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• 1. [Article] Twentieth-Century Trends in Runoff, Evapotranspiration, and Soil Moisture in the Western United States

  A physically based hydrology model is used to produce time series for the period 1916–2003 of evapotranspiration (ET), runoff, and soil moisture (SM) over the western United States from which long-term ...

  Citation × Citation Citation Abstract Copy Title: Twentieth-Century Trends in Runoff, Evapotranspiration, and Soil Moisture in the Western United States Author: Clark, Martyn P., Lettenmaier, Dennis P., Mote, Philip W., Hamlet, Alan F. A physically based hydrology model is used to produce time series for the period 1916–2003 of evapotranspiration (ET), runoff, and soil moisture (SM) over the western United States from which long-term trends are evaluated. The results show that trends in ET in spring and summer are determined primarily by trends in precipitation and snowmelt that determine water availability. From April to June, ET trends are mostly positive due primarily to earlier snowmelt and earlier emergence of snow-free ground, and secondarily to increasing trends in spring precipitation. From July to September trends in ET are more strongly influenced by precipitation trends, with the exception of areas (most notably California) that receive little summer precipitation and have experienced large changes in snowmelt timing. Trends in the seasonal timing of ET are modest, but during the period 1947–2003 when temperature trends are large, they reflect a shift of ET from midsummer to early summer and late spring. As in other studies, it is found that runoff is occurring earlier in spring, a trend that is related primarily to increasing temperature, and is most apparent during 1947–2003. Trends in the annual runoff ratio, a variable critical to western water management, are determined primarily by trends in cool season precipitation, rather than changes in the timing of runoff or ET. It was found that the signature of temperature-related trends in runoff and SM is strongly keyed to mean midwinter [December–February (DJF)] temperatures. Areas with warmer winter temperatures show increasing trends in the runoff fraction as early as February, and colder areas as late as June. Trends toward earlier spring SM recharge are apparent and increasing trends in SM on 1 April are evident over much of the region. The 1 July SM trends are less affected by snowmelt changes and are controlled more by precipitation trends. Title: Twentieth-Century Trends in Runoff, Evapotranspiration, and Soil Moisture in the Western United States Author: Clark, Martyn P., Lettenmaier, Dennis P., Mote, Philip W., Hamlet, Alan F. A physically based hydrology model is used to produce time series for the period 1916–2003 of evapotranspiration (ET), runoff, and soil moisture (SM) over the western United States from which long-term trends are evaluated. The results show that trends in ET in spring and summer are determined primarily by trends in precipitation and snowmelt that determine water availability. From April to June, ET trends are mostly positive due primarily to earlier snowmelt and earlier emergence of snow-free ground, and secondarily to increasing trends in spring precipitation. From July to September trends in ET are more strongly influenced by precipitation trends, with the exception of areas (most notably California) that receive little summer precipitation and have experienced large changes in snowmelt timing. Trends in the seasonal timing of ET are modest, but during the period 1947–2003 when temperature trends are large, they reflect a shift of ET from midsummer to early summer and late spring. As in other studies, it is found that runoff is occurring earlier in spring, a trend that is related primarily to increasing temperature, and is most apparent during 1947–2003. Trends in the annual runoff ratio, a variable critical to western water management, are determined primarily by trends in cool season precipitation, rather than changes in the timing of runoff or ET. It was found that the signature of temperature-related trends in runoff and SM is strongly keyed to mean midwinter [December–February (DJF)] temperatures. Areas with warmer winter temperatures show increasing trends in the runoff fraction as early as February, and colder areas as late as June. Trends toward earlier spring SM recharge are apparent and increasing trends in SM on 1 April are evident over much of the region. The 1 July SM trends are less affected by snowmelt changes and are controlled more by precipitation trends. Close

