Infiltration and runoff generation processes in fire‐affected soils

Post‐wildfire runoff was investigated by combining field measurements and modelling of infiltration into fire‐affected soils to predict time‐to‐start of runoff and peak runoff rate at the plot scale (1 m²). Time series of soil‐water content, rainfall and runoff were measured on a hillslope burned by the 2010 Fourmile Canyon Fire west of Boulder, Colorado during cyclonic and convective rainstorms in the spring and summer of 2011. Some of the field measurements and measured soil physical properties were used to calibrate a one‐dimensional post‐wildfire numerical model, which was then used as a ‘virtual instrument’ to provide estimates of the saturated hydraulic conductivity and high‐resolution (1 mm) estimates of the soil‐water profile and water fluxes within the unsaturated zone. Field and model estimates of the wetting‐front depth indicated that post‐wildfire infiltration was on average confined to shallow depths less than 30 mm. Model estimates of the effective saturated hydraulic conductivity, K_s, near the soil surface ranged from 0.1 to 5.2 mm h^?1. Because of the relatively small values of K_s, the time‐to‐start of runoff (measured from the start of rainfall), t_p, was found to depend only on the initial soil‐water saturation deficit (predicted by the model) and a measured characteristic of the rainfall profile (referred to as the average rainfall acceleration, equal to the initial rate of change in rainfall intensity). An analytical model was developed from the combined results and explained 92–97% of the variance of t_p, and the numerical infiltration model explained 74–91% of the variance of the peak runoff rates. These results are from one burned site, but they strongly suggest that t_p in fire‐affected soils (which often have low values of K_s) is probably controlled more by the storm profile and the initial soil‐water saturation deficit than by soil hydraulic properties. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

Effect of storms during drought on post‐wildfire recovery of channel sediment dynamics and habitat in the southern California chaparral,USA

Current global warming projections suggest a possible increase in wildfire and drought, augmenting the need to understand how drought following wildfire affects the recovery of stream channels in relation to sediment dynamics. We investigated post‐wildfire geomorphic responses caused by storms during a prolonged drought following the 2013 Springs Fire in southern California (USA), using multi‐temporal terrestrial laser scanning and detailed field measurements. After the fire, a dry‐season dry‐ravel sediment pulse contributed sand and small gravel to hillslope‐channel margins in Big Sycamore Creek and its tributaries. A small storm in WY 2014 generated sufficient flow to mobilize a portion of the sediment derived from the dry‐ravel pulse and deposited the fine sediment in the channel, totaling ~0.60 m³/m of volume per unit length of channel. The sediment deposit buried step‐pool habitat structure and reduced roughness by over 90%. These changes altered sediment transport characteristics of the bed material present before and after the storm; the ratio of available to critical shear stress (τ_o/τ_c) increased by five times. Storms during WY 2015 contributed additional fine sediment from tributaries and lower hillslopes and hyperconcentrated flow transported and deposited additional sediment in the channel. Together these sources delivered sediment on the order of six times that in 2014, further increasing τ_o/τ_c. These storms during multi‐year drought following wildfire transformed channel dynamics. The increased sediment transport capacity persisted during the drought period characterized by the longer residence time of relatively fine‐grained post‐fire channel sedimentation. This contrasts with wetter years, when post‐fire sediment is transported from the fluvial system during the same season as the post‐fire sediment pulse. Results of this short‐term study highlight the complex and substantial effects of multi‐year drought on geomorphic responses following wildfire. These responses influence pool habitat that is critical to longer‐term post‐wildfire riparian ecosystem recovery. Copyright © 2017 John Wiley & Sons, Ltd.     

