Winter Snowpack Research Articles

Winter snowpack loss increases warm-season compound hot-dry extremes

Ongoing warming intensifies snowpack extremes, posing significant hydroclimatic risks to socio-ecological systems. However, the relation between snowpack extremes and subsequent compound hydroclimatic extremes remains unclear. Here, we investigated the impact of snowpack extremes on warm-season compound hydroclimatic extremes in the Northern Hemisphere using multisource datasets from 1980 to 2022. We found widespread increases in deficient, short, and deficient-short snowpack extremes, triggering more compound hot-dry extremes within a month after snowpack disappearance (mean coincidence rate over 0.6, p < 0.05). The impact of compound snowpack extremes exceeded that of individual snowpack extremes in both areas (over 10%) and coincidence rates (over 0.2). Meanwhile, increased intensity, rather than frequency, of snowpack extremes drove mainly the occurrence of compound hydroclimatic extremes. Furthermore, background climate factors, followed by vegetation, topography, and soil, affected relations between snowpack and compound hydroclimatic extremes. These findings will deepen our understanding of the emerging consecutive extremes and improve their predictability.

Illuminating Snow Droughts: The Future of Western United States Snowpack in the SPEAR Large Ensemble

AbstractSeasonal snowpack in the Western United States (WUS) is vital for meeting summer hydrological demands, reducing the intensity and frequency of wildfires, and supporting snow‐tourism economies. While the frequency and severity of snow droughts (SD), that is, anomalously low snowpacks, are expected to increase under continued global warming, the uncertainty from internal climate variability remains challenging to quantify with observations alone. Using a 30‐member large ensemble from a state‐of‐the‐art global climate model, the Seamless System for Prediction and EArth System Research (SPEAR), and an observations‐based data set, we find WUS SD changes are already significant. By 2100, SPEAR projects SDs to be nearly 9 times more frequent under shared socioeconomic pathway 5‐8.5 (SSP5‐8.5) and 5 times more frequent under SSP2‐4.5, compared to a 1921–2011 average. By investigating the influence of the two primary drivers of SD, temperature and precipitation amount, we find the average WUS SD will become warmer and wetter. To assess how these changes affect future summer water availability, we track late winter and spring snowpack across WUS watersheds, finding differences in the onset time of a “no‐snow” threshold between regions and large internal variability within the ensemble that are both on the order of decades. We attribute the inter‐regional variability to differences in the regions' mean winter temperature and the intra‐regional variability to irreducible internal climate variability which is not well‐explained by temperature variations alone. Despite strong scenario forcing, internal climate variability will continue to drive variations in SD and no‐snow conditions through 2100.

Impact of atmospheric rivers on the winter snowpack in the headwaters of Euphrates-Tigris basin

Understanding the hydrometeorological impacts of atmospheric rivers (ARs) on mountain snowpack is crucial for water resources management in the snow-fed river basins such as the Euphrates-Tigris (ET). In this study, we investigate the contribution of wintertime (December-January–February) ARs to precipitation and snowpack in the headwater regions of the ET Basin for the period of 1979–2019 using a state-of-the-art AR catalog and ERA5 reanalysis data. The results show that AR days in the headwaters region could be warmer by up to 3 °C and wetter by over 5 mm day−1 compared to non-AR days. The contribution of ARs to the total winter precipitation varies from year to year, with a maximum contribution of over 80% in 2010 and an average contribution of 60% over the 40-year period. While snow accumulation on AR days shows spatial variability, the average snow contribution is 27% of the seasonal average, ranging from 12 to 57% for different years. The south-facing parts of the mountain range experience significant snowmelt, with contributions ranging from 15 to 80% for different years. The high total precipitation (60%) and low snowpack (27%) contribution can be attributed to the semi-arid characteristics of the region and the occurrence of rain-on-snow events, where rain falling on existing snow rapidly melts the snowpack. The findings have implications for water resource management and call for continued research to improve our knowledge of ARs and their interactions with the complex terrain of the ET Basin.

