Grantee Research Project Results
Final Report: Modeling of the Hydrochemical Response of High Elevation Watersheds to Climate Change and Atmospheric Deposition
EPA Grant Number: R834188Title: Modeling of the Hydrochemical Response of High Elevation Watersheds to Climate Change and Atmospheric Deposition
Investigators: Driscoll, Charles T. , Campbell, John L. , Pourmokhtarian, Afshin , Hayhoe, Katharine , Wu, Wei , Dong, Zheng
Institution: Syracuse University , University of Southern Mississippi , USDA , Towson University
Current Institution: Syracuse University , Towson University , USDA , University of Southern Mississippi
EPA Project Officer: Packard, Benjamin H
Project Period: August 1, 2009 through July 31, 2012 (Extended to July 31, 2014)
Project Amount: $800,000
RFA: Consequences of Global Change for Water Quality (2008) RFA Text | Recipients Lists
Research Category: Aquatic Ecosystems , Ecological Indicators/Assessment/Restoration , Watersheds , Water , Climate Change
Objective:
The impacts of changing climate on terrestrial ecosystems have been assessed by observational, gradient, laboratory and field studies. However, state-of-the-art biogeochemical models provide an important tool to investigate climatic perturbations to complex ecosystems.
The overarching goal for this study is to compare model calculations across high elevation watersheds in the United States with a range of climatic conditions to better understand and quantify how the quantity and quality of surface waters might respond to changing climate.
To achieve this goal, we used the hydrochemical model, PnET-BGC, driven by both past and future simulated climate, to assess the impact of climate change on watershed processes and water quantity and quality. Climate input scenarios are generated with well-documented statistical downscaling procedures based on simulations from outputs from the latest atmosphere ocean global circulation models (AOGCMs) available from the Intergovernmental Panel on Climate Change (IPCC). Using these climate input data, PnET-BGC was run at the 11 high elevation watersheds in the United States. This approach enabled us to evaluate hydrochemical responses of changing climate intensively at local scales and more broadly at regional and national scales. We worked with cooperating scientists from the intensive study sites to interpret model results and design additional analyses.
Summary/Accomplishments (Outputs/Outcomes):
The fully integrated coupled hydrological and biogeochemical model (PnET-BGC) was applied to evaluate the effects of climate change and increasing concentrations of atmospheric carbon dioxide at diverse, intensively studied, high-elevation watersheds and to evaluate aspects of these applications. Coarse scale projections from global circulation models were downscaled to local watersheds and applied as inputs to the biogeochemical model, PnET-BGC.
Streamflow responses to past and projected future changes in climate at the Hubbard Brook Experimental Forest, New Hampshire
Climate change has the potential to alter streamflow, which has important ecological, economic, and social implications. In the northeastern United States, it is unclear how climate change may affect surface water supply, which is of critical importance in this densely populated region. The impact of climate change on the timing and quantity of streamflow was evaluated at small watersheds at the Hubbard Brook Experimental Forest in New Hampshire. The site is well suited for this analysis because of the availability of long-term meteorological and hydrological records for analyzing past trends, and ample data to parameterize and test hydrological models used to predict future trends. Future streamflow projections were developed with the forest ecosystem model, PnET-BGC, driven by climate change scenarios from statistically downscaled atmospheric-ocean general circulation model outputs. It is projected that future climate change will cause earlier snowmelt, and with the diminishing snowpack is earlier timing and decreases in the magnitude of peak discharge associated with snowmelt at Hubbard Brook. Recent increases in precipitation have significantly increased annual water yield, a trend that is expected to continue under future climate change. Significant declines in evapotranspiration (loss of water to the atmosphere) have been observed over the long-term record, although the cause has not been identified. However, in the future evapotranspiration is expected to increase in response to a warmer and wetter environment. Future projections suggest that these increases in evapotranspiration largely offset increases in precipitation resulting in relatively little change in streamflow. Future work should aim to reduce uncertainty in the climate projections, particularly for precipitation, obtain a better understanding of the effect of carbon dioxide on vegetation, determine how climate change will influence tree species composition, and assess the impacts of changing hydrology on down-stream water supplies.
