Debris flows are a typical hazard on steep slopes after wildfire, but unlike debris flows that mobilize from landslides, most postwildfire debris flows are generated from water runoff. The majority of existing debris flow modeling has focused on landslide-triggered debris flows. In this study we explore the potential for using process-based rainfall-runoff models to simulate the timing of water flow and runoffgenerated debris flows in recently burned areas. Two different spatially distributed hydrologic models with differing levels of complexity were used: the full shallow water equations and the kinematic wave approximation. Model parameter values were calibrated in two different watersheds, spanning two orders of magnitude in drainage area. These watersheds were affected by the 2009 Station Fire in the San Gabriel Mountains, CA, USA. Input data for the numerical models were constrained by time series of soil moisture, flow stage, and rainfall collected at field sites, as well as high-resolution lidar-derived digital elevation models. The calibrated parameters were used to model a third watershed in the burn area, and the results show a good match with observed timing of flow peaks. The calibrated roughness parameter (Manning's n) was generally higher when using the kinematic wave approximation relative to the shallow water equations, and decreased with increasing spatial scale. The calibrated effective watershed hydraulic conductivity was low for both models, even for storms occurring several months after the fire, suggesting that wildfireinduced changes to soil-water infiltration were retained throughout that time. Overall, the two model simulations were quite similar suggesting that a kinematic wave model, which is simpler and more computationally efficient, is a suitable approach for predicting flood and debris flow timing in steep, burned watersheds. Key Points:Calibrated model parameters successfully modeled hydrographs with intermittent debris flows in an uncalibrated watershed Kinematic wave can be used rather than the full shallow water equations to predict flow timing Water-only models can simulate the timing of flow in postwildfire settings where floods transition to debris flows (2016), Model simulations of flood and debris flow timing in steep catchments after wildfire, Water Resour. Res., 52, 6041-6061, PUBLICATIONS processes controlling the hydrologic response of a watershed. For example, many process-based models are spatially explicit [Bl€ oschl et al., 2013]; therefore, these models can be used to understand the distribution of state-variables (e.g., flow velocity and depth) at different time steps during a rainstorm. In addition, process-based models often use parameters that reflect measurable landscape characteristics [Bl€ oschl et al., 2013], and thus parameter sensitivity testing can help to explain the overall process controls within a watershed. The structure of process-based models therefore offers an opportunity to test hypotheses, and to more fully explore the importance of different ...

Wildfire alters vegetation cover and soil hydrologic properties, substantially increasing the likelihood of debris flows in steep watersheds. Our understanding of initiation mechanisms of postwildfire debris flows is limited, in part, by a lack of direct observations and measurements. In particular, there is a need to understand temporal variations in debris‐flow likelihood following wildfire and how those variations relate to wildfire‐induced hydrologic and geomorphic changes. In this study, we use a combination of in situ measurements, hydrologic monitoring equipment, and numerical modeling to assess the impact of wildfire‐induced hydrologic and geomorphic changes on debris‐flow initiation during seven postwildfire rainstorms. We predict the impact of hillslope erosion on debris‐flow initiation by combining terrestrial laser scanning surveys of a hillslope burned during the 2016 Fish Fire with numerical modeling of sediment transport throughout a 0.12‐km2 basin in southern California. We use measurements of sediment thickness within the channel to constrain numerical experiments and to assess the role of channel sediment supply on debris‐flow initiation. Results demonstrate that debris flows initiated during rainstorms where hillslopes contributed minimally to the event sediment yield and suggest that large inputs of sediment from rill and gully networks are not essential for runoff‐generated debris flows. Simulations suggest that both the gradual entrainment of sediment and the mass failure of channel bed sediment can increase sediment concentration to levels associated with debris flows. Finally, postwildfire debris‐flow initiation appears closely linked to the same rainfall intensity‐duration threshold despite temporal changes in the sediment source, initiation processes, and hydraulic roughness.

