H51M-02
Runoff Response at Three Spatial Scale from a Burned Watershed
The hypothesis that the magnitude and timing of runoff from burned watersheds are functions of the properties of flow paths at multiple scales was investigated at three nested spatial scales within an area burned by the 2005 Harvard Fire near Burbank, California. Water depths were measured using pressure sensors: at the outlet of a subwatershed (10000 m2); in 3-inch Parshall flumes near the outlets of three mini-watersheds (820-1780 m2) within the subwatershed; and by 12 overland-flow detectors in 6 micro-watersheds (~11-15 m2) within one of the mini-watersheds. Rainfall intensities were measured using recording raingages deployed around the perimeter of the mini-watersheds and at the subwatershed outlet. Time-to-concentration, TC, and lag time, TL, were computed for the 15 largest of 30 rainstorms (maximum 30- minute intensities were 3.3-13.0 mm/h) between December 2005 and April 2006. TC , elapsed time from the beginning of the rain until the first increase in water depth, averaged 1.0 hours at the micro-scale, 1.7 hours at the mini-scale, and 1.5 hours at the subwatershed scale. TL is the lag time that produced the maximum cross- correlation coefficient between the time series of rainfall intensities and the series of water depths. TL averaged 0.15 hours at the micro-scale, 0.35 hours at the mini-scale, and 0.39 hours at the subwatershed scale. The coefficient was >0.50 for 43% (N=168) of the measurements at the micro-scale, for 61% (N=54) at the mini- scale, and for 67% (N=6) at the subwatershed scale indicating the runoff response lagged but was often well correlated with the time-varying rainfall intensity.
H51M-01
Differential Recovery Rates of Post Fire Runoff and Erosion: The Effects of Slope, Aspect and Vegetation
Response of natural ecosystems in Mediterranean regions to wildfires has been extensively studied, particularly in context with the relationship of vegetation denudation and runoff/erosion processes. Following a 120 ha wildfire that took place during April 2005, at the Carmel Mountain ridge, we constructed monitoring plots to assess vegetation dynamics, runoff and erosion rates. Post-fire runoff and sediment were collected after each rain event for two rainy seasons from 14 plots (~10 m 2) located in different physio-geographic settings. Changes in vegetation cover were monitored by taking "aerial photographs" of the plots, using a digital camera mounted on a 6 m pole. Images were rectified and classified using image analysis software, and GIS layers of the vegetation cover were generated. These three factors: vegetation cover, runoff and erosion were assessed against slope aspect (north vs. south), slope steepness (steep vs. moderate), and fire severity (low vs. high). Results indicated differential spatio-temporal changes in erosion, runoff and vegetation growth. By the end of the second winter percentage of vegetation cover was comparable with non-burned control plots. Vegetation in the burnt plots, however, was predominated by herbaceous cover, in contrast to woody vegetation in the control plots. Vegetation cover was significantly higher in north facing slopes compared to south facing ones. Both sediment and runoff yields were significantly higher on south facing slopes, steep slopes and high-severity burnt areas. Sediment yield decreased during the second year, and was correlated with the increase in vegetation cover (or the reciprocal decrease in exposed stoniness). In contrast to other studies, cumulative runoff yields were not lower during the second rain season compared to the first one, though runoff coefficients slightly decreased, but insignificantly; we attribute this to the presence of predominantly herbaceous vegetation. The sharp decrease in sediment yield was not proportional to the modest decrease in runoff rates. On south facing, steep slopes and severely burnt plots runoff values were correlated with maximum I 10 min rain intensity, while on north facing moderate slopes and low-severity burnt plots runoff was correlated with the amount of precipitation during a given event. We suggest that in steep and south facing slopes Hortonian processes dictate runoff generation, while at moderate and north facing slopes runoff generation is governed by non-Hortonian processes. Differential fire effects on various the physio-geographic settings resulted in different predominating runoff mechanisms which have implications for recovery processes of the burnt landscape.
