Recent Drought Research Articles

Seasonal and aridity influences on the relationships between drought indices and hydrological variables over China

In recent decades multiple drought indices have been developed to quantify drought characteristics and to examine drought effects. However, little is known about regional applicability of drought indices for monitoring seasonal droughts over China. Here we investigated the capacity of three commonly used drought indices (SPI, SPEI and scPDSI) for showing water deficit as compared to independent precipitation, root zone soil moisture (SM) and terrestrial water storage (TWS) estimates collected from satellite observation and model simulation, by using trend and correlation analysis of the drought indices and hydrological variables in different seasons and climatic zones from 2000 to 2019. Our results suggested that areas with a significant increasing trend in precipitation (p < 0.05) occupied 13.4–20% of the study area from spring to autumn. The corresponding area for SM varied from 20 to 26%. A wetting trend was dominant in drylands despite TWS depletion. A drying trend appeared in southern North China Plain and southwest China, where severe droughts in 2009 and 2011 led to substantial declines in SM and TWS. SPI and SPEI worked better than scPDSI in characterizing meteorological drought at the seasonal timescale, given that the inter-annual variation of precipitation was in consistent with that of SPI and SPEI in approximately 70–80% of China. scPDSI was suitable for agricultural drought monitoring in different seasons, and agricultural drought was well characterized by SPI and SPEI only in the summer. SPI, SPEI and scPDSI both showed worse performances in quantifying drought trend and magnitude in drier regions and seasons characterized by a low aridity index. Thus, potential future expansion of global drylands calls for an integration of human impact into hydrological drought modeling.

Tapping into the physiological responses to mistletoe infection during heat and drought stress.

Mistletoes are important co-contributors to tree mortality globally, particularly during droughts. In Australia, mistletoe distributions are expanding in temperate woodlands, while their hosts have experienced unprecedented heat and drought stress in recent years. We investigated whether the excessive water use of mistletoes increased the probability of xylem emboli in a mature woodland during the recent record drought that was compounded by multiple heatwaves. We continuously recorded transpiration ($T_{SLA}$) of infected and uninfected branches from two eucalypt species over two summers, monitored stem and leaf water potentials ($\Psi $) and used hydraulic vulnerability curves to estimate percent loss in conductivity (PLC) for each species. Variations in weather (vapor pressure deficit, photosynthetically active radiation, soil water content), host species and % mistletoe foliage explained 78% of hourly $T_{SLA}$. While mistletoe acted as an uncontrollable sink for water in the host even during typical summer days, daily $T_{SLA}$ increased up to 4-fold in infected branches on hot days, highlighting the previously overlooked importance of temperature stress in amplifying water loss in mistletoes. The increased water use of mistletoes resulted in significantly decreased host $\Psi _{\rm{leaf}}$ and $\Psi _{\rm{trunk}}$. It further translated to an estimated increase of up to 11% PLC for infected hosts, confirming greater hydraulic dysfunction of infected trees that place them at higher risk of hydraulic failure. However, uninfected branches of Eucalyptus fibrosa F.Muell. had much tighter controls on water loss than uninfected branches of Eucalyptus moluccana Roxb., which shifted the risk of hydraulic failure towards an increased risk of carbon starvation for E. fibrosa. The contrasting mechanistic responses to heat and drought stress between both co-occurring species demonstrates the complexity of host-parasite interactions and highlights the challenge in predicting species-specific responses to biotic agents in a warmer and drier climate.

Large tree mortality leads to major aboveground biomass decline in a tropical forest reserve.

