Wildfires raise risks of floods, debris flows, major geomorphologic and sedimentologic change, and water quality and quantity shifts. A principal control on the magnitude of these changes is ...field‐saturated hydraulic conductivity (Kfs), which dictates surface runoff generation and is a key input into numerical models. This work synthesizes 73 Kfs datasets from the literature in the first year following fire at the plot scale (≤ 10 m2). A meta‐analysis using a random effects analysis showed significant differences between burned and unburned Kfs. The reductions in Kfs after fire, expressed by the ratio of Kfs Burned/Kfs Unburned, were 0.46 (95% confidence interval of 0.31‐0.70) combining wildfire and prescribed fire and 0.3 (95% confidence interval of 0.13‐0.71) for wildfire. No significant differences for Kfs were observed between wildfire and prescribed fire or moderate and high fire severity. Both Kfs magnitude and variability depended more on measurement method than measurement support area at the plot scale, with methods applying head ≥0.5 cm producing larger estimates of Kfs. It is recommended that post‐fire efforts to characterize Kfs for modeling or process‐based interpretations use methods that reflect the dominant infiltration processes: tension infiltrometers and simulated rainfall methods when soil matrix flow dominates and ponded head methods when macropore flow is critical. Published 2019. This article is a U.S. Government work and is in the public domain in the USA.
Measured soil field‐saturated soil hydraulic conductivity in the first year after wildfire and prescribed fire in forests from 73 sites across the globe is synthesized and analyzed using meta‐analysis.
A series of virtual experiments were conducted using a one‐dimensional numerical model of unsaturated flow (Hydrus‐1D) to investigate the factors responsible for shifts in subsurface hydrologic ...response following wildfire. These virtual experiments used a series of well‐characterized experimental plots in the area affected by the 2010 Fourmile Canyon Fire near Boulder, CO, USA, that cover north and south facing slope aspects for plots both affected and unaffected by wildfire. Inverse estimation of soil‐hydraulic parameters allowed establishing “foundation simulations” that served as the basis for virtual experiments to test findings from interpretation of field and laboratory data and extend understanding beyond what could be gleaned from data alone. The numerical model virtual experiments showed that loss of transpiration because of vegetation combustion/mortality caused soils to be wetter at depths greater than 5 cm on both north and south facing slopes, which agrees with field observations. Loss of tree canopy interception contributed to wetter subsurface conditions and loss of litter/duff increased evaporation, drying soils, in the top few cm on north facing slopes. On south facing slopes, at depths shallower than 3 cm, the simulations did not reproduce the observed trends of drier soils after wildfire, which suggests that more robust methods are needed to simulate evaporation and soil‐water retention at soil‐water contents less than 0.05 cm3 cm−3. Simulated bottom boundary flux, a proxy for groundwater recharge, was greater in north facing burned plots by a factor of 3.5–5.2 and greater on south facing plots by a factor of 10.6–12.7, relative to unburned plots.
Key Points
Wildfire impacts on hydrology were simulated with an unsaturated flow model
Transpiration and canopy interception loss after wildfire led to wetter soils
Simulated bottom boundary flux from the soil was greater in fire‐affected plots
•Reasons for use of distributed process-based hydrological models are reviewed.•Avenues for developments of process-based hydrological models are presented.•Hydrology will depend on appropriate use ...of process-based models.
Process-based hydrological models have a long history dating back to the 1960s. Criticized by some as over-parameterized, overly complex, and difficult to use, a more nuanced view is that these tools are necessary in many situations and, in a certain class of problems, they are the most appropriate type of hydrological model. This is especially the case in situations where knowledge of flow paths or distributed state variables and/or preservation of physical constraints is important. Examples of this include: spatiotemporal variability of soil moisture, groundwater flow and runoff generation, sediment and contaminant transport, or when feedbacks among various Earth’s system processes or understanding the impacts of climate non-stationarity are of primary concern. These are situations where process-based models excel and other models are unverifiable. This article presents this pragmatic view in the context of existing literature to justify the approach where applicable and necessary. We review how improvements in data availability, computational resources and algorithms have made detailed hydrological simulations a reality. Avenues for the future of process-based hydrological models are presented suggesting their use as virtual laboratories, for design purposes, and with a powerful treatment of uncertainty.
This work examined the plot‐scale differences in soil‐water retention caused by wildfire in the area of the 2010 Fourmile Canyon Fire in the Colorado Front Range, United States. We measured ...soil‐water retention curves on intact cores and repacked samples, soil particle‐size distributions, and organic matter content. Estimates were also made of plant‐available water based on the soil‐water retention curves. Parameters for use in soil‐hydraulic property models were estimated; these parameters can be used in unsaturated flow modeling for comparing burned and unburned watersheds. The primary driver for measured differences in soil‐water retention in burned and unburned soils was organic matter content and not soil‐particle size distribution. The tendency for unburned south‐facing soils to have greater organic matter content than unburned north‐facing soils in this field area may explain why unburned south‐facing soils had greater soil‐water retention than unburned north‐facing soils. Our results suggest that high‐severity wildfire can “homogenize” soil‐water retention across the landscape by erasing soil‐water retention differences resulting from organic matter content, which for this site may be affected by slope aspect. This homogenization could have important implications for ecohydrology and plant succession/recovery in burned areas, which could be a factor in dictating the window of vulnerability of the landscape to flash floods and erosion that are a common consequence of wildfire.
