Santa Ynez Mountains Research Articles

The Sundowner Winds Experiment (SWEX) in Santa Barbara, California: Advancing Understanding and Predictability of Downslope Windstorms in Coastal Environments

Abstract Coastal Santa Barbara is among the most exposed communities to wildfire hazards in Southern California. Downslope, dry, and gusty windstorms are frequently observed on the south-facing slopes of the Santa Ynez Mountains that separate the Pacific Ocean from the Santa Ynez valley. These winds, known as “Sundowners,” peak after sunset and are strong throughout the night and early morning. The Sundowner Winds Experiment (SWEX) was a field campaign funded by the National Science Foundation that took place in Santa Barbara, California, between 1 April and 15 May 2022. It was a collaborative effort of 10 institutions to advance understanding and predictability of Sundowners, while providing rich datasets for developing new theories of downslope windstorms in coastal environments with similar geographic and climatic characteristics. Sundowner spatiotemporal characteristics are controlled by complex interactions among atmospheric processes occurring upstream (Santa Ynez valley), and downstream due to the influence of a cool and stable marine boundary layer. SWEX was designed to enhance spatial measurements to resolve local circulations and vertical structure from the surface to the midtroposphere and from the Santa Barbara Channel to the Santa Ynez valley. This article discusses how SWEX brought cutting-edge science and the strengths of multiple ground-based and mobile instrument platforms to bear on this important problem. Among them are flux towers, mobile and stationary lidars, wind profilers, ceilometers, radiosondes, and an aircraft equipped with three lidars and a dropsonde system. The unique features observed during SWEX using this network of sophisticated instruments are discussed here.

Status of Lompoc Yerba Santa Eriodictyon capitatum (Namaceae): an Endangered Plant in Santa Barbara County, Southern California

Lompoc yerba santa Eriodictyon capitatum (Namaceae) is an evergreen shrub endemic to west Santa Barbara County in coastal southern Calif. The species was listed as rare under the Calif. Endangered Species Act in 1979 and as endangered under the U.S. Endangered Species Act in 2000. At Federal listing in 2000, Lompoc yerba santa was known from four locations (five occurrences) in three geographic areas. In 2022, the species is known from 11 occurrences in four geographic populations of which 10 occurrences are considered extant and 1 occurrence likely extirpated: Orcutt Oil Field in the Solomon Hills, 2 occurrences; Burton Mesa on Vandenberg Space Force Base North, 6 occurrences; La Salle Canyon on Vandenberg Space Force Base South, 1 occurrence; and Santa Ynez Mountains, 2 occurrences. Lompoc yerba santa occurs mostly in chaparral and bishop pine (Pinus muricata) forest, and most occurrences are in sandy soil. The species stands up to 5 m tall and with vertical stems up to 10 cm diameter. Much of its reproduction appears to be vegetative by rhizome production, suggesting low dispersal ability, and it is a habitat specialist with apparent dependence on substrate disturbance and fire. Using all available information and international standards with a precautionary but realistic attitude to uncertainty, Lompoc yerba santa in 2022 meets IUCN criteria for endangered status: the small extent of occurrence is 433 km2 (<5,000 km2) and the small area of occupancy is 2.30 km2 (<10 km2), along with the geographic range being severely fragmented, and the quality of habitat is observed/inferred/projected to be in continuing decline (altered fire regime/fire management practices, climate change with severe drought and increased temperatures).

