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Hal E. Voepel
School of Geography and Environment, University of Southampton, Southampton, UK

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School of Geography and Environmental Science University of Southampton Highfield Campus Southampton, SO17 1BJ

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Article
Published: 15 July 2021 in Communications Earth & Environment
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Rising temperatures, rapid urbanization and soaring demand for natural resources threaten deltas worldwide and make them vulnerable to rising seas, subsidence, droughts, floods, and salt intrusion. However, climate change projections in deltas often address climate-driven stressors in isolation and disregard parallel anthropogenic processes, leading to insufficient socio-political drive. Here, using a combination of process-based numerical models that integrate both climatic and anthropogenic environmental stressors, we project salt intrusion within the Mekong mega-Delta, in the next three decades. We assess the relative effects of various drivers and show that anthropogenic forces such as groundwater extraction-induced subsidence and riverbed level incisions due to sediment starvation can increase the salinity-affected areas by 10–27% compared to the present-day situation, while future sea level rise adds another 6–19% increase. These projections provide crucial input for adaptation policy development in the Mekong Delta and the methodology inspires future systemic studies of environmental changes in other deltas.

ACS Style

Sepehr Eslami; Piet Hoekstra; Philip S. J. Minderhoud; Nam Nguyen Trung; Jannis M. Hoch; Edwin H. Sutanudjaja; Do Duc Dung; Tran Quang Tho; Hal E. Voepel; Marie-Noëlle Woillez; Maarten van der Vegt. Projections of salt intrusion in a mega-delta under climatic and anthropogenic stressors. Communications Earth & Environment 2021, 2, 1 -11.

AMA Style

Sepehr Eslami, Piet Hoekstra, Philip S. J. Minderhoud, Nam Nguyen Trung, Jannis M. Hoch, Edwin H. Sutanudjaja, Do Duc Dung, Tran Quang Tho, Hal E. Voepel, Marie-Noëlle Woillez, Maarten van der Vegt. Projections of salt intrusion in a mega-delta under climatic and anthropogenic stressors. Communications Earth & Environment. 2021; 2 (1):1-11.

Chicago/Turabian Style

Sepehr Eslami; Piet Hoekstra; Philip S. J. Minderhoud; Nam Nguyen Trung; Jannis M. Hoch; Edwin H. Sutanudjaja; Do Duc Dung; Tran Quang Tho; Hal E. Voepel; Marie-Noëlle Woillez; Maarten van der Vegt. 2021. "Projections of salt intrusion in a mega-delta under climatic and anthropogenic stressors." Communications Earth & Environment 2, no. 1: 1-11.

Journal article
Published: 15 May 2021 in Sustainability
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The development of a coherent and coordinated policy for the management of large socio-agricultural systems, such as the Mekong delta in southern Vietnam, is reliant on aligning the development, delivery, and implementation of policy on national to local scales. Effective decision making is linked to a coherent, broadly-shared vision of the strategic management of socio-agricultural systems. However, when policies are ambiguous, and at worst contradictory, long-term management and planning can consequently suffer. These potential adverse impacts may be compounded if stakeholders have divergent visions of the current and future states of socio-agricultural systems. Herein we used a transferable, scenario-based methodology which uses a standard quadrant matrix in order to explore both anticipated and idealized future states. Our case study was the Mekong delta. The scenario matrix was based upon two key strategic choices (axis) for the delta, derived from analysis of policy documents, literature, stakeholder engagement, and land use models. These are: (i) who will run agriculture in the future, agri-business or the established commune system; and (ii) to what degree sustainability will be incorporated into production. During a workshop meeting, stakeholders identified that agri-business will dominate future agricultural production in the delta but showed a clear concern that sustainability might consequently be undermined despite policy claims of the contrary. As such, our study highlights an important gap between national expectations and regional perspectives. Our results suggest that the new development plans for the Mekong delta (which comprise a new Master Plan and a new 5-year socio-economic development plan), which emphasize agro-business development, should adopt approaches that address concerns of sustainability as well as a more streamlined policy formulation and implementation that accounts for stakeholder concerns at both provincial and national levels.

ACS Style

Craig Hutton; Oliver Hensengerth; Tristan Berchoux; Van Tri; Thi Tong; Nghia Hung; Hal Voepel; Stephen Darby; Duong Bui; Thi Bui; Nguyen Huy; Daniel Parsons. Stakeholder Expectations of Future Policy Implementation Compared to Formal Policy Trajectories: Scenarios for Agricultural Food Systems in the Mekong Delta. Sustainability 2021, 13, 5534 .

AMA Style

Craig Hutton, Oliver Hensengerth, Tristan Berchoux, Van Tri, Thi Tong, Nghia Hung, Hal Voepel, Stephen Darby, Duong Bui, Thi Bui, Nguyen Huy, Daniel Parsons. Stakeholder Expectations of Future Policy Implementation Compared to Formal Policy Trajectories: Scenarios for Agricultural Food Systems in the Mekong Delta. Sustainability. 2021; 13 (10):5534.