Key Points
Organic matter reduction had the largest wildfire impact on soil‐water retention
Wildfire removes organic matter‐driven soil‐water retention differences
Aspect may contribute to pre‐wildfire differences in soil‐water retention
We collected soil‐hydraulic property data from the literature for wildfire‐affected soils, ash, and unburned soils. These data were used to calculate metrics and timescales of hydrologic response ...related to infiltration and surface runoff generation. Sorptivity (S) and wetting front potential (Ψf) were significantly different (lower) in burned soils compared with unburned soils, whereas field‐saturated hydraulic conductivity (Kfs) was not significantly different. The magnitude and duration of the influence of capillarity during infiltration was greatly reduced in burned soils, causing faster ponding times in response to rainfall. Ash had large values of S and Kfs but moderate values of Ψf, compared with unburned and burned soils, indicating ash has long ponding times in response to rainfall. The ratio of S2/Kfs was nearly constant (~100 mm) for unburned soils but more variable in burned soils, suggesting that unburned soils have a balance between gravity and capillarity contributions to infiltration that may depend on soil organic matter, whereas in burned soils the gravity contribution to infiltration is greater. Changes in S and Kfs in burned soils act synergistically to reduce infiltration and accelerate and amplify surface runoff generation. Synthesis of these findings identifies three key areas for future research. First, short timescales of capillary influences on infiltration indicate the need for better measurements of infiltration at times less than 1 min to accurately characterize S in burned soils. Second, using parameter values, such as Ψf, from unburned areas could produce substantial errors in hydrologic modeling when used without adjustment for wildfire effects, causing parameter compensation and resulting underestimation of Kfs. Third, more thorough measurement campaigns that capture soil‐structural changes, organic matter impacts, quantitative water repellency trends, and soil‐water content along with soil‐hydraulic properties could drive the development of better techniques for numerically simulating infiltration in burned areas.
Predictions of post‐wildfire flooding and debris flows are needed, typically with short lead times. Measurements of soil‐hydraulic properties necessary for model parameterization are, however, seldom ...available. This study quantified soil‐hydraulic properties, soil‐water retention, and selected soil physical properties within the perimeter of the 2017 Thomas Fire in California. The Thomas Fire burn scar produced catastrophic debris flows in January 2018, highlighting the need for improved prediction capability. Soil‐hydraulic properties were also indirectly estimated using relations tied to soil‐water retention. These measurements and estimates are examined in the context of parameterizing post‐wildfire hydrologic models. Tension infiltrometer measurements showed significant decreases (p < .05) in field‐saturated hydraulic conductivity (Kfs) and sorptivity (S) in burned areas relative to unburned areas. Wildfire effects on soil water‐retention were dominated by significant decreases in saturated soil‐water content (θS). The van Genuchten parameters α, N, and residual water content did not show significant wildfire effects. The impacts of the wildfire on hydraulic and physical soil properties were greatest in the top 1 cm, emphasizing that measurements of post‐fire soil properties should focus on the near‐surface. Reductions in Kfs, θs, and soil‐water retention in burned soils were attributed to fire‐induced decreases in soil structure evidenced by increases in dry bulk density. Sorptivity reductions in burned soils were attributed to increases in soil‐water repellency. Rapid post‐fire assessments of flash flood and debris flow hazards using physically‐based hydrologic models are facilitated by similarities between Kfs, S, and the Green–Ampt wetting front potential (ψf) with measurements at other southern CA burned sites. We suggest that ratios of burned to unburned Kfs (0.37), S (0.36), and ψf (0.66) could be used to scale unburned values for model parameterization. Alternatively, typical burned values (Kfs = 20 mm hr−1; S = 6 mm hr−0.5; ψf = 1.6 mm) could be used for model parameterization.
Post‐wildfire hydraulic properties were measured, showing major impacts for hydraulic conductivity, sorptivity, saturated soil water content, and bulk density. Fire effects were concentrated in the top 1 cm of soil.