Revision of Microasydates, New Nearctic Genus of Soft-Winged Flower Beetles (Coleoptera: Melyridae: Dasytinae: Listrini)

A group of four North American dasytine species is carved from the genus Trichochrous Motschulsky, 1860. The genus Microasydates Gimmel and Mayor, new genus (type species: Pristoscelis umbratus LeConte, 1866) is created for these species. This new genus is clearly defined based on several adult male characters, including the presence of a field of coarse black setae concentrated medially on abdominal ventrite 4, slender tibial spurs in combination with the head not distinctly enlarged, lack of a basal strut on abdominal sternite VIII, tegmen membranous apically, and ventral teeth on the median lobe of the aedeagus. Species are distinguished primarily by characters of pronotal shape, setation, female pygidium, and the median lobe of the male aedeagus. Previously described are Microasydates punctipennis (LeConte, 1866), new combination and Microasydates umbratus (LeConte, 1866), new combination. Newly described are Microasydates sanclemente Gimmel and Mayor, new species and Microasydates santabarbara Gimmel and Mayor, new species. The only widespread mainland species is M. umbratus, which occurs almost throughout California across the Great Basin into Utah, while the remainder of the diversity is in the California Channel Islands and Santa Ynez Mountains in Santa Barbara County. Upon examination of type material, Trichochrous indigens Casey, 1895, Trichochrous nubilatus Casey, 1895, Trichochrous separatus Casey, 1895, Trichochrous suffusus Casey, 1895, Trichochrous testaceus Casey, 1895, and Trichochrous pratensis Blaisdell, 1924 are all considered new junior subjective synonyms of M. umbratus. Trichochrous fulvovestitus Casey, 1895, status restored, and its junior synonym, Trichochrous propinquus Casey, 1895, are removed from synonymy with M. umbratus and retained in the genus Trichochrous. Adults of Microasydates are active during mid to late summer, and aggregate, often in large numbers, on the flowers of a wide variety of mostly perennial plants, where they mate and apparently feed on pollen, nectar, and occasionally small arthropods. Seasonality data and known host plant associations are provided along with species treatments.

Mesoscale patterns associated with two distinct heatwave events in coastal Santa Barbara, California, and their impact on local fire risk conditions

This study investigates mesoscale mechanisms associated with two extreme heatwaves affecting Santa Barbara County (SBC), southern California, and their implication for severe fire weather. We examine these issues using a database consisting of surface stations, radiosoundings, and 1 km grid spacing simulations with the Weather Research & Forecasting (WRF), including a climatology spanning 32 years. During the first heatwave event, synoptic conditions induced downslope winds on the southern-facing slopes of the Santa Ynez Mountains (SYM) on July 6, 2018. One surface station hit an all-time record, and nine surface stations exceeded 99.9th percentiles of surface temperatures. A wildfire (the Holiday Fire) erupted on the slopes of the SYM driven by high temperatures, low relative humidity, and strong winds. The nearby radiosonde registered temperatures at 850 hPa that exceeded the 95th percentile historical records (62 yrs). WRF simulations indicated that mountain wave activity contributed to the excessive surface temperatures on the south-facing slopes of the SYM, and explained the late evening timing for the maximum daily temperatures. The second heatwave broke all-time temperature records at 10 surface stations across SBC on September 6, 2020. Maximum temperatures for most of the SBC occurred during mid-afternoon, the highest observed temperature at the surface was 48.3 °C (118.9∘F), and the 850 hPa temperatures exceeded the 99th percentile. The September 2020 event occurred under weaker synoptic forcing (pressure gradients) than the July 2018 event, resulting in weaker winds in coastal Santa Barbara (including the slopes of the SYM) and Santa Ynez Valley. Nonetheless, the extreme heat and low relative humidity increased the Fosberg Fire Weather Index (FFWI) at critical values for a few hours in some sites when winds were moderate. To evaluate the relative importance of these extreme events in the historical context and to assess the region’s wildfire risk we propose a novel diagram based on the joint behavior of winds, temperature, humidity and FFWI. While no wildfires have broken out during the September 2020 heatwave, our analysis suggests that a combination of extreme heat with stronger winds would lead to unprecedented fire danger. These extreme conditions may become more common in a warming planet.

