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Research about California

Source-linked reports with geographic coverage including California.

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USGS2022

Methods for robust estimates of tree biomass from pollen accumulation rates: Quantifying paleoecological reconstruction uncertainty

Pollen accumulation rates (PAR, grains cm –2 year –1 ) have been shown to be a reliable but methodologically complex bioproxy for quantitative reconstruction of past tree abundance. In a prior study, we found that the PARs of major tree taxa – Pseudotsuga , Pinus , Notholithocarpus , and the pollen group TC (Taxaceae and Cupressaceae families) – were robust and precise estimators of contemporary tree biomass. This paper expands our earlier work. Here, we more fully evaluate the errors associated with biomass reconstructions to identify weaknesses and recommend improvements in PAR-based reconstructions of forest biomass. We account for uncertainty in our biomass proxy in a formal, coherent fashion. The greatest error was introduced by the age models, underscoring the need for improved statistical approaches to age-depth modeling. Documenting the uncertainty in pollen vegetation models should be standard practice in paleoecology. We also share insights gained from the delineation of the relevant source area of pollen, advances in Bayesian 210 Pb modeling, the importance of site selection, and the use of independent data to corroborate biomass estimates. Lastly, we demonstrate our workflow with a new dataset of reconstructed tree biomass between 1850 and 2018 AD from lakes in the Klamath Mountains, California. Our biomass records followed a broad trend of low mean biomass in the ∼1850s followed by large contemporary increases, consistent with expectations of forest densification due to twentieth century fire suppression policies in the American West. More recent reconstructed tree biomass estimates also corresponded with silviculture treatments occurring within the relevant source area of pollen of our lake sites.

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USGS2022

Whole-ecosystem experiment illustrates short timescale hydrodynamic, light, and nutrient control of primary production in a terminal slough

Estuaries are among the most productive of aquatic ecosystems. Yet the collective understanding of patterns and drivers of primary production in estuaries is incomplete, in part due to complex hydrodynamics and multiple controlling factors that vary at a range of temporal and spatial scales. A whole-ecosystem experiment was conducted in a deep, pelagically dominated terminal channel of the Sacramento-San Joaquin Delta (California, USA) that seasonally appears to become nitrogen limited, to test whether adding calcium nitrate would stimulate primary productivity or increase phytoplankton density. Production did not respond consistently to fertilization, in part because nitrate and phytoplankton were dispersed away from the manipulated area within 1–3 days. Temporal and spatial patterns of gross primary production were more strongly related to stratification and light availability (i.e., turbidity) than nitrogen, highlighting the role of hydrodynamics in regulating system production. Similarly, chlorophyll was positively related not only to stratification but also to nitrogen—with a positive interaction—suggesting stratification may trigger nutrient limitation. The average rate of primary production (4.3 g O 2 m −2 d −1 ), metabolic N demand (0.023 mg N L −1 d −1 ), and ambient dissolved inorganic nitrogen concentration (0.03 mg N L −1 ) indicate that nitrogen can become limiting in time and space, especially during episodic stratification events when phytoplankton are isolated within the photic zone, or farther upstream where water clarity increases, dispersive flux decreases, and stratification is stronger and more frequent. The role of hydrodynamics in organizing habitat connectivity and regulating physical and chemical processes at multiple temporal and spatial scales is critical for determining resource availability and evaluating biogeochemical processes in estuaries.

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USGS2022

The effects of prolonged drought on vegetation dieback and megafires in southern California chaparral

Drought contributed to extensive dieback of southern California chaparral, and normalized difference vegetation index before drought and near the end of the drought was used to estimate this dieback, after accounting for other disturbances recorded in aerial photographs. Within the perimeters of two megafires that occurred after the drought, the 2017 Thomas Fire and the 2018 Woolsey Fire, there had been extensive areas of dieback. Comparing dieback with Monitoring Trends in Burn Severity measures of fire severity, there was a highly significant negative relationship between drought-caused shrub dieback and fire-caused dieback as measured by fire severity. We interpret this as further support for our remote sensing methodology for prefire dieback. Models of fire behavior suggest that one means by which dieback contributes to fire size is through increasing the density and distance of spot fires, particularly under extreme wind conditions. Lower elevation chaparral associations appear to be most vulnerable and are closer to urban environments, which should be a concern to fire managers in regions subjected to extended droughts.

