Grantee Research Project Results
2011 Progress 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
Current 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 Period Covered by this Report: August 1, 2010 through July 31,2011
Project Amount: $800,000
RFA: Consequences of Global Change for Water Quality (2008) RFA Text | Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , Climate Change , Watersheds , Aquatic Ecosystems , Water
Objective:
Our overarching goal for this proposed 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 are using the hydrochemical model, PnET-BGC, driven by both past and future simulated climate, to assess the impact of climate change on water quantity and quality. Climate input scenarios are generated with a well-documented statistical downscaling procedure based on simulations from the latest AOGCM outputs available from the Intergovernmental Panel on Climate Change (IPCC) Working Group 1. Using these climate input data, PnET-BGC are being run at the 14 high elevation watersheds. This approach will enable us to evaluate hydrochemical responses intensively at local scales and more broadly at regional and national scales. We are working with cooperating scientists from the intensive study sites to interpret model results and design additional analyses.
Progress Summary:
The modeling results for Hubbard Brook Experimental Forest (HBEF), NH, suggest that spring (April-June) snowmelt will occur earlier and will be less extreme in the future. Low flows associated with enhanced evapotranspiration during the growing season, will extend earlier into the spring and later into the fall (October-December). Future streamflow in late fall and early winter (January-March) will increase because of less snowpack accumulation due to warmer air temperatures and concurrent declines in the ratio of snow to rain. Hydrologic simulations suggest substantial changes in the HBEF water budget, with shifts in seasonal and annual hydrology. These changes also alter biogeochemistry cycles and fluxes of relevant chemical elements in watershed. Under scenarios of climate change with carbon dioxide (CO2) effects on vegetation, the average annual runoff increased slightly in comparison with climate change alone. Under elevated CO2, changes in stomatal conductance decreased canopy transpiration that offset higher evaporation in the presence of higher temperatures. Although modeling results suggest increases in annual runoff, higher temperatures and an extended season of enhanced growth offset this increase to some extent due to increases in evapotranspiration.
Under PnET-BGC model runs without CO2 effects, warmer temperatures in the future caused a decrease in soil moisture and increase in vapor pressure deficit, despite the increase in precipitation. These factors decrease evapotranspiration and cause midsummer drought stress, the extent of which is dependent on the climate change scenario considered. Although wood net primary production (NPP) increased due to warmer temperatures and a longer growing season, repeated midsummer drought is projected to decrease maximum leaf area index, foliar NPP and litterfall and fine root NPP [Aber and Federer, 1992; Campbell et al., 2009, 2011]. Overall, these changes translate into less carbon (C) sequestration in foliage and fine roots, and more in wood. Because of slower decomposition rates associated with woody litter, the model simulates a decrease in C transfer to humus. The increase in wood NPP does not offset the decline in the litter inputs (sum of leaf litterfall and fine roots) to the soil organic matter (SOM) pool.
For model runs that considered CO2 effects, plant water use efficiency (WUE) increased and midsummer drought did not occur appreciably except under the two warmest scenarios (HadCM3-A1fi and GFDL-A1fi). The effect of elevated CO2 on stomatal conductance and increase in WUE offset the effect of higher temperatures by enhanced tree growth and higher nutrient uptake. Over the second half of this century under the two warmest scenarios (HadCM3-A1fi and GFDL-A1fi), the CO2 effect on vegetation was not able to offset the effect of temperature; midsummer droughts and water stress caused less uptake of nitrogen (N) and elevated availability of N followed by nitrification and elevated leachate of nitrate (NO3-). Increases in atmospheric CO2 resulted in increased tree growth and limited NO3- leaching over the first half of the 21st century, while tree growth remained constant or decreased over the second half of the century because of water stress. This pattern is due to the nonlinear response of photosynthesis to increasing atmospheric CO2. Over time, and especially under higher CO2 emission scenarios and warmer temperatures, the CO2 fertilization effect declines and N saturation occurs, as temperature becomes the dominant driver of N cycling. This work suggests that the legacy of accumulation of elevated N deposition in forest watersheds downwind of emission sources could have delayed deleterious effects on soil and surface water. If stores of N are mineralized under changing climate, the consequences of elevated NO3- leaching could be realized.
