Potential Future Conditions Research Articles

Future sea ice conditions and weather forecasts in the Arctic: Implications for Arctic shipping

The ability to forecast sea ice (both extent and thickness) and weather conditions are the major factors when it comes to safe marine transportation in the Arctic Ocean. This paper presents findings focusing on sea ice and weather prediction in the Arctic Ocean for navigation purposes, in particular along the Northeast Passage. Based on comparison with the observed sea ice concentrations for validation, the best performing Earth system models from the Intergovernmental Panel on Climate Change (IPCC) program (CMIP5—Coupled Model Intercomparison Project phase 5) were selected to provide ranges of potential future sea ice conditions. Our results showed that, despite a general tendency toward less sea ice cover in summer, internal variability will still be large and shipping along the Northeast Passage might still be hampered by sea ice blocking narrow passages. This will make sea ice forecasts on shorter time and space scales and Arctic weather prediction even more important.

A multi-scale ensemble-based framework for forecasting compound coastal-riverine flooding: The Hackensack-Passaic watershed and Newark Bay

Abstract Estuarine regions can experience compound impacts from coastal storm surge and riverine flooding. The challenges in forecasting flooding in such areas are multi-faceted due to uncertainties associated with meteorological drivers and interactions between hydrological and coastal processes. The objective of this work is to evaluate how uncertainties from meteorological predictions propagate through an ensemble-based flood prediction framework and translate into uncertainties in simulated inundation extents. A multi-scale framework, consisting of hydrologic, coastal and hydrodynamic models, was used to simulate two extreme flood events at the confluence of the Passaic and Hackensack rivers and Newark Bay. The events were Hurricane Irene (2011), a combination of inland flooding and coastal storm surge, and Hurricane Sandy (2012) where coastal storm surge was the dominant component. The hydrodynamic component of the framework was first forced with measured streamflow and ocean water level data to establish baseline inundation extents with the best available forcing data. The coastal and hydrologic models were then forced with meteorological predictions from 21 ensemble members of the Global Ensemble Forecast System (GEFS) to retrospectively represent potential future conditions up to 96 hours prior to the events. Inundation extents produced by the hydrodynamic model, forced with the 95th percentile of the ensemble-based coastal and hydrologic boundary conditions, were in good agreement with baseline conditions for both events. The USGS reanalysis of Hurricane Sandy inundation extents was encapsulated between the 50th and 95th percentile of the forecasted inundation extents, and that of Hurricane Irene was similar but with caveats associated with data availability and reliability. This work highlights the importance of accounting for meteorological uncertainty to represent a range of possible future inundation extents at high resolution (∼m).

Hydro-climatic variability and extremes over the Athabasca River basin: Historical trends and projected future occurrence

Since humans and ecosystems require adequate, reliable water supplies, hydro-climatic variability and extremes pose serious threats to society and the environment. Western Canada, including the Athabasca River Basin (ARB), is prone to considerable hydro-climatic variability including periodic extreme droughts and excessive moisture conditions. The main causes of these extremes are persistent, mid-tropospheric circulation patterns that disrupt expected precipitation and temperature. However, no investigation has specifically focused on the occurrence and causes of both past and future hydro-climatologic variability within the ARB. Assessment of the Standardized Precipitation Evapotranspiration Index (SPEI) reveals substantial inter-annual and decadal-scale variability over the entire ARB and its individual reaches, with no discernible trends during the last ~100 years. Evaluation of the 500-hPa circulation patterns associated with identified hydro-climatic extremes shows that major droughts are related to higher frequencies of distinctive ridging patterns over the ARB and fewer incidences of troughing. Excessive moisture conditions tend to have opposite patterns. Projections from several regional climate models reveal an average change toward more drought-like summer and slightly wetter annual conditions over the ARB, but there is a substantial range for individual models ranging from a considerable increase in drought with a higher degree of inter-annual variability, to relatively little change from current conditions. Simulated changes to key atmospheric circulations are consistent with these SPEI projections and indicate that those patterns associated with hydro-climatic extremes will continue in the future and in some cases, increase in frequency. Results from this analysis have quantified historical hydro-climatic variability in the ARB and have provided insight into potential future conditions as driven by changes to surface climate and synoptic-scale atmospheric circulation patterns.