Streamflow and suspended sediment yield following the 2000 Bobcat fire,Colorado

Postfire runoff and erosion are a concern, and more data are needed on the effects of wildfire at the watershed‐scale, especially in the Colorado Front Range. The goal of this study was to characterize and compare the streamflow and suspended sediment yield response of two watersheds (Bobcat Gulch and Jug Gulch) after the 2000 Bobcat fire. Bobcat Gulch had several erosion control treatments applied after the fire, including aerial seeding, contour log felling, mulching, and straw wattles. Jug Gulch was partially seeded. Study objectives were to: (1) measure precipitation, streamflow, and sediment yields; (2) assess the effect of rainfall intensity on peak discharges, storm runoff, and sediment yields; (3) evaluate short‐term hydrologic recovery. Two months after the fire, a storm with a maximum 30 min rainfall intensity I₃₀ of 42 mm h^?1 generated a peak discharge of 3900 l s^?1 km^?2 in Bobcat Gulch. The same storm produced less than 5 l s^?1 km^?2 in Jug Gulch, due to less rainfall and the low watershed response. In the second summer, storms with, I₃₀ of 23 mm h^?1 and 32 mm h^?1 generated peak discharges of 1100 l s^?1 km^?2 and 1700 l s^?1 km^?2 in the treated and untreated watersheds respectively. Maximum water yield efficiencies were 10% and 17% respectively, but 18 of the 23 storms returned ≤2% of the rainfall as runoff, effectively obscuring interpretation of the erosion control treatments. I₃₀ explained 86% of the variability in peak discharges, 74% of the variability in storm runoff, and >80% of the variability in sediment yields. Maximum single‐storm sediment yields in the second summer were 370 kg ha^?1 in the treated watershed and 950 kg ha^?1 in the untreated watershed. Copyright © 2005 John Wiley & Sons, Ltd.     

Reconstructing extreme post‐wildfire floods: a comparison of convective and mesoscale events

In much of western United States destructive floods after wildfire are frequently caused by localized, short‐duration convective thunderstorms; however, little is known about post‐fire flooding from longer‐duration, low‐intensity mesoscale storms. In this study we estimate and compare peak flows from convective and mesoscale floods following the 2012 High Park Fire in the ungaged 15.5 km² Skin Gulch basin in the northcentral Colorado Front Range. The convective storm on 6 July 2012 came just days after the wildfire was contained. Radar data indicated that the total rainfall was 20–47 mm, and the maximum rainfall intensities (upwards of 50 mm h^?1) were concentrated over portions of the watershed that burned at high severity. The mesoscale storm on 9–15 September 2013 produced 220–240 mm of rain but had maximum 15‐min intensities of only 25–32 mm h^?1. Peak flows for each flood were estimated using three independent techniques. Our best estimate using a 2D hydraulic model was 28 m³ s^?1 km^?2 for the flood following the convective storm, placing it among the largest rainfall‐runoff floods per unit area in the United States. In contrast, the flood associated with the mesoscale flood was only 6 m³ s^?1 km^?2, but the long‐duration flood caused extensive channel incision and widening, indicating that this storm was much more geomorphically effective. The peak flow estimates for the 2013 flood had a higher relative uncertainty and this stemmed from whether we used pre‐ or post‐flood channel topography. The results document the extent to which a high and moderate severity forest fire can greatly increase peak flows and alter channel morphology, illustrate how indirect peak flow estimates have larger errors than is generally assumed, and indicate that the magnitude of post‐fire floods and geomorphic change can be affected by the timing, magnitude, duration, and sequence of rainstorms. Copyright © 2017 John Wiley & Sons, Ltd.     

Post‐fire hydrological response and suspended sediment transport of a terraced Mediterranean catchment

In July 2013, a wildfire severely affected the western part of the island of Mallorca (Spain). During the first three post‐fire hydrological years, when the window of disturbance tends to be more open, the hydrological and sediment delivery processes and dynamics were assessed in a representative catchment intensively shaped by terracing that covered 37% of its surface area. A nested approach was applied with two gauging stations (covering 1.2 km² and 4.8 km²) built in September 2013 that took continuous measurements of rainfall, water and sediment yield. Average suspended sediment concentration (1503 mg L^?1) and the maximum peak (33 618 mg L^?1) were two orders of magnitude higher than those obtained in non‐burned terraced catchments of Mallorca. This factor may be related to changes in soils and the massive incorporation of ash into the suspended sediment flux during the most extreme post‐fire event; 50 mm of rainfall in 15 min, reaching an erosivity of 2886 MJ mm ha^?1 h^?1. Moreover, hysteretic counter‐clockwise loops were predominant (60%), probably related to the increased sensitivity of the landscape after wildfire perturbation. Though the study period was average in terms of total annual precipitation (even higher in intensities), minimal runoff (2%) and low sediment yield (6.3 t km^?2 y^?1) illustrated how the intrinsic characteristics of the catchment, i.e. calcareous soils, terraces and the application of post‐fire measures, limited the hydrosedimentary response despite the wildfire impact. Copyright © 2017 John Wiley & Sons, Ltd.     