Humans are transforming the ecology of the Earth through rapid changes in land use and climate. These changes can affect tropical forest structure, dynamics and diversity. While numerous studies have focused on diversity metrics, other aspects of forest function, such as long-term biomass dynamics, are often less considered. We evaluated plant community structure change (i.e., abundance, diversity, composition, and aboveground biomass) in a 2.25ha forest dynamics plot located within a ~ 365ha reserve in southern Costa Rica. We censused, mapped and identified to species all plants ≥ 5cm diameter at breast height (DBH) in three surveys spanning 2010-2020. While there were no changes in late-successional species diversity, there were marked changes in overall species composition and biomass. Abundance of large (≥ 40cm DBH) old-growth dense-wooded trees (e.g., Lauraceae, Rosaceae) decreased dramatically (27%), leading to major biomass decline over time, possibly driven by recent and recurrent drought events. Gaps created by large trees were colonized by early-successional species, but these recruits did not make up for the biomass lost. Finally, stem abundance increased by 20%, driven by increasing dominance of Hampea appendiculata. While results suggest this reserve may effectively conserve overall plant diversity, this may mask other key shifts such as large aboveground biomass loss. If this pattern is pervasive across tropical forest reserves, it could hamper efforts to preserve forest structure and ecosystem services (e.g., carbon storage). Monitoring programs could better assess carbon trends in reserves over time simply by tracking large tree dynamics.

Post‐Drought Groundwater Storage Recovery in California's Central Valley

AbstractGroundwater depletion is a major threat to agricultural and municipal water supply in California's Central Valley. Recent droughts during 2007–2009 and 2012–2016 exacerbated chronic groundwater depletion. However, it is unclear how much groundwater storage recovered from drought‐related overdrafts during post‐drought years, and how climatic conditions and water management affected recovery times. We estimated groundwater storage change in the Central Valley for April 2002 through September 2019 using four methods: GRACE satellite data, a water balance approach, a hydrologic simulation model, and monitoring wells. We also evaluated the sensitivity of drought recovery to different climate scenarios (recent climate ± droughts and future climate change scenarios: 20 GCMs and 2 RCPs) using water balance method and statistical sampling of historical climate data. Estimated Central Valley groundwater loss during the two droughts ranged from 19 km3 (2007–2009) to 28 km3 (2012–2016) (median of four methods). Median aquifer storage recovery was 34% and 19% of the overdraft during the 2010–2011 and 2017–2019 post‐drought years, respectively. Numerical experiments show that recovery times are sensitive to climate forcing, with longer recovery times for a future climate scenario that replicate historical climatology relative to historical forcing with no droughts. Overdraft recovery times decrease by ∼2× with implementation of pumping restrictions (30th to 50th percentiles of historical groundwater depletion) to constrain groundwater depletion relative to no restrictions with a no‐drought future climatology. This study highlights the importance of considering water management implications for future drought recoveries within the context of climate change scenarios.

An operational tool to detect non-native invasive grasses: Assessing temporal transferability and bias in fires to optimize performance for land management

Non-native invasive grasses have spread rapidly across the American West in recent years, significantly changing the ecology of the landscape by increasing fire frequency and degrading wildlife habitat. To meet the need for national forests in California to monitor the spread of these grasses into shrub-dominated ecosystems, we developed an operational tool that produces maps of herbaceous cover on a biennial basis at 30 m scale for southern California. Because the tool is planned for deployment for the remainder of the state, we present a rigorous analysis of temporal and spatial transferability of the model to demonstrate its suitability as a broad-scale technical application. Beta regression and random forest models were tested using Landsat 5, 7, and 8 imagery, including a variety of vegetation indices and spectral bands, as well as a predictor representing the time elapsed since the most recent fire. Comparing models created in six different years for which high resolution training data were available, we found that a random forest model trained using 2009 data performed better than models trained in other years in terms of variance explained by predictors (70% variance explained), inter-annual temporal transferability, cross-validation predictions for withheld data (root mean square error = 4.14), and minimal bias in burned v. unburned areas. Predictors in the final model included the maximum spring value for red-green angle (RGA), mean summer RGA, and standard deviation in the spring for the normalized burn ratio (NBR); these predictors leverage the distinct phenology of annual grasses as compared to evergreen shrubs that dominate the study area. Feasibility considerations for maintenance of an operational tool were an important component of model development, and we discuss 1) leveraging data processing capabilities using Google Earth Engine, 2) a cost-benefit comparison of model re-training for future years, and 3) an assessment of training methods that relied primarily on imagery-based assessments, while reserving field visitation for targeted assessments when conditions warrant further validation. Model outputs predicted to the full raster scale indicated non-native invasive grass spread has occurred at a moderate level over the last decade, with increases in cover observed mainly in burned areas, but with expansion and density likely suppressed by recent drought.