Glacier Meltwater Impacts to Late Summer Flow and Geochemistry of Tributaries in the Wind River Range, Wyoming, USA

Reductions in winter snowpack over the last few decades have affected water resource availability in the western United States. In regions where glaciers exist, local communities have an advantage over nonglaciated regions as water continues to flow once the snowpack has melted. Despite this advantage, relatively little is understood about the contributions of glacial meltwater quantity and quality to streamflow. Torrey Creek watershed in the Wind River Range, Wyoming, contains several glaciers that provide meltwater to supplement late summer streamflow to the Wind River. Despite its importance as a contributor to water resources in the region, Torrey Creek discharge is not regularly monitored. Field research conducted in August 2019 for this study, along with previous field work completed in August 2012 and 2014, provides some understanding of late summer meltwater quantity and quality within the Torrey Creek watershed. Measured Torrey Creek discharge during mid-August 2019 averaged 3.80 m³ s−1 with 37 percent of the discharge attributed to glacial ice melt from stable isotope analysis. Nutrient loading to Torrey Creek from glacial meltwater was dominated by NO2–NO3 at 179.99 kg per month followed by 21.86 kg of total P for August 2019.

Heterogeneity in post-fire thermal responses across Pacific Northwest streams: A multi-site study

Over the past century, water temperatures in many streams across the Pacific Northwest (PNW) have steadily risen, shrinking endangered salmonid habitats. The warming of PNW stream reaches can be further accelerated by wildfires burning forest stands that provide shade to streams. However, previous research on the effect of wildfires on stream water temperatures has focused on individual streams or burn events, limiting our understanding of the diversity in post-fire thermal responses across PNW streams. To bridge this knowledge gap, we assessed the impact of wildfires on daily summer water temperatures across 31 PNW stream sites, where 10–100% of their riparian area burned. To ensure robustness of our results, we employed multiple approaches to characterize and quantify fire effects on post-fire stream water temperature changes.Averaged across the 31 burned sites, wildfires corresponded to a 0.3 – 1°C increase in daily summer water temperatures over the subsequent three years. Nonetheless, post-fire summer thermal responses displayed extensive heterogeneity across burned sites where the likelihood and rate of a post-fire summer water temperature warming was higher for stream sites with greater proportion of their riparian area burned under high severity. Also, watershed features such as basin area, post-fire weather, bedrock permeability, pre-fire riparian forest cover, and winter snowpack depth were identified as strong predictors of the post-fire summer water temperature responses across burned sites. Our study offers a multi-site perspective on the effect of wildfires on summer stream temperatures in the PNW, providing insights that can inform freshwater management efforts beyond individual streams and basins.

Understanding hydrological processes of glacierized catchments in the western Himalayas by a multi‐year tracer‐based hydrograph separation analysis

AbstractSnow and glacier melt are significant contributors to streamflow in Himalayan catchments, and their increasing contributions serve as key indicators of climate change. Consequently, the quantification of these streamflow components holds significant importance for effective water resource management. In this study, we utilized the spatio‐temporal variability of isotopic signatures in stream water, rainfall, winter fresh snow, snowpack, glaciers, springs, and wells, in conjunction with hydrometeorological observations and Snow Cover Area (SCA) data, to identify water sources and develop a conceptual understanding of streamflow dynamics in three catchments (Lidder, Sindh, and Vishow) within the western Himalayas. The following results were obtained: (a) endmember contributions to the streamflow exhibit significant spatial and seasonal variability across the three catchments during 2018–2020; (b) snowmelt dominates streamflow, with average contributions across the entire catchment varying: 59% ± 9%, 55% ± 4%, 56% ± 6%, and 55% ± 9% in Lidder, 43% ± 6%, 38% ± 6%, 32% ± 4%, and 33% ± 5% in Sindh and 45% ± 8%, 40% ± 6%, 39% ± 6%, and 32% ± 5% in Vishow during spring, summer, autumn, and winter seasons, respectively; (c) glacier melt contributions can reach ~30% to streamflow near the source regions during peak summer; (d) The primary uncertainties in streamflow components are attributed to the spatiotemporal variability of tracer signatures of winter fresh snow/snowpack (±1.9% to ±20%); (e)regarding future streamflow components, if the glacier contribution were to disappear completely, the annual average streamflow in Lidder and Sindh could decrease up to ~20%. The depletion of the cryosphere in the region has led to a rapid increase in runoff (1980–1900), but it has also resulted in a significant streamflow reduction due to glacier mass loss and changes in peak streamflow over the past three decades (1990–2020). The findings highlight the significance of environmental isotope analysis, which provides insights into water resources and offers a critical indication of the streamflow response to glacier loss under a changing climate.