• 2. [Article] JonesJuliaCEOASChangingForestWater.pdf

  Citation × Citation Citation Abstract Copy Title: JonesJuliaCEOASChangingForestWater.pdf Author: Creed, Irena F., Jones, Julia A., et al., Spargo, Adam T. Title: JonesJuliaCEOASChangingForestWater.pdf Author: Creed, Irena F., Jones, Julia A., et al., Spargo, Adam T. Close

• 3. [Article] JonesJuliaCEOASChangingForestWater_TableS1.pdf

  Citation × Citation Citation Abstract Copy Title: JonesJuliaCEOASChangingForestWater_TableS1.pdf Author: Creed, Irena F., Jones, Julia A., et al., Spargo, Adam T. Title: JonesJuliaCEOASChangingForestWater_TableS1.pdf Author: Creed, Irena F., Jones, Julia A., et al., Spargo, Adam T. Close

• 4. [Article] Understanding ecohydrological connectivity in savannas: a system dynamics modelling approach

  To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. The published article is copyrighted by John Wiley & Sons, Ltd. and can be found at: ...

  Citation × Citation Citation Abstract Copy Title: Understanding ecohydrological connectivity in savannas: a system dynamics modelling approach Author: Bond, Barbara, Franz, Trenton E., Miller, Gretchen R., Cable, Jessica M., Zou, Chris B., Tyler, Anna P., McDonald, Alyson K., Scott, Russell L., Wang, Lixin, Gou, Si To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. The published article is copyrighted by John Wiley & Sons, Ltd. and can be found at: http:http://www.wiley.com/WileyCDA/ Title: Understanding ecohydrological connectivity in savannas: a system dynamics modelling approach Author: Bond, Barbara, Franz, Trenton E., Miller, Gretchen R., Cable, Jessica M., Zou, Chris B., Tyler, Anna P., McDonald, Alyson K., Scott, Russell L., Wang, Lixin, Gou, Si To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. The published article is copyrighted by John Wiley & Sons, Ltd. and can be found at: http:http://www.wiley.com/WileyCDA/ Close

• 5. [Article] Ecohydrological mediation of water budget partitioning and time scales of subsurface flow in a seasonally semi-arid grassland

  Understanding how the interactions and feedbacks between plant function, climate, and soils ultimately affects the terrestrial water balance and subsurface flow processes is major challenge in scientific ...

