Modeling the changes to the hydrological cycle after a wildfire
In the Western United States, we have seen increases in wildfire frequency and severity. The dry conditions from the onset of climate change has added fuel to the fire. The burning of tens to thousands of square kilometers has left profound changes to the landscape, primarily by removing vegetation, but also by leaving layers of ash and burnt soil. Undoubtedly, these changes to the landscape perturb the balance of hydrological processes. However, the complex feedbacks between these processes makes anticipating the changes to the hydrological cycle challenging.
In California, many of the wildfires occur in the Sierra Nevada mountains, which are the source of 70% of California’s water resources. Understanding the feedbacks and implications of disturbances on the hydrological cycle can help watershed managers plan for future scenarios with wildfires and climate extremes. To this end, Maina & Siirila‐Woodburn (2019) used a modeling approach to identify the changes in the hydrological cycle in a typical Californian catchment – the Cosumnes River – extending across the Sierra Nevada Mountains and the Central Valley (Figure 1). The study aimed to establish the impacts of wildfires on evapotranspiration, infiltration, snowpack, and surface water and groundwater storages, as well as the feedbacks between these processes.
The authors use the coupled ParFlow–Community Land Model, a high resolution, high fidelity, physically-based integrated model. This model can capture hydrological processes as well as account for spatially distributed vegetation processes, such as land cover changes post-wildfire.
To capture the effects of wildfires on the hydrological cycle, the authors ran the model using three different scenarios: (1) baseline, (2) post-fire in a wet year, and (3) post-fire in a dry year. Baseline, or pre-fire, was simulated using pre-wildfire land cover, whereas post-fire simulations incorporated burn scars represented as barren soil (complete loss of vegetation). Wet and dry years were selected to capture the extreme weather events expected to occur with climate change.
Generally, the study found that wildfires in the Cosumnes River catchment resulted in decreased evapotranspiration, a larger snowpack, and increased runoff and groundwater volume.
The decrease in evapotranspiration can be attributed to the loss of vegetation (Figure 2), as it will limit the volume of water transferred to the atmosphere. With the removal of vegetation from the fire, and thus the pathway to transpire water from plant stomata, the only pathway remaining is the evaporation of water from soils and other pooled sources of water.
In both climate scenarios, post-fire simulations show an increase in the accumulated snowpack when compared to pre-fire conditions. The authors found that the greater snow accumulation was due to the lack of interception of snowfall from the canopy. The spatial distribution of the changes in the snowpack is however not uniform across the burn scars, with larger magnitudes in areas of topographical depressions and areas receiving lower radiation from the sun (Figure 3).
The increase in simulated surface streamflow magnitudes are a consequence of the decrease in evapotranspiration and increase in snowpack volume. The spring freshet, the runoff from thawing of snow and ice, in post-fire scenarios is greater due to increased snowpack accumulation. The impacts of wildfires are not always limited to areas immediately adjacent to the burned areas. In post-fire scenarios, the water would typically flush much faster in the headwaters, causing increased flows in downstream areas, such as in the Central Valley (Figure 1). While the wet year simulation has a greater change in streamflow magnitudes, results show that in the post-fire scenarios the water levels in the Cosumnes River and its tributaries can rise to three meters at any given time in both climatic scenarios (Figure 4).
In both the wet and dry year simulations, groundwater storage, measured by pressure head in the groundwater layer, is greater in the post-fire scenarios than the baseline. The increase in groundwater storage occurs in the areas underlying the burn scars and in low lying areas where there is highly permeable rock (Figure 5). In the wet year scenario, groundwater storage also increased in the area surrounding the rivers. This increase is the result of the surface water storage creating a pressure gradient driving exchange between surface water and groundwater.
Dr. Fadji Zaouna Maina and Dr. Erica Siirila-Woodburn’s modeling study demonstrates the complex and sometimes counterintuitive effects of wildfires on a catchment’s hydrological cycle. With these results, we can begin to understand and identify areas that are vulnerable to wildfires in Californian catchments. Planning and managing for current and future changes to water resources in a water-scarce region like California is essential for sustainable water management, and this study provides a detailed look at what we might expect.
Dr. Fadji Zaouna Maina is a Postdoctoral Fellow in the Energy Geosciences Division at Lawrence Berkeley National Laboratory. In 2019 she was part of the Forbe 30 under 30 Class of 2020. Find her on Twitter at @Yafadj.
Citation: F. Z. Maina, E. R. Siirila‐Woodburn, Watersheds dynamics following wildfires: Nonlinear feedbacks and implications on hydrologic responses. Hydrol. Process. 34, 33–50 (2019).