Jerome R. Mayaud

Modeling the response of subglacial drainage at Paakitsoq, west Greenland, to 21st century climate change

Although the Greenland Ice Sheet (GrIS) is losing mass at an accelerating rate, much uncertainty remains about how surface runoff interacts with the subglacial drainage system and affects water pressures and ice velocities, both currently, and into the future. Here, we apply a physically based, subglacial hydrological model to the Paakitsoq region, west Greenland, and run it into the future to calculate patterns of daily subglacial water pressure fluctuations in response to climatic warming. The model is driven with moulin input hydrographs calculated by a surface routing model, forced with distributed runoff. Surface runoff and routing are simulated for a baseline year (2000), before the model is forced with future climate scenarios for the years 2025, 2050, and 2095, based on the IPCCs Representative Concentration Pathways (RCPs). Our results show that as runoff increases throughout the 21st century, and/or as RCP scenarios become more extreme, the subglacial drainage system makes an earlier transition from a less efficient network operating at high water pressures, to a more efficient network with lower pressures. This will likely cause an overall decrease in ice velocities for marginal areas of the GrIS. However, short-term variations in runoff, and therefore subglacial pressure, can still cause localized speedups, even after the system has become more efficient. If these short-term pressure fluctuations become more pronounced as future runoff increases, the associated late-season speedups may help to compensate for the drop in overall summer velocities, associated with earlier transitioning from a high to a low pressure system.

A field-based parameterization of wind flow recovery in the lee of dryland plants

ABSTRACT: Wind erosion is a key component of land degradation in vulnerable dryland regions. Despite a wealth of studies investigating the impact of vegetation and windbreaks on windflow in controlled wind‐tunnel and modelling environments, there is still a paucity of empirical field data for accurately parameterizing the effect of vegetation in wind and sediment transport models. The aim of this study is to present a general parameterization of wind flow recovery in the lee of typical dryland vegetation elements (grass clumps and shrubs), based on their height (h) and optical porosity (&thgr;). Spatial variations in mean wind velocity around eight isolated vegetation elements in Namibia (three grass clumps and five shrubs) were recorded at 0.30 m height, using a combination of sonic and cup anemometry sampled at a temporal frequency of 10 seconds. Wind flow recovery in the lee of the elements was parameterized in an exponential form, Symbol. The best‐fit parameters derived from the field data were u0 = uref(0.0146&thgr; − 0.4076) and b = 0.0105&thgr; + 0.1627. By comparing this parameterization to existing models, it is shown that wind recovery curves derived from two‐dimensional wind fence experiments may not be suitable analogues for describing airflow around more complex, three‐dimensional forms. Field‐derived parameterizations such as the one presented here are a crucial step for connecting plant‐scale windflow behaviour to dryland bedform development at landscape scales.

A coupled vegetation/sediment transport model for dryland environments

Dryland regions are characterized by patchy vegetation, erodible surfaces, and erosive aeolian processes. Understanding how these constituent factors interact and shape landscape evolution is critical for managing potential environmental and anthropogenic impacts in drylands. However, modeling wind erosion on partially vegetated surfaces is a complex problem that has remained challenging for researchers. We present the new, coupled cellular automaton Vegetation and Sediment TrAnsport (ViSTA) model, which is designed to address fundamental questions about the development of arid and semiarid landscapes in a spatially explicit way. The technical aspects of the ViSTA model are described, including a new method for directly imposing oblique wind and transport directions onto a cell-based domain. Verification tests for the model are reported, including stable state solutions, the impact of drought and fire stress, wake flow dynamics, temporal scaling issues, and the impact of feedbacks between sediment movement and vegetation growth on landscape morphology. The model is then used to simulate an equilibrium nebkha dune field, and the resultant bed forms are shown to have very similar size and spacing characteristics to nebkhas observed in the Skeleton Coast, Namibia. The ViSTA model is a versatile geomorphological tool that could be used to predict threshold-related transitions in a range of dryland ecogeomorphic systems.

Modelled responses of the Kalahari Desert to 21 st century climate and land use change

Drylands are home to over 2 billion people globally, many of whom use the land for agricultural and pastoral activities. These vulnerable livelihoods could be disrupted if desert dunefields become more active in response to climate and land use change. Despite increasing knowledge about the role that wind, moisture availability and vegetation cover play in shaping dryland landscapes, relatively little is known about how drylands might respond to climatic and population pressures over the 21st century. Here we use a newly developed numerical model, which fully couples vegetation and sediment-transport dynamics, to simulate potential landscape evolution at three locations in the Kalahari Desert, under two future emissions scenarios: stabilising (RCP 4.5) and high (RCP 8.5). Our simulations suggest that whilst our study sites will experience some climatically-induced landscape change, the impacts of climate change alone on vegetation cover and sediment mobility may be relatively small. However, human activity could strongly exacerbate certain landscape trajectories. Fire frequency has a primary impact on vegetation cover, and, together with grazing pressure, plays a significant role in modulating shrub encroachment and ensuing land degradation processes. Appropriate land management strategies must be implemented across the Kalahari Desert to avoid severe environmental and socio-economic consequences over the coming decades.