PnET-BGC simulations suggest that dissolved organic carbon (DOC) will decrease over the 21st century under all climate change scenarios. This modeled decline in DOC is associated with a decline in litterfall and decreased soil C mineralization rates. The trends in streamwater DOC were modified under climate change in the presence of CO2 fertilization. The higher productivity of the forest (NPP and net ecosystem production) due to CO2 fertilization increased litterfall in comparison to values from model simulations without CO2 effects on vegetation. An increase in the decomposition of the organic matter pool triggered by higher temperatures led to higher DOC concentrations in streamwater. Note that when CO2 effects on vegetation were included in the simulations, large increases in stream DOC were not evident. Our model simulations would seem to be inconsistent with the hypothesis that climate change is driving increases in surface water DOC.
The watershed responses of other major elements such as calcium (Ca2+) and chemical characteristics such as pH to changes in climate follow the same patterns as NO3-. The PnET-BGC simulations suggest that model projections also suggest marked decreases in soil exchangeable calcium, magnesium and potassium with simultaneous decline in soil base saturation and Ca/Al ratio over this century due to changing climate. These changes are mainly attributed to elevated leaching losses of NO3-.
The model simulations indicated that climate change may alter the hydrologic cycle and the seasonality of stream discharge. Since drainage strongly influences element transport (Likens and Bormann, 1995), seasonal changes in discharge may also alter the seasonal patterns of chemical constituents. We assessed changes in seasonal patterns of concentrations of NO3-, Ca2+, pH, and acid neutralizing capacity (ANC) under all climate change scenarios with and without CO2 effects on vegetation over the period of 2070-2100 and compared these with the seasonal patterns of measured values from 1970 to 2000. The timing, patterns and magnitude of streamwater NO3- concentrations are highly variable depending on the climate scenarios used. Since NO3- is the main driver of acid-base status of the ecosystem, Ca2+, pH, and ANC follow similar patterns. These results suggest that as climate change will likely alter the overall element concentrations and fluxes; these changes will be manifested in the seasonal patterns of element concentrations and fluxes and the future occurrence of these changes.
Annual element mass balances for each future climate change scenario were calculated to assess patterns in the fluxes and pools of major elements (NH4+-N, NO3--N, C, Ca2+, Al) and associated processes depicted in PnET-BGC. The mass balances show that increases in streamwater NO3- associated with higher temperature were mainly due to higher rates of N mineralization and nitrification. While ammonium (NH4+) uptake by vegetation declined slightly, NO3- uptake greatly increased, resulting in an increase in total N assimilated by plants and a decrease in the pool of N in humus. Mobilization of aluminum (Al) from soil was enhanced due to acidification caused by high NO3- concentrations. Mineralization of C, without considering CO2 effects on vegetation, decreased compared to the reference period, causing decreases in the humus C pool while the amount of C sequestrated in vegetation increased substantially. Uptake of Ca by vegetation declined, as did the humus and soil exchangeable pools; however, the total pool of Ca in plants increased. Note this work is detailed in Pourmokhtarian et al. (in review).
Cross site analysis between different watersheds indicated that historical land disturbances coupled with climate variability will influence future responses of watershed to climate change, and variability in hydrochemical response across sites is due to historical land disturbances and soil and geological characteristics.
In addition to the HBEF, model simulations are also being conducted for Huntington Forest, NY; Sleepers River, VT; Cone Pond, NH; Bear Brook, ME, Niwot Ridge, CO; and the Smoky Mountain National Park, TN, NC.
Journal Articles on this Report : 2 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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Supplemental Keywords:
Air, RFA, climate change, air pollution effects, atmosphere, global climate change, hydrologic models, atmospheric models, RFA, Air, climate change, Air Pollution Effects, Atmosphere, environmental monitoring, hydrologic models, atmospheric modelsProgress 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.