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USGS2022

Colored shaded-relief bathymetric map and surrounding aerial imagery of Whiskeytown Lake, California

The Carr wildfire began on July 23, 2018, and burned almost 300,000 acres (approximately half on Federal lands) in northern California during the subsequent 6-week period. Over 97 percent of the area within Whiskeytown National Recreation Area, California, burned during the 2018 Carr wildfire, including the entire landscape that surrounds and drains into Whiskeytown Lake. Shortly after the Carr wildfire ended, the U.S. Geological Survey began investigations into the landscape responses, such as changes in erosion and sediment deposition, that occurred after the fire. This study focused on the collection and processing of bathymetric data and onshore aerial imagery in and around Whiskeytown Lake, California, to support wildfire science after the fire.

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USGS2022

Reference genome of the California glossy snake, Arizona elegans occidentalis: A declining California Species of Special Concern

The glossy snake ( Arizona elegans ) is a polytypic species broadly distributed across southwestern North America. The species occupies habitats ranging from California’s coastal chaparral to the shortgrass prairies of Texas and southeastern Nebraska, to the extensive arid scrublands of central México. Three subspecies are currently recognized in California, one of which is afforded state-level protection based on the extensive loss and modification of its preferred alluvial coastal scrub and inland desert habitat. We report the first genome assembly of A. elegans occidentalis as part of the California Conservation Genomics Project (CCGP). Consistent with the reference genome strategy of the CCGP, we used Pacific Biosciences HiFi long reads and Hi-C chromatin-proximity sequencing technologies to produce a de novo assembled genome. The assembly comprises a total of 140 scaffolds spanning 1,842,602,218 base pairs, has a contig NG50 of 61 Mb, a scaffold NG50 of 136 Mb, and a BUSCO complete score of 95.9%, and is one of the most complete snake genome assemblies. The A. e. occidentalis genome will be a key tool for understanding the genomic diversity and the basis of adaptations within this species and close relatives within the hyperdiverse snake family Colubridae.

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USGS2022

Recent declines in genetic diversity with limited dispersal among coastal cactus wren populations in San Diego County, California

Habitat loss and fragmentation can lead to smaller and more isolated populations and reduce genetic diversity and evolutionary potential. Conservation programs can benefit from including monitoring of genetic factors in fragmented populations to help inform restoration and management. We assessed genetic diversity and structure among four major populations of the Cactus Wren ( Campylorhynchus brunneicapillus ) in San Diego County in 2011–2012 and again in 2017–2019, using 22 microsatellite loci. We found a significant decline in heterozygosity in one population (San Pasqual) and a decline in allelic richness and effective population size in another (Sweetwater). Genetic diversity in the remaining two populations was not significantly different over time. Local diversity declined despite evidence of dispersal among some populations. Approximately 12% of genetically determined family groups (parents, offspring, siblings) included one or more members sampled in different territories with distances ranging from 0.2 to 10 km. All but one inferred dispersal events occurred within the same genetic population. Population structure remained relatively stable, although genetic differentiation tended to increase in the later sampling period. Simulations suggest that at currently estimated effective sizes, populations of Cactus Wrens will continue to lose genetic diversity for many generations, even if gene flow among them is enhanced. However, the rate of loss of heterozygosity could be reduced with increased gene flow. Habitat restoration may help bolster local population sizes and allelic richness over the long term, whereas translocation efforts from source populations outside of San Diego may be needed to restore genetic diversity in the short term.