Study of the drought status and its impact on groundwater resources: Case study, Hamadan, Iran

In recent year's climate changes account for extreme droughts, and one of the consequences of the drought is the reduction in groundwater levels. The present study was conducted to evaluate the drought status and its impact on the groundwater resources of Hamadan, Iran. This descriptive study data were collected from Water Affairs departments from 1987 to 2011. The study of the rainfall patterns over the past 25 years shows the large fluctuations in rainfall. The consequence of indiscriminate depletion of underground water resources is creating about 19 sinkholes in the plains. Over the past 50 years, about 320 villages have been uninhabited and the drinking water for 100 villages was supplied by tankers. Digging the illegal wells, uncontrolled exploitation of groundwater that cannot be replenished by recharge, uncontrolled exploitation of groundwater by the Shahid Mofateh power plants, unfavorable spatial and temporal distribution of rainfall, and lack of modern irrigation methods are responsible for the recent drought in Hamadan.

Prolonged early to middle Holocene drought in the Pacific Northwest inferred from lacustrine carbonate oxygen isotope values and sedimentology

Sediment-based lake-level reconstructions and the oxygen isotopic composition of lacustrine carbonate (δ18OCO3) provide insight on hydroclimate conditions during the early Holocene in western North America and the Pacific Northwest. There are relatively few such records with decadal temporal resolution, and changes in the isotopic composition of meteoric water potentially obscure the hydroclimate signal (precipitation-evaporation) in lacustrine δ18OCO3 data. Here, we present a Holocene-length drought index developed by combining the δ18OCO3 record from Castor Lake, a closed-basin system located in north-central Washington (USA), with the δ18OCO3 record from Lime Lake, which has an open-basin configuration and is located in northeastern Washington. The lake sediment records are dated using primarily a combination of radiocarbon measurements on terrestrial macrofossils, Pb-210, Cs-137, and tephrochronology. The δ18OCO3 drought index, along with changes in Castor Lake sediment facies, suggest periods of prolonged drought in the Pacific Northwest between 11,200 and 6200 cal yr BP, a finding that is supported by other lacustrine proxy records of lake level and temperature change. Early Holocene drought likely resulted from higher than present summer isolation that promoted longer, hotter summers and greater evaporation. Lower than present winter insolation at this time likely altered the location and/or strength of the westerlies, reducing cold season precipitation/snowpack and intensifying the dry conditions. This study highlights the sensitivity of hydroclimate in an already drought- and fire-prone region to the magnitude and duration of summer warmth and precipitation and snowpack levels during the winter.

“We would be dead in the water without our social media!”: Women using entrepreneurial bricolage to mitigate drought impacts in rural Australia

ABSTRACT It is widely understood that climate change transforms rural communities’ economic activities that have historically relied on farming through off-farm income for economic stability and mitigation. Entrepreneurship has become a central feature of the diversification of rural labor, but there is still a need to document how rural women operationalize entrepreneurship in their communities. We use three narrative interviews to examine how settler farming women in the Murweh and Paroo Shires of outback Queensland in Australia have mitigated the devastating financial impact of the most recent drought through the lens of entrepreneurial bricolage for off-farm income. The findings, generated through grounded theory, highlight the complexity of entrepreneurial bricolage processes through women’s use of networking (digital and communal), learning digital technologies, repurposing of resources and places, and use of family labor. The paper contributes theoretically to place-based, gendered and context-specific understandings of drought mitigation, through identifying how entrepreneurial bricolage is employed.