Winter Snowpack and Its Role in Water Resource Management and Ecosystem Function

Winter Snowpack and Its Role in Water Resource Management and Ecosystem Function

Snow sampling strategy can bias estimation of meltwater fractions in isotope hydrograph separation

The Isotope based Hydrograph Separation (IHS) has been instrumental in understanding the partitioning of streamflow sources and processes. However, uncertainties persist in the accuracy of IHS estimations and the appropriate definition and sampling of endmembers. To address these uncertainties, we used field data of snowpack, snowfall, and snow meltwater isotopes (δ18O) from Pallas, Northern Finland to estimate the total meltwater contribution during the snowmelt period. We investigated the biases resulting from the application of different sampling strategies for event water endmember. The total meltwater contribution to streamflow was 59.6 % (±2% uncertainty) using the time-variant rolling runoff-corrected melt flux-weighted meltwater 18O isotope value. However, replacing it with either snowfall or winter snowpack 18O isotope weighted average values underestimated the meltwater contribution by 17.8 % or 22.6 %, respectively. Conversely, using time-variant instantaneous meltwater 18O isotope values overestimated the meltwater contribution by only 1.5 %. These discrepancies highlight the importance of choosing the appropriate endmember isotopes in IHS. The large differences in meltwater contribution for a 2-week peak discharge period based on different endmembers can lead to different interpretations of hydrological, ecohydrological, and biogeochemical processes. Thus, to better understand streamflow generation processes, we suggest using rolling runoff-corrected meltwater 18O or 2H isotope values in the IHS. In the absence of meltwater samples, the 18O or 2H isotope values of snowpack samples during the peak melt season may provide reasonable estimates of the meltwater contribution, with some minor underestimations. Our study highlights the importance of appropriate event meltwater endmember selection and sampling methodology for the IHS.

Drivers of Snowfall Accumulation in the Central Idaho Mountains Using Long-Term High-Resolution WRF Simulations

Abstract The western United States region, an economic and agricultural powerhouse, is highly dependent on winter snowpack from the mountain west. Coupled with increasing water and renewable electricity demands, the predictability and viability of snowpack resources in a changing climate are becoming increasingly important. In Idaho, specifically, up to 75% of the state’s electricity production comes from hydropower, which is dependent on the timing and volume of spring snowmelt. While we know that 1 April snowpack is declining from SNOTEL observations and is expected to continue to decline as indicated by GCM predictions, our ability to understand the variability of snowfall accumulation and distribution at the regional level is less robust. In this paper, we analyze snowfall events using 0.9-km-resolution WRF simulations to understand the variability of snowfall accumulation and distribution in the mountains of Idaho between 1 October 2016 and 31 April 2017. Various characteristics of snowfall events throughout the season are evaluated, including the spatial coverage, event durations, and snowfall rates, along with the relationship between cloud microphysical variables—particularly liquid and ice water content—on snowfall amounts. Our findings suggest that efficient snowfall conditions—for example, higher levels of elevated supercooled liquid water—can exist throughout the winter season but are more impactful when surface temperatures are near or below freezing. Inefficient snowfall events are common, exceeding 50% of the total snowfall events for the year, with some of those occurring in peak winter. For such events, glaciogenic cloud seeding could make a significant impact on snowpack development and viability in the region. Significance Statement The purpose and significance of this study is to better understand the variability of snowfall event accumulation and distribution in the Payette Mountains region of Idaho as it relates to the local topography, the drivers of snowfall events, the cloud microphysical properties, and what constitutes an efficient or inefficient snowfall event (i.e., its ability to convert atmospheric liquid water into snowfall). As part of this process, we identify how many snowfall events in a season are inefficient to determine the number of snowfall events in a season that are candidates for enhancement by glaciogenic cloud seeding.

