I am a Research Hydrologist in the USGS Colorado Water Science Center and Affiliate Faculty member of the Department of Geosciences at Colorado State University.
I investigate snow and hydrological processes in mountainous environments of the United States that are critically important for understanding water resources and availability for the nation. My current research uses a combination of field-based measurements, remote sensing observations, and physically based modeling over a range of spatial scales to better understand the spatial and temporal variability of snow water resources and how changes in snow processes are linked with changes to water availability, water budgets, and water quality. I received my PhD in Watershed Science from Colorado State University in 2016 and studied the importance of snow sublimation to seasonal snowpack variability. A list of my Science and Products are provided below and can also be viewed on my Google Scholar page.
Science and Products
Rocky Mountain Regional Snowpack Chemistry Monitoring Study
Estimating the Future Effects of Forest Disturbance on Snow Water Resources in a Changing Environment
Linking water, carbon, and nitrogen cycles in seasonally snow-covered catchments under changing land resource conditions
Snowpack Sublimation - Measurements and Modeling in the Colorado River Basin
High Resolution Current and Future Climate SnowModel Simulations in the Upper Colorado River Basin
Basin Characteristics and Streamflow Statistics for Selected Gages, Alaska, USA (ver. 2.0, September, 2022)
Historical simulated snowpack for the Lake Sherburne, MT watershed and vicinity, water years 1980-2019
National Hydrologic Model Alaska Domain parameter database, version 1
SnowModel simulations and supporting observations for the Rio Grande Headwaters, southwestern Colorado, United States, 1984 - 2017
Climatological data for the Loch Vale watershed in Rocky Mountain National Park, Colorado, water years 1992-2019
Geospatial Fabric for the National Hydrologic Model Alaska Domain, version 1
Data release in support of Runoff sensitivity to snow depletion curve representation within a continental scale hydrologic model
SnowModel simulations and supporting observations for the north-central Colorado Rocky Mountains during water years 2011 through 2015
High resolution SnowModel simulations reveal future elevation-dependent snow loss and earlier, flashier surface water input for the Upper Colorado River Basin
Upper Rio Grande Basin water-resource status and trends: Focus area study review and synthesis
Evaluating hydrologic region assignment techniques for ungaged basins in Alaska, USA
Black carbon dominated dust in recent radiative forcing on Rocky Mountain snowpacks
Snow depth retrieval with an autonomous UAV-mounted software-defined radar
Spatial variability in seasonal snowpack trends across the Rio Grande headwaters (1984 - 2017)
Changes in climate and land cover affect seasonal streamflow forecasts in the Rio Grande headwaters
Runoff sensitivity to snow depletion curve representation within a continental scale hydrologic model
Relating hydroclimatic change to streamflow, baseflow, and hydrologic partitioning in the Upper Rio Grande Basin, 1980 to 2015
Preferential elution of ionic solutes in melting snowpacks: Improving process understanding through field observations and modeling in the Rocky Mountains
A new sampler for the collection and retrieval of dry dust deposition
Science needs for continued development of total nitrogen deposition budgets in the United States
Science and Products
- Science
Rocky Mountain Regional Snowpack Chemistry Monitoring Study
Snowpacks collect atmospheric deposition throughout the snowfall season and offer a unique opportunity to obtain a composite sample of the chemistry of most of the annual precipitation at high elevations [> 1,800 meters]. The purpose of the snowpack network is to determine annual concentrations and depositional amounts of selected nutrients and other constituents in snow resulting from atmospheric...Estimating the Future Effects of Forest Disturbance on Snow Water Resources in a Changing Environment
In the Western U.S., approximately 65% of the water supply comes from forested regions with most of the water that feeds local rivers coming from snowmelt that originates in mountain forests. The Rio Grande headwaters (I.e. the primary water generating region of the Rio Grande river) is experiencing large changes to the landscape primarily from forest fires and bark beetle infestations. Already, 8Linking water, carbon, and nitrogen cycles in seasonally snow-covered catchments under changing land resource conditions