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USGS2022

Mapping structural control through analysis of land-surface deformation for the Rialto-Colton groundwater subbasin, San Bernardino County, California, 1992–2010

The locations of many faults in and near the Rialto-Colton groundwater subbasin are not precisely known because the spatial density of existing lithologic and hydrologic data used to infer the locations of faults can be sparse. The U.S. Geological Survey, in cooperation with the San Bernardino Valley Municipal Water District, analyzed structural control of groundwater flow in and near the Rialto-Colton groundwater subbasin using Interferometric Synthetic Aperture Radar (InSAR) methods. Faults commonly are barriers to groundwater flow, and the high spatial resolution of InSAR imagery can be used to infer the locations of buried faults where groundwater pumping occurs. InSAR results have revealed three areas in and near the Rialto-Colton groundwater subbasin where buried faults are interpreted as groundwater-flow barriers: the northwestern area about 3 miles northwest of the City of Rialto, the San Jacinto fault area west of the City of San Bernardino, and the southeastern area about 2 miles southeast of the City of Colton. The InSAR results were combined with knowledge gained from previous studies to better define the location and extent of faults acting as groundwater-flow barriers. New data about faults acting as groundwater-flow barriers can be incorporated into future conceptual and hydrologic models of the Rialto-Colton groundwater subbasin and provide water managers information to help effectively manage groundwater resources.

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USGS2022

Seasonal and long-term clarity trend assessment of Lake Tahoe, California–Nevada

The clarity of Lake Tahoe, observed using a Secchi disk on a regular basis since the late 1960s, continues to be a sentinel metric of lake health. Water clarity is influenced by physical and biological processes and has declined in the five decades of monitoring, revealing differences between summer (June–September) and winter (December–March). This document summarizes key findings of a study of Lake Tahoe water clarity, including long-term variability and the relative importance of several influencing variables and processes. This study, prepared in cooperation with the Nevada Division of Environmental Protection, focused on (1) an apparent divergence in clarity trends between summer and winter periods, (2) observed changes in in-lake physical and ecological variables that may influence or control seasonal and annual clarity trends, and (3) five research hypotheses regarding lake clarity that were developed by Lake Tahoe management agencies. Previously collected data were used to complete this study. Trend analysis confirmed that winter clarity stabilized (that is, there is no longer a statistically significant trend up or down) during the last 20 years. Evaluation of clarity for selected months in the 50-year Secchi disk clarity dataset showed that only two summer months, July and August, had statistically significant decreases in clarity from 2000–19. Different subsets of available data were analyzed to reveal the presence or absences of trends for each season, decade, and month. Five hypotheses related to lake clarity were part of the study described by this report. Hypothesis 1 stated that clarity is controlled predominantly by the distribution and volumetric density of fine particles in suspension. This hypothesis was studied using available data describing in-lake fine (0–20 micrometers) particles from 2008–19. Water clarity was negatively correlated with in-lake particle abundance, with particles in the 1.0-4.6 μm range having the greatest effect, consistent with light-scattering theory. Estimated abundances of diatoms of the genus Cyclotella also were found to be negatively correlated with clarity. Data limitations precluded a complete investigation of hypothesis 2, which stated that the observed improvements in winter water clarity are a response to decreasing fine suspended-sediment concentrations in the lake resulting from load reductions from upland sources in and near urbanized areas. Data describing fine-sediment loading from urban areas to the lake were only available since 2014, and only once or twice per month. A slight, statistically significant, negative correlation was identified between urban fine-particle loading and monthly lake clarity with a 4-month lag. Particle abundance in monitored streams is highly correlated with simultaneous particle abundance in the lake. Hypothesis 3 stated that changing hydrodynamic conditions in the lake are increasing thermal stability and resistance to mixing. Trend analyses performed on stability index and buoyancy frequency time series computed from long-term observations of lake temperatures support the hypothesis that hydrodynamic conditions have evolved since 1969 to increase the lake’s resistance to mixing. The date of maximum mixing in winter has become progressively earlier in the year. Lake density stratification, defined using the stability index, is commencing earlier in the year and extending a month longer than in the early years of the monitoring program. Hypothesis 4 stated that the trend of decreasing summer clarity is a result of earlier, prolonged, and more intense stratification. Statistically significant correlations were found between summer clarity and (1) date of onset of stratification, (2) duration of stratification, and (3) buoyancy frequency. Hypothesis 5 stated that ecological (food web) interactions are causing changes in the trends of seasonal or annual clarity; data supporting hypothesis 5 were limited to examples from other systems and to intermittent monitoring of Lake Tahoe and Emerald Bay. The resulting narrative assessment was motivated by a 6-year study of Mysis shrimp disappearance and return in Emerald Bay. The available data and a large body of published literature are consistent with the inference that Mysis shrimp-induced food web changes are causing changes in the trends of seasonal or annual clarity. This food-web study focused on the relations between introduced Mysis shrimp, the native cladocerans ( Daphnia and Bosmina ) that were largely eliminated following Mysis introduction, and the effect on fine particles within the lake. The records of Mysis and other zooplankton data for Lake Tahoe are episodic and have large gaps. Consequently, statistical analyses could not be conducted to compare zooplankton data with other variables. The long-term record, however, indicates that the key effect was a change to the phytoplankton assemblage, where larger diatoms disappeared, likely due to Mysis grazing, only to be replaced by Cyclotella that are an order-of-magnitude smaller and have increased the abundance and volumetric density of total fine particles in suspension (biotic and abiotic).