Stress histories control rock-breakdown trajectories in arid environments

Rock and boulder surfaces are often exposed to weathering and /or rock-breakdown processes for extremely long time periods. This is especially true for arid environments on Earth and on planetary bodies such as Mars. One important, but largely unexplored, gap in knowledge is the influence of past stress histories on the operation of present rock-breakdown processes. Do rocks in the same area with different stress histories respond equally to newly imposed environmental conditions? This study investigates the influence of different physical and chemical stress histories on the response of basalt to salt weathering. We designed a fourstage approach of pre-treatment, field exposure, weathering simulation, and post-treatment: (1) physical, chemical, or no pre-treatment in the laboratory; (2) 3 yr exposure in either a hyper-arid sandy or salt-pan environment in the Namib desert (Namibia); (4) 60 cycles of a hot desert salt weathering simulation; and (4) desalination. Salt uptake and rock breakdown was assessed at each stage through comparison with baseline observations of mass, internal strength (Dynamic Young’s modulus) and surface morphology (three-dimensional microscopy). Clear differences in block responses were found. Physically pre-treated blocks (especially those left in the salt-pan environment) experienced the highest loss of strength overall, chemically pre-treated blocks showed the greatest mass loss in the sandy environment, and freshly cut blocks gained strength during exposure in the desert and maintained this during the experiment. These results imply that stress history matters for predicting breakdown rates, with humid, arid, and saline legacies influencing subsequent breakdown in distinctive ways. INTRODUCTION Rock-breakdown processes such as physical and chemical weathering are important agents of geomorphic change, producing erodible sediment and influencing slope instability. Rates of rock breakdown in arid environments are generally slow (e.g., ~1 mm k.y.−1; Ryb et al., 2014), although ‘hot spots’ of locally wet, salty conditions have much higher breakdown rates (e.g., ~10–150 mm k.y.−1; Viles and Goudie, 2007). In arid environments on Earth, salt weathering is an important rock-breakdown process (Goudie, 1993; Warke, 2007), as are thermal stresses from differential insolation (identified as a likely cause of boulder cracking by Eppes et al. [2010, 2015], and shown experimentally to cause deterioration in pre-stressed blocks by Viles et al. [2010]) and wind abrasion. Similarly, experimental, observational, and modeling studies show thermal cycling to be an important cause of rock breakdown on dry planetary bodies such as Mars (Viles et al., 2010; Eppes et al., 2015; Molaro et al., 2015), in addition to eolian abrasion (Bridges et al., 2014) and salt weathering (Jagoutz, 2006). The relative importance of these different rock-breakdown processes and their dynamics over space and time have not yet been clearly evaluated. The term ‘stress history’ has been used to describe how the legacy of past processes influences response to current weathering (Warke, 2007). For example, rocks exposed to long periods of chemical weathering in wetter phases may respond more quickly to eolian abrasion in subsequent drier periods than rocks without that history. Or, rocks that have experienced extensive thermal cycling in arid conditions may break down more rapidly than other rocks when exposed to salt weathering associated with wetter conditions (Warke, 2007). Such stress histories may partially explain spatial and temporal patterning in rockbreakdown rates and styles in arid environments, and help explain variability of landscape evolution in geomorphic settings such as desert pavements (Viles and Goudie, 2013) and alluvial fans (Eppes and McFadden, 2008) over decadal to millennial time scales. What is lacking is empirical evidence of how different stress histories affect subsequent weathering trajectories. This paper evaluates the influence of stress histories on a relatively resilient rock type (basalt) found widely on Mars and in many Earth deserts (e.g., northern Namibia and Saudi Arabia). Specifically, we assess how legacies from past environmental conditions (wetter, drier, or more saline) influence breakdown rates. We utilize a novel methodology (combining sequential laboratory and field experiments) to address the following questions: (1) how do past histories of chemical weathering (by acid) or physical weathering (by thermal cycling) influence subsequent rock breakdown in eolian and salt-rich environments, (2) how does exposure in eolian or salt-rich environments influence subsequent salt weathering, and (3) how can such influences on weathering trajectories best be quantified experimentally?

Future access to essential services in a growing smart city: The case of Surrey, British Columbia

Abstract The concept of accessibility – the ease with which people can reach places or opportunities –lies at the heart of what makes cities livable, workable and sustainable. As urban populations shift over time, predicting the changes to accessibility demand for certain services becomes crucial for responsible and ‘smart’ urban planning and infrastructure investment. In this study, we investigate how projected population change could affect accessibility to essential services in the City of Surrey, one of the fastest growing cities in Canada. Our objectives are two-fold: first, to quantify the additional pressure that Surreys growing population will have on existing facilities; second, to investigate how changes in the spatial distribution of different age and income groups will impact accessibility equity across the city. We evaluated accessibility levels to healthcare facilities and schools across Surreys multimodal transport network using origin-destination matrices, and combined this information with high-resolution longitudinal census data. Paying close attention to two vulnerable population groups – children and youth (0–19 years of age) and seniors (65+ years of age) – we analyzed shifts in accessibility demand from 2016 to 2022. The results show that population growth both within and outside the catchments of existing facilities will have varying implications for future accessibility demand in different areas of the city. By 2022, the citys hospitals and walk-in clinics will be accessible to ~9000 and ~124,000 more people (respectively) within a predefined threshold of 30 min by public transport. Schools will also face increased demand, as ~8000 additional children/youth in 2022 will move to areas with access to at least half of the citys schools. Conversely, over 27,000 more people – almost half of them seniors – will not be able to access a hospital in under 30 min by 2022. Since low-income and senior residents moving into poorly connected areas tend to be more reliant on public transport, accessibility equity may decline in some rural communities. Our study highlights how open-source data and code can be leveraged to conduct in-depth analysis of accessibility demand across a city, which is key for ensuring inclusive and ‘smart’ urban investment strategies.