Changes in snowpack accumulation, distribution, and melt in high-elevation catchments are likely to have important impacts on water, carbon, and nitrogen cycles, which are tightly coupled through exchanges of energy and biogeochemical compounds between atmospheric, terrestrial, and aquatic environments. Our research helps to better understand how changes in climate will affect water availability...Snowpack Sublimation - Measurements and Modeling in the Colorado River Basin
Snow is an essential resource in the western United States (U.S.), providing water for drinking, irrigation, industry, energy production, and ecosystems across much of the region. In the mountains of the western U.S., most precipitation falls as snow, which accumulates in seasonal snowpacks that serve as a large natural reservoir. Snowpack sublimation, which is analogous to evaporation from land... - Data
High Resolution Current and Future Climate SnowModel Simulations in the Upper Colorado River Basin
This data release contains SnowModel snow evolution simulation output on a 100-meter (m) geospatial grid for a 311 kilometer (km) × 300 km model domain in Colorado, United States, encompassing the Colorado and Gunnison River Basin headwaters in the Upper Colorado River Basin. Weather Research and Forecasting (WRF) Model convection-permitting and orography-resolving (4-km grid spacing) regional cliBasin Characteristics and Streamflow Statistics for Selected Gages, Alaska, USA (ver. 2.0, September, 2022)
This data release documents the data used for the associated publication "Evaluating hydrologic region assignment techniques for ungaged watersheds in Alaska, USA" (Barnhart and others, 2022) The data sets within this release are stored in 14 files: (1) Streamflow observations and sites used. (2) Statistically estimated streamflow values computed for each site. (3) Streamflow statisticsHistorical simulated snowpack for the Lake Sherburne, MT watershed and vicinity, water years 1980-2019
Abstract This data release contains historical SnowModel (Liston and Elder, 2006) output for the Lake Sherburne, MT watershed and surrounding area. The two quantities simulated for this release were snow water equivalent depth (swed), the liquid water equivalent depth stored as snow in the simulation domain, and runoff (roff), which includes snowmelt at the snow-soil interface and rainfall on pixeNational Hydrologic Model Alaska Domain parameter database, version 1
This data release contains input data for hydrologic simulations of the Alaska Domain application of the U.S. Geological Survey (USGS) Precipitation Runoff Modelling System (PRMS) as implemented in the National Hydrologic Model (NHM) infrastructure (Regan and others, 2018). The NHM Alaska Domain parameter database consists of 114 parameter files in ASCII format (CSV), two files needed to run the ASnowModel simulations and supporting observations for the Rio Grande Headwaters, southwestern Colorado, United States, 1984 - 2017
This data release supports the study by Sexstone and others (2020) and contains simulation output from SnowModel (Liston and Elder, 2006), a well-validated process-based snow modeling system. Simulations are for water years 1984 through 2017 (October 1, 1983 through September 30, 2017) across a 11,200 square kilometer model domain in the San Juan Mountains of southwestern Colorado, United States tClimatological data for the Loch Vale watershed in Rocky Mountain National Park, Colorado, water years 1992-2019
This data release contains hourly means of climatological data collected by the U.S. Geological Survey (USGS) from 10/1/1991 to 9/30/2019 at three weather stations in the Loch Vale watershed in Rocky Mountain National Park (RMNP), Colorado. In order of increasing elevation, the three weather stations are Loch Vale meteorological station at RMNP, Colo. (Main weather station, USGS station 4017191053Geospatial Fabric for the National Hydrologic Model Alaska Domain, version 1