Enhanced groundwater recharge rates and altered recharge sensitivity to climate variability through subsurface heterogeneity

Our environment is heterogeneous. In hydrological sciences, the heterogeneity of subsurface properties, such as hydraulic conductivities or porosities, exerts an important control on water balance. This notably includes groundwater recharge, which is an important variable for efficient and sustainable groundwater resources management. Current large-scale hydrological models do not adequately consider this subsurface heterogeneity. Here we show that regions with strong subsurface heterogeneity have enhanced present and future recharge rates due to a different sensitivity of recharge to climate variability compared with regions with homogeneous subsurface properties. Our study domain comprises the carbonate rock regions of Europe, Northern Africa, and the Middle East, which cover ∼25% of the total land area. We compare the simulations of two large-scale hydrological models, one of them accounting for subsurface heterogeneity. Carbonate rock regions strongly exhibit "karstification," which is known to produce particularly strong subsurface heterogeneity. Aquifers from these regions contribute up to half of the drinking water supply for some European countries. Our results suggest that water management for these regions cannot rely on most of the presently available projections of groundwater recharge because spatially variable storages and spatial concentration of recharge result in actual recharge rates that are up to four times larger for present conditions and changes up to five times larger for potential future conditions than previously estimated. These differences in recharge rates for strongly heterogeneous regions suggest a need for groundwater management strategies that are adapted to the fast transit of water from the surface to the aquifers.

Modelling benthic macrofauna and seagrass distribution patterns in a North Sea tidal basin in response to 2050 climatic and environmental scenarios

Small-scale spatial distribution patterns of seven macrofauna species, seagrass beds and mixed mussel/oyster reefs were modelled for the Jade Bay (North Sea, Germany) in response to climatic and environmental scenarios (representing 2050). For the species distribution models four presence-absence modelling methods were merged within the ensemble forecasting platform ‘biomod2’. The present spatial distribution (representing 2009) was modelled by statistically related species presences, true species absences and six high-resolution environmental grids. The future spatial distribution was then predicted in response to expected climate change-induced ongoing (1) sea-level rise and (2) water temperature increase. Between 2009 and 2050, the present and future prediction maps revealed a significant range gain for two macrofauna species (Macoma balthica, Tubificoides benedii), whereas the species' range sizes of five macrofauna species remained relatively stable across space and time. The predicted probability of occurrence (PO) of two macrofauna species (Cerastoderma edule, Scoloplos armiger) decreased significantly under the potential future habitat conditions. In addition, a clear seagrass bed extension (Zostera noltii) on the lower intertidal flats (mixed sediments) and a decrease in the PO of mixed Mytilus edulis/Crassostrea gigas reefs was predicted for 2050. Until the mid-21st century, our future climatic and environmental scenario revealed significant changes in the range sizes (gains-losses) and/or the PO (increases-decreases) for seven of the 10 modelled species at the study site.

Climate changes and wildfire alter vegetation of Yellowstone National Park, but forest cover persists

AbstractWe present landscape simulation results contrasting effects of changing climates on forest vegetation and fire regimes in Yellowstone National Park, USA, by mid‐21st century. We simulated potential changes to fire dynamics and forest characteristics under three future climate projections representing a range of potential future conditions using the FireBGCv2 model. Under the future climate scenarios with moderate warming (>2°C) and moderate increases in precipitation (3–5%), model simulations resulted in 1.2–4.2 times more burned area, decreases in forest cover (10–44%), and reductions in basal area (14–60%). In these same scenarios, lodgepole pine (Pinus contorta) decreased in basal area (18–41%), while Douglas‐fir (Pseudotsuga menziesii) basal area increased (21–58%). Conversely, mild warming (<2°C) coupled with greater increases in precipitation (12–13%) suggested an increase in forest cover and basal area by mid‐century, with spruce and subalpine fir increasing in abundance. Overall, we found changes in forest tree species compositions were caused by the climate‐mediated changes in fire regime (56–315% increase in annual area burned). Simulated changes in forest composition and fire regime under warming climates portray a landscape that shifts from lodgepole pine to Douglas‐fir caused by the interaction between the magnitude and seasonality of future climate changes, by climate‐induced changes in the frequency and intensity of wildfires, and by tree species response.