Modeling Potential Hydrochemical Responses to Climate Change and Increasing carbon dioxide at the Hubbard Brook Experimental Forest Using a Dynamic Biogeochemical Model (PnET-BGC)
Dynamic hydrochemical models are useful tools for understanding and projecting the interactive effects of climate change, atmospheric carbon dioxide, and atmospheric deposition on the hydrology and water quality of forested watersheds. We used the biogeochemical model, PnET-BGC, to evaluate the effects of potential future changes in temperature, precipitation, solar radiation, and atmospheric carbon dioxide on pools, concentrations, and fluxes of major elements at the Hubbard Brook Experimental Forest in New Hampshire, United States. Future climate projections used to run PnET-BGC were generated specifically for the Hubbard Brook Experimental Forest with a statistical technique that downscales climate output (e.g., air temperature, precipitation, solar radiation) from atmosphere-ocean general circulation models (AOGCMs) to a finer temporal and spatial resolution. These climate projections suggest that over the twenty-first century, average air temperature will increase at the site by 1.7°C to 6.5°C with simultaneous increases in annual average precipitation ranging from 4 to 32 cm above the long-term mean (1970–2000). PnET-BGC simulations under future climate change show a shift in hydrology characterized by later snowpack development, earlier spring discharge (snowmelt), greater evapotranspiration, and a slight increase in annual water yield (associated with carbon dioxide effects on vegetation). Model results indicate that under elevated temperature, net soil nitrogen mineralization and nitrification markedly increase, resulting in acidification of soil and stream water, thereby altering the quality of water draining from forested watersheds. Invoking a carbon dioxide fertilization effect on vegetation under climate change substantially mitigates watershed nitrogen loss, highlighting the need for a more thorough understanding of carbon dioxide effects on forest vegetation.
The Influence of Downscaling Models and Observations on Modeled Future Hydrochemistry of Forest Watersheds
Most projections of climate change impacts on ecosystems rely on multiple climate model projections, but utilize only one downscaling approach trained on one set of observations. We explored the extent to which modeled biogeochemical responses to changing climate are affected by the selection of the climate downscaling method and training observations used at the Hubbard Brook Experimental Forest, New Hampshire, USA. We evaluated three different downscaling methods: the delta method (or the “change factor method”); monthly quantile mapping (Bias Correction-Spatial Disaggregation, or BCSD); and daily quantile regression (Asynchronous Regional Regression Model, or ARRM). Additionally, we trained outputs, from four atmosphere-ocean global circulation models (CCSM3, HadCM3, PCM, and GFDL-CM2.1) driven by higher (A1fi) and lower (B1) future emission scenarios, on two sets of observations (1/8th degree resolution grid vs. individual weather station) to generate the high-resolution climate input for the hydrochemical model PnET-BGC (ensemble of 48 runs). The choice of downscaling approach and spatial resolution of the observations used to train the downscaling model both had a major impact on modeled soil moisture and streamflow, which in turn affected forest growth, net nitrification and stream chemistry. Specifically, the delta method, the simplest downscaling approach evaluated, was highly sensitive to the observations used, resulting in projections that were significantly different than those simulated with the BCSD and ARRM methods. Using spatially smoothed gridded observations and/or methods that do not resolve sub-monthly shifts in the distribution of temperature and/or precipitation can produce poor results in model applications run at higher temporal and/or spatial resolutions. These results underscore the importance of carefully considering the observations and downscaling method used to generate climate change projections for smaller scale modeling studies.
Cross-Site Analysis of Seven Headwater Watersheds in the Northeastern U.S.
Considering the dynamic nature of climate change both in time and space, it is challenging to generalize the long-term climatic shifts across the Northeast. Nevertheless, comparing and contrasting an array of watersheds with a wide range of characteristics provide important insights on the potential range of responses across diverse ecosystems. We conducted cross-site analysis for seven headwater watersheds in the northeastern U.S. with different forest ecosystem types, range of climate conditions, historical land disturbance (e.g., clear cut, fire, ice storm) and biophysical characteristics (e.g., climate, latitude, longitude, elevation, different vegetation, soil types). Results indicate that vegetation plays an important role since it regulates the hydrological and biogeochemical cycles. All forest watershed sites are projected to significantly increase evapotranspiration under future climate change due to warmer temperatures and an extended growing season. Model projections for sites where snow is currently prevalent, indicate that the extent of snowpack accumulation will diminish significantly or disappear by the end of the 21st century. This change could impact local economies and businesses that depend on winter recreational activities, and put pressure on water supplies. The future streamflow projections are variable across sites with some showing significant increases in annual water yield, while at others decreases are anticipated. As a result, forests across the Northeast and their downstream urban centers could face risk of both increased flooding and drought. This variability in climate response challenges policy makers and water resource and forest managers. Model simulations project that under climate change, northern hardwood forests will experience drought and water stress during the growing season, which affect their productivity and the rate of carbon, nitrogen and nutrient assimilation. In contrast, the spruce-fir forests are susceptible to temperature stress due to their lower optimum temperature for photosynthesis than hardwood forests.
Table: Summary of climate projections of change in annual air temperature and annual precipitation from statistically downscaled atmospheric-ocean global circulation model output for watersheds considered in this study. The value shown for each scenario is the difference between the mean of measured values for the reference period (1970-2000) and the simulation period 2070-2100. Note that for sites that do not have measured values for the entire period of 1970-2000, measured data for a shorter period are used.