This metadata record documents a geospatial dataset for the U.S. Geological Survey Precipitation Runoff Modeling System (PRMS) used to drive the National Hydrologic Model (NHM). The Alaska Geospatial Fabric v1 is the spatial representation of the hydrologic response units (HRUs) used for the PRMS NHM Alaska domain. These HRUs were generated using the twelve-digit Hydrologic Unit Code (HUC12) waterData release in support of Runoff sensitivity to snow depletion curve representation within a continental scale hydrologic model
This data release includes simulation output from a modeling experiment conducted using the initial calibration of the conterminous United States (CONUS) application of the Precipitation-Runoff Modeling System (PRMS) (Hay, 2019) as implemented in the National Hydrologic Model (NHM) infrastructure (Regan et al, 2018). The study associated with this data release (Sexstone et al., 2019) used the sameSnowModel simulations and supporting observations for the north-central Colorado Rocky Mountains during water years 2011 through 2015
This data release includes simulation output from SnowModel (Liston and Elder, 2006), a well-validated process-based snow modeling system, and supporting snow, meteorological, and streamflow observations from the water years 2011 through 2015 (October 1, 2010, through September 30, 2015) across a 3,600 square kilometer model domain in the north-central Colorado Rocky Mountains. For each water year - Multimedia
- Publications
Filter Total Items: 18
High resolution SnowModel simulations reveal future elevation-dependent snow loss and earlier, flashier surface water input for the Upper Colorado River Basin
Continued climate warming is reducing seasonal snowpacks in the western United States, where >50% of historical water supplies were snowmelt-derived. In the Upper Colorado River Basin, declining snow water equivalent (SWE) and altered surface water input (SWI, rainfall and snowmelt available to enter the soil) timing and magnitude affect streamflow generation and water availability. To adapt effecAuthorsJohn C. Hammond, Graham A. Sexstone, Annie Laura Putman, Theodore B. Barnhart, David Rey, Jessica M. Driscoll, Glen Liston, Kristen L. Rasmussen, Daniel McGrath, Steven R. Fassnacht, Stephanie K. KampfUpper Rio Grande Basin water-resource status and trends: Focus area study review and synthesis
The Upper Rio Grande Basin (URGB) is a critical international water resource under pressure from a myriad of climatic, ecological, infrastructural, water-use, and legal constraints. The objective of this study is to provide a comprehensive assessment of the spatial distribution and temporal trends of selected water-budget components (snow processes, evapotranspiration (ET), streamflow processes, aAuthorsKyle R. Douglas-Mankin, Christine Rumsey, Graham A. Sexstone, Tamara I. Ivahnenko, Natalie Houston, Shaleene Chavarria, Gabriel B. Senay, Linzy K. Foster, Jonathan V. Thomas, Allison K. Flickinger, Amy E. Galanter, C. David Moeser, Toby L. Welborn, Diana E. Pedraza, Patrick M. Lambert, Michael Scott JohnsonEvaluating hydrologic region assignment techniques for ungaged basins in Alaska, USA
Building continental-scale hydrologic models in data-sparse regions requires an understanding of spatial variation in hydrologic processes. Extending these models to ungaged locations requires techniques to group ungaged locations with gaged ones to make process importance and model parameter transfer decisions to ungaged locations. This analysis (1) tested the utility of fundamental streamflow stAuthorsTheodore B. Barnhart, William Farmer, John C. Hammond, Graham A. Sexstone, Janet H. Curran, Joshua C. Koch, Jessica M. DriscollBlack carbon dominated dust in recent radiative forcing on Rocky Mountain snowpacks
The vast majority of surface water resources in the semi-arid western United States start as winter snowpack. Solar radiation is a primary driver of snowmelt, making snowpack water resources especially sensitive to even small increases in concentrations of light absorbing particles such as mineral dust and combustion-related black carbon (BC). Here we show, using fresh snow measurements and snowpaAuthorsKelly Gleason, Joseph R. McConnell, Monica Arienzo, Graham A. Sexstone, Stefan RahimiSnow depth retrieval with an autonomous UAV-mounted software-defined radar
We present results from a field campaign to measure seasonal snow depth at Cameron Pass, Colorado, using a synthetic ultrawideband software-defined radar (SDRadar) implemented in commercially available Universal Software Radio Peripheral (USRP) software-defined radio hardware and flown on a small hexacopter unmanned aerial vehicle (UAV). We coherently synthesize an ultrawideband signal from steppeAuthorsS. Prager, Graham A. Sexstone, Daniel J McGrath, John Fulton, Mahta MoghaddamSpatial variability in seasonal snowpack trends across the Rio Grande headwaters (1984 - 2017)