  Citation × Citation Citation Abstract Copy Title: Ecohydrological mediation of water budget partitioning and time scales of subsurface flow in a seasonally semi-arid grassland Author: Pangle, Luke A., 1981- Understanding how the interactions and feedbacks between plant function, climate, and soils ultimately affects the terrestrial water balance and subsurface flow processes is major challenge in scientific hydrology. This dissertation summarizes the findings of a manipulative climate warming experiment, an observational field study that utilized stable-isotope tracers, and associated modeling analyses that I used to examine the physiological and physical mechanisms by which grassland ecosystems mediate water-balance partitioning and transit times of water flowing through the subsurface. Utilizing the climate-controlled mesocosm experiment, I examined the responses of evapotranspiration, soil moisture, and potential groundwater recharge to a 3.5˚C temperature increase in a grassland ecosystem experiencing a Mediterranean climate. I hypothesized that a warmer climate would cause a shift in the soil-water balance toward greater evapotranspiration, and less recharge. The results showed that warming treatments enhanced evapotranspiration during the spring. However, this reduced soil moisture more rapidly, resulting in less evapotranspiration during the summer than occurred under ambient temperatures, and no difference when considered over the entire year. Groundwater recharge was reduced during late-spring storms relative to the ambient temperature treatment, but these reductions were a small fraction of the annual total, and were offset by slightly greater recharge in the fall under warming treatments. The results highlighted the potential for interactions between climate, vegetation, and soils to moderate the hydrological response to climate warming, particularly in environments where precipitation is seasonal and out of phase with the vegetation growing season. I conducted additional field studies that utilized three lysimeters with surface conditions ranging from bare soil through two stages of aggrading grassland vegetation. Using hydrometric data, stable-isotopes as conservative tracer, and two hydrograph-separation techniques I evaluated whether aggrading grassland vegetation and root systems alter time scales of subsurface flow, and how this alteration may influence potential groundwater recharge. I tested the hypothesis that soil structural change under aggrading vegetation would enhance the rapid infiltration of precipitation-event water, resulting in greater potential recharge during individual storms. Contrary to this expectation, results from both hydrograph-separation techniques showed that precipitation-event water comprised 0 - 6% of potential recharge among all the storm events I analyzed, being greatest under bare soil, and always zero in grasslands ranging in age from 3.8 - 5.9 years. These results contradicted my original hypothesis, and were attributed to the low intensity of local precipitation, large soil-water storage potential, and the predominantly shallow rooting tendency of the grassland vegetation. In a final analysis I used stable-isotope measurements and a linear-time-invariant convolution approach to model mean-transit times and transit-time distributions of subsurface flow under each surface condition, and over the entire water year. The results showed that mean-transit times were not significantly different in the presence of aggrading vegetation. From these analyses I concluded that physical alteration of the soil by aggrading plant root systems was not an ecohydrologically significant mechanism in this system. Ex post facto analyses showed that, at the time scale of individual storm events, the reduction of effective precipitation by interception and evaporative loss from the grassland canopies was of much greater importance--even with very low leaf area indices. At the annual time scale, root expansion enabled much greater exploitation of soil water during the summer drought, causing a shift from a recharge-dominated to an evapotranspiration-dominated soil-water balance within the