Hydroclimatology of the Volta River Basin in West Africa: Trends and variability from 1901 to 2002

Long-term historical records of rainfall (P), runoff (Q) and other climatic factors were used to investigate hydrological variability and trends in the Volta River Basin over the period 1901-2002. Potential (E_p) and actual evaporation (E), rainfall variability index (δ), Budyko’s aridity index (I_A), evaporation ratio (C_E) and runoff ratio (C_Q) were estimated from the available hydroclimatological records. Mann-Kendall trend analysis and non-parametric Sen’s slope estimates were performed on the respective time series variables to detect monotonic trend direction and magnitude of change over time.Rainfall variability index showed that 1968 was the wettest year (δ = +1.75) while 1983 was the driest (δ = −3.03), with the last three decades being drier than any other comparable period in the hydrological history of the Volta. An increase of 0.2 mm/yr² (P < 0.05) was observed in E_p for the 1901-1969 sub-series while an increased of 1.8 mm/yr² (P < 0.01) was recorded since 1970. Rainfall increased at the rate of 0.7 mm/yr² or 49 mm/yr between 1901 and 1969, whereas a decrease of 0.2 mm/yr² (6 mm/yr) was estimated for 1970-2002 sub-series. Runoff increased significantly at the rate of 0.8 mm/yr (23 mm/yr) since 1970. Runoff before dam construction was higher (87.5 mm/yr) and more varied (CV = 41.5%) than the post-dam period with value of 73.5 mm/yr (CV = 23.9%). A 10% relative decrease in P resulted in a 16% decrease in Q between 1936 and 1998. Since 1970, all the months showed increasing runoff trends with significant slopes (P < 0.05) in 9 out of the 12 months. Possible causes, such as climate change and land cover change, on the detected changes in hydroclimatology are briefly discussed.     

Fire effects on rangeland hydrology and erosion in a steep sagebrush‐dominated landscape

Post‐fire runoff and erosion from wildlands has been well researched, but few studies have researched the degree of control exerted by fire on rangeland hydrology and erosion processes. Furthermore, the spatial continuity and temporal persistence of wildfire impacts on rangeland hydrology and erosion are not well understood. Small‐plot rainfall and concentrated flow simulations were applied to unburned and severely burned hillslopes to determine the spatial continuity and persistence of fire‐induced impacts on runoff and erosion by interrill and rill processes on steep sagebrush‐dominated sites. Runoff and erosion were measured immediately following and each of 3 years post‐wildfire. Spatial and temporal variability in post‐fire hydrologic and erosional responses were compared with runoff and erosion measured under unburned conditions. Results from interrill simulations indicate fire‐induced impacts were predominantly on coppice microsites and that fire influenced interrill sediment yield more than runoff. Interrill runoff was nearly unchanged by burning, but 3‐year cumulative interrill sediment yield on burned hillslopes (50 g m^?2) was twice that of unburned hillslopes (25 g m^?2). The greatest impact of fire was on the dynamics of runoff once overland flow began. Reduced ground cover on burned hillslopes allowed overland flow to concentrate into rills. The 3‐year cumulative runoff from concentrated flow simulations on burned hillslopes (298 l) was nearly 20 times that measured on unburned hillslopes (16 l). The 3‐year cumulative sediment yield from concentrated flow on burned and unburned hillslopes was 20 400 g m^?2 and 6 g m^?2 respectively. Fire effects on runoff generation and sediment were greatly reduced, but remained, 3 years post‐fire. The results indicate that the impacts of fire on runoff and erosion from severely burned steep sagebrush landscapes vary significantly by microsite and process, exhibiting seasonal fluctuation in degree, and that fire‐induced increases in runoff and erosion may require more than 3 years to return to background levels. Published in 2008 by John Wiley & Sons, Ltd.     