The Influence of Winter Snowpack on the Use of Summer Rains in Montane Pine Forests Across the Southwest U.S.

AbstractA two decade‐long megadrought, with likely anthropogenic causes, has impacted forest growth and mortality across the southwestern U.S. Given this event, and the future likelihood of similar climate challenges, it is important to understand how different water resources are used by semi‐arid forests in this region. Within the geographic domain of the North American Monsoon climate system, we studied seasonal water‐use in eight different Pinus ponderosa montane forests distributed across a climate gradient with varying contributions from winter and summer precipitation. We collected oxygen isotopes from precipitation, soil, and xylem water during two contrasting hydrologic years to determine how trees differentially use winter versus summer precipitation sources. Most trees switched from using snowmelt water as the primary source during the early‐summer hyper‐arid period, to monsoon rainwater during the late‐summer. However, during the low snowpack year, which represents the most common climate phenomenon during the megadrought, trees at all sites used less summer rain when compared to the higher snowpack year, demonstrating a drought‐induced antecedent influence of winter precipitation on the uptake of summer rain. A possible mechanism to explain the antecedent effect is an earlier snow disappearance during the low snowpack year weakening hydrologic connectivity within the soil profile, decreasing the soil infiltration of summer rains. However, in years with higher snowpack, the snow lasts longer, and this can improve the hydrologic connectivity within the soil profile. As a result, there is more infiltration of summer rains into the soils. This can enhance the maintenance of active shallow fine‐root biomass during the period when snowpack disappears, and monsoon rains have yet to arrive. These findings provide insight into how the seasonal interactions between major seasonal climate systems influence forest tree water use in the face of an extreme megadrought.

Multi-locus genomic signatures of local adaptation to snow across the landscape in California populations of a willow leaf beetle.

Organisms living in mountains contend with extreme climatic conditions, including short growing seasons and long winters with extensive snow cover. Anthropogenic climate change is driving unprecedented, rapid warming of montane regions across the globe, resulting in reduced winter snowpack. Loss of snow as a thermal buffer may have serious consequences for animals overwintering in soil, yet little is known about how variability in snowpack acts as a selective agent in montane ecosystems. Here, we examine genomic variation in California populations of the leaf beetle Chrysomela aeneicollis, an emerging natural model system for understanding how organisms respond to climate change. We used a genotype-environment association approach to identify genomic signatures of local adaptation to microclimate in populations from three montane regions with variable snowpack and a coastal region with no snow. We found that both winter-associated environmental variation and geographical distance contribute to overall genomic variation across the landscape. We identified non-synonymous variation in novel candidate loci associated with cytoskeletal function, ion transport and membrane stability, cellular processes associated with cold tolerance in other insects. These findings provide intriguing evidence that variation in snowpack imposes selective gradients in montane ecosystems.

Towards a conceptualization of the hydrological processes behind changes of young water fraction with elevation: a focus on mountainous alpine catchments

