John Francis Knowles

John Francis Knowles
California State University, Chico · Department of Earth and Environmental Sciences

About

54
Publications
14,413
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1,069
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Introduction
My research couples eddy covariance and field-based observations with remote sensing and modeling techniques, in order to investigate the multi-scale impacts of climate change and management practices on water and nutrient cycling at Earth’s surface.
Education
January 2010 - August 2015
University of Colorado Boulder
Field of study
  • Geography
September 1999 - May 2003
Vassar College
Field of study
  • Liberal Arts

Publications

Publications (54)
Article
Full-text available
High-elevation montane forests are disproportionately important to carbon sequestration in semi-arid climates where low elevations are dry and characterized by low carbon density ecosystems. However, these ecosystems are increasingly threatened by climate change with seasonal implications for photosynthesis and forest growth. As a result, we levera...
Article
Knowing how evapotranspiration (ET) is mediated by abiotic and biotic pathways is essential to understanding how water affects ecosystem productivity. Recent studies have investigated the average transpiration fraction (T/ET) across sites and biomes, but the temporal variability of the partitioning and its controls are less understood. Here, we exa...
Article
Mounting evidence indicates dryland ecosystems play an important role in driving the interannual variability and trend of the terrestrial carbon sink. Nevertheless, our understanding of the seasonal dynamics of dryland ecosystem carbon uptake through photosynthesis [gross primary productivity (GPP)] remains relatively limited due in part to the lim...
Article
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Fires increasingly impact forested watersheds, with uncertain water resources impacts. While research has revealed higher peak flows, longer‐term yields may increase or decrease following fire, and the mechanisms regulating post‐fire streamflow are little explored. Hydrologic response to disturbance is poorly understood in the Lower Colorado River...
Article
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Earth’s ecosystems are increasingly threatened by “hot drought,” which occurs when hot air temperatures coincide with precipitation deficits, intensifying the hydrological, physiological, and ecological effects of drought by enhancing evaporative losses of soil moisture and increasing plant stress due to higher vapor pressure deficit (VPD). Drought...
Article
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Current understanding of the dynamic and slow flow paths that support streamflow in mountain headwater catchments is inhibited by the lack of long-term hydrogeochemical data and the frequent use of short residence time age tracers. To address this, the current study combined the traditional mean transit time and the state-of-the-art fraction of you...
Article
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Plant canopy temperature (Tc) is partly regulated by evaporation and transpiration from the canopy surface and can be used to infer changes in stomatal regulation and vegetation water stress. In this study, we used a thermal Unmanned Aircraft Systems in conjunction with eddy covariance, sap flow, and spectral reflectance data to assess the diurnal...
Preprint
Full-text available
Current understanding of the dynamic flow paths and subsurface water storages that support streamflow in mountain catchments is inhibited by the lack of long-term hydrologic data and the frequent use of single age tracers that are not applicable to older groundwater reservoirs. To address this, the current study used both multiple metrics and trace...
Article
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Hydraulic redistribution is the transport of water from wet to dry soil layers, upward or downward, through plant roots. Often in savanna and woodland ecosystems, deep‐rooted trees, and shallow‐rooted grasses coexist. The degree to which these different species compete for or share soil‐water derived from precipitation or groundwater, as well as ho...
Article
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Large datasets of greenhouse gas and energy surface-atmosphere fluxes measured with the eddy-covariance technique (e.g., FLUXNET2015, AmeriFlux BASE) are widely used to benchmark models and remote-sensing products. This study addresses one of the major challenges facing model-data integration: To what spatial extent do flux measurements taken at in...
Article
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Catchment‐scale response functions, such as transit time distribution (TTD) and evapotranspiration time distribution (ETTD), are considered fundamental descriptors of a catchment's hydrologic and ecohydrologic responses to spatially and temporally varying precipitation inputs. Yet, estimating these functions is challenging, especially in headwater...
Article
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Anticipating the ability of ecosystems to maintain functional integrity across predicted altered precipitation regimes remains a grand ecohydrological challenge. Overstory trees and understory grasses within semiarid savannas vary in their structure and sensitivity to environmental pressures, underscoring the need to examine the ecohydrological imp...
Article
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At the seasonal timescale, daily photochemical reflectance index (PRI) measurements track changes in photoprotective pigment pools as plants respond to seasonally variable environmental conditions. As such, remotely‐sensed PRI products present opportunities to study seasonal processes in evergreen conifer forests, where complex vegetation dynamics...
Article
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Semiarid forests in the southwestern United States are generally restricted to mountain regions where complex terrain adds to the challenge of characterizing stand productivity. Among the heterogeneous features of these ecosystems, topography represents an important control on system-level processes including snow accumulation and melt. This basic...
Article
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We used the weighted wavelet method to perform spectral analysis of observed long-term precipitation, streamflow, actual evapotranspiration, and soil water storage at a sub-humid mountain catchment near Tucson, Arizona, USA. Fractal scaling in precipitation and the daily change in soil water storage occurred up to a period of 14 days and correspond...