first year of growth. Title: Ecohydrological mediation of water budget partitioning and time scales of subsurface flow in a seasonally semi-arid grassland Author: Pangle, Luke A., 1981- Understanding how the interactions and feedbacks between plant function, climate, and soils ultimately affects the terrestrial water balance and subsurface flow processes is major challenge in scientific hydrology. This dissertation summarizes the findings of a manipulative climate warming experiment, an observational field study that utilized stable-isotope tracers, and associated modeling analyses that I used to examine the physiological and physical mechanisms by which grassland ecosystems mediate water-balance partitioning and transit times of water flowing through the subsurface. Utilizing the climate-controlled mesocosm experiment, I examined the responses of evapotranspiration, soil moisture, and potential groundwater recharge to a 3.5˚C temperature increase in a grassland ecosystem experiencing a Mediterranean climate. I hypothesized that a warmer climate would cause a shift in the soil-water balance toward greater evapotranspiration, and less recharge. The results showed that warming treatments enhanced evapotranspiration during the spring. However, this reduced soil moisture more rapidly, resulting in less evapotranspiration during the summer than occurred under ambient temperatures, and no difference when considered over the entire year. Groundwater recharge was reduced during late-spring storms relative to the ambient temperature treatment, but these reductions were a small fraction of the annual total, and were offset by slightly greater recharge in the fall under warming treatments. The results highlighted the potential for interactions between climate, vegetation, and soils to moderate the hydrological response to climate warming, particularly in environments where precipitation is seasonal and out of phase with the vegetation growing season. I conducted additional field studies that utilized three lysimeters with surface conditions ranging from bare soil through two stages of aggrading grassland vegetation. Using hydrometric data, stable-isotopes as conservative tracer, and two hydrograph-separation techniques I evaluated whether aggrading grassland vegetation and root systems alter time scales of subsurface flow, and how this alteration may influence potential groundwater recharge. I tested the hypothesis that soil structural change under aggrading vegetation would enhance the rapid infiltration of precipitation-event water, resulting in greater potential recharge during individual storms. Contrary to this expectation, results from both hydrograph-separation techniques showed that precipitation-event water comprised 0 - 6% of potential recharge among all the storm events I analyzed, being greatest under bare soil, and always zero in grasslands ranging in age from 3.8 - 5.9 years. These results contradicted my original hypothesis, and were attributed to the low intensity of local precipitation, large soil-water storage potential, and the predominantly shallow rooting tendency of the grassland vegetation. In a final analysis I used stable-isotope measurements and a linear-time-invariant convolution approach to model mean-transit times and transit-time distributions of subsurface flow under each surface condition, and over the entire water year. The results showed that mean-transit times were not significantly different in the presence of aggrading vegetation. From these analyses I concluded that physical alteration of the soil by aggrading plant root systems was not an ecohydrologically significant mechanism in this system. Ex post facto analyses showed that, at the time scale of individual storm events, the reduction of effective precipitation by interception and evaporative loss from the grassland canopies was of much greater importance--even with very low leaf area indices. At the annual time scale, root expansion enabled much greater exploitation of soil water during the summer drought, causing a shift from a recharge-dominated to an evapotranspiration-dominated soil-water balance within the first year of growth. Close

