About

Jon Pelletier is a professor in the Department of Geosciences at the University of Arizona. His primary research goal is to combine field measurements, analyses of digital data, and mathematical modeling to understand landform evolution. His work encompasses all major landform types, including hillslope, fluvial, aeolian, glacial, and coastal environments. Pelletier's research spans a wide range of spatial scales from microtopographic features (~10^-3 meters) to global landforms (~10^7 meters), and temporal scales from individual events such as rainstorms (~10^-4 years) to mountain building and decay (~10^8 years). He has been recognized as a Geological Society of America Fellow in 2015 and a Galileo Circle Fellow at the College of Science, University of Arizona, in 2011.

Research topics

• Geology
• Geomorphology
• Computer Science
• Physics
• Mathematics
• Geotechnical engineering
• Physical geography
• Paleontology
• Geochemistry
• Geography
• Geometry
• Earth science
• Mechanics
• Mineralogy

Selected publications

• An evaluation of flow-routing algorithms for calculating contributing area on regular grids

  Earth Surface Dynamics · 2025-03-19 · 3 citations

  articleOpen access

  Abstract. Calculating contributing area (often used as a proxy for surface water discharge) within a digital elevation model (DEM) or landscape evolution model (LEM) is a fundamental operation in geomorphology. Here we document the fact that a commonly used multiple-flow-direction algorithm for calculating contributing area, i.e., D∞ of Tarboton (1997), is sufficiently biased along the cardinal and ordinal directions that it is unsuitable for some standard applications of flow-routing algorithms. We revisit the purported excess dispersion of the multiple-flow-direction (MFD) algorithm of Freeman (1991) that motivated the development of D∞ and demonstrate that MFD is superior to D∞ when tested against analytic solutions for the contributing areas of idealized landforms and the predictions of the shallow-water equation solver FLO-2D for more complex landforms in which the water surface slope is closely approximated by the bed slope. We also introduce a new flow-routing algorithm entitled IDS (in reference to the iterative depth- and slope-dependent nature of the algorithm) that is more suitable than MFD for applications in which the bed and water surface slopes differ substantially. IDS solves for water flow depths under steady hydrologic conditions by distributing the discharge delivered to each grid point from upslope to its downslope neighbors in rank order of elevation (highest to lowest) and in proportion to a power-law function of the square root of the water surface slope and the five-thirds power of the water depth, mimicking the relationships among water discharge, depth, and surface slope in Manning's equation. IDS is iterative in two ways: (1) water depths are added in small increments so that the water surface slope can gradually differ from the bed slope, facilitating the spreading of water in areas of laterally unconfined flow, and (2) the partitioning of discharge from high to low elevations can be repeated, improving the accuracy of the solution as the water depths of downslope grid points become more well approximated with each successive iteration. We assess the performance of IDS by comparing its results to those of FLO-2D for a variety of real and idealized landforms and to an analytic solution of the shallow-water equations. We also demonstrate how IDS can be modified to solve other fluid-dynamical nonlinear partial differential equations arising in Earth surface processes, such as the Boussinesq equation for the height of the water table in an unconfined aquifer.