Article
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“Sky islands” are characteristic of sequential mountain-valley terrain where mountains form an island archipelago rising from surrounding valleys of desert “sea”. At high elevations in the Madrean sky islands of the southwestern United States (USA) and Mexico, mixed evergreen conifer forests occur near the latitudinal extent of their distribution....
Article
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This study coupled long‐term hydrometric and stable water isotope data to identify links between subsurface water storage and vegetation in a subhumid mountain catchment in Arizona, USA. Specific observations included catchment‐scale hydrologic fluxes and soil water storage, and stable water isotopes from stream water, soil water, groundwater, and...
Article
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Predicting fluid biogeochemistry in the vadose zone is difficult because of time‐dependent variation in multiple controlling factors, such as temperature, moisture, and biological activity. Furthermore, soils are multicomponent, heterogeneous porous media where manifold reactions may be affecting solution chemistry. We postulated that ecosystem‐sca...
Article
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High-latitude warming is capable of accelerating permafrost degradation and the decomposition of previously frozen carbon. The existence of an analogous high-altitude feedback, however, has yet to be directly evaluated. We address this knowledge gap by coupling a radiocarbon-based model to 7 years (2008-2014) of continuous eddy covariance data from...
Article
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Growing season length (GSL) is a key unifying concept in ecology that can be estimated from eddy covariance-derived estimates of net ecosystem production (NEP). Previous studies disagree on how increasing GSLs may affect NEP in evergreen coniferous forests, potentially due to the variety of methods used to quantify GSL from NEP. We calculated GSL a...
Article
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Most land surface models (LSMs) used in Earth System Models produce a lower ratio of transpiration (T) to evapotranspiration (ET) than field observations, degrading the credibility of Earth System Model-projected ecosystem responses and feedbacks to climate change. To interpret this model deficiency, we conducted a pair of model experiments using a...
Article
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Future projections of declining snowpack and increasing potential evaporation are predicted to advance the timing of snowmelt in mountain ecosystems globally with unknown implications for snowmelt-driven forest productivity. Accordingly, this study combined satellite- and tower-based observations to investigate the forest productivity response to s...
Article
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An understanding of surface and subsurface water contributions to streamflow is essential for accurate predictions of water supply from mountain watersheds that often serve as water towers for downstream communities. As such, this study used the end-member mixing analysis (EMMA) technique to investigate source water contributions and hydrologic flo...
Article
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Snow sublimation is an important component of the snow mass balance, but the spatial and temporal variability of this process is not well understood in mountain environments. This study combines a process-based snow model (SnowModel) with eddy covariance (EC) measurements to investigate (1) the spatio-temporal variability of simulated snow sublimat...
Article
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Precipitation changes the physiological characteristics of an ecosystem. Because land-surface models are often used to project changes in the hydrological cycle, modeling the effect of precipitation on the latent heat flux λE is an important aspect of land-surface models. Here, we contrast conditionally-sampled diel composites of the eddy-covarianc...
Article
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We used multiple sources of remotely sensed and ground based information to evaluate the spatio-temporal variability of snowpack accumulation, potential evapotranspiration (PET), and Normalized Difference Vegetation Index (NDVI) throughout the Southern Rocky Mountain ecoregion, USA. Relationships between these variables were used to establish basel...
Article
We used End-Member Mixing Analysis (EMMA) to investigate the spatiotemporal variability of source water contributions to streamflow generation from three headwater catchments that span a precipitation and ecosystem type gradient across ∼1500 m elevation in the Colorado Front Range, USA. We additionally characterized the magnitude and type (rain ver...
Article
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Abiotic factors structure plant community composition and ecosystem function across many different spatial scales. Often, such variation is considered at regional or global scales, but here we ask whether ecosystem-scale simulations can be used to better understand landscape-level variation that might be particularly important in complex terrain, s...
Article
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Declining mountain snowpack and earlier snowmelt across the western United States has implications for downstream communities. We present a possible mechanism linking snowmelt rate and streamflow generation using a gridded implementation of the Budyko framework. We computed an ensemble of Budyko streamflow anomalies (BSA) using Variable Infiltratio...
Article
Full-text available
The transpiration (T) fraction of total terrestrial evapotranspiration (ET), T/ET, can vary across ecosystems between 20-95% with a global average of ∼60%. The wide range may either reflect true heterogeneity between ecosystems and/or uncertainties in the techniques used to derive this property. Here we compared independent approaches to estimate T...
Article
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We measured soil respiration across a soil moisture gradient ranging from dry to wet snow-scoured alpine tundra soils throughout three winters and two summers. In the absence of snow accumulation, soil moisture variability was principally determined by the combination of meso-topographical hydrological focusing and shallow subsurface permeability,...