Scenarios for investigating the future of Canada’s oceans and marine fisheries under environmental and socioeconomic change

There is a critical need to develop effective strategies for the long-term sustainability of Canada’s oceans. However, this is challenged by uncertainty over future impacts of global environmental and socioeconomic change on marine ecosystems, and how coastal communities will respond to these changes. Scenario analysis can address this uncertainty by exploring alternative futures for Canadian oceans under different pathways of climate change, economic development, social and policy changes. However, there has, to date, been no scenario analysis of Canada’s future ocean sustainability at a national scale. To facilitate this process, we review whether the literature on existing scenarios of Canada’s fisheries and marine ecosystems provides an integrative, social-ecological perspective about potential future conditions. Overall, there is sufficient national-level oceanographic data and application of ecosystem, biophysical, and socioeconomic models to generate projections of future ocean and socioeconomic trends in Canada. However, we find that the majority of marine-related scenario analyses in Canada focus on climate scenarios and the associated oceanographic and ecological changes. There is a gap in the incorporation of social, economic, and governance drivers in scenarios, as well as a lack of scenarios which consider the economic and social impact of future change. Moreover, available marine scenario studies mostly do not cover all three Canadian oceans simultaneously. To address these gaps, we propose to develop national-level scenarios using a matrix framework following the concept of Shared Socioeconomic Pathways, which would allow a social-ecological examination of Canada’s oceans in terms of the state of future uncertainties.

The competitive dynamics of toxic Alexandrium fundyense and non-toxic Alexandrium tamarense: The role of temperature

The dinoflagellate Alexandrium produces paralytic shellfish poisoning toxins. The genus is globally distributed, with Scottish waters being of particular interest due to the co-occurrence of different species and strains. In Scottish waters, Alexandrium was historically thought to be dominated by the highly toxic (Group I) Alexandrium fundyense. However, the morphologically indistinguishable (Group III) Alexandrium tamarense has recently also been found to co-occur, raising important questions in relation to Alexandrium biogeography. To begin to address these, we investigated Alexandrium growth, yield and toxin production in a range of temperature conditions characteristics of present and potential future conditions, using a recently developed flow cytometry method that allowed, for the first time, simultaneous enumeration of the cryptic species in co-culture. Experiments were undertaken in a range of temperatures (12, 15, 18 and 21°C) in the phosphate (P) limiting conditions that promotes A. fundyense toxicity. Cell/biomass yield was greater for A. tamarense at all temperatures, with observed growth rates varying with temperature. Growth rather and yield were different in mono- and co-culture with the outcome of these interactions also being temperature dependent. For toxic A. fundyense, GTX-3, STX and NEO were the dominant analogues, but total toxicity, toxicity per cell and the number of, and relative proportion of, toxin analogues changed in relation to the onset of P limitation and also as a function of temperature, with the highest toxin concentrations per cell being observed at 12°C. Toxin concentrations were approximately double in P limited stationary phase compared to exponential growth. Toxin concentrations were lower in the co-cultures, indicating inhibition of production in the presence of non-toxic A. tamarense. The strong performance of A. tamarense is in co-culture at odds with the historical understanding that Scottish waters were dominated by A. fundyense and indicates that changes in water temperatures, and also potentially alleopathic interactions, will influence Alexandrium populations and hence the PSP toxicity threat to humans from shellfish.

Placental, Matrilineal, and Epigenetic Mechanisms Promoting Environmentally Adaptive Development of the Mammalian Brain.