  PublisherDOI

• Climate‐Driven Changes to Suspended‐Sediment Yields by the End of the Century

  Earth s Future · 2025-08-28 · 1 citations

  articleOpen accessSenior author

  Abstract Anticipated changes in climate by the end of this century are likely to modify suspended‐sediment yields ( S y ) in diverse ways. Past work has shown how hydrological non‐stationarity may alter water discharges and hence S y , but less attention has been given to the impact of likely future changes in upland sediment‐detachment rates on downstream S y . In certain environments, climatically driven changes in vegetation cover on upland hillslopes may more than counteract the effects of changing runoff on S y . Changes in precipitation, temperature, and vegetation may, therefore, interact in nonlinear ways to produce unexpected changes. In this work, we simulated future changes to background S y (i.e., changes unrelated to land‐use changes and dams) with climatological and vegetative data output from an ensemble of CMIP6 Earth System Model (ESM) simulations. Depending on the future scenario, the cumulative annual sediment flux of 780 globally distributed rivers increases by between 2.3% and 8.4%. Significant deviations from historical S y are projected at high latitudes in response to each forcing variable, while low‐latitude responses are regionally varied. In regions where ensemble members agree on future changes in forcing variables, large S y changes are forecast with high confidence (e.g., >200% S y increase for several northeastern U.S. rivers at the 95% level). In contrast, ensemble variability in vegetation projections results in considerable uncertainty in the projected S y of rivers in other regions. Further improvements to the vegetation components of ESMs will help to reduce regional uncertainties in projected changes to S y .

  PublisherOA PDFDOI

• Climate-driven changes to suspended sediment yields by the end of the century

  2025-02-25

  preprintOpen accessSenior author

  Anticipated changes in climate by the end of this century are likely to modify suspended sediment yields (Sy) in diverse ways. Past work has shown how hydrological non-stationarity may alter water discharges and Sy, but less attention has been given to the impact of likely future changes in upland sediment detachment on downstream Sy. In certain environments, potential changes in vegetation cover on upland hillslopes associated with a change in rainfall and runoff may more than counteract the effects of runoff on Sy. Changes in precipitation, temperature, and vegetation may, therefore, interact in nonlinear ways to produce unexpected changes. In this work, we simulated future changes to background Sy (i.e., changes unrelated to land-use changes and dams) with climatological and vegetative data output from an ensemble of CMIP6 Earth System Model (ESM) simulations. Depending on the future scenario, the cumulative annual sediment flux of 780 globally distributed rivers increases by between 2.3% and 8.4%. Projected high-latitude changes in each forcing variable result in significant latitudinally averaged deviations from historical Sy, while lower latitudes show diverse regional responses. In regions where ensemble members agree on future changes in forcing variables, large Sy changes are forecast with high confidence (e.g., >200% Sy increase for several northeastern U.S. rivers at the 95% level). In contrast, ensemble variability in vegetation projections results in considerable uncertainty in the projected Sy of rivers in other regions. Further improvements to the vegetation components of ESMs will help to reduce regional uncertainties in projected changes to Sy.

  PublisherOA PDFDOI

• Geometric constraints on tributary fluvial network junction angles

  Earth Surface Dynamics · 2025-03-12 · 2 citations

  articleOpen access1st authorCorresponding

  Abstract. The intersection of two non-parallel planes is a line. Howard (1990), following Horton (1932), proposed that the orientation and slope of a fluvial valley bottom within a tributary network are geometrically constrained by the orientation and slope of the line formed by the intersection of planar approximations to the topography upslope from the tributary junction along the two tributary directions. Previously published analyses of junction angle data support this geometric model, yet junction angles have also been proposed to be controlled by climate and/or optimality principles (e.g., minimum power expenditure). In this paper, we document a test of the Howard (1990) model using ∼107 fluvial network junctions in the conterminous US and a portion of the Loess Plateau, China. Junction angles are consistent with the predictions of the Howard (1990) model when the orientations and slopes are computed for the drainage basins whose outlets are the main valley and each upstream tributary rather than in the traditional way using valley-bottom segments near tributary junctions. When computed in the traditional way, junction angles are a function of slope ratios (as the Howard, 1990, model predicts), but data deviate systematically from the Howard (1990) model. We map the mean junction angles computed along valley bottoms within each 2.5 km×2.5 km pixel of the conterminous USA and document lower mean junction angles in incised Late Cenozoic alluvial piedmont deposits compared to those of incised bedrock/older deposits. We demonstrate using numerical modeling that lower ratios of the small-scale roughness of the initial pre-incision surface to the large-scale/regional slope of a landscape can contribute to lower mean junction angles. Using modern analogs, we demonstrate that Late Cenozoic alluvial piedmonts likely had ratios of mean microtopographic slope to large-scale slope/tilt that were lower (i.e., ∼1) prior to tributary drainage network development than the same ratios of bedrock/older deposits (≫1). This finding provides a means of understanding how the geometric model of Howard (1990) contributes to the result that incised Late Cenozoic alluvial piedmont deposits have lower mean tributary fluvial network junction angles, on average, compared to those of incised bedrock/older deposits. This work demonstrates that the topography of a landscape prior to fluvial incision may exert a key constraint on tributary fluvial network junction angles. This work adds to the list of possible controls on fluvial network junction angles, including climate- and optimality-based models for junction angles that have been the primary focus of research during the past decade.