Estimation of wildfire wind conditions via perimeter and surface area optimization

This paper shows that the prediction capability of wildfire progression can be improved by estimation of a single prevailing wind vector parametrized by a wind speed and a wind direction to drive a wildfire simulation created by FARSITE. Estimations of these wind vectors are achieved in this work by a gradient-free optimization via a grid search that compares wildfire model simulations with measured wildfire perimeters, where noisy observations are modeled as uncertainties on the locations of the vertices of the measured wildfire perimeters. Two optimizations are established to acquire the optimal wind speed and wind direction. To formulate a perimeter optimization, an uncertainty-weighted least-squares error is computed between the vertices of the simulated and measured wildfire perimeter. The challenge in this approach is to match the number of vertices on the simulated and measured wildfire perimeter via interpolation of perimeter points and their uncertainties. For a surface area optimization, an uncertainty-weighted surface area error is introduced to capture the surface of the union minus the intersection of the simulated and measured wildfire perimeter. The challenge in this approach is to formulate a surface area error, weighted by the uncertainties on the locations of the vertices of the measured wildfire perimeter. The optimization in this paper is based on an iterative refinement of a grid of the wind vector and provides robustness to intermittent erroneous results produced by FARSITE, while allowing parallel execution of wildfire model calculations. This paper is an extension of the work in Tan et al., (2021). Results on wind vector estimation are illustrated on two historical wildfire events: the 2019 Maria Fire that burned south of the community of Santa Paula in the area of Somis, CA, and the 2019 Cave Fire that started in the Santa Ynez Mountains of Santa Barbara County.

Effects of Wildfires and Ash Leaching on Stream Chemistry in the Santa Ynez Mountains of Southern California

Wildfires can change ecosystems by altering solutes in streams. We examined major cations in streams draining a chaparral-dominated watershed in the Santa Ynez Mountains (California, USA) following a wildfire that burned 75 km2 from July 8 to October 5, 2017. We identified changes in solute concentrations, and postulated a relation between these changes and ash leached by rainwater following the wildfire. Collectively, K+ leached from ash samples exceeded that of all other major cations combined. After the wildfire, the concentrations of all major cations increased in stream water sampled near the fire perimeter following the first storm of the season: K+ increased 12-fold, Na+ and Ca2+ increased 1.4-fold, and Mg2+ increased 1.6-fold. Our results suggested that the 12-fold increase in K+ in stream water resulted from K+ leached from ash in the fire scar. Both C and N were measured in the ash samples. The low N content of the ash indicated either high volatilization of N relative to C occurred, or burned material contained less N.

Extreme winds and fire weather in coastal Santa Barbara County,CA: An observational analysis

AbstractCoastal Santa Barbara (SB) County in Southern California, characterized by a Mediterranean climate and complex topography, is a region prone to downslope windstorms that create critical fire weather conditions and rapidly spread wildfires. The Santa Ynez Mountains, oriented from east to west, rise abruptly from the coast, separating air masses from the ocean and the Santa Ynez Valley. The juxtaposition of these geographic features generates spatiotemporally variable wind regimes. This study analyzes diurnal‐to‐seasonal wind cycles and extremes in this region using hourly data from eight weather stations and four buoys for the period 1998–2019. Data from a vertical wind profiler at the Santa Barbara airport in Goleta, CA was extracted from August 2016 to September 2020. Air temperature, dew point temperature, and the Fosberg fire weather index are examined at land stations. We show that cycles in wind speed vary spatiotemporally; mountain (valley and coastal) stations exhibit a pronounced semiannual (annual) cycle, and wind maxima is observed during the evening (afternoon) at mountain (valley and coastal) stations. Differences in wind speed percentiles were evident among stations, particularly at and above the 75th percentile. Strong winds recorded at buoys were significantly correlated (betweenr = 0.3–0.5) to land stations. However, cross‐correlational analysis did not reveal any temporal lags between mountain stations and buoys. Distributions of temperature and dew point during extreme winds differed between east and west mountain stations. Significant fire weather conditions were most frequent at mountain stations in Refugio and Montecito, with 5% occurrence in the spring and over 3% occurrence in fall. Weaker summertime winds lowered fire weather conditions at Montecito in the summer.