This study evaluated the spatial variability of trends in simulated snowpack properties across the Rio Grande headwaters of Colorado using the SnowModel snow evolution modeling system. SnowModel simulations were performed using a grid resolution of 100 m and 3-hourly time step over a 34-yr period (1984–2017). Atmospheric forcing was provided by phase 2 of the North American Land Data AssimilationAuthorsGraham A. Sexstone, Colin A. Penn, Glen Liston, Kelly Gleason, C. David Moeser, David W. ClowChanges in climate and land cover affect seasonal streamflow forecasts in the Rio Grande headwaters
Seasonal streamflow forecast bias, changes in climate, snowpack, and land cover, and the effects of these changes on relations between basin‐wide snowpack, SNOw TELemetry (SNOTEL) station snowpack, and seasonal streamflow were evaluated in the headwaters of the Rio Grande, Colorado. Results indicate that shifts in the seasonality of precipitation and changing climatology are consistent with periodAuthorsColin A. Penn, David W. Clow, Graham A. Sexstone, Sheila F. MurphyRunoff sensitivity to snow depletion curve representation within a continental scale hydrologic model
The spatial variability of snow water equivalent (SWE) can exert a strong influence on the timing and magnitude of snowmelt delivery to a watershed. Therefore, the representation of subgrid or subwatershed snow variability in hydrologic models is important for accurately simulating snowmelt dynamics and runoff response. The U.S. Geological Survey National Hydrologic Model infrastructure with the PAuthorsGraham A. Sexstone, Jessica M. Driscoll, Lauren Hay, John C. Hammond, Theodore B. BarnhartRelating hydroclimatic change to streamflow, baseflow, and hydrologic partitioning in the Upper Rio Grande Basin, 1980 to 2015
Understanding how changing climatic conditions affect streamflow volume and timing is critical for effective water management. In the Rio Grande Basin of the southwest U.S., decreasing snowpack, increasing minimum temperatures, and decreasing streamflow have been observed in recent decades, but the effects of hydroclimatic changes on baseflow, or groundwater discharge to streams, have not been invAuthorsChristine Rumsey, Matthew P. Miller, Graham A. SexstonePreferential elution of ionic solutes in melting snowpacks: Improving process understanding through field observations and modeling in the Rocky Mountains
The preferential elution of ions from melting snowpacks is a complex problem that has been linked to temporary acidification of water bodies. However, the understanding of these processes in snowpacks around the world, including the polar regions that are experiencing unprecedented warming and melting, remains limited despite being instrumental in supporting climate change adaptation.In this studyAuthorsDiogo Costa, Graham A. Sexstone, J.W. Pomeroy, Donald H. Campbell, David W. Clow, Alisa MastA new sampler for the collection and retrieval of dry dust deposition
Atmospheric dust can influence biogeochemical cycles, accelerate snowmelt, and affect air, water quality, and human health. Yet, the bulk of atmospherically transported material remains poorly quantified in terms of total mass fluxes and composition. This lack of information stems in part from the challenges associated with measuring dust deposition. Here we report on the design and efficacy of aAuthorsJ. Brahney, Gregory A. Wetherbee, Graham A. Sexstone, C. Youngbull, P. Strong, Ruth C. HeindelScience needs for continued development of total nitrogen deposition budgets in the United States
The objectives of this white paper are to describe the state of the science with respect to total Nr deposition budgets in North America and the research needed to improve these budgets from both a measurement and modeling standpoint. The document is intended to serve as a plan for TDep research activities but also, more broadly, to provide program managers, natural resource managers, policy makeAuthorsGregory A. Wetherbee, Pamela H. Templar, Richard V. Pouyat, Stephen M. Decina, Brian M. Kerschner, Thomas H. Whitlow, Pamela E. Padgett, Donna B. Schwede, Jill Baron, David Clow, Alisa Mast, Graham A. Sexstone, Kristi H. Morris