reduction is one of the potential benefits of the reduction of forest overstocking in the Sierra Nevada. Roche et al. pointed out that estimating ET flux variation in the Sierra Nevada, a region with frequent fire disturbances, could help predict forest trajectories during this age of climate change.

Vegetation Communities in the Foothills of Southern California

In contrasts to most regions described above, southern California requires more efforts to study the trends of evapotranspiration because of the increased frequency of wildfire disturbances. The main vegetation communities in the foothills of southern California include scrub-type vegetation, oak woodland, riparian vegetation, and grassland (City of Monrovia 2014). Distance from the coast, elevation, position on either the desert or coastal side of the mountains, and topography are some of the most significant factors that contribute to the type of vegetation at specific parts of the foothills (Stephenson and Calcarone 1999). All these vegetation communities are composed of different plants, and thus, have different rates of evapotranspiration, and responses to wildfire disturbance. Scrub-type vegetation in southern California includes chaparral, coastal sage scrub, and others (City of Monrovia 2014). For chaparral species, fire is a critical component of its life cycle, and more chaparrals would replace the burned ones. Coastal sage scrub vegetation could have fire behavior earlier in the year than chaparral and exhibit more post-fire responses than chaparrals. Even though most shrub types are adaptive to wildfires, too-frequent fire can still cause permanent damages to them. Compared to other vegetation, they also have more resistance towards droughts, especially coastal sage scrubs, thus, participating in the hydrological cycle in unique ways.

Oak woodland, often considered one of the iconic landscapes of California, is prevalent at the base of the San Gabriel Mountains. However, this type of vegetation is decreasing due to disease, development, and fragmentation. Native Americans have been setting periodic fires in oak woodlands to increase acorn yields (City of Monrovia 2014). Oak trees in these vegetation communities have developed fire resistance through developing thick barks. Oak saplings and seedlings are often killed by severe fires, and it takes about ten years for a typical oak woodland to recover after a severe fire. Riparian vegetation communities, simply the ecological communities along the edge of freshwater streams and rivers, are usually barriers to wildfire spread. The high susceptibility to invasive species in these communities has made them more prone to fire. These communities also possess unique characteristics in terms of hydrological cycles.

Grassland communities are not typically native to the foothills in southern California; instead, formation of the most of them is a result of grading, grazing, or other human-induced disturbances. Grasslands are highly prone to fires, yet they also come back the easiest since grassland fires are usually not as severe. Fire regimes in the foothill grassland communities have posed serious threats to native grass species that are already rare in the area.

The diversity and complexities of these communities require sophisticated planning to manage the increasing wildfires in southern California. Besides, considering the link between the California drought and the increased frequency of wildfire disturbances, scientists should spend more time studying the hydrological cycles in different vegetation communities as well. Especially those conducting post-fire researches should focus on understanding how fires have changed hydrological processes, which affect the composition and wellbeing of these vegetation communities in southern California.

Future Research and Policy Implications

Understanding hydrological cycles is critical to studying the ecological effects of wildfire disturbances. The hydrological cycle is an essential process to the wellbeing of any ecosystem, and such cycles operate differently depending on the biotic and abiotic factors. A wildfire disturbance is likely to dramatically change the physical conditions and ecological compositions of the affected area, thus altering the hydrological cycle as well. At the same time, hydrological cycles also vary based on many temporal and larger regional factors as well as climate change, and these cycles could also affect the composition of different vegetation communities including their proneness to wildfires. Therefore, areas that are prone to wildfires require close monitoring of their hydrological cycles. Hydrology in southern California needs more studies because the multi-year drought starting in 2011 has changed hydrological cycles in the region.

Many studies have monitored evapotranspiration (ET), an important aspect of the hydrological cycle, as an effective way to study the correlation between wildfires and hydrology. The United States Department of Agriculture (USDA) has a long history of tracking ET data for agricultural purposes and has published hydrological analyses of post-fire conditions (2016). With its prior experiences and studies, USDA should continue to fund researches to gain more sophisticated comprehension of hydrology in post-fire conditions, especially in southern California, a region that is lacking such studies. USDA should also collaborate with the United States Forest Services (USFS) to develop fire management and prevention plans that take the effects of and impacts on hydrology into consideration. Lastly, considering the diversity and complexities of the vegetation communities in the southern California foothills, local city governments along the foothills should also consult USDA and USFS to comply their specific plans for wildfire prevention and management with deeper understandings of the role hydrological cycles in wildfire disturbances.

Reference

Bond-Lamberty, B., S. D. Peckham, S. T., Gower and B. E. Ewers (2009). Effects of fire on regional evapotranspiration in the central Canadian boreal forest. Global Change Biology, Vol. 15, Issue 5, 1057-1363. doi: 10.1111/j.1365-2486.2008.01776.x

City of Monrovia (2014). Community Wildfire Protection Plan. https://www.cityofmonrovia.org/your-government/fire-department/prevention-permits-codes/community-wildfire-protection-plan

Kettridge, N., M. C. Lukenbach, K. J. Hokanson, C. Hopkinson, K. J. Devito, R. M. Petrone, C. A. Mendoza, and J. M. Waddington (2017). Low evapotranspiration enhances the resilience of peatland carbon stocks to fire. Geophysical Research Letters, 44, 9341–9349. https://doi.org/10.1002/2017GL074186

Poon, P. K., and A. M. Kinoshita (2018). Spatial and temporal evapotranspiration trends after wildfire in semi-arid landscapes. Journal of Hydrology, 559, 71-83. https://doi.org/10.1016/j.jhydrol.2018.02.023

Roche, J. W., M. L. Goulden, and R. C. Bales (2018). Estimating evapotranspiration change due to forest treatment and fire at the basin scale in the Sierra Nevada, California. Ecohydrology, Vol. 11, Issue 7. https://doi.org/10.1002/eco.1978

Stephenson, J. R., G. M. Calcarone (1999). Southern California mountains and foothills assessment: habitat and species conservation issues. General Technical Report GTR-PSW-175. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture; 402 p. https://www.fs.fed.us/psw/publications/documents/gtr-172/

United States Department of Agriculture (USDA). 2016. Hydrologic Analyses of Post-Wildfire Conditions. Hydrology Technical Note, 4. https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=39877.wba