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USGS2022

Groundwater quality of the Lucerne Valley groundwater basin, California

Anthropogenic activities, including groundwater withdrawals, return flow from irrigated agriculture, and treated wastewater-effluent disposal have the potential to affect groundwater quality in the Lucerne Valley groundwater basin, located in the southwest Mojave Desert. Questions regarding the current state and potential future of groundwater quality in this basin were addressed by (1) considering groundwater data from and findings of historical water-quality studies, (2) evaluating recent (1990–2021) U.S. Geological Survey water-quality and geochemical-tracer data, and (3) assessing groundwater-quality results from samples collected in 2021 to better understand the transport of applied treated wastewater effluent in the subsurface and associated effects of this practice on water quality. As observed by previous studies, differences in groundwater quality existed among the upper, middle, and lower aquifers of the Lucerne Valley groundwater basin, with the lower aquifer characterized by high dissolved-solid content relative to the middle and upper aquifers. Stable and radioisotope tracers indicate that most of the groundwater sampled in the basin was recharged during cooler, wetter climate conditions than those of the present day (2022). Analyses of the 2021 samples collected to examine the subsurface transport of applied treated wastewater effluent were not conclusive but indicate that water from applied treated wastewater effluent is currently (2022) limited to the upper aquifer and likely to remain so given the extensive confining unit below the upper aquifer.

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USGS2022

New Idria serpentinite protrusion, Diablo Range, California: From upper mantle to the surface

The New Idria serpentinite body in the Coast Ranges of California is a diapir that resulted from the interaction of the migrating Mendocino trench-ridge-transform fault triple junction, transpression, metasomatic fluids, and previously subducted oceanic crust and mantle. Northward propagation of the San Andreas fault progressively eliminated the original subduction zone, allowing seawater to penetrate into the formerly subducting abyssal peridotite mantle, triggering serpentinization. The associated physical changes in density, volume, and strength yielded an expanding, buoyantly rising serpentinite protrusion, facilitated by transpression along the San Andreas fault. Sedimentary facies and intrusion of minor cross cutting syenite and alkali basalt dikes indicate that the serpentinization-driven diapir buoyantly rose and widely breached the surface by ca. 14 Ma, attending migration of the Mendocino Triple Junction past the latitude of New Idria.

