Status of an endangered subspecies: the peninsular pronghorn at Baja California

During November 1993, we traversed by ground and by air the Vizcaíno Desert, the last redoubt of the peninsular pronghorn (Antilocapra americana peninsularis). A total of 29 h of aerial census was performed and a total of 1900 km²were covered by ground, totalling 400 man hours of survey. At present the peninsular pronghorn population consists of 175 individuals, and inhabits an area of approximately 5000 km². Because of the impossibility of controlling its activities over such an area and because of the present small population size, we propose to manage part of the population in semi-captivity at Mesa de la Choya island, with the objective of assuring the survival of this subspecies, the most endangered mammal in Mexico.     

The Role of Mid-Latitude Pacific Cyclones in the Winter Precipitation of California*

The characteristics of the mid-latitude Pacific cyclones that produce precipitation over California are investigated statistically to determine their role in governing precipitation receipt. From all cyclones occurring over the Pacific Ocean from December through March and between 1965–1990 inclusive, we identify a subset of cyclones likely to produce precipitation over California. The characteristics analyzed are track, frequency, duration, speed, central pressure, and proximity. These are related to monthly precipitation for the entire period and are also used to explain the difference in precipitation received during the earlier (wet) and latter (dry) halves of the 1980s. Results indicate that the winter cyclones responsible for precipitation over California originate in the southeast quadrant of the region influenced by the Aleutian Low and decay south of the Gulf of Alaska. The extent of cyclonic activity over the east Pacific Ocean near California diminished during the dry period. There were fewer cyclones, they were significantly weaker, and they traveled along a more meridional track during the dry period.     

Probabilistic approach to short- and intermediate-term earthquake forecasting: II test application to southern California

A method is proposed for assessing the modifying effects of precursory observations on long-term probabilities of strong earthquakes (M6). Estimated short-and intermediate-term probabilities so estimated rely on the mean precursory time and its uncertainty as a function of the mainshock magnitude and epicentral position. Short-and intermediate-term modification of long-term probabilities within a 120,000 km² circular area covering most of Southern California are estimated for one or more observations of strain, resistivity, and ground water anomalies centered near Palmdale in the central Transverse Ranges. The principal assumptions employed are log normal distribution of globally reported waiting periods and assumed predictive reliability of precursory phenomena. As expected, the significance of short-and intermediate-term modifications scale with the reliability of precursory observations, and are particularly enhanced within window lengths of a precursory characteristic time.     

美国南加州地壳、上地幔三维P波速度成像-莫霍面起伏的影响

应用南加州地震台网记录的6 347次区域地震的146 422个P波到时数据，并结合其它地震学方法所得到的莫霍面深度资料，在考虑莫霍面起伏变化下，确定了该区0～35 km深度内的精细三维P波速度模型．检测板分辨率实验结果说明了反演结果的可靠性．该模型提供了有关区域结构的重要信息，浅层P波速度结构与地表的地质特征相关，非常准确地反映了山脉盆地、地形地貌的差异；各深度层上的速度结构表现为分块特征，反映了以大型活动断裂为界勾画出的分区，如洛杉矶盆地、文图拉盆地、莫哈韦沙漠、半岛山脉、圣华金山谷、内华达山区及思尔顿凹槽区域整体变化的特征．圣安德列斯断层成为研究区内明显的边界，几个深度层上可以看到两侧的速度存在明显的差异，而横跨该断裂的几个剖面图上则显示了位于断层西南侧的太平洋板块区域速度较高、不均匀性较大，地震活动较深；位于断层东北部的北美板块速度偏低，非均匀性稍弱，地震活动性则较弱．比较平缓莫霍界面和起伏莫霍界面下反演得到的结果可以说明:莫霍界面的起伏对中下地壳层位速度结构的影响是明显的，尤其是在莫霍面深度变化较大的部位，这种影响更为强烈．带莫霍面形状的模型能更精确地计算射线路径和理论到时，从而使反演后的残差更小、结果更精确．      

P-wave tomography of crust and upper mantle under Southern California: Influence of topography of Moho discontinuity