The Effect of Upstream Orography on the Onset of Sundowner Winds in Coastal Santa Barbara, CA

AbstractThe impact of upstream terrain on the diurnal variability of downslope windstorms on the south‐facing slopes of the Santa Ynez Mountains (SYM) is investigated using numerical simulations. These windstorms, called Sundowners due to their typical onset around sunset, have intensified all major wildfires in the area. This study investigates the role of the orography upstream of the SYM in the diurnal behavior of Sundowners. Two types of Sundowners are examined: western sundowners (winds with dominant northwesterly direction) and eastern Sundowners (winds with dominant northeasterly direction). By using semi‐idealized simulations, in which we progressively reduce the upstream terrain, we show that the onset of the lee slope jet occurs in the late afternoon only when the flow approaches the SYM from the northeast, after interacting with a considerably higher mountain barrier. We demonstrate that during the eastern regime, the progressive reduction of the upstream terrain results in strong lee slope winds throughout the day. Conversely, the diurnal cycle of downslope winds during the western regime is less sensitive to the reduction of the upstream terrain. The Sundowner diurnal cycle during the eastern regime can be explained by boundary‐layer processes in the valley and the blocking effect of high mountains upstream of the SYM. These results contribute to a better understanding of the influence of upstream orography in the cycle and intensity of downslope windstorms in coastal mountains.

Observations and Analyses of the 9 January 2018 Debris-Flow Disaster, Santa Barbara County, California

ABSTRACTThe post–Thomas Fire debris flows of 9 January 2018 killed 23 people, damaged 558 structures, and caused severe damage to infrastructure in Montecito and Carpinteria, CA. U.S. Highway 101 was closed for 13 days, significantly impacting transportation and commerce in the region. A narrow cold frontal rain band generated extreme rainfall rates within the western burn area, triggering runoff-driven debris flows that inundated 5.6 km2 of coastal land in eastern Santa Barbara County. Collectively, this series of debris flows is comparable in magnitude to the largest documented post-fire debris flows in the state and cost over a billion dollars in debris removal and damages to homes and infrastructure. This study summarizes observations and analyses on the extent and magnitude of inundation areas, debris-flow velocity and volume, and sources of debris-flow material on the south flank of the Santa Ynez Mountains. Additionally, we describe the atmospheric conditions that generated intense rainfall and use precipitation data to compare debris-flow source areas with spatially associated peak 15 minute rainfall amounts. We then couple the physical characterization of the event with a compilation of debris-flow damages to summarize economic impacts.

Climatology of Sundowner winds in coastal Santa Barbara, California, based on 30 yr high resolution WRF downscaling

Sundowner winds are a type of downslope windstorms observed on the southern slopes of the Santa Ynez Mountains (SYM) in Santa Barbara, southern California. The name “Sundowner” is due to the onset of strong winds near sunset. Sundowner winds can reach gale force and are extremely dangerous during wildfires. This study presents a climatology of Sundowners based on a 30 yr high spatiotemporal resolution dataset obtained with the Weather Research and Forecasting model. Combined empirical orthogonal function (EOF) analysis of hourly zonal and meridional winds (10 m) with 1 km grid spacing is performed to characterize the diurnal variability of surface winds. The first two eigenmodes (EOF-1, EOF-2) (58% of total variance) characterize the variability of surface winds in the western and eastern parts of the SYM, respectively. The hourly percentiles of the frequency distribution of the first two time coefficients (PC1, PC2) are used to identify three types of Sundowner regimes. The western (eastern) regime occurs when surface winds project strongly on EOF-1 (EOF-2) but not on EOF-2 (EOF-1). The western regime is characterized by a strong coastal jet around Point Conception with northwesterly winds extending into the Santa Barbara Channel. In contrast, a weak coastal jet not extending into the Santa Barbara Channel is observed in the eastern regime. In the Santa Barbara (SBA) regime, hourly surface winds project strongly on both EOF-1 and EOF-2. The western regime occurs more frequently than the eastern and SBA regimes and peaks during March-May. The eastern regime has maximum in January-February, whereas the SBA regime has a seasonal cycle similar to the western regime. Horizontal and vertical structures of winds and potential temperature show that the lee slope jet and mountain wave activity on the SYM exhibit strong spatial variations in the three regimes.