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USGS2022

Assessing small-mammal trapping design using spatially explicit capture recapture (SECR) modeling on long-term monitoring data

Few studies have evaluated the optimal sampling design for tracking small mammal population trends, especially for rare or difficult to detect species. Spatially explicit capture-recapture (SECR) models present an advancement over non-spatial models by accounting for individual movement when estimating density. The salt marsh harvest mouse (SMHM; Reithrodontomys raviventris ) is a federal and California state listed endangered species endemic to the San Francisco Bay-Delta estuary, California, USA; where a population in a subembayment has been continually monitored over an 18-year period using mark-recapture methods. We analyzed capture data within a SECR modeling framework that allowed us to account for differences in detection and movement between sexes. We compared the full dataset to subsampling scenarios to evaluate how the grid size (area) of the trap design, trap density (spacing), and number of consecutive trapping occasions (duration) influenced density estimates. To validate the subsampling methods, we ran Monte Carlo simulations based on the true parameter estimates for each specific year. We found that reducing the area of the trapping design by more than 36% resulted in the inability of the SECR model to replicate density estimates within the SE of the original density estimates. However, when trapping occasions were reduced from 4 to 3-nights the density estimates were indistinguishable from the full dataset. Furthermore, reducing trap density by 50% also resulted in density estimates comparable to the full dataset and was a substantially better model than reducing the trap area by 50%. Overall, our results indicated that moderate reductions in the number of trapping occasions or trap density could yield similar density estimates when using a SECR approach. This approach allows the optimization of field trapping efforts and designs by reducing field efforts while maintaining the same population estimate compared to the full dataset. Using a SECR approach may help other wildlife programs identify sampling efficiencies without sacrificing data integrity for long term monitoring of population densities.

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USGS2022

Comparative susceptibilities of selected California Chinook salmon and steelhead populations to isolates of L Genogroup Infectious Hematopoietic Necrosis Virus (IHNV)

Salmonid species demonstrate varied susceptibility to the viral pathogen infectious hematopoietic necrosis virus (IHNV). In California conservation hatcheries, juvenile Chinook salmon ( Oncorhynchus tshawytscha) have experienced disease outbreaks due to L genogroup IHNV since the 1940s, while indigenous steelhead (anadromous O. mykiss) appear relatively resistant. To characterize factors contributing to the losses of California salmonid fish due to IHNV, three populations of Chinook salmon and two populations of steelhead native to California watersheds were compared in controlled waterborne challenges with California L genogroup IHNV isolates at viral doses of 10 4 –10 6 pfu mL −1 . Chinook salmon fry were moderately to highly susceptible (CPM = 47–87%) when exposed to subgroup LI and LII IHNV. Susceptibility to mortality decreased with increasing age and also with a higher temperature. Mortality for steelhead fry exposed to two IHNV isolates was low (CPM = 1.3–33%). There was little intraspecies variation in susceptibility among populations of Chinook salmon and no differences in virulence between viruses strains. Viral persistence was demonstrated by the isolation of low levels of infectious IHNV from the skin of two juvenile Chinook salmon at 215 d post exposure. The persistence of the virus among Chinook salmon used for stocking into Lake Oroville may be an explanation for the severe epidemics of IHN at the Feather River hatchery in 1998–2002.

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USGS2022

Waveform signatures of earthquakes located close to the subducted Gorda Plate interface