Conference Paper
We used the eddy covariance method to continuously measure the net ecosystem exchange of carbon dioxide for seven years from a snow-scoured alpine tundra meadow on Niwot Ridge in Colorado, USA that may be underlain by sporadic permafrost. On average, the alpine tundra was a net annual source of 232 g C m-2 to the atmosphere, and the source strength...
Conference Paper
2015 AGU Fall Meeting: http://fallmeeting.agu.org/2015/ C41F-02 Rapid Snowmelt Leads to Greater Streamflow Across the Western United States Thursday, 17 December 2015: 08:15 3005 (Moscone West) Theodore B Barnhart1, Ben Livneh2, Noah P Molotch1, John F Knowles1, Adrian Adam Harpold3 and Dominik Schneider1, (1)University of Colorado at Boulder, Geog...
Conference Paper
In the geoscience community, Earth system models are typically used to investigate carbon cycle – climate feedbacks at global scales, but increasingly this class of models applies ecological understanding to ask biologically relevant questions. Fewer studies, however, explore the utility and feasibility of using Earth system models to generate and...
Article
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Background: The eddy covariance (EC) technique provides a direct measure of water vapour and carbon dioxide fluxes between ecosystems and the atmosphere. Aims: This review article highlights the findings of various studies that have integrated EC observations into basic meteorological, hydrological and ecological research questions in two ecosystem...
Article
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The alpine tundra landscape is a patchwork of co-mingled ecosystems that vary due to meso-topographical (<100 m) landscape position, shallow subsurface heterogeneity, and subsequent soil moisture availability. This results in hotspots of biological activity, variable carbon cycling over short horizontal distances, and confounds predictions of the a...
Article
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Climate change is affecting the hydrology of high-elevation mountain ecosystems, with implications for ecosystem functioning and water availability to downstream populations. We directly and continuously measured precipitation and evapotranspiration (ET) from both subalpine forest and alpine tundra portions of a single catchment, as well as dischar...
Conference Paper
The alpine tundra is a mosaic of comingled vegetation communities that vary predominantly as a function of landscape position, micro-scale topography, subsurface permeability, and resultant soil moisture availability. We characterized the spatio-temporal variability of soil respiration from 17 alpine tundra sites across an irregular soil moisture g...
Conference Paper
Snowmelt is the primary source of surface water in the western United States and many other regions on Earth. Climate warming is forecast to impact the amount of precipitation that falls as snow and forms the mountain snowpack. Climate change induced alterations to snowpack translate to changes in snowpack magnitude, the timing of snowmelt, and cha...
Article
Full-text available
Snowpack temperatures from a subalpine forest below Niwot Ridge, Colorado, are examined with respect to atmospheric conditions and the 30-min above-canopy and subcanopy eddy covariance fluxes of sensible Qh and latent Qe heat. In the lower snowpack, daily snow temperature changes greater than 1°C day−1 occurred about 1–2 times in late winter and ea...
Conference Paper
Background/Question/Methods Atmospheric carbon dioxide (CO2) and methane (CH4) have been increasing since the pre-industrial era, resulting in climate warming that has increased flooding, permafrost melt, hurricanes, and the loss of sea-ice and glaciers. In order to accurately characterize and predict fluxes of these trace gases in the future, it...
Conference Paper
The eddy covariance method was used to measure the surface energy balance and latent heat flux (LE) above high-elevation (3480 m above sea level) alpine tundra at Niwot Ridge, Colorado over three years from 2007 through 2009. In spite of 955 mm mean annual total precipitation, the 24-hour mean evaporative fraction was 0.39, typical of dry grassland...
Conference Paper
Here we evaluate how evapotranspiration (ET) and deep percolation (DP) impact the precipitation-runoff response, aquifer recharge, and linked nutrient-water cycling at the 664-ha sub-alpine Como Creek drainage in the Colorado Front Range. ET is measured continuously using eddy covariance, soil moisture (SM) is measured using 2-m vertical sensor arr...
Conference Paper
Background/Question/Methods Global soils contain more C than terrestrial vegetation or the atmosphere, thus changes to patterns of soil C storage and/or efflux have particularly important C cycling ramifications. Alpine tundra soils may provide an exceptional opportunity to study the effects of climate perturbation on soil CO2 efflux since they fos...
Conference Paper
Background/Question/Methods A characteristic of alpine ecosystems is the high degree of spatial and temporal variation in the variables that regulate surface-atmosphere CO2 and H2O exchanges. A a result, it has been difficult to directly measure ecosystem-level exchanges, especially continuously throughout all seasons including winter. Therefore,...
Conference Paper
Ecosystems in topographically complex (mountainous) terrain are likely responsible for a majority of land-atmosphere CO2 exchange (net ecosystem exchange; NEE) across the western United States due to high inputs of winter precipitation as snowfall. NEE in these regions has been historically difficult to quantify using the eddy covariance (EC) metho...
Article
Full-text available
Eddy covariance measurements of the surface energy balance and carbon dioxide exchange above high-elevation (3,480m above sea level) alpine tundra located near Niwot Ridge, Colorado, were compared to simultaneous measurements made over an adjacent subalpine forest over two summers and one winter, from June 9, 2007 to July 3, 2008. The surface energ...
Conference Paper
Several studies have shown that alpine treeline (timberline) is especially sensitive to environmental changes, and is therefore a strong "early-warning" indicator of regional climate change. Many of these changes may be induced by factors including local land use changes, and understanding the difference and dynamics of water and carbon dioxide flu...

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