The evolution of intrauterine development, vivipary, and placentation in eutherian mammals has introduced new possibilities and constraints in the regulation of neural plasticity and development which promote neural function that is adaptive to the environment that a developing brain is likely to encounter in the future. A range of evolutionary adaptations associated with placentation transfers disproportionate control of this process to the matriline, a period unique in mammalian development in that there are three matrilineal genomes interacting in the same organism at the same time (maternal, foetal, and postmeiotic oocytes). The interactions between the maternal and developing foetal hypothalamus and placenta can provide a template by which a mother can transmit potentially adaptive information concerning potential future environmental conditions to the developing brain. In conjunction with genomic imprinting, it also provides a template to integrate epigenetic information from both maternal and paternal lineages. Placentation also hands ultimate control of genomic imprinting and intergenerational epigenetic information transfer to the matriline as epigenetic markers undergo erasure and reprogramming in the developing oocyte. These developments, in conjunction with an expanded neocortex, provide a unique evolutionary template by which matrilineal transfer of maternal care, resources, and culture can be used to promote brain development and infant survival.

Sustainability Review of Water-Supply Options in the Los Angeles Region

AbstractThe City of Los Angeles, with a population reaching 4 million people in 2013, imports nearly 90% of its water from sources outside the city. However, climate change, population growth, competing demands, water quality concerns, and environmental restoration projects all have a large impact on Los Angeles’ dependency on future water importation. In this study, a system characterization was performed to assess each of the water sources that supply Los Angeles and the factors affecting them under present and potential future conditions. Additionally, water demand and conservation methods in Los Angeles were examined to understand their impact on the overall water supply system. Of the five main water sources that supply Los Angeles—Los Angeles Aqueduct, Colorado River, California Aqueduct, local groundwater, and reclaimed water—a majority will be impacted by climate change, water quality, energy, and cost constraints. While the expansion of water demand management and agricultural water transfers can...

Statistically downscaled projections of snow/rain partitioning for Alaska

Adaptation planning in Alaska, as in other snowy parts of the world, will require snow projections, yet snow is a challenging variable to measure, simulate and downscale. Here we describe the construction and evaluation of 771-m-resolution gridded historical and statistically downscaled projections of snow/rain partitioning for the state of Alaska at decadal temporal resolution. The method developed here uses observational data to describe the relationship between average monthly temperature and the fraction of wet days in that month receiving snow, the snow-day fraction. Regionally and seasonally specific equations were developed to accommodate variability in synoptic scale climatology of rain and snow events. These equations were then applied to gridded decadal temperature data and projections. The gridded products provide a reasonable characterization of snow-day fraction throughout the state. However, there are local deviations from the regional relationships, particularly in the topographically complex areas ringing the Gulf of Alaska and Cook Inlet. When applied to questions about changing precipitation regimes in northern, western and south-eastern Alaska, these data demonstrate the potential for marked changes from snow-dominated to mixed precipitation regimes and also exhibit a wide range of potential future conditions. Copyright © 2013 John Wiley & Sons, Ltd.

Simulations of Potential Future Conditions in the Cache Critical Groundwater Area, Arkansas

A three-dimensional finite-difference model for part of the Mississippi River Valley alluvial aquifer in the Cache Critical Groundwater Area of eastern Arkansas was constructed to simulate potential future conditions of groundwater flow. The objectives of this study were to test different pilot point distributions to find reasonable estimates of aquifer properties for the alluvial aquifer, to simulate flux from rivers, and to demonstrate how changes in pumping rates for different scenarios affect areas of long-term water-level declines over time. The model was calibrated using the parameter estimation code. Additional calibration was achieved using pilot points with regularization and singular value decomposition. Pilot point parameter values were estimated at a number of discrete locations in the study area to obtain reasonable estimates of aquifer properties. Nine pumping scenarios for the years 2011 to 2020 were tested and compared to the simulated water-level heads from 2010. Hydraulic conductivity values from pilot point calibration ranged between 42 and 173 m/d. Specific yield values ranged between 0.19 and 0.337. Recharge rates ranged between 0.00009 and 0.0006 m/d. The model was calibrated using 2,322 hydraulic head measurements for the years 2000 to 2010 from 150 observation wells located in the study area. For all scenarios, the volume of water depleted ranged between 5.7 and 23.3 percent, except in Scenario 2 (minimum pumping rates), in which the volume increased by 2.5 percent.