  PublisherOA PDFDOI

• Adverse Safety Events in Emergency Medical Services Care of Children With Out-of-Hospital Cardiac Arrest

  JAMA Network Open · 2024-01-12 · 15 citations

  articleOpen access

  Importance: Survival for children with out-of-hospital cardiac arrest (OHCA) remains poor despite improvements in adult OHCA survival. Objective: To characterize the frequency of and factors associated with adverse safety events (ASEs) in pediatric OHCA. Design, Setting, and Participants: This population-based retrospective cohort study examined patient care reports from 51 emergency medical services (EMS) agencies in California, Georgia, Oregon, Pennsylvania, Texas, and Wisconsin for children younger than 18 years with an OHCA in which resuscitation was attempted by EMS personnel between 2013 and 2019. Medical record review was conducted from January 2019 to April 2022 and data analysis from October 2022 to February 2023. Main Outcomes and Measure: Severe ASEs during the patient encounter (eg, failure to give an indicated medication, 10-fold medication overdose). Results: A total of 1019 encounters of EMS-treated pediatric OHCA were evaluated; 465 patients (46%) were younger than 12 months. At least 1 severe ASE occurred in 610 patients (60%), and 310 patients (30%) had 2 or more. Neonates had the highest frequency of ASEs. The most common severe ASEs involved epinephrine administration (332 [30%]), vascular access (212 [19%]), and ventilation (160 [14%]). In multivariable logistic regression, the only factor associated with severe ASEs was young age. Neonates with birth-related and non-birth-related OHCA had greater odds of a severe ASE compared with adolescents (birth-related: odds ratio [OR], 7.0; 95% CI, 3.1-16.1; non-birth-related: OR, 3.4; 95% CI, 1.2-9.6). Conclusions and Relevance: In this large geographically diverse cohort of children with EMS-treated OHCA, 60% of all patients experienced at least 1 severe ASE. The odds of a severe ASE were higher for neonates than adolescents and even higher when the cardiac arrest was birth related. Given the national increase in out-of-hospital births and ongoing poor outcomes of OHCA in young children, these findings represent an urgent call to action to improve care delivery and training for this population.