Chicago/Turabian Style

Craig Hutton; Oliver Hensengerth; Tristan Berchoux; Van Tri; Thi Tong; Nghia Hung; Hal Voepel; Stephen Darby; Duong Bui; Thi Bui; Nguyen Huy; Daniel Parsons. 2021. "Stakeholder Expectations of Future Policy Implementation Compared to Formal Policy Trajectories: Scenarios for Agricultural Food Systems in the Mekong Delta." Sustainability 13, no. 10: 5534.

Journal article
Published: 06 April 2019 in Science of The Total Environment
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The Mekong delta is recognised as one of the world's most vulnerable mega-deltas, being subject to a range of environmental pressures including sea level rise, increasing population, and changes in flows and nutrients from its upland catchment. With changing climate and socioeconomics there is a need to assess how the Mekong catchment will be affected in terms of the delivery of water and nutrients into the delta system. Here we apply the Integrated Catchment model (INCA) to the whole Mekong River Basin to simulate flow and water quality, including nitrate, ammonia, total phosphorus and soluble reactive phosphorus. The impacts of climate change on all these variables have been assessed across 24 river reaches ranging from the Himalayas down to the delta in Vietnam. We used the UK Met Office PRECIS regionally coupled climate model to downscale precipitation and temperature to the Mekong catchment. This was accomplished using the Global Circulation Model GFDL-CM to provide the boundary conditions under two carbon control strategies, namely representative concentration pathways (RCP) 4.5 and a RCP 8.5 scenario. The RCP 4.5 scenario represents the carbon strategy required to meet the Paris Accord, which aims to limit peak global temperatures to below a 2 °C rise whilst seeking to pursue options that limit temperature rise to 1.5 °C. The RCP 8.5 scenario is associated with a larger 3–4 °C rise. In addition, we also constructed a range of socio-economic scenarios to investigate the potential impacts of changing population, atmospheric pollution, economic growth and land use change up to the 2050s. Results of INCA simulations indicate increases in mean flows of up to 24%, with flood flows in the monsoon period increasing by up to 27%, but with increasing periods of drought up to 2050. A shift in the timing of the monsoon is also simulated, with a 4 week advance in the onset of monsoon flows on average. Decreases in nitrogen and phosphorus concentrations occur primarily due to flow dilution, but fluxes of these nutrients also increase by 5%, which reflects the changing flow, land use change and population changes.

ACS Style

P.G. Whitehead; L. Jin; G. Bussi; Hal Voepel; Stephen Darby; Grigorios Vasilopoulos; R. Manley; H. Rodda; Craig Hutton; Christopher Hackney; Van Pham Dang Tri; N.N. Hung. Water quality modelling of the Mekong River basin: Climate change and socioeconomics drive flow and nutrient flux changes to the Mekong Delta. Science of The Total Environment 2019, 673, 218 -229.

AMA Style

P.G. Whitehead, L. Jin, G. Bussi, Hal Voepel, Stephen Darby, Grigorios Vasilopoulos, R. Manley, H. Rodda, Craig Hutton, Christopher Hackney, Van Pham Dang Tri, N.N. Hung. Water quality modelling of the Mekong River basin: Climate change and socioeconomics drive flow and nutrient flux changes to the Mekong Delta. Science of The Total Environment. 2019; 673 ():218-229.

Chicago/Turabian Style

P.G. Whitehead; L. Jin; G. Bussi; Hal Voepel; Stephen Darby; Grigorios Vasilopoulos; R. Manley; H. Rodda; Craig Hutton; Christopher Hackney; Van Pham Dang Tri; N.N. Hung. 2019. "Water quality modelling of the Mekong River basin: Climate change and socioeconomics drive flow and nutrient flux changes to the Mekong Delta." Science of The Total Environment 673, no. : 218-229.