We also provided a new framework for assessing climate change impacts based on incremental projected increases in temperature. Across all sites, model projections of decreases in soil carbon and nitrogen pools showed a strong negative relationship with temperature. The responses of other state variables (e.g., net primary production, streamwater discharge) are nonlinear due to effects of precipitation quantity and soil water and linkages with temperature change. Nevertheless, this framework could help guide policy makers and managers to make appropriate decisions to mitigate effects and ensure the continuation of ecosystem services and facilitate adaptation to changing climate. Finally, it is essential that experimental forested watersheds such as those investigated be maintained and preserved. Long-term monitoring and measurements of meteorology, biomass, hydrology, and stream chemistry at these intensive sites is not only necessary for model parameterization and testing but it is essential to detect climate change signals over time.
Vegetation and Hydrochemical Response to Projected Future Changes in Climate at the Coweeta Basin, North Carolina
Climate change will impact ecosystem structure and function, such as the primary productivity, soil processes, hydrologic response of watersheds, and the composition of surface water. The degree of impact is difficult to quantify and generalize due to the complex and nonlinear interactions among the ecosystem processes involved, and their significant spatial and temporal heterogeneities. We selected Coweeta Hydrological Laboratory as a study site in the Southeast, due to the availability of long-term meteorological, hydrological, vegetation, soil and chemistry data, making the model parameterization and predictions more robust than the other locations.
Future climate projections used to run PnET-BGC were generated specifically for the Coweeta Basin with a statistical technique that downscales climate output (e.g., air temperature, precipitation, solar radiation) from global atmosphere-ocean general circulation models (AOGCMs) to finer watershed scale. These climate projections suggest that over the twenty-first century, average air temperature will increase at the site by 1.4°C to 6.2°C with simultaneous increases in annual average precipitation ranging from 1.0 cm to 35.3 cm above the long-term mean (1970–2000).
The impact of climate change on vegetation shows strong seasonal variability. In late fall, winter and early spring (November – March), both gross and net primary productivity are predicted to increase under the changing climate scenarios due to milder temperature. In contrast, they show decreases in the other seasons (April – October) with the largest decreases occurring in summer (June to August) due to combined stresses of water and temperature. Respiration rate of vegetation is projected to increase across all the seasons due to higher temperature under climate change compared to the base climate scenario.
The overall uptake of nitrogen and other nutrients (magnesium, potassium, calcium, and sulfate) by vegetation is projected to decrease under climate change due to lower primary production in growing seasons. Furthermore, vegetation tends to assimilate much more ammonium than nitrate under future climate change compared to the base climate scenario, resulting in more nitrate in the soil and potential acidification of soil and water. Stream discharge is projected to increase under climate change due to higher precipitation, with higher frequency of flooding events (> 30 cm/month) in winter and more frequent drought events (< 1.5 cm/month) in summer. The general increase of discharge shows a dilution effect on calcium concentration in stream water. As the magnitude of magnesium, potassium and sulfate leaching is greater than the increase of discharge, their streamwater losses are projected to increase under climate change. With these changes in the processing of nutrients, the ability of Coweeta watersheds to neutralize acid deposition is projected to decrease in spring (April – June) under climate change, which could affect stream biota substantially as spring is onset of growing season for a variety of species.
The complex interactions among vegetation, soil and surface water show nonlinear and unbalanced effects of climate change on different ecosystem processes at different seasons, and the application of the PnET-BGC largely facilitated the projections of the impacts.
Streamflow and Net Primary Production Responses to Future Climate Change in High-Elevation Watersheds at Niwot Ridge and Loch Vale, Colorado and Andrews Experimental Forest, Oregon
Climate projections were made for three intensively studied watersheds in the West, including Niwot Ridge and Loch Vale, Colorado and Andrews Experimental Forest, Oregon. Effects of climate change on streamflow in high-elevation alpine-tundra and subalpine forest watersheds of the Rockies are distinct from those in northern hardwood forests in the East, which are due to a combination of differences in the inherent climate, vegetation functional traits and topography of those watersheds. The 220-ha Green Lake 4 watershed at Niwot Ridge and the 660-ha Loch Vale watershed are both located above 3000 m in elevation and have relief of 600 m and 1000 m respectively. Marked increases in temperature and modest increases in precipitation are projected for both sites. It is projected that future climate change will prolong the snowmelt period due to the melting of ice and snowpack at the colder high-elevation area within the watersheds. Future increases in temperature will extend the length of growing season for alpine tundra, which is expected to increase evapotranspiration and decrease peak discharge in summer. However, this increase in evapotranspiration is not enough to offset the increase in snowmelt, and future annual discharge is projected to increase at Niwot Ridge and Loch Vale. The response of aboveground net primary production to future climate change in alpine tundra sites varies depending on atmospheric-ocean global circulation model (AOGCM) considered and the magnitude of climate change projected. Most AOGCM projections indicate increasing future aboveground net primary production in alpine tundra sites due to a combination of increases in temperature and the extended growing season.