Runoff initiation in a preserved semiarid Caatinga small watershed,Northeastern Brazil

This study analyses some hydrological driving forces and their interrelation with surface‐flow initiation in a semiarid Caatinga basin (12 km²), Northeastern Brazil. During the analysis period (2005 – 2014), 118 events with precipitation higher than 10 mm were monitored, providing 45 events with runoff, 25 with negligible runoff and 49 without runoff. To verify the dominant processes, 179 on‐site measurements of saturated hydraulic conductivity (Ksat) were conducted. The results showed that annual runoff coefficient lay below 0.5% and discharge at the outlet has only occurred four days per annum on average, providing an insight to the surface‐water scarcity of the Caatinga biome. The most relevant variables to explain runoff initiation were total precipitation and maximum 60‐min rainfall intensity (I₆₀). Runoff always occurred when rainfall surpassed 31 mm, but it never occurred for rainfall below 14 mm or for I₆₀ below 12 mm h^?1. The fact that the duration of the critical intensity is similar to the basin concentration time (65 min) and that the infiltration threshold value approaches the river‐bank saturated hydraulic conductivity support the assumption that Hortonian runoff prevails. However, none of the analysed variables (total or precedent precipitation, soil moisture content, rainfall intensities or rainfall duration) has been able to explain the runoff initiation in all monitored events: the best criteria, e.g. failed to explain 27% of the events. It is possible that surface‐flow initiation in the Caatinga biome is strongly influenced by the root‐system dynamics, which changes macro‐porosity status and, therefore, initial abstraction. Copyright © 2016 John Wiley & Sons, Ltd.     

Linking runoff response to burn severity after a wildfire

Extreme floods often follow wildfire in mountainous watersheds. However, a quantitative relation between the runoff response and burn severity at the watershed scale has not been established. Runoff response was measured as the runoff coefficient C, which is equal to the peak discharge per unit drainage area divided by the average maximum 30 min rainfall intensity during each rain storm. The magnitude of the burn severity was expressed as the change in the normalized burn ratio. A new burn severity variable, hydraulic functional connectivity Φ was developed and incorporates both the magnitude of the burn severity and the spatial sequence of the burn severity along hillslope flow paths. The runoff response and the burn severity were measured in seven subwatersheds (0·24 to 0·85 km²) in the upper part of Rendija Canyon burned by the 2000 Cerro Grande Fire near Los Alamos, New Mexico, USA. A rainfall–discharge relation was determined for four of the subwatersheds with nearly the same burn severity. The peak discharge per unit drainage area was a linear function of the maximum 30 min rainfall intensity I₃₀. This function predicted a rainfall intensity threshold of 8·5 mm h^?1 below which no runoff was generated. The runoff coefficient was a linear function of the mean hydraulic functional connectivity of the subwatersheds. Moreover, the variability of the mean hydraulic functional connectivity was related to the variability of the mean runoff coefficient, and this relation provides physical insight into why the runoff response from the same subwatershed can vary for different rainstorms with the same rainfall intensity. Copyright © 2007 John Wiley & Sons, Ltd.     

Magnetic enhancement in wildfire‐affected soil and its potential for sediment‐source ascription

Intense rainfall following wildfire can cause substantial soil and sediment redistribution. With concern for the increasing magnitude and frequency of wildfire events, research needs to focus on hydrogeomorphological impacts of fire, particularly downstream fluxes of sediment and nutrients. Here, we investigate variation in magnetic enhancement of soil by fire in burnt eucalypt forest slopes to explore its potential as a post‐fire sediment tracer. Low‐frequency magnetic susceptibility values (χ_lf) of <10 µm material sourced from burnt slopes (c. 8·0–10·4 × 10^?6 m³ kg^?1) are an order of magnitude greater than those of <10 µm material derived from long‐unburnt areas (0·8 × 10^?6 m³ kg^?1). Susceptibility of anhysteretic remanent magnetization (χARM) and saturation isothermal remanent magnetization (SIRM) values are similarly enhanced. Signatures are strongly influenced by soil and sediment particle size and storage of previously burnt material in footslope areas. Whilst observations indicate that signatures based on magnetic enhancement show promise for post‐fire sediment tracing, problems arise with the lack of dimensionality in such data. Magnetic grain size indicators χ_fd%, χARM/SIRM and χ_fd/χARM offer further discrimination of source material but cannot be included in numerical unmixing models owing to non‐linear additivity. This leads to complications in quantitatively ascribing downstream sediment to source areas of contrasting burn severity since sources represent numerical multiples of each other, indicating the need to involve additional indicators, such as geochemical evidence, to allow a more robust discrimination. Copyright © 2005 John Wiley & Sons, Ltd.     

