Evapotranspiration in a subarctic agroecosystem: field measurements, modeling and sustainability perspectives

Thesis (Ph.D.) University of Alaska Fairbanks, 2015Northern latitudes are known to be the most vulnerable regions already witnessing the impacts of climate change. These impacts have not only affected a broad spectrum of ecological conditions but also physical and socio-economic functions and activities across the region. Uncertainties in climate change and its progression exposes agroecosystem development and sustainability to a great risk. Yet, not fully understood, climate feedbacks and influencing factors such as human population growth and consumption imposes economical and financial stress in the sustainability of agroecosystem activities. On the opposite direction, trends in this activity can drive regional modifications to climate to an extent that is still unknown and not yet forecasted. Over time, as the acreages of agricultural lands increase from conversion of natural lands such as boreal forests, unexpected changes in surface energetics and particularly overturning of evapotranspiration rates and changes in soil moisture regime may potentially accentuate regional climate change. These changes therefore are expected to introduce new challenges for Alaskan agriculturists because of increasing vulnerabilities and affecting conditions that shape resilience of agricultural systems and production. This research focused on improving understanding of surface energetics in an agroecosystem of Interior Alaska. A synthesis study was conducted combining the analysis of intensive field experiments including direct measurements of micrometeorological, hydrological, meteorological variables and computational modelling during the summer growing season. The evaluation of evapotranspiration (ET) dynamical regime and surface energy processes showed that ET represented a large portion of surface energy balance with similar aspects to surface fluxing levels in Arctic tundra, and in contrast, with more abundant flux levels than in subarctic boreal forest. Surface heterogeneities due to soil moisture and temperature regime drive differences in energy balance closure as a function of spatial scales despite the mostly flat surfaces and stationary atmospheric surface layer flows in the experimental area. A fully coupled numerical simulation was performed to model fluxes at the land-atmosphere interface and compared to independent observations of surface energy. A final assessment of experimental methodologies and numerical modeling is presented in preparation for integrative data fusion analysis and studies involving new satellite remote sensing capabilities, physical modeling and network field observations

Evapotranspiration in Northern Agro-Ecosystems: Numerical Simulation and Experimental Comparison

Evapotranspiration and near-surface soil moisture dynamics are key-entangled variables regulating flux at the surface-atmosphere interface. Both are central in improving mass and energy balances in agro ecosystems. However, under the extreme conditions of high-latitude soils and weather pattern variability, the implementation of such coupled liquid and vapor phase numerical simulation remain to be tested. We consider the nonisothermal solution of the vapor flux equation that accounts for the thermally driven water vapor transport and phase changes. Fully coupled flux model outputs are compared and contrasted against field measurements of soil temperature, heat flux, water content, and evaporation in a subarctic agroecosystem in Alaska. Two well-defined hydro-meteorological situations were selected: dry and wet periods. Numerical simulation was forced by time series of incoming global solar radiation and atmospheric surface layer thermodynamic parameters: surface wind speed, ambient temperature, relative humidity, precipitation, and soil temperature and soil moisture. In this simulation, soil parameters changing in depth and time are considered as dynamically adjusted boundary conditions for solving the set of coupled differential equations. Results from this evaluation give good correlation of modeled and observed data in net radiation (Rnet) (R2 of 0.92, root mean square error (RMSE) of 45 W m−2), latent heat (0.70, RMSE of 53 W m−2), and sensible heat (R2 = 0.63, RMSE = 32 W m−2) during the dry period. On the other hand, a poor agreement was obtained in the radiative fluxes and turbulent fluxes during the wet period due to the lack of representation in the radiation field and differences in soil dynamics across the landscape

Diurnal cycle of radiative and turbulent fluxes during clear sky conditions.

<p>Case of 30 July (Day of Year 211) at the experiment site. Horizontal axis is in AKST time in [hrs.] and vertical axis is in W m^-2. <i>R</i>_<i>net</i> = net radiation, <i>LE</i> = latent heat flux, <i>H</i> = sensible heat flux, <i>G</i> = ground heat flux.</p

Mean summer values of the energy balance partitioning for Arctic and Subarctic ecosystems calculated and/or collected from various published data sources.