Abstract. The young water fraction (Fyw*), defined as the fraction of catchment outflow with transit times of less than 2–3 months, is increasingly used in hydrological studies that exploit the potential of isotope tracers. The use of this new metric in catchment intercomparison studies is helpful to understand and conceptualize the relevant processes controlling catchment functioning. Previous studies have shown surprising evidence that mountainous catchments worldwide yield low Fyw*. These low values have been partially explained by isolated hydrological processes, including deep vertical infiltration and long groundwater flow paths. However, a thorough framework illustrating the relevant mechanisms leading to a low Fyw* in mountainous catchments is missing. The main aim of this paper is to give an overview of what drives Fyw* variations according to elevation, thus clarifying why it generally decreases at high elevation. For this purpose, we assembled a data set of 27 study catchments, located in both Switzerland and Italy, for which we calculateFyw*. We assume that this decrease can be explained by the groundwater storage potential, quantified by the areal extent of Quaternary deposits over a catchment (Fqd), and the low-flow duration (LFD) throughout the period of isotope sampling (PoS). In snow-dominated systems, LFD is strictly related to the snowpack persistence, quantified through the mean fractional snow cover area (FSCA). The drivers are related to the catchment storage contribution to the stream that we quantify by applying a cutting-edge baseflow separation method to the discharge time series of the study sites and by estimating the mean baseflow fraction (Fbf) over the PoS. Our results suggest that Quaternary deposits could play a role in modulating Fyw* elevation gradients via their capacity to store groundwater, but subsequent confirmation with further, more detailed geological information is necessary. LFD indicates the proportion of PoS in which the stream is sustained and dominated by stored water coming from the catchment storage. Accordingly, our results reveal that the increase of LFD at high elevations, to a large extent driven by the persistence of winter snowpacks and the simultaneous lack of a liquid water input to the catchments, results in lower Fyw*. In our data set, Fbf reveals a strong complementarity with Fyw*, suggesting that the latter could be estimated as Fyw*≃1-Fbf for catchments without stable water isotope measurements. As a conclusion, we develop a perceptual model that integrates all the results of our analysis into a framework for how hydrological processes control Fyw* according to elevation. This lays the foundations for an improvement of the theory-driven models.

Selection processes of Arctic seasonal glacier snowpack bacterial communities

BackgroundArctic snowpack microbial communities are continually subject to dynamic chemical and microbial input from the atmosphere. As such, the factors that contribute to structuring their microbial communities are complex and have yet to be completely resolved. These snowpack communities can be used to evaluate whether they fit niche-based or neutral assembly theories.MethodsWe sampled snow from 22 glacier sites on 7 glaciers across Svalbard in April during the maximum snow accumulation period and prior to the melt period to evaluate the factors that drive snowpack metataxonomy. These snowpacks were seasonal, accumulating in early winter on bare ice and firn and completely melting out in autumn. Using a Bayesian fitting strategy to evaluate Hubbell’s Unified Neutral Theory of Biodiversity at multiple sites, we tested for neutrality and defined immigration rates at different taxonomic levels. Bacterial abundance and diversity were measured and the amount of potential ice-nucleating bacteria was calculated. The chemical composition (anions, cations, organic acids) and particulate impurity load (elemental and organic carbon) of the winter and spring snowpack were also characterized. We used these data in addition to geographical information to assess possible niche-based effects on snow microbial communities using multivariate and variable partitioning analysis.ResultsWhile certain taxonomic signals were found to fit the neutral assembly model, clear evidence of niche-based selection was observed at most sites. Inorganic chemistry was not linked directly to diversity, but helped to identify predominant colonization sources and predict microbial abundance, which was tightly linked to sea spray. Organic acids were the most significant predictors of microbial diversity. At low organic acid concentrations, the snow microbial structure represented the seeding community closely, and evolved away from it at higher organic acid concentrations, with concomitant increases in bacterial numbers.ConclusionsThese results indicate that environmental selection plays a significant role in structuring snow microbial communities and that future studies should focus on activity and growth.A4vcHcKNh9Gahh9Z6pYiCtVideo

Using landscape genomics to delineate future adaptive potential for climate change in the Yosemite toad (Anaxyrus canorus).