We use 146 422 P-wave arrival times from 6 347 local earthquakes recorded by the Southern California SeismicNetwork to determine a detailed three-dimensional P-wave velocity structure at 0～35 km depth. We have takeninto account the Moho depth variations, which were obtained by seismological methods. Checkerboard tests sug-gest that our inversion results are reliable. Our models provide new information on regional geological structuresof Southern California. At shallow depths P-wave velocity structure correlates with surface geological features andexpresses well variations of surface topography of the mountains and basins. The velocity structure at each layer ischaracterized by block structures bounded by large faults. Ventura Basin, Los Angeles Basin, Mojave Desert, Pen-insular Ranges, San Joaquin Valley, Sierra Nevada, and Salton Trough show respectively all-round block. SanAndreas Fault becomes an obvious boundary of the region. To its southwest, the velocity is higher, and there arestrong heterogeneity and deeper seismicity; but to its northeast, the velocity is lower and shows less variation thanto the southwest, the seismicity is shallower. To investigate the effect of the Moho geometry we conducted inver-sions for two cases: one for flat Moho geometry, another for a Moho with lateral depth variations. We found thatthe topography of the Moho greatly affects the velocity structure of the middle and lower crust. When the Mohotopography is considered, a more reasonable tomographic result can be obtained and the resulting 3-D velocitymodel fits the data better.     

Fluvial system dynamics derived from distributed sediment budgets: perspectives from an uncertainty‐bounded application

The utility of sediment budget analysis is explored in revealing spatio‐temporal changes in the sediment dynamics and morphological responses of a fluvial system subject to significant human impacts during the recent Anthropocene. Sediment budgets require a data‐intensive approach to represent spatially‐differentiated impacts adequately and are subject to numerous estimation uncertainties. Here, field and topographic surveys, historical data, numerical modelling and a representative‐area extrapolation method are integrated to construct a distributed, process‐based sediment budget that addresses historical legacy factors for the highly regulated Lagunitas Creek (213 km²), California, USA, for the period 1983–2008. Independent corroboration methods and error propagation analysis produce an uncertainty assessment unique to a catchment of this size. Current sediment yields of ~20 000 t a^‐1 ± 6000 t a^‐1 equate to unit rates of ~300 t km^‐2 a^‐1 ± 90 t km^‐2 a^‐1 over the effective sediment contributing area of 64 km². This is comparable with yields associated with early Euro‐American settlement in the catchment, despite loss of sediment supply upstream of the two large dams. It occurs because ~57% of the sediment is now derived from incision‐related channel erosion. Further, the highly efficient routing of channel‐derived sediments in these incised channels suggests an efflux of 84% of contemporary sediment production, contrasting with the efflux of ≈10–30% reported for unregulated agricultural catchments. The results highlight that sediment budgets for regulated rivers must accommodate channel morphological responses to avoid significantly misrepresenting catchment yields, and that volumetric precision in sediment budgets may best be improved by repeat, spatially dense, channel cross‐section surveys. Human activities have impacted every aspect of the sediment dynamics of Lagunitas Creek (production, storage, transfer, rates of movement through storage), confirming that, while distributed sediment budgets are data demanding and subject to numerous error sources, the approach can provide valuable insights into Anthropocene fluvial geomorphology. Copyright © 2017 John Wiley & Sons, Ltd.     

Revisiting hydrological drought propagation and recovery considering water quantity and quality

Climate extremes, in particular droughts, are significant driving forces towards riverine ecosystem disturbance. Drought impacts on stream ecosystems include losses that can be either direct (e.g., destruction of habitat for aquatic species) or indirect (e.g., deterioration of water quality, soil quality, and increased chance of wildfires). This paper combines hydrologic drought and water quality changes during droughts and represents a multistage framework to detect and characterize hydrological droughts while considering water quality parameters. This method is applied to 52 streamflow stations in the state of California, USA, over the study period of 1950–2010. The framework is assessed and validated based on two drought events declared by the state in 2002 and 2008. Results show that there are two opposite drought propagation patterns in northern and southern California. In general, northern California indicates more frequent droughts with shorter time to recover. Chronology of drought shows that stations located in southern California have not followed a specific pattern but they experienced longer drought episodes with prolonged drought recovery. When considering water quality, results show that droughts either deteriorate or enhance water systems, depending on the parameter of interest. Undesirable changes (e.g., increased temperature and decreased dissolved oxygen) are observed during droughts. In contrast, decreased turbidity is detected in rivers during drought episodes, which is desirable in water systems. Nevertheless, water quality deteriorates during drought recovery, even after drought termination. Depending on climatic and streamflow characteristics of the watersheds, it was found that it would take nearly 2 months on average for water quality to recover after drought termination.     