Phenological Water Balance Applications for Trend Analyses and Risk Management

The overarching goal of this work is to develop and demonstrate methods that support effective agro-pastoral risk management in a changing climate. Disaster mitigation strategies, such as the Sendai Framework for Disaster Risk Reduction (SFDRR), emphasize the need to address underlying causes of disaster risk and to prevent the emergence of new risks. Such assessments can be difficult, because they require transforming changes in meteorological outcomes into sector-specific impact. While it is common to examine trends in seasonal precipitation and precipitation extremes, it is much less common to study how these trends interact with crop and pasture water needs. Here, we show that the Water Requirement (WR) component of the widely used Water Requirement Satisfaction Index (WRSI) can be used to enhance the interpretation of precipitation changes. The WR helps answer a key question: was the amount of rainfall received in a given season enough to satisfy a crop or pasture's water needs? Our first results section focuses on analyzing spatial patterns of climate change. We show how WR values can be used to translate east African rainfall declines into estimates of crop and rangeland water deficits. We also show that increases in WR, during recent droughts, has intensified aridity in arid regions. In addition, using the PWB, we also show that precipitation increases in humid areas of western east Africa have been producing increasingly frequent excessive rainfall seasons. The second portion of our paper focuses on assessing temporal outcomes for a fixed location (Kenya) to support drought-management scenario development. Kenyan rainfall is decreasing and population is increasing. How can we translate this data into actionable information? The United Nations and World Meteorological Organization advise nations to proactively plan for agro-hydrologic shocks by setting aside sufficient grain and financial resources to help buffer inevitable low-crop production years. We show how precipitation, WR, crop statistics, and population data can be used to help guide 1-in-10 and 1-in-25-year low crop yield scenarios, which could be used to guide Kenya's drought management planning and development. The first and second research components share a common objective: using the PWB to translate rainfall data into more actionable information that can inform disaster risk management and development planning.

Archival Weather Conditions at Siding Spring Observatory

Weather conditions are a key factor in deciding where to build telescope facilities. It is also a critical factor when planning and designing large surveys. Siding Spring Observatory, located in Northern New South Wales, Australia, has been a national hub for optical astronomy for decades. Weather data have been gathered from multiple sources on site and analyzed to show long term trends in wind, humidity, temperatures, and observing conditions on site. A gradual decrease in humidity was observed due to the recent drought, a trend that is expected to continue into the future. Downtime due to high humidity occurs most in winter, with 17.5% of observing time sitting at >90%. Wind direction and speeds are highly variable across the mountain. Predominant winds are NNW in summer (December–February), and S in winter (June–August). Strong gusts in excess of 55 ms−1 can rarely be experienced near the apex of the mountain, however, the majority of the mountain does not experience this, and the median wind speed is 3.4 ms−1. Temperatures are consistent across the mountain, with a median of 14.1°C. Diurnal temperature variations are largest in summer, at 11°C. 46% of nights are cloudy, with a further 9%–29% having poor viewing conditions depending on wavelength range.

Urban household uptake of water sensitive urban design source control measures: an exploratory comparative survey across Cape Town and Pretoria, South Africa

ABSTRACT Water Sensitive Urban Design (WSUD) presents a process towards the objective of a water-sensitive city. An integrated approach requires community acceptance to implement WSUD at household level. An exploratory comparative survey was conducted amongst 250 households across Cape Town and Pretoria, South Africa, to determine actual uptake of rainwater harvesting, greywater recycling systems and permeable paving, factors affecting uptake, and the perceived effectiveness of assistance to implement WSUD relative to other water demand-side management instruments. Actual uptake of rainwater harvesting and greywater reuse is noticeable amongst water-saving middle-to-high income home owners. Although loglinear analyses suggest that Cape Town’s recent drought is unlikely to contribute towards more established levels of uptake, assistance to implement WSUD is nevertheless perceived as the second most effective instrument. Three directions for more systematic research are proposed to better inform the practical implementation of WSUD at household level.