Avicennia germinans (black mangrove) vessel architecture is linked to chilling and salinity tolerance in the Gulf of Mexico.

Over the last several decades, the distribution of the black mangrove Avicennia germinans in the Gulf of Mexico has expanded, in part because it can survive the occasional freeze events and high soil salinities characteristic of the area. Vessel architecture may influence mangrove chilling and salinity tolerance. We surveyed populations of A. germinans throughout the Gulf to determine if vessel architecture was linked to field environmental conditions. We measured vessel density, hydraulically weighted vessel diameter, potential conductance capacity, and maximum tensile fracture stress. At each sampling site we recorded mangrove canopy height and soil salinity, and determined average minimum winter temperature from archived weather records. At a subset of sites, we measured carbon fixation rates using a LI-COR 6400XT Portable Photosynthesis System. Populations of A. germinans from cooler areas (Texas and Louisiana) had narrower vessels, likely reducing the risk of freeze-induced embolisms but also decreasing water conductance capacity. Vessels were also narrower in regions with high soil salinity, including Texas, USA and tidal flats in Veracruz, Mexico. Vessel density did not consistently vary with temperature or soil salinity. In abiotically stressful areas, A. germinans had a safe hydraulic architecture with narrower vessels that may increase local survival. This safe architecture appears to come at a substantial physiological cost in terms of reduction in conductance capacity and carbon fixation potential, likely contributing to lower canopy heights. The current distribution of A. germinans in the Gulf is influenced by the complex interplay between temperature, salinity, and vessel architecture. Given the plasticity of A. germinans vessel characters, it is likely that this mangrove species will be able to adapt to a wide range of potential future environmental conditions, and continue its expansion in the Gulf of Mexico in response to near-term climate change.

Optimal planning of water supply system for long-term sustainability

Developing a long-term system plan for sustainable water supply is a challenging task due to system complexity and future uncertainties in water demands and source availability. Here a coupled optimization model is proposed for water supply system design and long-term operations by deciding system component sizes and water flow allocations simultaneously. The objective is to minimize overall system costs (i.e., sum of capital and operation costs) while meeting water demands and operational constraints. The economic costs include initial component construction costs and operation expenditure over pre-defined operation years. The proposed model integrates a genetic algorithm with a linear programming model to optimize water infrastructure investments and annual water transfers satisfying flow constraints. The coupled model was applied to a simplified water supply network composed of multiple water sources and users. For the application network, various qualities of water from different sources could be supplied to different users. Plausible future scenarios with time varying water demands were simulated representing potential future conditions. Application results show that the proposed coupled model is beneficial in decision making process to design structural components in near future and prepare long-term policies for water shortage and water right issues in upcoming years. The model can be tailored to a specific system and various regulations and conditions can be incorporated within the model without adding complexity to the optimization framework.

Challenges in integrative approaches to modelling the marine ecosystems of the North Atlantic: Physics to fish and coasts to ocean

It has long been recognised that there are strong interactions and feedbacks between climate, upper ocean biogeochemistry and marine food webs, and also that food web structure and phytoplankton community distribution are important determinants of variability in carbon production and export from the euphotic zone. Numerical models provide a vital tool to explore these interactions, given their capability to investigate multiple connected components of the system and the sensitivity to multiple drivers, including potential future conditions. A major driver for ecosystem model development is the demand for quantitative tools to support ecosystem-based management initiatives. The purpose of this paper is to review approaches to the modelling of marine ecosystems with a focus on the North Atlantic Ocean and its adjacent shelf seas, and to highlight the challenges they face and suggest ways forward. We consider the state of the art in simulating oceans and shelf sea physics, planktonic and higher trophic level ecosystems, and look towards building an integrative approach with these existing tools. We note how the different approaches have evolved historically and that many of the previous obstacles to harmonisation may no longer be present. We illustrate this with examples from the on-going and planned modelling effort in the Integrative Modelling Work Package of the EURO-BASIN programme.