Research article
Published: 15 March 2019 in Earth Surface Processes and Landforms
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Sediment transport equations typically produce transport rates that are biased by orders of magnitude. A causal component of this inaccuracy is the inability to represent complex grain‐scale interactions controlling entrainment. Grain‐scale incipient motion has long been modelled using geometric relationships based on simplified particle geometry and two‐dimensional (2D) force or moment balances. However, this approach neglects many complexities of real grains, including grain shape, cohesion and the angle of entrainment relative to flow direction. To better represent this complexity, we develop the first vector‐based, fully three‐dimensional (3D) grain rotation entrainment model that can be used to resolve any entrainment formulation in 3D, and which also includes the effect of matrix cohesion. To apply this model we use X‐ray computed tomography to quantify the 3D structure of water‐worked river grains. We compare our 3D model results with those derived from application of a 2D entrainment model. We find that the 2D approach produces estimates of dimensionless critical shear stress ( ) that are an order of magnitude lower than our 3D model. We demonstrate that it is more appropriate to use the c‐axis when calculating 2D projections, which increases values of to more closely match our 3D estimates. The 3D model reveals that the main controls on critical shear stress in our samples are projection of grains, cohesive effects from a fine‐grained matrix, and bearing angle for the plane of rotation (the lateral angle of departure from downstream flow that, in part, defines the grain's direction of pivot about an axis formed by two contact points in 3D). The structural precision of our 3D model demonstrates sources of geometric error inherent in 2D models. By improving flow properties to better replicate local hydraulics in our 3D model, entrainment modelling of scanned riverbed grains has the potential for benchmarking 2D model enhancements. This article is protected by copyright. All rights reserved.

ACS Style

Hal Voepel; Julian Leyland; Rebecca A. Hodge; Sharif Ahmed; David Sear. Development of a vector‐based 3D grain entrainment model with application to X‐ray computed tomography scanned riverbed sediment. Earth Surface Processes and Landforms 2019, 44, 3057 -3077.

AMA Style

Hal Voepel, Julian Leyland, Rebecca A. Hodge, Sharif Ahmed, David Sear. Development of a vector‐based 3D grain entrainment model with application to X‐ray computed tomography scanned riverbed sediment. Earth Surface Processes and Landforms. 2019; 44 (15):3057-3077.

Chicago/Turabian Style

Hal Voepel; Julian Leyland; Rebecca A. Hodge; Sharif Ahmed; David Sear. 2019. "Development of a vector‐based 3D grain entrainment model with application to X‐ray computed tomography scanned riverbed sediment." Earth Surface Processes and Landforms 44, no. 15: 3057-3077.

Journal article
Published: 01 August 2014 in Geomorphology
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ACS Style

Marwan A. Hassan; Samuel V.J. Robinson; Hal Voepel; Jack Lewis; Thomas E. Lisle. Modeling temporal trends in bedload transport in gravel-bed streams using hierarchical mixed-effects models. Geomorphology 2014, 219, 260 -269.

AMA Style

Marwan A. Hassan, Samuel V.J. Robinson, Hal Voepel, Jack Lewis, Thomas E. Lisle. Modeling temporal trends in bedload transport in gravel-bed streams using hierarchical mixed-effects models. Geomorphology. 2014; 219 ():260-269.

Chicago/Turabian Style

Marwan A. Hassan; Samuel V.J. Robinson; Hal Voepel; Jack Lewis; Thomas E. Lisle. 2014. "Modeling temporal trends in bedload transport in gravel-bed streams using hierarchical mixed-effects models." Geomorphology 219, no. : 260-269.

Journal article
Published: 12 June 2014 in Transport in Porous Media
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A laboratory experiment of transient thermal convection in a 1-m-high cell was conducted to compare the length and time scales of plume development to theory. The temperature field was resolved to less than 1 mm and was measured by dissolving a solution of thermochromic crystals into the water–glycerin working fluid. The time-dependent experiment was run by applying heat at the bottom boundary that eventually was \(6\,^\circ \) C above the background temperature of the fluid. After development of a thermal boundary layer, the instability became visible at 26 min, with the development of 11, 3–4 cm width plumes growing from the boundary layer. The initially rapid growth rate reached a limiting velocity of approximately 0.5 cm min \(^{-1}\) , and then decelerated throughout the experiment. Plumes interacted primarily by merging together; by the end of the experiment only three plumes were present. The Nusselt number at the onset of convection was 10, although it dropped to 4 after 45 min, which would be expected of a barely unstable system.

ACS Style

C. A. Cooper; J. B. Crews; R. Schumer; R. J. Breitmeyer; Hal Voepel; D. L. Decker. Experimental Investigation of Transient Thermal Convection in Porous Media. Transport in Porous Media 2014, 104, 335 -347.

AMA Style

C. A. Cooper, J. B. Crews, R. Schumer, R. J. Breitmeyer, Hal Voepel, D. L. Decker. Experimental Investigation of Transient Thermal Convection in Porous Media. Transport in Porous Media. 2014; 104 (2):335-347.

Chicago/Turabian Style

C. A. Cooper; J. B. Crews; R. Schumer; R. J. Breitmeyer; Hal Voepel; D. L. Decker. 2014. "Experimental Investigation of Transient Thermal Convection in Porous Media." Transport in Porous Media 104, no. 2: 335-347.

Journal article
Published: 24 December 2013 in Journal of Geophysical Research: Earth Surface
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ACS Style

Hal Voepel; Rina Schumer; Marwan A. Hassan. Sediment residence time distributions: Theory and application from bed elevation measurements. Journal of Geophysical Research: Earth Surface 2013, 118, 2557 -2567.