When usage matters: time-of-use analysis of Cape Town's Day Zero drought response.

Poor resource management and infrastructure limitations make the effects of drought worse for cities in developing countries. One way to alleviate the impact without large investments is targeted demand management. This has worked well in studies that focused on some of the recent droughts, including Cape Town's 'Day Zero' drought of 2016-2018. Many studies have measured demand response to a drought at a coarse time resolution, but few have measured it at an hourly resolution or compared weekday with weekend use. In this study we evaluated households' hourly time-of-use behaviour in response to the Cape Town drought at two prominent inflection points identified by previous studies: the announcement of the Critical Water Shortages Disaster Plan in October 2017 and the introduction of Level 6B restrictions in February 2018. The first major reduction was caused by residents reducing their usage by about a third in the early morning and evening hours on weekdays, and the second, even larger, reduction was achieved in the mid-morning hours on weekdays when home owners were not at home but ensured that domestic workers used water sparingly.

The potential of data driven approaches for quantifying hydrological extremes

Recent droughts in Europe have shown that national water systems are facing increasing challenges when dealing with drought impacts. Especially the Netherlands has seen an increasing need to adapt their water management to improve preparedness for future drought events. Ideally, the necessary information needed for operational water management decisions should be readily available ahead of time and/or computed flexibly and efficiently to ensure sufficient time to evaluate the various management actions. In this study, we show that in addition to physically based hydrological models, the upcoming and promising trend of incorporating machine learning (ML) in hydrology can provide the basis for future efforts in supporting national operational water management by providing the needed information efficiently and with the required accuracy. As a precursor for their use in a forecasting system, we assessed the ability of five different data driven methods to simulate hydrological variables at a national-scale. We developed a unified workflow where we use limited information on hydro-meteorological variables and general water management policies to simulate historic timeseries of discharge, groundwater levels, surface water levels and surface water temperatures. We find that all ML methods, ranging from very simple to more complex ones, showed a generally good performance for stations and target levels which are closely linked to the input data and location (e.g. stations along main river network). For downstream stations and small rivers, the Random Forest method outperforms the other methods both for discharge and surface water levels. For surface water temperature no location dependency was observed and for groundwater levels, all methods were performing comparable with most stations ranging in nRMSE 0.2-0.3. Generally, the best performances were reached by the more advanced Random Forest and LSTM methods, which was also seen when simulating high and low flow events. High flow events were slightly better captured than low flow events but overall simulating extreme events based on a simple input data set remains challenging. Specific training sets, including event related information and additional input variables, could like improve future assessments. Including the feature importance of the methods allowed us to detect how and where water management influence played an important role. The addition of information on water management in the ML routines increases overall performance, although limited. We conclude that ML and other data driven approaches have potential in predicting different hydrological variables. We were able to capture and incorporate water management aspects in our analysis, creating a base for future experiments where scenario analysis might reveal ML based mitigation strategies. The combination of limited input data requirement and short computation times makes this new framework suitable for forecasting purposes.

تاثیر خشکسالی اقلیمی دودهه (1390-1370) برناپایداری توسعه در دهستان هشیوارشهرستان داراب

تاثیر خشکسالی اقلیمی دودهه (1390-1370) برناپایداری توسعه در دهستان هشیوارشهرستان داراب

Valuing the impacts of forest disturbances on ecosystem services: An examination of recreation and climate regulation services in U.S. national forests