• 6. [Article] Impact of exotic Ponderosa pine (Pinus ponderosa Doug. ex Laws.) plantations on water resources in northwestern Patagonia, Argentina.

  Exotic ponderosa pine (Pinus ponderosa Doug. ex Lawson) plantations are being planted within the natural distribution area of cordilleran cypress (Austrocedrus chilensis (D.Don) Pic. Ser. et Bizzarri) ...

  Citation × Citation Citation Abstract Copy Title: Impact of exotic Ponderosa pine (Pinus ponderosa Doug. ex Laws.) plantations on water resources in northwestern Patagonia, Argentina. Author: Licata, Julian A. Exotic ponderosa pine (Pinus ponderosa Doug. ex Lawson) plantations are being planted within the natural distribution area of cordilleran cypress (Austrocedrus chilensis (D.Don) Pic. Ser. et Bizzarri) in Patagonia, Argentina. The productivity of these exotic plantations is much greater than that of native forests, suggesting greater water use. Before these plantations become widespread, it is crucial to determine what effects ponderosa pine plantations may have on scarce water resources in the region. In the first study, I examined the seasonal variation in transpiration and soil water depletion for four forest stands: high and low density ponderosa pine plantations, and high and low density cordilleran cypress stands. Measurements were taken during two growing seasons with contrasting amounts of precipitation. Transpiration was greater in ponderosa pine than in cordilleran cypress stands of similar density. The high density ponderosa pine plot used a greater amount of water from deeper soil layers compared to all the other plots. In the second study, I conducted measurements of above- and below-canopy rainfall, and water storage capacity of branches to estimate the proportion of water that it is intercepted by the canopy and evaporates back into the atmosphere without reaching the soil. A simple analytical model was used to estimate annual interception losses by the four forest stands. Cordilleran cypress stands showed much larger interception losses than ponderosa pine stands of similar density. Thus, the negative effects of increased transpiration in ponderosa pine may be offset by their smaller interception losses when compared to cordilleran cypress. In the third study, I used a detailed process-based model to close the annual water budget of the stands, and evaluate the balance between ET and precipitation under several climate change scenarios, based on the projections made in the IPCC’s fourth assessment report. Model predictions suggested that high density ponderosa pine plantations are not hydrologically sustainable (ET > precipitation) under current conditions, and that none of the high density stands would be hydrologically sustainable under future scenarios. Management of stand density in both native cordilleran cypress and ponderosa pine plantations would be necessary to reduce the impacts of future climate change on water resources. Title: Impact of exotic Ponderosa pine (Pinus ponderosa Doug. ex Laws.) plantations on water resources in northwestern Patagonia, Argentina. Author: Licata, Julian A. Exotic ponderosa pine (Pinus ponderosa Doug. ex Lawson) plantations are being planted within the natural distribution area of cordilleran cypress (Austrocedrus chilensis (D.Don) Pic. Ser. et Bizzarri) in Patagonia, Argentina. The productivity of these exotic plantations is much greater than that of native forests, suggesting greater water use. Before these plantations become widespread, it is crucial to determine what effects ponderosa pine plantations may have on scarce water resources in the region. In the first study, I examined the seasonal variation in transpiration and soil water depletion for four forest stands: high and low density ponderosa pine plantations, and high and low density cordilleran cypress stands. Measurements were taken during two growing seasons with contrasting amounts of precipitation. Transpiration was greater in ponderosa pine than in cordilleran cypress stands of similar density. The high density ponderosa pine plot used a greater amount of water from deeper soil layers compared to all the other plots. In the second study, I conducted measurements of above- and below-canopy rainfall, and water storage capacity of branches to estimate the proportion of water that it is intercepted by the canopy and evaporates back into the atmosphere without reaching the soil. A simple analytical model was used to estimate annual interception losses by the four forest stands. Cordilleran cypress stands showed much larger interception losses than ponderosa pine stands of similar density. Thus, the negative effects of increased transpiration in ponderosa pine may be offset by their smaller interception losses when compared to cordilleran cypress. In the third study, I used a detailed process-based model to close the annual water budget of the stands, and evaluate the balance between ET and precipitation under several climate change scenarios, based on the projections made in the IPCC’s fourth assessment report. Model predictions suggested that high density ponderosa pine plantations are not hydrologically sustainable (ET > precipitation) under current conditions, and that none of the high density stands would be hydrologically sustainable under future scenarios. Management of stand density in both native cordilleran cypress and ponderosa pine plantations would be necessary to reduce the impacts of future climate change on water resources. Close