Asymmetric hillslope erosion following wildfire in Fourmile Canyon,Colorado

Infrequent, high‐magnitude events cause a disproportionate amount of sediment transport on steep hillslopes, but few quantitative data are available that capture these processes. Here we study the influence of wildfire and hillslope aspect on soil erosion in Fourmile Canyon, Colorado. This region experienced the Fourmile Fire of 2010, strong summer convective storms in 2011 and 2012, and extreme flooding in September 2013. We sampled soils shortly after these events and use fallout radionuclides to trace erosion on polar‐ and equatorial‐facing burned slopes and on a polar‐facing unburned slope. Because these radionuclides are concentrated in the upper decimeter of soil, soil inventories are sensitive to erosion by surface runoff. The polar‐facing burned slope had significantly lower cesium‐137 (¹³⁷Cs) and lead‐210 (²¹⁰Pb) inventories (p < 0.05) than either the polar‐facing unburned slope or equatorial‐facing burned slope. Local slope magnitude does not appear to control the erosional response to wildfire, as relatively gently sloping (~20%) polar‐facing positions were severely eroded in the most intensively burned area. Field evidence and soil profile analyses indicate up to 4 cm of local soil erosion on the polar‐facing burned slope, but radionuclide mass balance indicates that much of this was trapped nearby. Using a ¹³⁷Cs‐based erosion model, we find that the burned polar‐facing slope had a net mean sediment loss of 2 mm (~1 kg m^?2) over a one to three year period, which is one to two orders of magnitude higher than longer‐term erosion rates reported for this region. In this part of the Colorado Front Range, strong hillslope asymmetry controls soil moisture and vegetation; polar‐facing slopes support significantly denser pine and fir stands, which fuels more intense wildfires. We conclude that polar‐facing slopes experience the most severe surface erosion following wildfires in this region, indicating that landscape‐scale aridity can control the geomorphic response of hillslopes to wildfires. Copyright © 2018 John Wiley & Sons, Ltd.     

Is rainfall intensity significant in the rainfall–runoff process within tropical rainforests of northeast Queensland? The Hewlett regression analyses revisited

Following the statistical analyses of long‐term rainfall‐runoff records from research basins in humid temperate latitudes, Hewlett and co‐workers extended the global challenge to disprove their findings that rainfall intensity was non‐significant. This paper responds to Hewlett's challenge as no preceding analyses have involved forested basins in a tropical cyclone‐prone area. Based on a 7 year rainfall‐runoff record, quickflow (QF), peak flow (QP) and quickflow response ratios (QRR) were regressed as dependent variables against rainfall parameters (intensity, P_i, amount, P), storm duration, D and antecedent flow, I. These data sets were categorised into total streamflow (Q) classes and stratified into three seasons, (monsoon, post‐monsoon and dry) for forested and cleared catchments. Where rainfall variable collinearity met acceptable levels, the addition of P_i to regression models including P, D, I contributed up to 9% and 66% of the respective variations in quickflow and peak flow. For the highest Q storm classes (monsoon), P_i alone accounted for up to 67% and 91% of the variation in Q_F and Q_P respectively and was the dominant influence on Q_P for all seasons. The very high rainfall intensities experienced in the monsoon season is a causal factor why these results differ from those of other research drainage basins. Surprisingly, P_i continued to have a significant influence on Q_F for dry season classes when less‐intense rainfall occurs. Further the results were similar for both catchments across all seasons. P was the dominant independent variable affecting Q_F above a threshold Q of 50 mm (monsoon), as rainfall contributes directly to saturation overland flow and return flow under saturated conditions. Further although QRR increased with increasing Q for each season, the regression results for that parameter were poor possibly due to the non‐linearity of the rainfall‐runoff relationship. Copyright © 2010 John Wiley & Sons, Ltd.     