Hydrologic recovery after wildfire is critical for restoring the ecosystem services of protecting of human lives and infrastructure from hazards and delivering water supply of sufficient quality and ...quantity. Recovery of soil‐hydraulic properties, such as field‐saturated hydraulic conductivity (Kfs), is a key factor for assessing the duration of watershed‐scale flash flood and debris flow risks after wildfire. Despite the crucial role of Kfs in parameterizing numerical hydrologic models to predict the magnitude of postwildfire run‐off and erosion, existing quantitative relations to predict Kfs recovery with time since wildfire are lacking. Here, we conduct meta‐analyses of 5 datasets from the literature that measure or estimate Kfs with time since wildfire for longer than 3‐year duration. The meta‐analyses focus on fitting 2 quantitative relations (linear and non‐linear logistic) to explain trends in Kfs temporal recovery. The 2 relations adequately described temporal recovery except for 1 site where macropore flow dominated infiltration and Kfs recovery. This work also suggests that Kfs can have low hydrologic resistance (large postfire changes), and moderate to high hydrologic stability (recovery time relative to disturbance recurrence interval) and resilience (recovery of hydrologic function and provision of ecosystem services). Future Kfs relations could more explicitly incorporate processes such as soil‐water repellency, ground cover and soil structure regeneration, macropore recovery, and vegetation regrowth.
Upper magnitude limits and scaling with basin size for post-wildfire floods are unknown. An envelope curve was estimated defining post-wildfire flood upper limits as a function of basin area. We show ...the importance of separating peak flows by floods versus debris flows. Post-wildfire flood maxima are a constant 43 m
s
km
for basins from 0.01 to 23 to 34 km
and then declining with added basin area according to a power law relation. Intense rainfall spatial scaling may cause the envelope curve threshold at 23 to 34 km
. Post-wildfire flood maxima are smaller than unburned flood maxima for similar basin area. Rainstorm comparisons indicate that post-wildfire floods are triggered by smaller precipitation depths than unburned floods. Post-wildfire exceptional floods are driven by extreme rainfall rates, in contrast to post-wildfire debris flows. Runoff rates for post-wildfire envelope floods are consistent with infiltration-excess runoff. Future increases in precipitation intensity or wildfire frequency and extent could increase post-wildfire flood upper limits.
Permafrost thaw alters subsurface flow in boreal regions that in turn influences the magnitude, seasonality, and chemical composition of streamflow. Prediction of these changes is challenged by ...incomplete knowledge of timing, flowpath depth, and amount of groundwater discharge to streams in response to thaw. One important phenomenon that may affect flow and transport through boreal hillslopes is development of lateral perennial thaw zones (PTZs), the existence of which is here supported by geophysical observations and cryohydrogeologic modeling. Model results link thaw to enhanced and seasonally-extended baseflow, which have implications for mobilization of soluble constituents. Results demonstrate the sensitivity of PTZ development to organic layer thickness and near-surface factors that mediate heat exchange at the atmosphere/ground-surface interface. Study findings suggest that PTZs serve as a detectable precursor to accelerated permafrost degradation. This study provides important contextual insight on a fundamental thermo-hydrologic process that can enhance terrestrial-to-aquatic transfer of permafrost carbon, nitrogen, and mercury previously sequestered in thawing watersheds.
Burn severity influences on post-fire recovery of soil-hydraulic properties controlling runoff generation are poorly understood despite the importance for parameterizing infiltration models. We ...measured soil-hydraulic properties of field-saturated hydraulic conductivity (Kfs), sorptivity (S), and wetting front potential (ψf) for four years after the 2013 Black Forest Fire, Colorado, USA, at six sites across a gradient of initial remotely sensed burn severity using the change in the normalized burn ratio (dNBR). These measurements were correlated with soil-physical property measurements of bulk density (ρb), loss on ignition (LOI, a measure of soil organic matter), and ground cover composition to provide insight into causal factors for temporal changes in Kfs, S, and ψf. Modeled infiltration using the Smith-Parlange approach parameterized with measured Kfs, S, and ψf further discerned the role of precipitation intensity on runoff generation.
Temporal trends of soil-physical properties and ground cover showed influences from initial burn severity. Trends in soil-hydraulic properties, surprisingly, were not strongly influenced by initial burn severity despite inferred effects of ρb, LOI, and ground cover on trends in Kfs and S. Calculations of dNBR at the time of sampling showed strong correlations with Kfs and S, demonstrating a new approach for estimating long-unburned Kfs and S values, infiltration model parameters after fire, and assessing the time of return to pre-fire values. Simulated infiltration-excess runoff, in contrast, did depend on initial burn severity. Time series of the ratio S2/Kfs ≈ ψf tended to converge between 1 and 10 mm four years after wildfire, potentially (i) defining a long-unburned forest domain of S2/Kfs and ψf from 1 to 10 mm with relatively high Kfs values, and (ii) providing a new post-fire soil-hydraulic property recovery metric (i.e. S2/Kfs ≈ ψf in the range of 1 to 10 mm) for sites in the Rocky Mountains of the USA.
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•Physical/hydraulic property measurements elucidate burn severity effects.•Physical property trends did depend on initial burn severity.•Hydraulic property trends did not depend on initial burn severity.•Sorptivity stabilized faster than hydraulic conductivity after wildfire.•Functional recovery and statistical recovery of infiltration may not align.
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