• 7. [Article] Changing forest water yields in response to climate warming: results from long-term experimental watershed sites across North America

  To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by the author(s) ...

  Citation × Citation Citation Abstract Copy Title: Changing forest water yields in response to climate warming: results from long-term experimental watershed sites across North America Author: Creed, Irena F., Jones, Julia A., et al., Spargo, Adam T. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by John Wiley & Sons Ltd. The published article can be found at: http://onlinelibrary.wiley.com/journal/10.1111/%28ISSN%291365-2486. Title: Changing forest water yields in response to climate warming: results from long-term experimental watershed sites across North America Author: Creed, Irena F., Jones, Julia A., et al., Spargo, Adam T. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by John Wiley & Sons Ltd. The published article can be found at: http://onlinelibrary.wiley.com/journal/10.1111/%28ISSN%291365-2486. Close

• 8. [Article] Trends in streamflow from old growth forested watersheds in the western Cascades

  Increasing temperatures in western North America are expected to result in a decline in winter snowpacks, earlier snowmelt, and a shift in the timing of streamflows, with an increasing fraction of streamflows ...

  Citation × Citation Citation Abstract Copy Title: Trends in streamflow from old growth forested watersheds in the western Cascades Author: Moore, Kathleen M Increasing temperatures in western North America are expected to result in a decline in winter snowpacks, earlier snowmelt, and a shift in the timing of streamflows, with an increasing fraction of streamflows occurring earlier in the water year and drier conditions during the summer. However, few streamflow datasets have associated climate and vegetation records adequate to interpret changes in climate, forest processes, and their consequences for streamflows. This study examined long-term streamflow records from three headwater watersheds in old growth forest at the H. J. Andrews Experimental Forest, Oregon, to seek evidence of trends and investigate possible explanations for these changes. The three small (8.5-60 ha) watersheds (WS2, WS8, and WS9) range in elevation from 432-1182 m and have streamflow records dating back to 1953, 1964, and 1969 respectively. Spring and summer average minimum temperatures have increased over the study period, while April 1st SWE at Santiam Junction has declined significantly. Although precipitation has remained unchanged over the period of study, runoff ratios have declined in spring (March through May) at rates ranging from 25.4% (WS2) to 41.2% (WS8) over the 40- to 50-year periods of streamflow record. Total declines in spring runoff equate to 1.13, 2.13, and 1.23 mm day-1 at WS2, WS8, and WS9, respectively. However, neither winter nor summer runoff ratios have changed significantly, nor has there been a significant shift in the date of water year median flow over the period of record. Winter and spring runoff ratios are negatively related to vapor pressure deficit and/or temperature in the corresponding season, while summer runoff 2 ratios are positively related to vapor pressure deficit and/or temperature. Overall, the findings of this study most strongly support the hypothesis that warming temperatures have resulted in a reduction in spring snowpacks, with an earlier onset of evapotranspiration in the spring and a reduction in evapotranspiration during the summer, due to stomatal closure when VPD becomes sufficiently large or soil moisture becomes limited. Title: Trends in streamflow from old growth forested watersheds in the western Cascades Author: Moore, Kathleen M Increasing temperatures in western North America are expected to result in a decline in winter snowpacks, earlier snowmelt, and a shift in the timing of streamflows, with an increasing fraction of streamflows occurring earlier in the water year and drier conditions during the summer. However, few streamflow datasets have associated climate and vegetation records adequate to interpret changes in climate, forest processes, and their consequences for streamflows. This study examined long-term streamflow records from three headwater watersheds in old growth forest at the H. J. Andrews Experimental Forest, Oregon, to seek evidence of trends and investigate possible explanations for these changes. The three small (8.5-60 ha) watersheds (WS2, WS8, and WS9) range in elevation from 432-1182 m and have streamflow records dating back to 1953, 1964, and 1969 respectively. Spring and summer average minimum temperatures have increased over the study period, while April 1st SWE at Santiam Junction has declined significantly. Although precipitation has remained unchanged over the period of study, runoff ratios have declined in spring (March through May) at rates ranging from 25.4% (WS2) to 41.2% (WS8) over the 40- to 50-year periods of streamflow record. Total declines in spring runoff equate to 1.13, 2.13, and 1.23 mm day-1 at WS2, WS8, and WS9, respectively. However, neither winter nor summer runoff ratios have changed significantly, nor has there been a significant shift in the date of water year median flow over the period of record. Winter and spring runoff ratios are negatively related to vapor pressure deficit and/or temperature in the corresponding season, while summer runoff 2 ratios are positively related to vapor pressure deficit and/or temperature. Overall, the findings of this study most strongly support the hypothesis that warming temperatures have resulted in a reduction in spring snowpacks, with an earlier onset of evapotranspiration in the spring and a reduction in evapotranspiration during the summer, due to stomatal closure when VPD becomes sufficiently large or soil moisture becomes limited. Close

• 9. [Article] A forest vulnerability index based on drought and high temperatures

  To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier ...

  Citation × Citation Citation Abstract Copy Title: A forest vulnerability index based on drought and high temperatures Author: Mildrexler, David, Yang, Zhiqiang, Bell, David M., Cohen, Warren B. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/remote-sensing-of-environment/ Title: A forest vulnerability index based on drought and high temperatures Author: Mildrexler, David, Yang, Zhiqiang, Bell, David M., Cohen, Warren B. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/remote-sensing-of-environment/ Close

• 10. [Article] Ecohydrological consequences of drought- and infestationtriggered tree die-off: insights and hypotheses

  To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by John Wiley ...

  Citation × Citation Citation Abstract Copy Title: Ecohydrological consequences of drought- and infestationtriggered tree die-off: insights and hypotheses Author: Smith, Alistair M. S., Huxman, Travis E., Weiler, Markus, Hale, V. Cody, Allen, Craig D., Luce, Charles H., Adams, Henry D., Breshears, David D. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by John Wiley & Sons Ltd. and can be found at: http://onlinelibrary.wiley.com/. Title: Ecohydrological consequences of drought- and infestationtriggered tree die-off: insights and hypotheses Author: Smith, Alistair M. S., Huxman, Travis E., Weiler, Markus, Hale, V. Cody, Allen, Craig D., Luce, Charles H., Adams, Henry D., Breshears, David D. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by John Wiley & Sons Ltd. and can be found at: http://onlinelibrary.wiley.com/. Close