Soil‐mantled pole‐facing hillslopes on Earth tend to be steeper, wetter, and have more vegetation cover compared with adjacent equator‐facing hillslopes. These and other slope aspect controls are often the consequence of feedbacks among hydrologic, ecologic, pedogenic, and geomorphic processes triggered by spatial variations in mean annual insolation. In this paper we review the state of knowledge on slope aspect controls of Critical Zone (CZ) processes using the latitudinal and elevational dependence of topographic asymmetry as a motivating observation. At relatively low latitudes and elevations, pole‐facing hillslopes tend to be steeper. At higher latitudes and elevations this pattern reverses. We reproduce this pattern using an empirical model based on parsimonious functions of latitude, an aridity index, mean‐annual temperature, and slope gradient. Using this empirical model and the literature as guides, we present a conceptual model for the slope‐aspect‐driven CZ feedbacks that generate asymmetry in water‐limited and temperature‐limited end‐member cases. In this conceptual model the dominant factor driving slope aspect differences at relatively low latitudes and elevations is the difference in mean‐annual soil moisture. The dominant factor at higher latitudes and elevations is temperature limitation on vegetation growth. In water‐limited cases, we propose that higher mean‐annual soil moisture on pole‐facing hillslopes drives higher soil production rates, higher water storage potential, more vegetation cover, faster dust deposition, and lower erosional efficiency in a positive feedback. At higher latitudes and elevations, pole‐facing hillslopes tend to have less vegetation cover, greater erosional efficiency, and gentler slopes, thus reversing the pattern of asymmetry found at lower latitudes and elevations. Our conceptual model emphasizes the linkages among short‐ and long‐timescale processes and across CZ sub‐disciplines; it also points to opportunities to further understand how CZ processes interact. We also demonstrate the importance of paleoclimatic conditions and non‐climatic factors in influencing slope aspect variations. Copyright © 2017 John Wiley & Sons, Ltd.

Postwildfire debris flows are frequently triggered by runoff following high‐intensity rainfall, but the physical mechanisms by which water‐dominated flows transition to debris flows are poorly understood relative to debris flow initiation from shallow landslides. In this study, we combined a numerical model with high‐resolution hydrologic and geomorphic data sets to test two different hypotheses for debris flow initiation during a rainfall event that produced numerous debris flows within a recently burned drainage basin. Based on simulations, large volumes of sediment eroded from the hillslopes were redeposited within the channel network throughout the storm, leading to the initiation of numerous debris flows as a result of the mass failure of sediment dams that built up within the channel. More generally, results provide a quantitative framework for assessing the potential of runoff‐generated debris flows based on sediment supply and hydrologic conditions.

In the semiarid Southwestern USA, wildfires are commonly followed by runoff-generated debris flows because wildfires remove vegetation and ground cover, which reduces soil infiltration capacity and increases soil erodibility. At a study site in Southern California, we initially observed runoff-generated debris flows in the first year following fire. However, at the same site three years after the fire, the mass-wasting response to a long-duration rainstorm with high rainfall intensity peaks was shallow landsliding rather than runoff-generated debris flows. Moreover, the same storm caused landslides on unburned hillslopes as well as on slopes burned 5 years prior to the storm and areas burned by successive wildfires, 10 years and 3 years before the rainstorm. The landslide density was the highest on the hillslopes that had burned 3 years beforehand, and the hillslopes burned 5 years prior to the storm had low landslide densities, similar to unburned areas. We also found that reburning (i.e., two wildfires within the past 10 years) had little influence on landslide density. Our results indicate that landscape susceptibility to shallow landslides might return to that of unburned conditions after as little as 5 years of vegetation recovery. Moreover, most of the landslide activity was on steep, equatorial-facing slopes that receive higher solar radiation and had slower rates of vegetation regrowth, which further implicates vegetation as a controlling factor on post-fire landslide susceptibility. Finally, the total volume of sediment mobilized by the year 3 landslides was much smaller than the year 1 runoff-generated debris flows, and the landslides were orders of magnitude less mobile than the runoff-generated debris flows.