The increasing frequency and severity of forest disturbances challenge the sustainable provision of ecosystem services by forests, and this is true even in regions with net forest growth. The combination of ecosystem service losses from forest disturbances that are immediate as well as the slower post-disturbance process of forest regrowth could result in long-lasting social, economic, and environmental costs. Using economic and geospatial analysis, we estimate the value of recreational services losses due to drought and quantify the social cost of carbon emissions due to wildfires and bark beetle damage in three National Forests in the Sierra Nevada region, California (Inyo National Forest, Tahoe National Forest, and Lake Tahoe Basin Management Unit). Our findings indicate that recent droughts resulted in an annual reduction of about $73 million, $36 million, and $15 million for Lake Tahoe Basin Management Unit, Tahoe, and Inyo National Forests, respectively. Tree biomass loss due to wildfires and bark beetle damage generated around 10 Mt CO2 of total emissions from 2003 and 2012. The estimated cumulative social costs of such emissions were around $0.8 million for fire and $2.4 million for bark beetle (using a 3% discount rate for related climate change damage). The social costs almost tripled under extreme climate damage projections. Our analysis could inform adaptation and management strategies to conserve or enhance the health and function of publicly managed forests, and to protect their flow of ecosystem services for present and future generations.

Solar forcing of early Holocene droughts on the Yucat\xe1n peninsula

A speleothem record from the north-eastern Yucatán peninsula (Mexico) provides new insights into the tropical hydro-climate of the Americas between 11,040 and 9520 a BP on up to sub-decadal scale. Despite the complex atmospheric reorganization during the end of the last deglaciation, the dominant internal leading modes of precipitation variability during the late Holocene were also active during the time of record. While multi-decadal variations were not persistent, Mesoamerican precipitation was dominated by changes on the decadal- and centennial scale, which may be attributed to ENSO activity driven by solar forcing. Freshwater fluxes from the remnant Laurentide ice sheet into the Gulf of Mexico and the North Atlantic have additionally modulated the regional evaporation/precipitation balance. In particular, this study underlines the importance of solar activity on tropical and subtropical climate variability through forcing of the tropical Pacific, providing a plausible scenario for observed recurrent droughts on the decadal scale throughout the Holocene.

Lessons in Policy Incoherence: A Review of Recent Water Policies, Water Planning and Drought Policy in Australia

The purpose of this paper was to consider the extent to which drought policy in Australia is consistent with water policy and to also reflect on the coherence of government planning in the Murray–Darling Basin. At the outset, it is contended that the gains from an efficiency‐enhancing water policy are likely to be substantially weakened if responses to drought and basin planning run counter to this. The analysis centres on the federal government's response to the Productivity Commission in April 2019, the legacy of interventions in the Murray–Darling Basin and the subsequent “Australian Government Drought Response, Resilience and Preparedness Plan” (the Drought Plan) released in late 2019. Collectively, the analysis adds to the literature that considers coherence within environmental management but also highlights where weaknesses could be given more attention.

Using Cellular Automata Approach to Optimize the Hydropower Reservoir Operation of Folsom Dam

While hydropower in California is one the main sources of renewable energy, population growth has continuously increased demand for energy. In addition, recent droughts reduced the amount of available water behind the hydropower dams to provide the water head needed to run the turbines in hydropower plants. A more sustainable alternative, instead of developing new infrastructure, is to enhance the daily operation of reservoirs to support hydropower generation. This study suggests a new optimal operation policy for Folsom Reservoir in California and hydropower plants, which maximizes hydropower generation and reduces flood risk. This study demonstrates the application of the cellular automata (CeA) approach to optimize the daily hydropower operation of Folsom Reservoir. The reservoir operation is a nonlinear problem, where the hydropower generation and elevation-area-storage functions are the main nonlinearity to accurately represent the daily operation of the system. Moreover, the performance of the CeA approach under two extreme climate conditions, wet and dry, was evaluated and compared to the operation during normal conditions. Results showed that the CeA approach provides more efficient solutions in comparison to the commonly used evolutionary optimization algorithms. For the size of the non-linear optimization problem designed in this study, CeA outperformed genetic algorithm for finding optimal solutions for different climate conditions. Results of CeA showed that although the annual average inflow to the reservoir during the dry period was about 30% less than the normal condition, CeA offered about a 20% reduction in average hydropower generation. The new operation policy offered by CeA can partly compensate for the loss of the snowpack in California’s Sierra Nevada under a warming climate. The approach and its outcomes support an informed decision-making process and provide practical reservoir operational guideline to remediate the adverse effects of hydroclimatic changes in the future.