  PublisherOA PDFDOI

• Author response to referee comments on egusphere-2024-1153

  2024-07-04

  peer-reviewOpen access1st authorCorresponding

  <strong class="journal-contentHeaderColor">Abstract.</strong> The intersection of two non-parallel planes is a line. Howard (1990), following Horton (1932), proposed that the orientation and slope of a fluvial valley within a tributary network are geometrically constrained by the orientation and slope of the line formed by the intersection of planar approximations to the topography upslope from the tributary junction along the two tributary directions. Previously published analyses of junction-angle data support this geometric model, yet junction angles have also been proposed to be controlled by climate and/or optimality principles (e.g., minimum-power expenditure). In this paper, we document a test of the Howard (1990) model using ~10⁷ fluvial network junctions in the conterminous U.S. and a portion of the Loess Plateau, China. Junction angles are consistent with the predictions of the Howard (1990) model when the orientations and slopes are computed using drainage basins rather than in the traditional way using valley-bottom segments near tributary junctions. When computed in the traditional way, junction angles are a function of slope ratios (as the Howard (1990) model) predicts, but data deviate from the Howard (1990) model in a manner that we propose is the result of valley-bottom meandering/tortuosity. We map the mean junction angles computed along valley bottoms within each 2.5 km x 2.5 km pixel of the conterminous U.S.A. and document lower mean junction angles in incised late-Cenozoic alluvial piedmont deposits compared to those of incised bedrock/older deposits. To understand how this finding relates to the geometric model of Howard (1990), we demonstrate that, for an idealized model of an initially unincised landform, i.e., a tilted plane with random microtopography, lower ratios of the mean microtopographic slope to the large-scale slope/tilt are associated with lower mean junction angles compared to landforms with higher such ratios. Using modern analogs, we demonstrate that unincised late-Cenozoic alluvial piedmonts likely had ratios of mean microtopographic slope to large-scale slope/tilt that were lower (i.e., ~1) prior to tributary drainage network development than the same ratios of bedrock/older deposits (≫1). This finding provides a means of understanding how the geometric model of Howard (1990) results in incised late Cenozoic alluvial piedmont deposits with lower mean tributary fluvial network junction angles, on average, compared to those of incised bedrock/older deposits. This work demonstrates that the topography of a landscape prior to fluvial incision exerts a key constraint on tributary fluvial network junction angles via a fundamental geometric principle that is independent of any climate- or optimality-based principle.

  PublisherDOI

• Comment on egusphere-2024-1153

  2024-06-07

  peer-reviewOpen access1st authorCorresponding

  <strong class="journal-contentHeaderColor">Abstract.</strong> The intersection of two non-parallel planes is a line. Howard (1990), following Horton (1932), proposed that the orientation and slope of a fluvial valley within a tributary network are geometrically constrained by the orientation and slope of the line formed by the intersection of planar approximations to the topography upslope from the tributary junction along the two tributary directions. Previously published analyses of junction-angle data support this geometric model, yet junction angles have also been proposed to be controlled by climate and/or optimality principles (e.g., minimum-power expenditure). In this paper, we document a test of the Howard (1990) model using ~10⁷ fluvial network junctions in the conterminous U.S. and a portion of the Loess Plateau, China. Junction angles are consistent with the predictions of the Howard (1990) model when the orientations and slopes are computed using drainage basins rather than in the traditional way using valley-bottom segments near tributary junctions. When computed in the traditional way, junction angles are a function of slope ratios (as the Howard (1990) model) predicts, but data deviate from the Howard (1990) model in a manner that we propose is the result of valley-bottom meandering/tortuosity. We map the mean junction angles computed along valley bottoms within each 2.5 km x 2.5 km pixel of the conterminous U.S.A. and document lower mean junction angles in incised late-Cenozoic alluvial piedmont deposits compared to those of incised bedrock/older deposits. To understand how this finding relates to the geometric model of Howard (1990), we demonstrate that, for an idealized model of an initially unincised landform, i.e., a tilted plane with random microtopography, lower ratios of the mean microtopographic slope to the large-scale slope/tilt are associated with lower mean junction angles compared to landforms with higher such ratios. Using modern analogs, we demonstrate that unincised late-Cenozoic alluvial piedmonts likely had ratios of mean microtopographic slope to large-scale slope/tilt that were lower (i.e., ~1) prior to tributary drainage network development than the same ratios of bedrock/older deposits (≫1). This finding provides a means of understanding how the geometric model of Howard (1990) results in incised late Cenozoic alluvial piedmont deposits with lower mean tributary fluvial network junction angles, on average, compared to those of incised bedrock/older deposits. This work demonstrates that the topography of a landscape prior to fluvial incision exerts a key constraint on tributary fluvial network junction angles via a fundamental geometric principle that is independent of any climate- or optimality-based principle.