A scenario-based analysis of building energy performance

Inherent to the sustainability discourse is concern about the long-term future. If sustainable, cities must deliver positive benefits over their possibly long lifetime. Yet in formulating guidance and negotiating design choices for urban development, the consideration of uncertainty and potential future adverse conditions leading to failure is difficult to deal with. In this paper an approach to this issue that can inform the planning and/or urban design process is presented. It is based on a scenario analysis methodology (futures analysis), which herein is used to appraise the energy-efficiency strategies currently recommended in the UK planning system and best practice. The built environment is one of the major consumers of energy, and its energy efficiency is thereby central to any attempt to reduce carbon dioxide emissions. Through the analysis, some important factors that could undermine good energy performance emerge. Such findings can lead to the making of decisions that can enhance urban resilience. The first part of the paper provides an overview of the energy-efficient strategies examined, as well as current approaches for considering the evolution of present conditions when planning. The second part presents the futures analysis and its significance is demonstrated through a case study.

Enhancement of photosynthetic carbon assimilation efficiency by phytoplankton in the future coastal ocean

Abstract. A mesocosm experiment was conducted to evaluate the influence of photosynthetic performance on the energetic balance of coastal phytoplankton, in relation to community production and autotrophic phytoplankton biomass in future coastal oceans. Natural phytoplankton assemblages were incubated in field mesocosms under ambient condition (control: ca. 400 μatm CO2 and ambient temperature), and two sets of potential future ocean conditions (acidification: ca. 900 μatm CO2 and ambient temperature; greenhouse: ca. 900 μatm CO2 and 3 °C warmer). The photosynthetic performances were estimated by in vivo fluorometry (effective quantum yield (ΦPSII), steady-state light response curves (LCs)) and in situ incorporation of 14C (photosynthesis-irradiance curves). The ΦPSII and rETRm,LC (relative maximum electron transport rate) clearly reduced under acidification, in particular, when phytoplankton were exposed to high light levels. However, PBmax (maximum photosynthetic rate) was the same in the ambient and acidification conditions. Thus, phytoplankton utilized less light under acidification condition, but could still assimilate a similar amount of carbon compared to the ambient condition. The PBmax and α (photosynthetic efficiency) under greenhouse condition were significantly higher than those under ambient condition without any difference in ΦPSII, rETRm,LC and α,LC (electron transport efficiency) between the treatments. Therefore, phytoplankton utilized the same amount of light under greenhouse condition, but could assimilate more carbon than under ambient condition. As a result, Chl a normalized primary production was higher in greenhouse than in other conditions. Nevertheless, the community production did not change between the experimental treatments. The main reason for the lack of a change in primary production under future climate conditions is the control of autotrophic phytoplankton biomass by grazing. Consequently, acidification and greenhouse environments have a potential to increase growth and primary production of phytoplankton by enhancing inorganic carbon assimilation efficiency when top-down regulation is negligible.

Forest soil decomposition and its contribution to heterotrophic respiration: A case study based on Canada

Soil carbon (C) stocks are large C reservoirs that are characterized by turnover times of decades to centuries. The ability to predict how long C remains in soils requires an understanding of soil decomposition and the influence of climate change on destabilization processes. This study examined forest soil decomposition and quantified the influence of the choice of decomposition parameters on national-scale heterotrophic respiration (Rh) estimates for the managed forest of Canada.Soil carbon (C) stock estimates from 597 ground plots were used to optimize five decomposition-related parameters of the two slowly decaying C pools of the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3). An exhaustive grid search method found multiple optimal solutions, i.e. combinations of base decay rates and temperature quotients for the slowly decaying C pools. Impacts on the Rh for the 2.3 × 106 km2 of Canada's managed forest were estimated using national inventory and disturbance data for ten optimal combinations of base decay rates and temperature quotients. The Rh was estimated in contemporary (1990) and potential future (2099) temperature conditions for which an increase of 4 °C in mean annual air temperature was assumed. The selection of optimal parameter combination had little impact on the resultant estimates of Rh, with a maximum difference of 1.7 g C m−2 yr−1 or 4.0 Tg C yr−1 (0.54% of total Rh) in 1990 and a maximum difference of 8.2 g C m−2 yr−1 or 18.8 Tg C yr−1 (2.3% of total Rh) in 2099.The slow pool within the mineral soil accounted for 54% of the total dead organic matter C stock. It had a disproportionately small contribution of 8.7% to the total contemporary national-scale Rh estimate and was relatively insensitive to temperature changes. If we accept the space-for-time substitution of temperature sensitivities used in the model parameterization, then these results suggest that the mineral soil decomposition modeled by the CBM-CFS3 will result in a weaker positive feedback in response to an increase in global temperatures than is currently anticipated in other models. However, the majority of the Rh comes from relatively small, but quickly decaying pools. The overall temperature quotient was estimated to be 2.15, which would result in an increase in Rh in response to an increase in global temperatures.