An essential goal in conservation biology is delineating population units that maximize the probability of species persisting into the future and adapting to future environmental change. However, future-facing conservation concerns are often addressed using retrospective patterns that could be irrelevant. We recommend a novel landscape genomics framework for delineating future "Geminate Evolutionary Units" (GEUs) in a focal species: (1) identify loci under environmental selection, (2) model and map adaptive conservation units that may spawn future lineages, (3) forecast relative selection pressures on each future lineage, and (4) estimate their fitness and likelihood of persistence using geo-genomic simulations. Using this process, we delineated conservation units for the Yosemite toad (Anaxyrus canorus), a U.S. federally threatened species that is highly vulnerable to climate change. We used a genome-wide dataset, redundancy analysis, and Bayesian association methods to identify 24 candidate loci responding to climatic selection (R 2 ranging from 0.09 to 0.52), after controlling for demographic structure. Candidate loci included genes such as MAP3K5, involved in cellular response to environmental change. We then forecasted future genomic response to climate change using the multivariate machine learning algorithm Gradient Forests. Based on all available evidence, we found three GEUs in Yosemite National Park, reflecting contrasting adaptive optima: YF-North (high winter snowpack with moderate summer rainfall), YF-East (low to moderate snowpack with high summer rainfall), and YF-Low-Elevation (low snowpack and rainfall). Simulations under the RCP 8.5 climate change scenario suggest that the species will decline by 29% over 90 years, but the highly diverse YF-East lineage will be least impacted for two reasons: (1) geographically it will be sheltered from the largest climatic selection pressures, and (2) its standing genetic diversity will promote a faster adaptive response. Our approach provides a comprehensive strategy for protecting imperiled non-model species with genomic data alone and has wide applicability to other declining species.

Impact of Shifts in Vegetation Phenology on the Carbon Balance of a Semiarid Sagebrush Ecosystem

Dryland ecosystems are critical in regulating the interannual variability of the global terrestrial carbon cycle. The responses of such ecosystems to weather and environmental conditions remain important factors that limit the accurate projections of carbon balance under future climate change. Here, we investigated how shifts in vegetation phenology resulting from changes in weather and environmental conditions influenced ecosystem carbon cycling in one semiarid ecosystem in the Hanford area of central Washington, United States. We examined two years of measurements of the phenology camera, eddy covariance, and soil chamber from an upland semiarid sagebrush ecosystem. Both years had contrasting diel and seasonal patterns of CO2 fluxes, primarily driven by differences in vegetation phenology. The net ecosystem exchange of CO2 (NEE) and evapotranspiration (ET) in 2019 were enlarged by shifted vegetation phenology, as a cold and snow-covered winter and warm and dry winter in 2020 resulted in constrained magnitudes of NEE and ET during the summer months. The annual gross primary productivity (GPP) was much higher in 2019 than in 2020 (−211 vs. −112 gC m−2), whereas ecosystem respiration was comparable in these two years (164 vs. 144 gC m−2). Thus, the annual NEE in 2019 was negative (−47 gC m−2) with the sagebrush ecosystem functioning as a carbon sink, while the positive annual NEE in 2020 indicated that the sagebrush ecosystem functioned as a carbon source. Our results demonstrate that winter snowpack can be a critical driver of annual carbon uptake in semiarid sagebrush ecosystems.

Growth trends and environmental drivers of major tree species of the northern hardwood forest of eastern North America

The future health and productivity of tree species in the northern hardwood forest of eastern North America are uncertain considering changes in climate and pollution loading there. To better understand the trajectory of the northern hardwood forest, we studied the growth of three tree species emblematic of it: sugar maple (Acer saccharum Marsh), American beech (Fagus grandifolia Ehrh.), and yellow birch (Betula alleghaniensis Britton), plus a fourth species, red maple (Acer rubrum L.), whose abundance has increased in the region. We also analyzed the link between growth and several factors for 690 trees in 45 plots throughout Vermont, USA: tree age and size, site elevation, and climate and acid deposition variables. Throughout their chronologies (1945–2014), all four species exhibited increasing growth followed by plateaued growth indicative of a maturing forest. For all species, summer moisture was positively correlated with growth, summer temperature was negatively associated with growth, and winter moisture or snow were positively correlated with growth. This last association was expected for sugar maple. However, our data suggest that winter snowpack may be more broadly relevant in sustaining tree growth in a region where snow has historically insulated the soil from freezing that can damage roots and lead to reduced aboveground growth. Measures of pollution deposition were also correlated with growth for all species except American beech—a species with documented tolerance to pollutant inputs. Of the four species studied, red maple had the fewest associations with environmental variables, which suggests that it may be less susceptible to growth reductions as the climate changes.