Projections of future climate change in Watershed 2 (undisturbed, old-growth, Douglas fir forest) at the Andrews Experimental Forest suggest increases in temperature coupled with slight increases in precipitation. Projections of streamflow are characterized by later snow development and earlier snowmelt that result from increasing temperature. Spring streamflow will decrease due to the decreasing depth of snow pack in winter. Projections of future annual discharge at Andrews Forest vary depending on AOGCM considered, with most models projecting increases in annual streamflow. Decreases in foliar biomass are projected at Andrews Forest caused by severe increases in summer drought.
Conclusions:
Future climate projections suggest increases in temperature and mostly increases in precipitation will occur at high elevation watersheds in the United States over the next century.
Projections of effects of changing climate on the hydrochemistry response of watersheds are challenged by uncertainty in emission scenarios, in projections made by atmosphere-ocean global circulation models (AOGCM), by approaches to downscale coarse scale AOGCM climate projections to local scale watersheds in complex terrain and limitations of the hydrochemical model used to make watershed simulations.
In the Northeast, future climate change is projected to decrease snowpack accumulation in high elevation forest watersheds, resulting a pattern of earlier stream discharge. In addition there will be an extension of the growing season, which coupled with increases in temperature will increase evapotranspiration losses. These shifts result in decreases in soil moisture and more frequent drought stress to forest vegetation during the summer growing season. This loss of vegetation function enhances soil mineralization, decreasing soil organic matter and increasing stream losses of nitrogen. This general forest ecosystem response to climate projections varies across the landscape and the region. Greater projected increases in precipitation and a fertilization effect of increases in forest production associated with increases in atmospheric carbon dioxide mitigate this hydrochemical response.
Limited model applications to the Southeast and West, show variations on the responses evident for the Northeast. For example, at Coweeta, North Carolina increases in summer drought stress and extreme variation in hydrologic events are projected under future climate conditions. In the Mountain West of Colorado shifts in stream hydrology are projected, but annual discharge is expected to increase despite increases in evapotranspiration due to melting of accumulations of snow and ice in the basins. The hydrochemical response for Andrews Forest in the Pacific Northwest is more similar to those projected for the Northeast due to the projection of the loss of seasonal snowpack. Note it is difficult to generalize a climate change response for regions outside of the Northeast on the basis of this research, because the model was applied to few sites in other regions.
Journal Articles on this Report : 5 Displayed | Download in RIS Format
| Other project views: | All 26 publications | 7 publications in selected types | All 6 journal articles |
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Campbell JL, Driscoll CT, Pourmokhtarian A, Hayhoe K. Streamflow responses to past and projected future changes in climate at the Hubbard Brook Experimental Forest, New Hampshire, United States. Water Resources Research 2011;47(2):W02514 (15 pp.). |
R834188 (2010) R834188 (2011) R834188 (2013) R834188 (Final) |
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Pourmokhtarian A, Driscoll CT, Campbell JL, Hayhoe K. Modeling potential hydrochemical responses to climate change and increasing CO2 at the Hubbard Brook Experimental Forest using a dynamic biogeochemical model (PnET-BGC). Water Resources Research 2012;48(7):W07514 (13 pp.). |
R834188 (2011) R834188 (2012) R834188 (2013) R834188 (Final) |
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Pourmokhtarian A, Driscoll CT, Campbell JL, Hayhoe K, Stoner AM. The effects of climate downscaling technique and observational data set on modeled ecological responses. Ecological Applications 2016;26(5):1321-1337. |
R834188 (Final) |
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Pourmokhtarian A, Driscoll CT, Campbell JL, Hayhoe K, Stoner AM, Adams MB, Burns D, Fernandez I, Mitchell MJ, Shanley JB. Modeled ecohydrological responses to climate change at seven small watersheds in the northeastern United States. Global Change Biology 2017;23(2):840-856. |
R834188 (Final) |
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Wu W, Driscoll CT. Impact of climate change on three-dimensional dynamic critical load functions. Environmental Science & Technology 2010;44(2):720-726. |
R834188 (Final) |
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Supplemental Keywords:
Climate change, evapotranspiration, hydrology, stream nutrient loss, nutrient uptake, primary productivity, soil organic matter, stream flow quantity and distribution, watersheds , RFA, Air, Atmosphere, Air Pollution Effects, climate change, Global Climate Change, hydrologic models, environmental monitoringProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.