  PublisherOA PDFDOI

• Geometric constraints on tributary fluvial network junction angles

  2024-04-25

  preprintOpen access1st authorCorresponding

  Abstract. The intersection of two non-parallel planes is a line. Howard (1990), following Horton (1932), proposed that the orientation and slope of a fluvial valley within a tributary network are geometrically constrained by the orientation and slope of the line formed by the intersection of planar approximations to the topography upslope from the tributary junction along the two tributary directions. Previously published analyses of junction-angle data support this geometric model, yet junction angles have also been proposed to be controlled by climate and/or optimality principles (e.g., minimum-power expenditure). In this paper, we document a test of the Howard (1990) model using ~107 fluvial network junctions in the conterminous U.S. and a portion of the Loess Plateau, China. Junction angles are consistent with the predictions of the Howard (1990) model when the orientations and slopes are computed using drainage basins rather than in the traditional way using valley-bottom segments near tributary junctions. When computed in the traditional way, junction angles are a function of slope ratios (as the Howard (1990) model) predicts, but data deviate from the Howard (1990) model in a manner that we propose is the result of valley-bottom meandering/tortuosity. We map the mean junction angles computed along valley bottoms within each 2.5 km x 2.5 km pixel of the conterminous U.S.A. and document lower mean junction angles in incised late-Cenozoic alluvial piedmont deposits compared to those of incised bedrock/older deposits. To understand how this finding relates to the geometric model of Howard (1990), we demonstrate that, for an idealized model of an initially unincised landform, i.e., a tilted plane with random microtopography, lower ratios of the mean microtopographic slope to the large-scale slope/tilt are associated with lower mean junction angles compared to landforms with higher such ratios. Using modern analogs, we demonstrate that unincised late-Cenozoic alluvial piedmonts likely had ratios of mean microtopographic slope to large-scale slope/tilt that were lower (i.e., ~1) prior to tributary drainage network development than the same ratios of bedrock/older deposits (≫1). This finding provides a means of understanding how the geometric model of Howard (1990) results in incised late Cenozoic alluvial piedmont deposits with lower mean tributary fluvial network junction angles, on average, compared to those of incised bedrock/older deposits. This work demonstrates that the topography of a landscape prior to fluvial incision exerts a key constraint on tributary fluvial network junction angles via a fundamental geometric principle that is independent of any climate- or optimality-based principle.

  PublisherDOI

• An evaluation of flow-routing algorithms for calculating contributing area on regular grids

  2024-04-22 · 1 citations

  preprintOpen accessCorresponding

  Abstract. Calculating contributing area (often used as a proxy for surface water discharge) within a Digital Elevation Model (DEM) or Landscape Evolution Model (LEM) is a fundamental operation in geomorphology. Here we document that a commonly used multiple-flow-direction algorithm for calculating contributing area, i.e., D∞ of Tarboton (1997), is sufficiently biased along the cardinal and ordinal directions that it is unsuitable for some standard applications of flow-routing algorithms. We revisit the purported excess dispersion of the MFD algorithm of Freeman (1991) that motivated the development of D∞ and demonstrate that MFD is superior to D∞ when tested against analytic solutions for the contributing areas of idealized landforms and the predictions of the shallow-water-equation solver FLO-2D for more complex landforms in which the water-surface slope is closely approximated by the bed slope. We also introduce a new flow-routing algorithm entitled IDS (in reference to the iterative depth-and-slope-dependent nature of the algorithm) that is more suitable than MFD for applications in which the bed and water-surface slopes differ substantially. IDS solves for water flow depths under steady hydrologic conditions by distributing the discharge delivered to each grid point from upslope to its downslope neighbors in rank order of elevation (highest to lowest) and in proportion to a power-law function of the square root of the water-surface slope and the five-thirds power of the water depth, mimicking the relationships among water discharge, depth, and surface slope in Manning’s equation. IDS is iterative in two ways: 1) water depths are added in small increments so that the water-surface slope can gradually differ from the bed slope, facilitating the spreading of water in areas of laterally unconfined flow, and 2) the partitioning of discharge from high to low elevations can be repeated, improving the accuracy of the solution as the water depths of downslope grid points become more well approximated with each successive iteration. We assess the performance of IDS by comparing its results to those of FLO-2D for a variety of real and idealized landforms and to an analytic solution of the shallow-water equations. We also demonstrate how IDS can be modified to solve other fluid-dynamical nonlinear partial differential equations arising in Earth-surface processes, such as the Boussinesq equation for the height of the water table in an unconfined aquifer.