Complex seismic velocity structure near the earthquake source can affect rupture dynamics and strongly modify the seismic waveforms recorded near the fault. Fault‐zone waves are commonly observed in continental crustal settings but are less clear in subduction zones due to the spatial separation between seismic stations and the plate boundary fault. We observed anomalously long duration S waves from earthquake clusters located near the interface of the subducted Gorda plate north of the Mendocino triple junction. In contrast, earthquakes located just a few kilometers below each cluster show impulsive S waves. A nodal array experiment was conducted around the Northern California Seismic Network station KCT for two months to investigate the origin of the complex S waves. Beamforming analysis shows that the S waves contain three arrivals that have different horizontal slownesses, which we term S 1, S 2, and S coda. Similar analysis on P waves also show two arrivals with different horizontal slownesses, which we term P 1 and P 2. P 1 and S 1 have larger horizontal slowness than P 2 and S 2, respectively, indicating that the phase pairs are body waves with different ray paths. Building upon a seismic refraction profile, we construct 1D velocity models and test different thicknesses and V P / V S ratios for the subducted oceanic crust. The arrival times and relative slownesses of P 1/ P 2 and S 1/ S 2 phases indicate that they are the direct and the Moho reflected phases, respectively. Their properties are consistent with a crustal thickness of ∼6 km and a moderate V P / V S ratio (∼1.8). The S coda is more difficult to characterize but has a clear dominant frequency that likely reflects the near‐source velocity and attenuation structure. Our study indicates that waveforms from earthquakes near the interface of the subducted slab can be used to infer detailed structural information about the plate‐boundary zone at seismogenic depths.

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USGS2022

Subspecies differentiation and range-wide genetic structure are driven by climate in the California gnatcatcher, a flagship species for coastal sage scrub conservation

Understanding genetic structure and diversity within species can uncover associations with environmental and geographic attributes that highlight adaptive potential and inform conservation and management. The California gnatcatcher, Polioptila californica , is a small songbird found in desert and coastal scrub habitats from the southern end of Baja California Sur to Ventura County, California. Lack of congruence among morphological subspecies hypotheses and lack of measurable genetic structure found in a few genetic markers led to questions about the validity of subspecies within P. californica and the listing status of the coastal California gnatcatcher, P. c. californica . As a U.S. federally threatened subspecies, P. c. californica is recognized as a flagship for coastal sage scrub conservation throughout southern California. We used restriction site-associated DNA sequencing to develop a genomic dataset for the California gnatcatcher. We sampled throughout the species' range, examined genetic structure, gene–environment associations, and demographic history, and tested for concordance between genetic structure and morphological subspecies groups. Our data support two distinct genetic groups with evidence of restricted movement and gene flow near the U.S.- Mexico international border. We found that climate-associated outlier loci were more strongly differentiated than climate neutral loci, suggesting that local climate adaptation may have helped to drive differentiation after Holocene range expansions. Patterns of habitat loss and fragmentation are also concordant with genetic substructure throughout the southern California portion of the range. Finally, our genetic data supported the morphologically defined P. c. californica as a distinct group, but there was little evidence of genetic differentiation among other previously hypothesized subspecies in Baja California. Our data suggest that retaining and restoring connectivity, and protecting populations, particularly at the northern range edge, could help preserve existing adaptive potential to allow for future range expansion and long-term persistence of the California gnatcatcher.

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USGS2022

Hydrogeology and simulation of groundwater flow in the Lucerne Valley groundwater basin, California