Uncertainty and variability of infiltration at Yucca Mountain: Part 2. Model results and corroboration

[2] The U.S. Nuclear Regulatory Commission actively investigated climate and infiltration at Yucca Mountain for two decades to (i) understand important controls and uncertainties influencing percolation through the unsaturated zone on multimillennial time scales and (ii) provide flux boundary conditions for up to 1 million years in performance assessment models of the proposed Yucca Mountain repository. This second part of a two-part series describes site-scale model results for present and potential future conditions and confirmatory analyses for present-day conditions. At both the grid-cell and site-average scale, the calculated uncertainty distribution for net infiltration is approximately lognormal, and the coefficient of variation decreases with increasing net infiltration. Smaller relative but larger absolute responses to climate change occur where net infiltration is large. Comparisons of distributed model estimates with temperature and geochemical observations from the unsaturated zone suggest that average estimates are generally consistent but exhibit significant variability. An observed seepage event in the South Ramp of the Exploratory Studies Facility, combined with related subsurface observations across the site, suggests that subsurface spreading from zones of high infiltration to zones of low infiltration may occur in stratabound fractures, laterally extensive discontinuities, or at transitions between welded and nonwelded tuff units. Two conceptual models for unsaturated-zone flow each explain the subsurface observations, collectively providing bounding estimates for net infiltration. Model-predicted uncertainty distribution for decadal-average site-scale net infiltration is generally consistent with estimated percolation fluxes using the bounding hypotheses, suggesting that the model-calculated uncertainty is reasonably consistent with the uncertainty in interpreting site observations.

Implications of nonrandom seed abscission and global stilling for migration of wind‐dispersed plant species

Migration of plant populations is a potential survival response to climate change that depends critically on seed dispersal. Biological and physical factors determine dispersal and migration of wind-dispersed species. Recent field and wind tunnel studies demonstrate biological adaptations that bias seed release toward conditions of higher wind velocity, promoting longer dispersal distances and faster migration. However, another suite of international studies also recently highlighted a global decrease in near-surface wind speeds, or 'global stilling'. This study assessed the implications of both factors on potential plant population migration rates, using a mechanistic modeling framework. Nonrandom abscission was investigated using models of three seed release mechanisms: (i) a simple drag model; (ii) a seed deflection model; and (iii) a 'wear and tear' model. The models generated a single functional relationship between the frequency of seed release and statistics of the near-surface wind environment, independent of the abscission mechanism. An Inertial-Particle, Coupled Eulerian-Lagrangian Closure model (IP-CELC) was used to investigate abscission effects on seed dispersal kernels and plant population migration rates under contemporary and potential future wind conditions (based on reported global stilling trends). The results confirm that nonrandom seed abscission increased dispersal distances, particularly for light seeds. The increases were mitigated by two physical feedbacks: (i) although nonrandom abscission increased the initial acceleration of seeds from rest, the sensitivity of the seed dispersal to this initial condition declined as the wind speed increased; and (ii) while nonrandom abscission increased the mean dispersal length, it reduced the kurtosis of seasonal dispersal kernels, and thus the chance of long-distance dispersal. Wind stilling greatly reduced the modeled migration rates under biased seed release conditions. Thus, species that require high wind velocities for seed abscission could experience threshold-like reductions in dispersal and migration potential if near-surface wind speeds continue to decline.