The Sundowner Winds Experiment (SWEX) Pilot Study: Understanding Downslope Windstorms in the Santa Ynez Mountains, Santa Barbara, California

AbstractSundowner winds are downslope gusty winds often observed on the southern slopes of the Santa Ynez Mountains (SYM) in coastal Santa Barbara (SB), California. They typically peak near sunset and exhibit characteristics of downslope windstorms through the evening. They are SB’s most critical fire weather in all seasons and represent a major hazard for aviation. The Sundowner Winds Experiment Pilot Study was designed to evaluate vertical profiles of winds, temperature, humidity, and stability leeward of the SYM during a Sundowner event. This was accomplished by launching 3-hourly radiosondes during a significant Sundowner event on 28–29 April 2018. This study showed that winds in the lee of the SYM exhibit complex spatial and temporal patterns. Vertical profiles showed a transition from humid onshore winds from morning to midafternoon to very pronounced offshore winds during the evening after sunset. These winds accompanied mountain waves and a northerly nocturnal lee jet with variable temporal behavior. Around sunset, the jet was characterized by strong wind speeds enhanced by mountain-wave breaking. Winds weakened considerably at 2300 PDT 29 April but enhanced dramatically at 0200 PDT 29 April at much lower elevations. These transitions were accompanied by changes in stability profiles and in the Richardson number. A simulation with the Weather Research and Forecasting (WRF) Model at 1-km grid spacing was examined to evaluate the skill of the model in capturing the observed winds and stability profiles and to assess mesoscale processes associated with this event. These results advanced understanding on Sundowner’s spatiotemporal characteristics and driving mechanisms.

Provenance of detrital sediments in Santa Barbara Basin, California, USA: Changes in source contributions between the Last Glacial Maximum and Holocene

AbstractDetrital terrestrial sediments preserved in near-shore marine basins bear distinctive geochemical identifiers that can be used to identify the on-shore sediment sources and sediment routing through time. Santa Barbara Basin (SBB), offshore of southern California, USA, contains a well-known, continuous, high-resolution Holocene flood record that can provide insights into the frequency and changes in on-shore sources across time for such events. Here SBB-adjacent stream bed sediments are characterized using mineralogical, elemental, and radiogenic strontium and neodymium isotopic compositions. Modern and Holocene SBB flood deposits and Last Glacial Maximum (LGM) sediments were similarly analyzed. The Southern Slopes of the Santa Ynez Mountains and Topatopa Mountains account for ∼85% of SBB Holocene flood deposit sediments, as calculated from Sr-Nd isotope mixing models. During the LGM sea level low stand, the Southern Slopes contribution increased (to ≥90%), while relative sediment contribution from Santa Clara River diminished. This loss was likely compensated, however, by increased sediment flux from the Southern Slopes and the Channel Islands.

Simulating Sundowner Winds in Coastal Santa Barbara: Model Validation and Sensitivity

This study investigates the influence of planetary boundary layer (PBL) schemes and land surface models (LSMs) on the performance of the Weather Research &amp; Forecasting model in simulating the development of downslope windstorms in the lee of the Santa Ynez Mountains in Santa Barbara, California (known as Sundowner winds). Using surface stations, a vertical wind profiler, and a multi-physics ensemble approach, we found that most of the wind speed biases are controlled by the roughness length z 0 , and so by the choice of LSM. While timing characteristics of Sundowners are insensitive to both LSM and PBL schemes, a clear sensitivity in the horizontal extent of strong surface winds is found for both PBL parameterization and z 0 , which are related to patterns of self-induced wave-breaking near the mountaintop, and the erosion of the marine layer. These results suggest that LSMs with relatively high values of z 0 , and TKE-based or hybrid PBL schemes adequately simulate downslope windstorms in the lee of mountain ranges, specifically in areas where downslope windstorms interact with the marine boundary layer with stably stratified characteristics.