The Lucerne Valley is in the southwestern part of the Mojave Desert and is about 75 miles northeast of Los Angeles, California. The Lucerne Valley groundwater basin encompasses about 230 square miles and is separated from the Upper Mojave Valley groundwater basin by splays of the Helendale Fault. Since its settlement, groundwater has been the primary source of water for agricultural, industrial, municipal, and domestic uses. Groundwater withdrawal from pumping has exceeded the amount of water recharged to the basin, causing groundwater declines of more than 100 feet between 1917 and 2016 in the center of the basin. The continued withdrawal has resulted in an increase in pumping costs, reduced well efficiency, and land subsidence near Lucerne Lake. Although the volume of pumping has declined in recent years, there is concern that new agricultural growth and limits on imported water will continue to strain the sustainability of the groundwater system. To address these concerns, the U.S. Geological Survey entered into a cooperative agreement with the Mojave Water Agency to develop a better understanding of the Lucerne Valley hydrogeologic system and provide tools to help evaluate and manage the effects of future development in the Lucerne Valley. The objectives of this study were to (1) improve the understanding of the aquifer system, (2) improve the understanding of subsidence in the basin, and (3) incorporate the understanding into a groundwater-flow model that can be used to help manage the groundwater resources in the Lucerne Valley. The model developed for this study covers the period of 1942–2016 and can help evaluate various proposed water-management scenarios during different climatic and hydrologic conditions. The aquifer system consists of a shallow aquifer, a confining unit, and middle and lower aquifers. These layered water-bearing units were identified based on geologic units of the mostly unconsolidated sediments and hydrologic properties. These alluvial deposits consist of clay, silt, sand, and gravel; some places also contain clay and silty clay lacustrine deposits. Several faults act, at least in part, as barriers to groundwater flow on the eastern, southern, and western edges of the basin. Present-day natural recharge is primarily from the infiltration of runoff from the San Bernardino Mountains to the south; however, stable and radioactive isotopes show that groundwater from the middle of the Lucerne Valley was older than about 10,000 years and probably was recharged as infiltration from streams draining the mountains in the Mojave Desert to the north, which probably does not occur under present-day climatic conditions. The annual average natural recharge for 1942–2016, estimated by a Basin Characterization Model, was about 635 acre-feet per year; the average amount of treated wastewater effluent transferred to the Lucerne Valley for artificial recharge annually ranged from about 1,500 to 4,000 acre-feet per year during 1980–2016. Pumpage estimates for 1942–2016 ranged from about 3,000 acre-feet in 1942 to about 18,300 acre-feet in 1984. The total cumulative amount of groundwater removed from the basin by pumping between 1942 and 2016 was estimated to be about 700,000 acre-feet, which was about 10 times greater than the cumulative amount of recharge to the entire Lucerne Valley groundwater basin. Before groundwater development, the direction of groundwater flow was from the southern part of the basin northward to discharge areas near Lucerne Lake, where it discharged through springs along the Helendale Fault and by evapotranspiration. Since the early 1900s, groundwater-level declines have mostly eliminated the areas where natural discharge occurred and exceeded 100 feet in the middle of the basin between the early 1950s and mid-1990s, and as much as 25 feet near the margins from about the mid-1950s to 2000s. A decrease in the rate of pumping after the mid-1990s lessened the hydraulic stress on the middle and lower aquifers and enabled hydraulic heads in the middle of the basin to recover slightly as groundwater near the margins of the basin moved toward the pumping depression. Although trends in groundwater levels in the center of the basin have reversed since the mid-1990s, levels at the basin margins continue to decline as the movement of groundwater from the margins fills the pumping depression and gradually flattens the groundwater table throughout the basin. The long-term extraction of groundwater and associated dewatering of the fine-grained sediments present within the aquifer system has resulted in aquifer compaction and consequently land subsidence, primarily near Lucerne Lake. Analysis of interferometric synthetic aperture radar data shows that almost 11 inches of land subsidence has occurred south of Lucerne Lake between April 1992 and November 2009; less subsidence occurred elsewhere in the basin during this period. This differential land subsidence has caused fissures and cracks in the ground surface, which have buckled the pavement and undercut roads in several locations. The Lucerne Valley Hydrologic Model was developed using the finite-difference groundwater modeling software One Water Hydrologic Model to represent the hydrologic conditions and stresses during 1942–2016. The model has a uniform grid of approximately 92 acres per cell (2,000 feet by 2,000 feet) and has four layers representing the water-bearing units. The results from the calibrated model simulations indicated that groundwater pumpage exceeded recharge, resulting in an estimated net cumulative depletion of groundwater storage (discharge minus recharge) of about 465,000 acre-feet from 1942 to 2016. The model simulated as much as 7.5 feet (90 inches; 2,286 millimeters) of aquifer compaction, which indicates the extensive fine-grained deposits and measured subsidence near Lucerne Lake.