H51M-03
Post-fire Erosion and Recovery in Chaparral Steeplands, Southern California
In fire-prone southern California chaparral environments, wildfire is a significant disturbance event. It incinerates vegetation, alters soil properties, and renders the landscape susceptible to the agents of erosion. Accelerated erosion can cause site degradation, can extirpate refugia populations of endangered species, and can harm human communities at the wildland/urban interface. The San Dimas Experimental Forest (SDEF) is a nearly 7000 ha research preserve located in the San Gabriel Mountains. Native vegetation in the SDEF consists primarily of mixed chaparral. Management treatments following a wildfire in 1960 involved the vegetation type-conversion of some native chaparral watersheds to a mixture of perennial grasses. In 1994, a study was initiated to quantify sediment fluxes through several small (1-3 ha) headwater catchments in the SDEF under both brush and grass vegetation. Several of these watersheds burned in a prescribed fire in May 2001. The remainder burned in a wildfire in September 2002. These burns provided a unique opportunity to quantify post-fire erosion on the same sites for which there were extensive pre- fire measurements. Hillslope erosion was sampled using sheet metal collector traps with a 30 cm aperture. Seventy-five traps were placed on unbounded plots scattered throughout each of four watersheds. Sediment yield was measured behind earthen dams in 17 small watersheds. A centrally located raingage recorded precipitation amounts and intensities. During this study period, the SDEF experienced both the wettest (2005) and driest (2007) years in its 74-year history. The values for the 15-minute maximum rainfall intensity show no relation to annual erosion (hillslope or small watershed) for either vegetation type before or after fire. Both hillslope erosion and small watershed sediment yield display remarkably similar patterns of post-fire erosion response: a one to two order of magnitude increase in first-year erosion followed by a relatively rapid recovery (2-3 years) to baseline levels. Post-fire erosion was very similar whether the watersheds burned in a prescribed fire or a wildfire. Although rainfall is a necessary driver, post-burn erosion is governed more by the fire-induced increased landscape sensitivity to the agents of erosion.
H51M-04
Calculation of flow, sediment transport, and channel erosion in burned watersheds prone to debris flows
The loss of vegetation by fire leaves recently burned watersheds vulnerable to flash flooding and debris flows. Quantifying the flow and sediment transport conditions that lead to the formation of debris flows in these watersheds is essential for improving predictions of the likely occurrence of these events. Unfortunately, conventional water and sediment gaging methods are not well suited for monitoring recently burned watersheds because these methods require measurements that are difficult or impossible to obtain owing to the rapid and potentially dangerous response of burned basins to rainfall. In an effort to provide accurate flow and sediment transport data in burned basins for use in warning systems and for testing models of debris-flow initiation, a fluid- mechanically based method for defining water and sediment stage-discharge relations has been adapted for use in the steep mountain channels characteristic of debris-flow-prone watersheds. The approach uses a fully predictive flow model to convert non-contact measurements of flow stage into estimates of water and sediment discharge. The model does not use empirical roughness coefficients, but rather determines channel roughness directly from detailed field measurements of the topography and the physical roughness elements on the bed and banks of the channel. The model also determines the component of boundary shear stress appropriate for sediment transport calculations on the entire wetted area of the surveyed reach. This calculation gives the model the unique capability of predicting vertical and lateral channel erosion during an event, which is an important component because channels are usually the largest source of sediment exported from burned basins. Completed field tests of the method in four steep mountain channels show the theoretical stage-water discharge relations produced by the model are in excellent agreement with direct measurements of discharge. Additional testing in progress is focused on (1) evaluating model predictions of channel erosion and (2) continued verification of the flow component of the model using surface velocity measurements obtained through video imagery. Initial results from this phase of testing show the model adequately reproduces the reach-averaged cross-sectional geometry of a gully formed during a flash flood on a burned hillslope.
H51M-05
The Role of Vegetative Ash in Post-Fire Erosion and the Generation of Progressively-Bulked Debris Flows
It has been proposed that the creation and deposition of a layer of ash on the ground surface can enhance erosion by surface runoff and might even be a necessary condition for the generation of progressively bulked debris flows. We conducted flume experiments to investigate the role of ash in increasing the volume and transport capacity of runoff. We found that the presence of ash on the soil surface reduced the ability of flowing water to infiltrate; this effect was even greater when the ash had been pre-wetted. In addition, we found that the transport capacity of flowing water is enhanced by the incorporation of ash into the flow because of the decrease in settling velocity and the increase in fluid density. We note, however, that the addition of ash reduces the boundary Reynolds number (Re*) such that, at high ash concentrations and with fine-grained sediment, sediment tranport declines as the flow becomes hydraulically smooth. We determined that the Meyer-Peter- Muller bedload transport equation can be used to estimate transport rates in ash-laden slurries when Re* is greater than 35. Finally, we propose that ash slurries may evolve into progressively bulked debris flows through a positive feedback process between transport capacity and erosivity.