Wildfire significantly alters the hydrologic properties of a burned area, leading to increases in overland flow, erosion, and the potential for runoff‐generated debris flows. The initiation of debris flows in recently burned areas is well characterized by rainfall intensity‐duration (ID) thresholds. However, there is currently a paucity of data quantifying the rainfall intensities required to trigger post‐wildfire debris flows, which limits our understanding of how and why rainfall ID thresholds vary in different climatic and geologic settings. In this study, we monitored debris‐flow activity following the Pinal Fire in central Arizona, which differs from both a climatic and hydrogeomorphic perspective from other regions in the western United States where ID thresholds for post‐wildfire debris flows are well established, namely the Transverse Ranges of southern California. Since the peak rainfall intensity within a rainstorm may exceed the rainfall intensity required to trigger a debris flow, the development of robust rainfall ID thresholds requires knowledge of the timing of debris flows within rainstorms. Existing post‐wildfire debris‐flow studies in Arizona only constrain the peak rainfall intensity within debris‐flow‐producing storms, which may far exceed the intensity that actually triggered the observed debris flow. In this study, we used pressure transducers within five burned drainage basins to constrain the timing of debris flows within rainstorms. Rainfall ID thresholds derived here from triggering rainfall intensities are, on average, 22 mm h−1 lower than ID thresholds derived under the assumption that the triggering intensity is equal to the maximum rainfall intensity recorded during a rainstorm. We then use a hydrologic model to demonstrate that the magnitude of the 15‐min rainfall ID threshold at the Pinal Fire site is associated with the rainfall intensity required to exceed a recently proposed dimensionless discharge threshold for debris‐flow initiation. Model results further suggest that previously observed differences in regional ID thresholds between Arizona and the San Gabriel Mountains of southern California may be attributed, in large part, to differences in the hydraulic properties of burned soils. © 2019 John Wiley & Sons, Ltd.

Wildfire alters the hydrologic and geomorphic responses of burned areas relative to nearby unburned areas, making them more prone to runoff, erosion, and debris flow. In post-wildfire settings, debris flows often initiate when runoff concentrates on steep slopes and rapidly mobilizes sediment. Rainfall intensityduration (ID) thresholds have been proven useful for assessing post-fire debris-flow potential but can vary substantially from one location to another as a result of hydrologic factors that control rainfall-runoff partitioning. Debris-flow initiation thresholds based on a slope-dependent dimensionless discharge criterion, which have the theoretical benefit of being consistent from site to site, have also been proposed but not extensively tested. We monitored debris-flow activity in 12 small (< 1 km 2 ) watersheds burned by the 2018 Buzzard Fire in New Mexico, USA, documenting 24 debris flows during the first several months following the wildfire. We use a recently proposed dimensionlessdischarge threshold in combination with rainfall-runoff modeling to estimate basin-specific rainfall ID thresholds for debris-flow initiation. These model-derived thresholds compare well with observations. Areas burned at low severity are characterized by higher infiltration capacity, rainfall interception, and hydraulic roughness relative to areas burned at moderate or high severity, but differences in rainfall ID thresholds between these two areas can be predominantly attributed to wildfire-induced changes in hydraulic roughness. Results highlight the utility of thresholds based on dimensionless discharge relative to those based on rainfall intensity and also provide additional data that will help constrain general models for the prediction of rainfall ID thresholds.

Topographic asymmetry, that is, differences in the morphology of landscapes as a function of slope aspect, can be used to infer ecohydrogeomorphic feedback relationships. In this study, we document the dependence of topographic gradients and drainage densities on slope aspect and time/age in four Quaternary cinder cone fields in Arizona, Oregon, and California. Cinder cones are particularly useful as natural experiments in geomorphic evolution because they begin their evolution at a known time in the past (many have been radiometrically dated) and because they often have simple, well‐constrained initial morphologies. North‐facing portions of cinder cones have steeper topographic gradients and higher mean vegetation cover (i.e., Normalized Difference Vegetation Index, or NDVI, values) under current climatic conditions compared with corresponding south‐facing portions of cones within each volcanic field. Drainage density is also higher on north‐facing portions of cones in three of the four volcanic fields. These differences in topography were not present initially but developed progressively over time, indicating that the asymmetry is a result of post‐eruption geomorphic processes. To test alternative hypotheses for the slope‐aspect control of topography, we developed a numerical model for cinder cone evolution and a methodology for estimating local paleovegetation cover as a function of elevation, slope aspect, and time within the Quaternary. The numerical model results demonstrate that rates of colluvial transport were higher on south‐facing hillslopes in at least three of the four cinder cones fields. Our paleovegetation analysis suggests that in the two Arizona volcanic fields we studied, higher rates of colluvial transport on south‐facing hillslopes were the result of greater time‐averaged vegetation cover and hence higher rates of sediment transport by floral bioturbation. Our results illustrate the profound impact that relatively small variations in solar insolation can have on landscapes via feedbacks among hydrology, vegetation cover, and sediment transport.