Upscaling of saturated conductivity for Hortonian runoff modelling

Stochastic and deterministic upscaling techniques are developed that upscale saturated conductivity at the support of 0.04 m² to representative actual infiltration (I_b) for support units (blocks) of 10¹–10⁴ m², as a function of steady state rainfall and runon to the block, under Hortonian runoff (infiltration excess overland flow). Parameters in the upscaling techniques represent the surface runoff flow pattern and the spatial probability distribution of saturated conductivity within the 10¹–10⁴ m² block. The stochastic upscaling technique represents the spatial process of infiltration and runoff using a simple process-imitating model, estimating I_b using Monte Carlo simulation. The deterministic upscaling technique aggregates these processes by a deterministic function relating rainfall and runon to I_b. The stochastic upscaling technique is shown to be capable to upscale saturated conductivity derived from ring infiltrometers to I_b values of plots (1 m²) corresponding to measured I_b values using rainfall simulators. It is shown that both upscaling techniques can be used to estimate I_b for each time step and each block in transient rainfall–runoff models, giving better estimates of cumulative runoff from a hillslope and a small catchment than model runs that do not use upscaling techniques.     

Seasonal rainfall–runoff relationships in a lowland forested watershed in the southeastern USA

Hydrological processes of lowland watersheds of the southern USA are not well understood compared to a hilly landscape due to their unique topography, soil compositions, and climate. This study describes the seasonal relationships between rainfall patterns and runoff (sum of storm flow and base flow) using 13 years (1964–1976) of rainfall and stream flow data for a low‐gradient, third‐order forested watershed. It was hypothesized that runoff–rainfall ratios (R/P) are smaller during the dry periods (summer and fall) and greater during the wet periods (winter and spring). We found a large seasonal variability in event R/P potentially due to differences in forest evapotranspiration that affected seasonal soil moisture conditions. Linear regression analysis results revealed a significant relationship between rainfall and runoff for wet (r² = 0·68; p < 0·01) and dry (r² = 0·19; p = 0·02) periods. Rainfall‐runoff relationships based on a 5‐day antecedent precipitation index (API) showed significant (r² = 0·39; p < 0·01) correspondence for wet but not (r² = 0·02; p = 0·56) for dry conditions. The same was true for rainfall‐runoff relationships based on 30‐day API (r² = 0·39; p < 0·01 for wet and r² = 0·00; p = 0·79 for dry). Stepwise regression analyses suggested that runoff was controlled mainly by rainfall amount and initial soil moisture conditions as represented by the initial flow rate of a storm event. Mean event R/P were higher for the wet period (R/P = 0·33), and the wet antecedent soil moisture condition based on 5‐day (R/P = 0·25) and 30‐day (R/P = 0·26) prior API than those for the dry period conditions. This study suggests that soil water status, i.e. antecedent soil moisture and groundwater table level, is important besides the rainfall to seasonal runoff generation in the coastal plain region with shallow soil argillic horizons. Copyright © 2011 John Wiley & Sons, Ltd.     

Characterization of rainfall generated by dripper‐type rainfall simulator using piezoelectric transducers and its impact on splash soil erosion