Aeolian sand transport at the Lanphere Dunes,Northern California

Aeolian sand transport was studied at the Lanphere Dunes, a coastal dune complex in northern California, by comparing slipface advance rates with transport predicted based on local wind data. The slipfaces of a 2·5 m high transverse ridge and 10 m high parabolic dune were monitored over a period of three months to estimate sand discharge. The study was performed during the dry season, which has the maximum sand‐driving potential. Over the three month study period, average sand discharge was 12·5 m³ per m width per year at the transverse ridge and 8·8 m³ per m width per year at the parabolic dune. A method was developed for modelling slipfaces that are sinuous and where sediment transport rates are not constant across the width of the slipface. Field measurements were used to generate three‐dimensional representations of dune slipfaces. Periodic measurements over the course of three months were used to compute the volume of displaced sediment. Theoretical sand transport was computed from local wind data using the Bagnold model and compared with the observed transport rates. Predicted rates were substantially lower than observed rates. Wind velocities rarely exceeded the threshold velocity. Discrepancies between the observed and predicted values appear to be caused by a combination of wind data recording procedures and differences between wind velocities at the anemometer location and the site where sand transport was measured. Wind data collected by weather bureaux have been utilized in numerous studies for modelling sediment transport. Such data typically have sample intervals of one hour or greater and are often averaged prior to reporting. The effect of averaging was investigated by comparing sand transport estimates based on daily average wind velocities with those based on the original hourly observations. The daily average data were depleted of high velocity winds and sand transport estimates were accordingly much lower than those based on the hourly data. Copyright © 2000 John Wiley & Sons, Ltd.     

Stormwater plume detection by MODIS imagery in the southern California coastal ocean

Stormwater plumes in the southern California coastal ocean were detected by MODIS-Aqua satellite imagery and compared to ship-based data on surface salinity and fecal indicator bacterial (FIB) counts collected during the Bight'03 Regional Water Quality Program surveys in February–March of 2004 and 2005. MODIS imagery was processed using a combined near-infrared/shortwave-infrared (NIR-SWIR) atmospheric correction method, which substantially improved normalized water-leaving radiation (nLw) optical spectra in coastal waters with high turbidity. Plumes were detected using a minimum-distance supervised classification method based on nLw spectra averaged within the training areas, defined as circular zones of 1.5–5.0-km radii around field stations with a surface salinity of S < 32.0 (“plume”) and S > 33.0 (“ocean”). The plume optical signatures (i.e., the nLw differences between “plume” and “ocean”) were most evident during the first 2 days after the rainstorms. To assess the accuracy of plume detection, stations were classified into “plume” and “ocean” using two criteria: (1) “plume” included the stations with salinity below a certain threshold estimated from the maximum accuracy of plume detection; and (2) FIB counts in “plume” exceeded the California State Water Board standards. The salinity threshold between “plume” and “ocean” was estimated as 32.2. The total accuracy of plume detection in terms of surface salinity was not high (68% on average), seemingly because of imperfect correlation between plume salinity and ocean color. The accuracy of plume detection in terms of FIB exceedances was even lower (64% on average), resulting from low correlation between ocean color and bacterial contamination. Nevertheless, satellite imagery was shown to be a useful tool for the estimation of the extent of potentially polluted plumes, which was hardly achievable by direct sampling methods (in particular, because the grids of ship-based stations covered only small parts of the plumes detected via synoptic MODIS imagery). In most southern California coastal areas, the zones of bacterial contamination were much smaller than the areas of turbid plumes; an exception was the plume of the Tijuana River, where the zone of bacterial contamination was comparable with the zone of plume detected by ocean color.     

Marsh expansion at Calaveras Point Marsh, South San Francisco Bay, California

Studies of shoreline progradation along low-energy vegetated shorelines have been limited, as these environments are generally experiencing erosion rather than deposition, with extreme erosion rates frequently found. This study examined yearly changes along a vegetated shoreline at Calaveras Point Marsh, South San Francisco Bay, California, using aerial photography, to determine the roles of climatic, watershed, and coastal process in driving shoreline changes. In addition, sediment accumulation was monitored on a yearly basis at 48 locations across the marsh to determine the role of geomorphic factors in promoting accumulation. Calaveras Point Marsh was found to have expanded from 49.26 ± 5.2 to 165.7 ± 4.7 ha between 1975 and 2005. Although the rate of marsh expansion was not positively correlated with yearly variability in precipitation, local streamflow, delta outflow, water level observations, population growth, or ENSO indices, marsh growth was greater during years of higher than average temperatures. Warmer temperatures may have promoted the recruitment and growth of Spartina foliosa, a C₄ grass known to be highly responsive to temperature. Other factors, such as the formation of a coastal barrier, a recent change in the location of the mouth of the Guadalupe River, and channel readjustment in response to diking are credited with driving the bulk of the marsh expansion. Sediment accumulation was found to be high closest to channels and to the shoreline, at low elevations and in recently vegetated marsh. Globally, the pace of sea level rise exerts the primary control on wetland development and persistence. However, at local geographic scales, factors such as tectonic events, modifications to natural sediment transport pathways or land use changes may overwhelm the effects of regional sea level rise, and allow for wetlands to develop, expand and persist despite rapid sea level rise.