Brief Communication: Synoptic-scale differences between Sundowner and Santa Ana wind regimes in the Santa Ynez Mountains, California

Abstract. Downslope Sundowner winds in southern California's Santa Ynez Mountains favor wildfire growth. To explore differences between Sundowners and Santa Ana winds (SAWs), we use surface observations from 1979 to 2014 to develop a climatology of extreme Sundowner days. The climatology was compared to an existing SAW index from 1979 to 2012. Sundowner (SAW) occurrence peaks in late spring (winter). SAWs demonstrate amplified 500 hPa geopotential heights over western North America and anomalous positive inland mean sea-level pressures. Sundowner-only conditions display zonal 500 hPa flow and negative inland sea-level pressure anomalies. A low-level northerly coastal jet is present during Sundowners but not SAWs.

Mechanical models favor a ramp geometry for the Ventura‐pitas point fault, California

AbstractRecent investigations have provided new and significantly revised constraints on the subsurface structure of the Ventura‐Pitas Point fault system in southern California; however, few data directly constrain fault surfaces below ~6 km depth. Here, we use geometrically complex three‐dimensional mechanical models driven by current geodetic strain rates to test two proposed subsurface models of the fault system. We find that the model that incorporates a ramp geometry for the Ventura‐Pitas Point fault better reproduces both the regional long term geologic slip rate data and interseismic GPS observations of uplift in the Santa Ynez Mountains. The model‐calculated average reverse slip rate for the Ventura‐Pitas Point fault is 3.5 ± 0.3 mm/yr, although slip rates are spatially variable on the fault surface with &gt; 8 mm/yr predicted on portions of the lower ramp section at depth.

Comparing endmember selection techniques for accurate mapping of plant species and land cover using imaging spectrometer data

Building representative spectral libraries and quantitatively selecting a subset of spectra for mapping plant species and land cover/land use within remotely sensed imagery remain challenging for accurate classification. Multiple Endmember Spectral Mixture Analysis (MESMA) can be used for both classification and modeling fractional composition, and has been applied to map multiple biogeophysical variables. Our major objectives in this research were to 1) test a sampling design for building independent and representative training and validation spectral libraries; 2) compare endmember selection by a combination of two established techniques (count-based selection (CoB) and endmember average root mean square error (EAR)) with a recently introduced one (iterative endmember selection (IES)); and 3) develop and test a hybrid method, which combines the strengths of the previous two methods. We applied CoB/EAR, IES, and the new hybrid technique to mapping plant species and cover types in the Santa Ynez Mountains and Santa Barbara urban area, California, USA, using Airborne Visible Infrared Imaging Spectrometer (AVIRIS) data. For all endmember selection techniques, the number of selected endmembers varied across 25 random training samples. IES was consistently more accurate than CoB/EAR, but resulted in spectral libraries more than twice as large and failed to model rare species. The hybrid endmember selection technique resulted in the highest overall accuracy and kappa values and proved to be least sensitive to the random sampling protocols, but also produced the largest spectral libraries. A modified hybrid method, in which the number of endmembers selected was limited, produced the second highest accuracies, combining the strengths of the more parsimonious endmember selection by CoB/EAR with improved endmember selection by IES. Both IES and the hybrid methods selected endmembers that successfully classified a wide range of plant species and cover types, indicating their usefulness for these applications.