<p>First column represents the ecosystem types, second column is the location of measuring site, third to fifth columns are energy partitioning values for <i>LE</i> /<i>R</i>_<i>net</i>, <i>H</i> /<i>R</i>_<i>net</i>, <i>G</i> /<i>R</i>_<i>net</i> (derived from daily midday flux averages), sixth column is VPD (kPa) for each ecosystem type, seventh column is the Bowen ratio (β), eigth column is the measuring method used for energy budget components measured, and nineth column is the reference for data. EC = Eddy covariance method. BREB = Bowen ratio-energy balance method.</p><p>^aAverage over two years growing season data in 2012 and 2013 from this present work during 1 June—20 September.</p><p>Mean summer values of the energy balance partitioning for Arctic and Subarctic ecosystems calculated and/or collected from various published data sources.</p

Frequency distribution of the wind speed and direction during summer.

<p>Left panel 2012 and right panel 2013 at the experiment site during the period of study at 2 m height.</p

Seasonal means of surface energy partitions, Bowen ratio (β), vapor pressure deficit (VPD), Priestley Taylor alpha coefficient (α) and energy balance closure (<i>C</i>_<i>F</i>) at the FEF.

<p>Values are calculated between 1 June to 15 September in 2012 and 2013 growing season. Average midday (1100–1500 AKST) energy balance energy partitioning obtained from a total 352 and 364 samples in 2012 and 2013, respectively. An average over the two-year period was calculated based on 716 data-points shown in the fourth column.</p><p>Seasonal means of surface energy partitions, Bowen ratio (β), vapor pressure deficit (VPD), Priestley Taylor alpha coefficient (α) and energy balance closure (<i>C</i>_<i>F</i>) at the FEF.</p

Time-series of soil temperatures.

<p>Soil temperature at 15 cm depth at the experiment site during 1 June to 17 September 2012 (black trace) and 2013 (gray trace). The horizontal axis represents fractional Julian day in local AKST.</p

Seasonal means of major microclimate variables at FEF during the growing seasons under study.

<p>R_net: net radiation (W m^-2), G_EC: ground heat flux (W m^-2) at EC site, bare field (G_bare), brome grass field (G_grass), and barley field (G_barley), LE: latent heat flux (W m^-2), VPD: vapor pressure deficit (kPa), θ_ly: volumetric soil moisture content (m³ m^-3) in irrigated vegetated lysimeter at 15 cm depth average from three lysimeters in summer 2012 and averaged from 0–20 cm depths from three lysimeters in summer 2013, θ_unly: volumetric soil moisture content (m³ m^-3) in unvegetated lysimeter in summer 2013, θ_FEF: an average volumetric soil moisture content (m³ m^-3) at 15 cm depth from brome grass, barley and bare field, T_s: an average soil temperature (°C) at 15 cm from brome grass, barley and bare field, and U: wind speed (m s^-1) at 2 m height at meteorological station. First column represents the major variables measured, second and third column are mean ± standard error of each variable for the 2012 and 2013 growing season.</p><p>^a More than two significant digits are needed for volumetric soil moisture content</p><p>^b volumetric soil moisture content data were available from June –27July, 2012</p><p>^c volumetric soil moisture content in a vegetated lysimeter for 2013 growing season</p><p>^d volumetric soil moisture content in an unvegetated lysimeter for 2013 growing season</p><p>Seasonal means of major microclimate variables at FEF during the growing seasons under study.</p

The annual and summer hydrological balance characteristics for Arctic and Sub-arctic regions compiled from various published data sources.

<p>^a Precipitation+Irrigation</p><p>^b Summer hydrological balance</p><p>The annual and summer hydrological balance characteristics for Arctic and Sub-arctic regions compiled from various published data sources.</p