Ekaterina Rakhmatulina

Findings: Over 47 years of wildfires has little impacts on vegetation and soil moisture of one basin but has great impacts on another basin.

We used satellite imagery to quantify vegetation transitions (Figure 1) during the 47 years of managed wildfire in SCB (1971-2018) and compared these transitions to the changes observed in ICB (Boisramé et al, 2018).

Figure 1: Vegetation transitions throughout wildfire management in SCB (top row) and SCB (bottom row).

Both spatially distributed and in situ soil moisture data were collected in SCB and ICB and categorized by different vegetation types (mixed conifer, dense meadow, sparse meadow, and shrub). Dense meadows had the highest soil moisture values, but there was little difference between all other cover types in SCB (Figure 2).

Figure 2: Distribution of soil moisture (in terms of volumetric water content) for each site-date-vegetation class combination in SCB.

Fire-driven changes in dominant vegetation type (observed from aerial imagery analysis) and forest structure (observed from forestry plot data) were minimal at SCB, especially when compared to ICB (Figure 3).

Figure 3: Percent of the total vegetated area covered by each vegetation class for both Illilouette Creek Basin (ICB) and Sugarloaf Creek Basin (SCB).

This can be partially explained by SCB's lower productivity (Figure 4) associated with lower precipitation totals (56-88% of ICB's precipitation). We infer that the managed fire regime in SCB has not led to large changes in soil moisture regimes in the basin.

Figure 4: Normalized difference vegetation index (NDVI; averaged across a given basin for a given date), a proxy for productivity, was consistently higher in Illilouette Creek Basin than Sugarloaf Creek Basin.

Hydrological benefits of historically observed wildfire management on a medium sized watershed are expected to have similar effects in future climate.

The hydrological effects of restoring wildfire in Sierra Nevada forests are of great interest to managers, given the importance of these forests to California's water supply. Data analysis and past modeling studies of the Illilouette Creek Basin (ICB) found that managed wildfire increased streamflow yields from the basin by approximately 5% compared to during fire suppression, and did so during a period when streamflow in other regional basins was declining (Boisramé et al. 2017, Boisramé et al. 2019). However, the changes in the ICB fire management policy were initiated nearly 50 years ago. Fire management changes implemented now and over the coming years in Sierra Nevada basins will occur under a different climate and fire regime than what was experienced during the 20th Century (Figure 1).

Figure 1: The historical modeling time period is from 1972 to 2012 (A), denoting observed 40 years since the end of fire suppression policies in ICB. Future modeling time period is from 2030 to 2070 (B) and two climate scenarios are used. Blue, orange, and red vertical lines denote historical, 30%, and 60% increase in frequency of fire events, respectively.

Figure 2 (right hand panel) shows how basin-scale streamflow and soil moisture storage change following the restoration of a fire regime under historical, rcp 4.5, and rcp 8.5 climate scenarios. The results indicate that despite minor differences in the timeseries, broadly there is no difference in the impact of fire on ICB hydrology under future versus historical climates: in both cases, restoring fire regimes in the basin leads to an increase of 15 mm/year specific discharge and a maximum increase of 32-33 mm soil water storage. This streamflow increase equates to ~$1.5-2.3 million of hydropower revenue and $6.0-9.2 million in water sales per year.

Figure 2: Modeling results of streamflow and soil moisture changes; positive change indicatesa relative increase in the variable as a result of fires. Left panel shows results for different fire frequency regimes under rcp 4.5 climate and right panel shows historical fire frequencysimulations under three different climate scenarios. Shaded areas indicate the 95% confidenceinterval for the simulations. Vertical orange lines represent timing of observed historical fires.

Wildfires are a cause of soil water repellency (hydrophobicity), which reduces infiltration while increasing erosion and flooding from post-fire rainfall. Post-fire soil water repellency degrades over time, often in response to repeated wetting and drying of the soil. In mountainous fire prone forests such as those in the Western USA, however, the fire season often terminates in a cold and wet winter, during which soils not only wet and dry, but also freeze and thaw. Little is know about the effect of repeated freezing and thawing of soil on the breakdown of post-fire hydrophobicity. This study characterized the changes in hydrophobicity of Sierra Nevada mountain soils exposed to different combinations of wet-dry and freeze-thaw cycling (Figure 1). Following each cycle, hydrophobicity was measured using the Molarity of Ethanol test. Hydrophobicity declined similarly across all experiments that included a wetting cycle. Repeated freezing and thawing of dry soil did not degrade soil water repellency (Figure 1).

Hydrophobicity as measured by MED test for eleven cycles of five treatments; the treatments are dry/freeze/thaw (DFT), wet/freeze/thaw (WFT), wet/dry (WD), wet/dry/freeze/thaw (WDFT), and wet/freeze/thaw/dry (WFTD). Actual measurements are shown as dots, and mean of six replicas is presented as a line. Average MED of non-burned soil is indicated by a gray dash line. Average MED for a reference soil that was burned but without treatment application measured throughout the experiment is indicated by a black dashed line.

Total soil organic matter content was not different between soils of contrasting hydrophobicity. Macroscopic changes such as fissures and cracks were observed to form as soil hydrophobicity decayed (Figure 2). Microscopic changes revealed by scanning electron microscope imagery suggest different levels of soil aggregation occurred in samples with distinct hydrophobicities, although the size of aggregates was not clearly correlated to the change in water repellency due to wet-dry and freeze-thaw cycling (Figure 3).

Figure 2: Images of eleven cycles of wet/freeze/thaw treatment for one of the replicas. Cycle ``0" is heated but untreated soil sample. Visible racking on soil surface is highlighted in white.

Fuel moisture is a crucial determinant of fire behavior and risk. Fuel moisture is usually described in terms of live fuels (living plants) and dead fuels. Although fuel moisture is often observed to provide input into fire risk assessments, it is usually not possible to measure it everywhere, forcing fire managers to rely on proxy fuel moisture indices. Mostly, these indices draw on atmospheric conditions. However, dead fuels are typically in hydraulic contact with soils, and a body of previous work suggested that soil moisture influenced fuel moisture, potentially creating another mechanistic link between fire and water cycles. In this project, I establish whether the relationship exists between soil moisture and dead fuel moisture across multiple locations and moisture gradients.