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USGS2022

Multi-stage soil-hydraulic recovery and limited ravel accumulations following the 2017 Nuns and Tubbs wildfires in Northern California

Wildfire can impact soil-hydraulic properties by reducing saturated hydraulic conductivity and sorptivity, making recently burned landscapes prone to debris flows and flash floods. The post-fire hazard window can range from years to decades. In Northern California, where wildfire frequency is steadily increasing, the impact and soil-hydraulic recovery from wildfires is unknown. Following the October 2017 Nuns and Tubbs fires in the Northern Bay Area of California, we established 41 monitoring sites for repeat tension-disc infiltrometer measurements of field-saturated hydraulic conductivity ( K fs ) over 3.5 years. Our site arrays, which encompass grasslands, chaparral, and oak and conifer forests across a range in lithology, show a marked decrease in K fs following the wildfires and a swift partial recovery following the initial post-fire rainy season. Our time series reveals a complex path to soil-hydraulic recovery marked by distinct seasonal stages. Analysis of changing K fs , sorptivity, and infiltration model residuals collectively suggests that these stages are related to transitions between soil-hydraulic processes like structural soil sealing from rainsplash, thermal cracking of bare soil, and vegetation regrowth. While soil infiltration rates were strongly impacted by the 2017 fires, dry ravel estimates are an order of magnitude less for similar slopes than the 2009 Station fire in the San Gabriel mountains of Southern California, suggesting that limited ravel flux may insufficiently load channels for debris flows that initiate from within-channel failure. Our analysis suggests that burned landscapes in the Northern Bay Area of California may experience rapid soil-hydraulic recovery and limited pathways toward post-fire debris flow initiation.

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USGS2022

Gill-net selectivity for fifteen fish species of the upper San Francisco Estuary

Gill-net size selectivity for 15 fish species occurring in the upper San Francisco Estuary was estimated from a data set compiled from multiple studies which together contained 7,096 individual fish observations from 882 gill net sets. The gill nets considered in this study closely resembled the American Fisheries Society’s recommended standardized experimental gill nets for sampling inland waters. Relationships between gill-net mesh sizes and the sizes for each fish species retained in them were estimated indirectly using generalized linear modeling and maximum likelihood. Selectivity curves are provided for each species to inform researchers about population characteristics of fishes sampled with similar gill nets.

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USGS2022

Flower plantings support wild bee reproduction and may also mitigate pesticide exposure effects

Sustainable agriculture relies on pollinators, and wild bees benefit yield of multiple crops. However, the combined exposure to pesticides and loss of flower resources, driven by agricultural intensification, contribute to declining diversity and abundance of many bee taxa. Flower plantings along the margins of agricultural fields offer diverse food resources not directly treated with pesticides. To investigate the potential of flower plantings to mitigate bee pesticide exposure effects and support bee reproduction, we selected replicated sites in intensively farmed landscapes where half contained flower plantings. We assessed solitary bee Osmia lignaria and bumble bee Bombus vosnesenskii nesting and reproduction throughout the season in these landscapes. We also quantified local and landscape flower resources and used bee-collected pollen to determine forage resource use and pesticide exposure and risk. Flower plantings, and their local flower resources, increased O. lignaria nesting probability. Bombus vosnesenskii reproduction was more strongly related to landscape than local flower resources. Bees at sites with and without flower plantings experienced similar pesticide risk, and the local flowers, alongside flowers in the landscape, were sources of pesticide exposure particularly for O. lignaria . However, local flower resources mitigated negative pesticide effects on B. vosnesenskii reproduction. Synthesis and applications . Bees in agricultural landscapes are threatened by pesticide exposure and loss of flower resources through agricultural intensification. Therefore, finding solutions to mitigate negative effects of pesticide use and flower deficiency is urgent. Our findings point towards flower plantings as a potential solution to support bee populations by mitigating pesticide exposure effects and providing key forage. Further investigation of the balance between forage benefits and added pesticide risk is needed to reveal contexts where net benefits occur.

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