Impacts of seasonally frozen ground on streamflow recession in the Red River of the North Basin

AbstractThe projected rising temperatures due to climate change can alter the frozen ground's duration and thickness in the cold regions of the Northern Hemisphere, leading to changes in surface and subsurface contributions to streamflow. The initial step in understanding the impacts of the changing climate on the hydrologic response of the cold climate watersheds is to investigate the effects of the cold climate processes (e.g., freeze–thaw processes) on watershed response. However, complexities associated with the spatiotemporal data collection of cold climate processes have limited the capabilities of hydrologic models in simulating cold climate watershed responses. This study aims to evaluate the impacts of seasonally frozen soil on the streamflow recession in the Red River of the North Basin (RRB). To the best of our knowledge, this study is one of the first attempts to quantify the impacts of frozen soil on the storage‐discharge relationship of the RRB. We define a set of hypotheses to evaluate how cold climate processes influence the surface and subsurface flow paths and affect the linearity/non‐linearity of the storage‐discharge relationship in spring and summer. The results confirmed that the cold climate processes control the recession behavior of the RRB. It was found that the streamflow discharge is linearly related to the storage during spring, whereas during summer, the relationship was less linear. Statistical analyses revealed that the antecedent winter soil temperature and snowpack control the seasonal dynamics of hydrograph recession. Particularly, results indicated a more linear storage‐discharge relationship for antecedent winters characterized by colder soil temperatures and larger snowpack. These findings highlight the importance of the cold climate hydrologic processes in cold climate basins such as the RRB.

Interpreting Sentinel-1 SAR Backscatter Signals of Snowpack Surface Melt/Freeze, Warming, and Ripening, through Field Measurements and Physically-Based SnowModel

The transition of a cold winter snowpack to one that is ripe and contributing to runoff is crucial to gauge for water resource management, but is highly variable in space and time. Snow surface melt/freeze cycles, associated with diurnal fluctuations in radiative inputs, are hallmarks of this transition. C-band synthetic aperture radar (SAR) reliably detects meltwater in the snowpack. Sentinel-1 (S1) C-band SAR offers consistent acquisition patterns that allow for diurnal investigations of melting snow. We used over 50 snow pit observations from 2020 in Grand Mesa, Colorado, USA, to track temperature and wetness in the snowpack as a function of depth and time during snowpack phases of warming, ripening, and runoff. We also ran the physically-based SnowModel, which provided a spatially and temporally continuous independent indication of snowpack conditions. Snowpack phases were identified and corroborated by comparing field measurements with SnowModel outputs. Knowledge of snowpack warming, ripening, and runoff phases was used to interpret diurnal changes in S1 backscatter values. Both field measurements and SnowModel simulations suggested that S1 SAR was not sensitive to the initial snowpack warming phase on Grand Mesa. In the ripening and runoff phases, the diurnal cycle in S1 SAR co-polarized backscatter was affected by both surface melt/freeze as well as the conditions of the snowpack underneath (ripening or ripe). The ripening phase was associated with significant increases in morning backscatter values, likely due to volume scattering from surface melt/freeze crusts, as well as significant decreases in evening backscatter values associated with snowmelt. During the runoff phase, both morning and evening backscatter decreased compared to reference values. These unique S1 diurnal signatures, and their interpretations using field measurements and SnowModel outputs, highlight the capacities and limitations of S1 SAR to understand snow surface states and bulk phases, which may offer runoff forecasting or energy balance model validation or parameterization, especially useful in remote or sparsely-gauged alpine basins.