H51M-06
The Effects of Repeated Fires on Vegetation Communities Structure and Implications for Geomorphological Processes
Forestfires have been recognized to be an inherent component of Mediterranean ecosystems. It has been suggested that fires have been human induced for eons as a management practice in the eastern part of the Mediterranean Basin. By the beginning of the 20th century, however, most of the open space in the nowadays region of Israel were denuded of vegetation. During the 1920's and through the 1950's major afforestation efforts took place in the region. Evidence suggests that in the Mediterranean Basin, as well as in Israel, the number of wildfires has increased dramatically over the last several decades, due to human activities. We propose, however, that the areas consumed by the fires increased as a result from the maturation of the maquis and forested regions. The Carmel Ridge in Israel is taken as a case study, where we use satellite image analysis to monitor vegetation changes in areas repeatedly burned during the last 22 years. An extended vegetation study of the region was conducted during 1985, and serves as a baseline for the state of the vegetation. Satellite images from 1990 (following a 1989 fire), 1995, 2000 (following 1998, 1999 fires) and 2006 (following a 2005 fire) were used to classify the different vegetation classes for each year, based on spectral and derived vegetation indices. The resulted classifications revealed changes in the spatial distribution of pine, broad-leaves and shrubby vegetation classes. Transition probabilities among the vegetation communities are being used to construct a Markov based transition matrix, which also accounts for aspect dependent vegetation dynamics (north VS. south facing slopes). In turn, a simulation model is being used to assess the effects of different fire regimes on long term vegetation changes. Preliminary results suggest that recurring fires within short time intervals may significantly alter the long-term structure of the vegetation communities. The pine stands may be replaced by a mixed maquis-shrub community, as pine stands do not have a sufficient time to develop a large enough of a seed base, following the frequent fires. These long-term changes may have implications for geomorphological processes, including the hydrological system and soil loss.
H51M-07
Resource homogenization in degraded arid landscapes induced by fire – erosion interactions
Hydrological and aeolian processes are major drivers in the dynamics of arid landscapes in that they redistribute soil resources with important implications on the composition and spatial patterns of dryland vegetation. These processes are thought to play a major role in the conversion of disturbed desert grasslands into shrublands, with possible impacts on regional climate and desertification. At its early stages the grassland-to-shrubland transition can be still reversible and fires have been shown to contribute to the reversibility of the system. Even though fires are know to interact both with wind and water erosion, an understanding of these interactions and of their effect on aridland degradation is still missing. Here we use field manipulation experiments in a grass-shrub transition zone in the Chihuahuan desert to show how the interaction of fires with erosion processes may affect the distribution of soil resources with consequent effects on the pace of land degradation processes. Using microtopography measurements and isotopic analyses, we provide experimental evidence for the occurrence of post-fire enhancement of soil erosion, and relate this effect to the weakening of interparticle bonding forces associated with the emergence of fire-induced soil hydrophobicity. We also show how this effect favors the reversibility of the early stages of shrub-to-grass transition through the redistribution of soil resources from the fertile shrub-dominated areas (or "fertility islands") to the bare soil interspaces.
H51M-08
Watershed-Scale Post-Fire Treatment Effects on Runoff and Erosion After the Hayman Fire, Colorado
Wildfires can cause large increases in runoff and erosion. Although post-fire treatments are used to mitigate these effects, their effectiveness is not well documented. A study was initiated after the 2002 Hayman Fire to determine natural post-fire recovery rates, measure the effectiveness of contour-felled logs, straw mulch, and hydromulch, and determine effects of post-fire salvage logging. Sediment traps and weirs were installed in six severely burned small watersheds (3-5 ha) at 2 sites in the Hayman Fire. The above treatments were applied to four watersheds while one watershed at each site was left untreated as a control. Precipitation, runoff, and sediment yields were measured through 2007. Between 2002 and 2006, 11 rain events produced runoff in the contour-felled log site and 12 events produced runoff in the mulch site. The minimum rainfall required to produce runoff was 4.3 mm, while the 10-minute maximum intensity that produced runoff ranged from 9.1 to 72 mm hr-1. Snow melt did not produce runoff in any of the watersheds. Runoff from all six watersheds was flashy and heavily sediment-laden. Event runoff in the control watersheds was between 0 and 8.6 mm in the first year after the fire and between 0 and 0.6 mm 5 years later. The maximum peak flow rate in a control watershed occurred two years after the fire and was 7.1 m3 s-1 km-2. The mean annual sediment yields in the control watersheds were 23 Mg ha-1 yr-1 in 2003 and 22 Mg ha- 1 yr-1 in 2004. Only one site produced sediment from one event each in 2005 and 2006; the sediment yields from the controls for these events were 5.1 and 2.1 Mg ha-1, respectively. The contour-felled log and straw mulch watersheds produced less runoff, lower peak flows, and less sediment than their controls. The hydromulch watershed responses were closer to, if not greater than, those in its control. Salvage logging did not produce any measurable increases in runoff or sediment yields relative to its control. With large hydrologic responses occurring in these watersheds 5 years after the fire, it is clear these sites have not yet recovered to their pre-fire condition.