Initial impacts of a wildfire on hydrology and suspended sediment and nutrient export in California chaparral watersheds

AbstractStream and rainfall gauging and runoff sampling were used to determine changes in hydrology and export of nutrients and suspended sediment from a June 2004 wildfire that burned 3010 ha in chaparral coastal watersheds of the Santa Ynez Mountains, California. Precipitation during water year 2005 exceeded average precipitation by 200–260%. Burned watersheds had order of magnitude higher peak discharge compared with unburned watersheds but similar annual runoff. Suspended sediment export of 181 mt ha−1 from a burned watershed was approximately ten times greater than from unburned watersheds. Ammonium export from burned watersheds largely occurred during the first three storms and was 32 times greater than from unburned watersheds. Nitrate, dissolved organic nitrogen, and phosphate export from burned watersheds increased by 5.5, 2.8, and 2.2 times, respectively, compared with unburned chaparral watersheds. Storm runoff and peak discharge increase in burned compared with unburned sites were greatest during early season storms when enhanced runoff occurred. As the winter progressed, closely spaced storms and above average precipitation reduced the fire‐related impacts that resulted in significant increases in annual post‐fire runoff and export in other studies in southern California chaparral. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Postfire debris flows occur quickly after rainfall starts

Areas damaged by forest fires can be vulnerable to debris flows because vegetation is no longer holding dirt and rocks in place, and debris flows in burned areas can be triggered by much less rainfall than would be needed to trigger a debris flow in an unburned area. Such debris flows are common in burned steep terrain in Southern California and sometimes cause significant damage to property and even loss of life—for instance, 16 people died in debris flows above San Bernardino on 25 December 2003. To better understand the conditions that lead to debris flows, Kean et al. measured properties including rainfall, channel bed pore fluid pressure, and hillslope soil water content for 24 debris flow events that occurred in five different watersheds that burned in the 2009 Station and Jesusita fires in the San Gabriel and Santa Ynez mountains. They looked at the timing of rainfall and debris flow events.

In situ measurements of post-fire debris flows in southern California: Comparisons of the timing and magnitude of 24 debris-flow events with rainfall and soil moisture conditions

[1] Debris flows often occur in burned steeplands of southern California, sometimes causing property damage and loss of life. In an effort to better understand the hydrologic controls on post-fire debris-flow initiation, timing and magnitude, we measured the flow stage, rainfall, channel bed pore fluid pressure and hillslope soil-moisture accompanying 24 debris flows recorded in five different watersheds burned in the 2009 Station and Jesusita Fires (San Gabriel and Santa Ynez Mountains). The measurements show substantial differences in debris-flow dynamics between sites and between sequential events at the same site. Despite these differences, the timing and magnitude of all events were consistently associated with local peaks in short duration (< = 30 min) rainfall intensity. Overall, debris-flow stage was best cross-correlated with time series of 5-min rainfall intensity, and lagged the rainfall by an average of just 5 min. An index of debris-flow volume was also best correlated with short-duration rainfall intensity, but found to be poorly correlated with storm cumulative rainfall and hillslope soil water content. Post-event observations of erosion and slope stability modeling suggest that the debris flows initiated primarily by processes related to surface water runoff, rather than shallow landslides. By identifying the storm characteristics most closely associated with post-fire debris flows, these measurements provide valuable guidance for warning operations and important constraints for developing and testing models of post-fire debris flows.

Early Settlement Systems in the Middle Santa Ynez Valley, Santa Barbara County, California

Although still scanty, recent archaeological investigation in the central Santa Ynez Valley of Santa Barbara County, California, has revealed evidence of occupation dating prior to 5000 cal BP. The most definitive evidence comes from a site overlooking the Lake Cachuma reservoir and another 1.5 km north of the Santa Ynez River. More circumstantial evidence comes from a series of sites along the Lake Cachuma shoreline and a site overlooking a southern tributary of the Santa Ynez River. Three alternative settlement system models can account for the existing data, each concerning the relationship between valley sites and coastal sites on the other side of the Santa Ynez Mountains. First, separate populations occupied coastal and valley territories. Second, populations occupied large territories that included both coastal and valley residential bases. Third, populations centered on the coast occupied valley camps for such enterprises as hunting. No basis exists for rejecting any of the alternatives, and it is possible that all three settlement system types existed at different times and places prior to 5000 cal BP.