Mineralogic controls are harbingers of hydrological controls on soil organic matter content in warmer boreal forests

Boreal forests contain ∼ 30 % of the global forest soil organic carbon (SOC) stock within a region vulnerable to climate change, yet regional studies on the climate relevant controls of this large reservoir are lacking. The Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect represents a boreal climate gradient of sites ranging in temperature and precipitation similar in extent to changes expected within the region over the next century. Though forest and soil type are similar across the transect, geological parent material upon which soils have developed vary across the region. Despite a > 50 % increase in litterfall and a 3-fold increase in the dissolved organic matter inputs to the mineral soil with decreasing latitude, average surface horizon mineral SOC and N content are similar among regions. To explain this, the relationships between mineral soil characteristics and other ecosystem parameters with SOC and N content were assessed using an information theoretic approach to rank models. Rankings were used to determine the most relevant variables explaining regional variance in surface mineral SOC. Unsurprisingly, organo-metal complexes (OMCs), a weathering product, were a major component of all plausible models. However, Al complexes ranked higher than Fe, likely because of the greater solubility of reduced Fe in reducing microsites, particularly beneath the winter snowpack or within the saturated soils during the spring melt. These results highlight the potential role of seasonal hydrology on the formation of metal-stabilized C and N in boreal forests and suggest that the role of Fe OMC in C sequestration may be enhanced with warmer winters. The realization and effect of these seasonally controlled OMC mechanisms require further understanding of mineral soil characteristics and hydrology and their response to climate warming. Such efforts will improve predictions of the impact of climate change on soil C and N content in these forests.

Update On A Long-term Winter Operational Cloud Seeding Program In Central/Southern Utah

North American Weather Consultants, Inc. (NAWC) has conducted operational winter cloud seeding programs in the mountainous areas of Central and Southern Utah since 1974. Beginning in 1988, seeding has also been conducted in three additional mountainous target areas within the State. The goal of these programs has been to enhance winter snowpack accumulation in the target areas, which now include most of the mountainous areas of the State. Studies have demonstrated that a large majority of the annual runoff in Utah streams and rivers is derived from melting snowpack, which explains the focus on wintertime seeding (within the November – April period). Augmented water supplies are typically used for irrigated agriculture or municipal water supplies. Programs are typically funded at the county level with cost sharing grants from the Utah Division of Water Resources (UDWR) and the three Lower Colorado River Basin States of Arizona, California, and Nevada, since 2007. An earlier WMA paper (Griffith et al. 2009) provided a summary of seeding operations for the water years of 1974 through 2007 for the four target areas. This paper is focused on the Central and Southern Utah program which is both the largest target area and the longest running program in the state. It covers all but three water years from 1974 through 2021 and is one of the three or four longest operational winter cloud seeding programs that have been conducted in the United States. The target area encompasses several mountain ranges in Central and Southern Utah. NAWC has defined the target area boundaries as those locations that are above 7,000 feet MSL. This is a large area of approximately10,000 square miles. Cloud seeding is accomplished using networks of ground-based, manually operated silver iodide nuclei generators located in valley or foothill locations upwind of the intended target mountain barriers. As such, these programs are classified as orographic winter cloud seeding programs. Orographic winter cloud seeding programs are typically categorized as those with the highest level of scientific support based upon capability statements of such organizations as the American Meteorological Society and the Weather Modification Association. NAWC historical target/control evaluations of this program indicate an average increase in December-March target area precipitation of 12% or an average increase in precipitation of 1.3”. These results were significant at the 0.06 level from a one-tailed Student’s t-test. The UDWR has conducted periodic studies to estimate the increases in annual streamflow resulting from the estimated increases in April 1st snow water content produced by this seeding program. The most recent study (UDWR 2018) indicated an estimated average annual streamflow increase of about 84,000 acre-feet for the Central and Southern Utah target area. Factoring in the cost of conducting this program resulted in an estimate of the average cost of the augmented runoff to be $2.02 per acre-foot.