Rainfall erosivity represents the primary driver for particle detachment in splash soil erosion. Several raindrop erosivity indices have been developed in order to quantify the potential of rainfall to cause soil erosion. Different types of rainfall simulators have been used to relate rainfall characteristics to soil detachment. However, rainfall produced by different rainfall simulators has different characteristics, specifically different relationships between rainfall intensity and rainfall erosivity. For this reason, the effect of rainfall characteristics produced by a dripper‐type rainfall simulator on splash soil erosion (D_s) has been investigated. The simulated rainfall kinetic energy (KE) and drop size distribution (DSD) were measured using piezoelectric transducers, modified from the Vaisala RAINCAP^® rain sensor. The soil splash was evaluated under various simulated rainfall intensities ranging from 10 to 100 mm h^?1 using the splash‐cup method. The simulated rainfall intensity (I) and kinetic energy relationship (I–KE) was found to be different from natural rainfall. The simulated rainfall intensity and splash soil erosion relationship (I–D_s) also followed this same trend. The I–KE relationship was found to follow the natural rainfall trend until the rainfall intensity reached 30 mm h^?1 and above this limit the KE started to decrease. This emphasizes the importance of the I–KE relationship in determining the I‐D_s relationship, which can differ from one rainfall simulator to another. D_s was found to be highly correlated with KE (r = 0·85, P < 0·001), when data produced by the rainfall intensity ranged from 10 to 100 mm h^?1. However, when the threshold rainfall intensity (30 mm h^?1) was considered, the correlation coefficient further improved (r = 0·89, P = 0·001). Accordingly, to improve the soil splash estimation of simulated rainfall under various rainfall intensities the I–KE characterization relationship for rainfall simulators has to be taken into account. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis of soil‐hydraulic properties and infiltration timescales in wildfire‐affected soils

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 (Ψ_f) were significantly different (lower) in burned soils compared with unburned soils, whereas field‐saturated hydraulic conductivity (K_fs) 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 K_fs but moderate values of Ψ_f, compared with unburned and burned soils, indicating ash has long ponding times in response to rainfall. The ratio of S²/K_fs 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 K_fs 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 Ψ_f, from unburned areas could produce substantial errors in hydrologic modeling when used without adjustment for wildfire effects, causing parameter compensation and resulting underestimation of K_fs. 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.     

Comparing theoretically supported rainfall‐runoff erosivity factors at the Sparacia (South Italy) experimental site

Interpreting rainfall‐runoff erosivity by a process‐oriented scheme allows to conjugate the physical approach to soil loss estimate with the empirical one. Including the effect of runoff in the model permits to distinguish between detachment and transport in the soil erosion process. In this paper, at first, a general definition of the rainfall‐runoff erosivity factor REF_e including the power of both event runoff coefficient Q_R and event rainfall erosivity index EI₃₀ of the Universal Soil Loss Equation (USLE) is proposed. The REF_e factor is applicable to all USLE‐based models (USLE, Modified USLE [USLE‐M] and Modified USLE‐M [USLE‐MM]) and it allows to distinguish between purely empirical models (e.g., Modified USLE‐M [USLE‐MM]) and those supported by applying theoretical dimensional analysis and self‐similarity to Wischmeier and Smith scheme. This last model category includes USLE, USLE‐M, and a new model, named USLE‐M based (USLE‐MB), that uses a rainfall‐runoff erosivity factor in which a power of runoff coefficient multiplies EI₃₀. Using the database of Sparacia experimental site, the USLE‐MB is parameterized and a comparison with soil loss data is carried out. The developed analysis shows that USLE‐MB (characterized by a Nash–Sutcliffe Efficiency Index NSEI equal to 0.73 and a root mean square error RMSE = 11.7 Mg ha^?1) has very similar soil loss estimate performances as compared with the USLE‐M (NSEI = 0.72 and RMSE = 12.0 Mg ha^?1). However, the USLE‐MB yields a maximum discrepancy factor between predicted and measured soil loss values (176) that is much lower than that of USLE‐M (291). In conclusion, the USLE‐MB should be preferred in the context of theoretically supported USLE type models.     

Measurements of the initiation of post‐wildfire runoff during rainstorms using in situ overland flow detectors