Drought onset and propagation into soil moisture and grassland vegetation responses during the 2012–2019 major drought in Southern California

Abstract. Despite clear signals of regional impacts of the recent severe drought in California, e.g., within Californian Central Valley groundwater storage and Sierra Nevada forests, our understanding of how this drought affected soil moisture and vegetation responses in lowland grasslands is limited. In order to better understand the resulting vulnerability of these landscapes to fire and ecosystem degradation, we aimed to generalize drought-induced changes in subsurface soil moisture and to explore its effects within grassland ecosystems of Southern California. We used a high-resolution in situ dataset of climate and soil moisture from two grassland sites (coastal and inland), alongside greenness (Normalized Difference Vegetation Index) data from Landsat imagery, to explore drought dynamics in environments with similar precipitation but contrasting evaporative demand over the period 2008–2019. We show that negative impacts of prolonged precipitation deficits on vegetation at the coastal site were buffered by fog and moderate temperatures. During the drought, the Santa Barbara region experienced an early onset of the dry season in mid-March instead of April, resulting in premature senescence of grasses by mid-April. We developed a parsimonious soil moisture balance model that captures dynamic vegetation–evapotranspiration feedbacks and analyzed the links between climate, soil moisture, and vegetation greenness over several years of simulated drought conditions, exploring the impacts of plausible climate change scenarios that reflect changes to precipitation amounts, their seasonal distribution, and evaporative demand. The redistribution of precipitation over a shortened rainy season highlighted a strong coupling of evapotranspiration to incoming precipitation at the coastal site, while the lower water-holding capacity of soils at the inland site resulted in additional drainage occurring under this scenario. The loss of spring rains due to a shortening of the rainy season also revealed a greater impact on the inland site, suggesting less resilience to low moisture at a time when plant development is about to start. The results also suggest that the coastal site would suffer disproportionally from extended dry periods, effectively driving these areas into more extreme drought than previously seen. These sensitivities suggest potential future increases in the risk of wildfires under climate change, as well as increased grassland ecosystem vulnerability.

Assessing the distribution and abundance of larval Longfin Smelt: What can a larval monitoring program tell us about the distribution of a rare species?

Following its listing as threatened under the California Endangered Species Act in 2009, Longfin Smelt (Spirinchus thaleichthys) became a focus of resource managers in the San Francisco Estuary. Water exports were identified as one of the factors affecting Longfin Smelt abundance, and managers were challenged with balancing freshwater flows through the Sacramento-San Joaquin River Delta between human and ecosystem needs. This balance becomes especially challenging during the winter and spring when Longfin Smelt are spawning. Resource managers identified that the impact associated with entrainment of larval Longfin Smelt in the winter was uncertain, and to understand and manage this risk, new data was needed. In 2009 the Smelt Larva Survey was implemented and has since sampled newly hatched larvae from January–March. Here, I analyze this data and ask specific questions regarding distribution and densities of the larvae throughout five regions of the Upper Estuary – Napa River, Suisun, Confluence, Northern Delta, and Southern Delta – with the goal of understanding the spatial and temporal patterns of larval distribution since 2009. I found that larvae were most prevalent in the Suisun, Confluence, and Northern Delta regions, and less common in the Southern Delta and Napa River regions. Larval Longfin Smelt densities changed following a recent drought and record low population abundances. Median per-station averaged densities ranged from 154 to 274 fish per 1,000 m3 between 2009 and 2013 but declined to 1 to 65 fish per 1,000 m3 from 2014 to 2019. This survey data demonstrates that Longfin Smelt reproductive output has declined since their listing in 2009 and that their distribution into the Southern Delta is low relative to the rest of the Upper Estuary. These results reaffirm the species’ continued decline since its listing, and that improving the abundance of spawning adults is one of the many important steps needed for long-term recovery and resilience.