AMA Style

Hal Voepel, Rina Schumer, Marwan A. Hassan. Sediment residence time distributions: Theory and application from bed elevation measurements. Journal of Geophysical Research: Earth Surface. 2013; 118 (4):2557-2567.

Chicago/Turabian Style

Hal Voepel; Rina Schumer; Marwan A. Hassan. 2013. "Sediment residence time distributions: Theory and application from bed elevation measurements." Journal of Geophysical Research: Earth Surface 118, no. 4: 2557-2567.

Journal article
Published: 25 February 2013 in Journal of Geophysical Research: Earth Surface
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[1] Previous work highlights the need for data collection to identify appropriate models for temporal evolution of tracer dispersal in rivers. Results of 64 gravel‐bed field tracer experiments covering a wide range of flow and sediment supply regimes are compiled here to determine the probabilistic character of gravel transport. We focus on whether particle travel distances and waits are thin‐ or heavy‐tailed. While heavy‐tailed travel distance distributions are observed between successive monitoring events in different hydrological and sediment supply regimes, heavy‐tailedness does not persist through total travel distance over multiple monitoring events, suggesting that individual monitoring events occur before particle travel distance exceeds the characteristic correlation length for the channel (such that particles that start in fast paths remain in fast paths and particles in slow paths remain in slow paths). After a large number of transport events, super‐diffusive spreading was not observed at any of the gravel bed streams. Continuous‐time tracking of x, y, z coordinates of tracers in natural streams is necessary to capture exact step and waiting time distributions.

ACS Style

Marwan A. Hassan; Hal Voepel; Rina Schumer; Gary Parker; Luigi Fraccarollo. Displacement characteristics of coarse fluvial bed sediment. Journal of Geophysical Research: Earth Surface 2013, 118, 155 -165.

AMA Style

Marwan A. Hassan, Hal Voepel, Rina Schumer, Gary Parker, Luigi Fraccarollo. Displacement characteristics of coarse fluvial bed sediment. Journal of Geophysical Research: Earth Surface. 2013; 118 (1):155-165.

Chicago/Turabian Style

Marwan A. Hassan; Hal Voepel; Rina Schumer; Gary Parker; Luigi Fraccarollo. 2013. "Displacement characteristics of coarse fluvial bed sediment." Journal of Geophysical Research: Earth Surface 118, no. 1: 155-165.

Journal article
Published: 10 August 2011 in Water Resources Research
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[1] There is no consensus on how changes in both temperature and precipitation will affect regional vegetation. We investigated controls on hydrologic partitioning at the catchment scale across many different ecoregions, and compared the resulting estimates of catchment wetting and vaporization (evapotranspiration) to remotely sensed indices of vegetation greenness. The fraction of catchment wetting vaporized by plants, known as the Horton index, is strongly related to the ratio of available energy to available water at the Earth's surface, the aridity index. Here we show that the Horton index is also a function of catchment mean slope and elevation, and is thus related to landscape characteristics that control how much and how long water is retained in a catchment. We compared the power of the components of the water and energy balance, as well as landscape characteristics, to predict Normalized Difference Vegetation Index (NDVI), a surrogate for vegetation productivity, at 312 Model Parameter Estimation Experiment (MOPEX) catchments across the United States. Statistical analysis revealed that the Horton index provides more precision in predicting maximum annual NDVI for all catchments than mean annual precipitation, potential evapotranspiration, or their ratio, the aridity index. Models of vegetation productivity should emphasize plant‐available water, rather than just precipitation, by incorporating the interaction of climate and landscape. Major findings related to the Horton index are: (1) it is a catchment signature that is relatively constant from year‐to‐year; (2) it is related to specific landscape characteristics; (3) it can be used to create catchment typologies; and (4) it is related to overall catchment greenness.

ACS Style

Hal Voepel; Benjamin Ruddell; Rina Schumer; Peter A. Troch; Paul Brooks; Andrew Neal; Matej Durcik; Murugesu Sivapalan. Quantifying the role of climate and landscape characteristics on hydrologic partitioning and vegetation response. Water Resources Research 2011, 47, 1 .

AMA Style

Hal Voepel, Benjamin Ruddell, Rina Schumer, Peter A. Troch, Paul Brooks, Andrew Neal, Matej Durcik, Murugesu Sivapalan. Quantifying the role of climate and landscape characteristics on hydrologic partitioning and vegetation response. Water Resources Research. 2011; 47 (10):1.

Chicago/Turabian Style

Hal Voepel; Benjamin Ruddell; Rina Schumer; Peter A. Troch; Paul Brooks; Andrew Neal; Matej Durcik; Murugesu Sivapalan. 2011. "Quantifying the role of climate and landscape characteristics on hydrologic partitioning and vegetation response." Water Resources Research 47, no. 10: 1.