Overland flow detectors (OFDs) were deployed in 2012 on a hillslope burned by the 2010 Fourmile Canyon fire near Boulder, Colorado, USA. These detectors were simple, electrical resistor‐type instruments that output a voltage (0–2·5 V) and were designed to measure and record the time of runoff initiation, a signal proportional to water depth, and the runoff hydrograph during natural convective rainstorms. Initiation of runoff was found to be spatially complex and began at different times in different locations on the hillslope. Runoff started first at upstream detectors 56% of the time, at the mid‐stream detectors 6%, and at the downstream detectors 38% of the time. Initiation of post‐wildfire runoff depended on the time‐to‐ponding, travel time between points, and the time to fill surface depression storage. These times ranged from 0·5–54, 0·4–1·1, and 0·2–14 minutes, respectively, indicating the importance of the ponding process in controlling the initiation of runoff at this site. Time‐to‐ponding was modeled as a function of the rainfall acceleration (i.e. the rate of change of rainfall intensity) and either the cumulative rainfall at the start of runoff or the soil–water deficit. Measurements made by the OFDs provided physical insight into the spatial and temporal initiation of post‐wildfire runoff during unsteady flow in response to time varying natural rainfall. They also provided data that can be telemetered and used to determine critical input parameters for hydrologic rainfall–runoff models. Copyright © 2015 John Wiley & Sons, Ltd.     

Sediment and solute transport on soil slope under simultaneous influence of rainfall impact and scouring flow

Soil erosion and nutrient losses with surface runoff in the loess plateau in China cause severe soil quality degradation and water pollution. It is driven by both rainfall impact and runoff flow that usually take place simultaneously during a rainfall event. However, the interactive effect of these two processes on soil erosion has received limited attention. The objectives of this study were to better understand the mechanism of soil erosion, solute transport in runoff, and hydraulic characteristics of flow under the simultaneous influence of rainfall and shallow clear‐water flow scouring. Laboratory flume experiments with three rainfall intensities (0, 60, and 120 mm h⁻¹) and four scouring inflow rates (10, 20, 30, and 40 l min⁻¹) were conducted to evaluate their interactive effect on runoff. Results indicate that both rainfall intensity and scouring inflow rate play important roles on runoff formation, soil erosion, and solute transport in the surface runoff. A rainfall splash and water scouring interactive effect on the transport of sediment and solute in runoff were observed at the rainfall intensity of 60 mm h⁻¹ and scouring inflow rates of 20 l min⁻¹. Cumulative sediment mass loss (Ms) was found to be a linear function of cumulative runoff volume (Wr) for each treatment. Solute transport was also affected by both rainfall intensity and scouring inflow rate, and the decrease in bromide concentration in the runoff with time fitted to a power function well. Reynolds number (Re) was a key hydraulic parameter to determine erodability on loess slopes. The Darcy–Weisbach friction coefficients (f) decreased with the Reynolds numbers (Re), and the average soil and water loss rate (M_l) increased with the Reynolds numbers (Re) on loess slope for both scenarios with or without rainfall impact. Copyright © 2010 John Wiley & Sons, Ltd.     

Soil degradation in central Spain due to sheet water erosion by low‐intensity rainfall events

Runoff and sediment lost due to water erosion were recorded for 36 (1 m²) plots with varying types of vegetative cover located on sloping gypsiferous fields in the South of Madrid. 75% of the events had maximum 30‐minute intensity (I₃₀) less than 10 mm h^?1 in the period studied (1994–2005). As for the vegetative cover, maximum correlation between runoff and soil loss was found in the least protected plots (0–40% cover) during the most intense rainfall events; however, a significant positive correlation was also observed in plots with greater coverage (40–60%). If coverage exceeded 60%, rainfall erosivity declined. The average amount of sediment produced in high‐intensity events was significantly greater (approximately 7 g m^?2 per I₃₀ event >10 mm h^?1) than that produced in the rest of the moderate‐intensity events (approximately 3 g m^?2 per I₃₀ event <10 mm h^?1), but due to the high rate of occurrence of the latter throughout the year sediment loss during the period studied totaled 128 g m^?2. By comparison, only 40 g m^?2 was produced by the I₃₀ events greater than 10 mm h^?1. Even though the amount of soil lost is relatively insignificant from a quantitative standpoint, the organic matter content lost in the sediment (six times more than in the soil) is a permanent loss that threatens the development of the surface of the soil in this area when the vegetative cover is less than 40%. The soil here experiences a chronic loss of 0·02 mm annually as a consequence of frequent, moderate events, in addition to any loss produced by extraordinary events, which, though less frequent, are much more erosive. If moderate events are ignored, an important part of soil loss will be lost in the long run. Copyright © 2007 John Wiley & Sons, Ltd.