  PublisherDOI

• Comment on egusphere-2024-1153

  2024-06-07

  peer-reviewOpen access1st authorCorresponding

  <strong class="journal-contentHeaderColor">Abstract.</strong> The intersection of two non-parallel planes is a line. Howard (1990), following Horton (1932), proposed that the orientation and slope of a fluvial valley within a tributary network are geometrically constrained by the orientation and slope of the line formed by the intersection of planar approximations to the topography upslope from the tributary junction along the two tributary directions. Previously published analyses of junction-angle data support this geometric model, yet junction angles have also been proposed to be controlled by climate and/or optimality principles (e.g., minimum-power expenditure). In this paper, we document a test of the Howard (1990) model using ~10⁷ fluvial network junctions in the conterminous U.S. and a portion of the Loess Plateau, China. Junction angles are consistent with the predictions of the Howard (1990) model when the orientations and slopes are computed using drainage basins rather than in the traditional way using valley-bottom segments near tributary junctions. When computed in the traditional way, junction angles are a function of slope ratios (as the Howard (1990) model) predicts, but data deviate from the Howard (1990) model in a manner that we propose is the result of valley-bottom meandering/tortuosity. We map the mean junction angles computed along valley bottoms within each 2.5 km x 2.5 km pixel of the conterminous U.S.A. and document lower mean junction angles in incised late-Cenozoic alluvial piedmont deposits compared to those of incised bedrock/older deposits. To understand how this finding relates to the geometric model of Howard (1990), we demonstrate that, for an idealized model of an initially unincised landform, i.e., a tilted plane with random microtopography, lower ratios of the mean microtopographic slope to the large-scale slope/tilt are associated with lower mean junction angles compared to landforms with higher such ratios. Using modern analogs, we demonstrate that unincised late-Cenozoic alluvial piedmonts likely had ratios of mean microtopographic slope to large-scale slope/tilt that were lower (i.e., ~1) prior to tributary drainage network development than the same ratios of bedrock/older deposits (≫1). This finding provides a means of understanding how the geometric model of Howard (1990) results in incised late Cenozoic alluvial piedmont deposits with lower mean tributary fluvial network junction angles, on average, compared to those of incised bedrock/older deposits. This work demonstrates that the topography of a landscape prior to fluvial incision exerts a key constraint on tributary fluvial network junction angles via a fundamental geometric principle that is independent of any climate- or optimality-based principle.

  PublisherDOI

Recent grants

• A workshop to assess and advance the prediction of land-surface response to climate and land-use changes

  NSF · $30k · 2013–2014

• Wind over rock: Quantifying the feedbacks among wind flow, bedrock erosion, and tectonic uplift

  NSF · $329k · 2013–2018

Frequent coauthors

• Craig Rasmussen

  42 shared

• Jon Chorover

  University of Arizona

  30 shared

• P. A. Troch

  University of Arizona

  30 shared

• Stephen B. DeLong

  United States Geological Survey

  27 shared

• Luke A. McGuire

  23 shared

• Marcel G. Schaap

  University of Arizona

  19 shared

• Guo‐Yue Niu

  University of Arizona

  19 shared

• P. D. Brooks

  University of Cambridge

  17 shared

Awards & honors

• 2015 Geological Society of America Fellow
• 2011 Galileo Circle Fellow, College of Science, University o…