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Climates of Vegetated Surfaces

From Boundary Layer Climates, T.R. OKE

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Climates of Vegetated Surfaces

  2. 2. CONTENTS 1. Special features (a) Energy and water storage in vegetation systems (b) Photosynthesis and carbon dioxide exchange (c) Effects of stand architecture 2. Leaves (a) Radiation budget (b) Energy balance (c) Climate 3. Plant covers and crops (a) Mass balances (b) Radiation budget (c) Energy balance (d) Climate 4. Orchards and forests (a) Mass balances (b) Radiation budget (c) Energy balance (d) Climate 5. Comparison of low plant and forest water use
  3. 3. Schematic depiction of fluxes involved in (a) the energy and (b) the water balances of a soil-plant-air volume 1. SPECIAL FEATURES Energy and water storage in vegetation systems  Energy balance: 𝑄∗ = 𝑄 𝐻 + 𝑄 𝐸 + ∆𝑄 𝑆 + ∆𝑄 𝑃 + ∆𝑄 𝐴, Where, ∆𝑄 𝑃 = Biochemical energy storage due to photosynthesis ∆𝑄 𝐴 = net horizontal energy transfer due to advection  Water balance: 𝑝 = 𝐸 + ∆𝑟 + ∆𝑆 + ∆𝐴, Where, ∆𝑆 = net water storage in the air and soil ∆𝐴 = net horizontal moisture exchange due to advection
  4. 4. 1. SPECIAL FEATURES Photosynthesis and carbon dioxide exchange Plant growth is tied to the supply of solar radiation and carbon dioxide through the processes of photosynthesis and respiration.  Gross photosynthesis (𝑃) [kg m-2 s-1] 𝐶𝑂2 + 𝐻2 𝑂 + 𝐿𝑖𝑔ℎ𝑡 𝑒𝑛𝑒𝑟𝑔𝑦 → 𝐶𝐻2 𝑂 𝑜𝑟 𝐶6 𝐻12 𝑂6 + 𝑂2  Respiration (𝑅) [kg m-2 s-1] 𝐶𝐻4 𝑜𝑟 𝐶6 𝐻12 𝑂6 + 𝑂2 → 𝐶𝑂2 + 𝐻2 𝑂 + 𝐶𝑜𝑚𝑏𝑢𝑠𝑡𝑖𝑜𝑛 𝑒𝑛𝑒𝑟𝑔𝑦  Net rate of photosynthesis, ∆𝑃 = 𝑃 − 𝑅 [kg m-2 s-1]  Energy stored due to net photosynthesis, ∆𝑄 𝑃 = ∅∆𝑃, Where ∅ = Heat of assimilation of carbon ≈ 1.15 × 107 𝐽 𝑘𝑔−1 ≈ 3.2 𝑊 𝑚−2 𝑝𝑒𝑟 𝑔𝑚−2ℎ−1 of 𝐶𝑂2 assimilation  By day, the crop is a net 𝐶𝑂2 sink because 𝑃 > 𝑅 and ∆𝑃 is positive  By night, the crop is a net 𝐶𝑂2 source because 𝑃 < 𝑅 and ∆𝑃 is negative
  5. 5. 1. SPECIAL FEATURES  Leaf stomate: passageway between the atmosphere and the interior of the plant and tree  Transpiration – latent heat is a major means of dissipating the energy load on leaves  The length of stomata depend on species (10 – 30 μm)  Width of stomata: 0 when closed to 10 μm when fully open  Area covered by stomata: 0.3-1% of the total leaf area  Stomata density: 50 – 500 per mm2 (a) View of a partially open stomate on a wheat leaf, (b) Schematic cross- section through a portion of a leaf illustrating the exchanges of water vapour and CO2 through a stomate, and of heat from the leaf Photosynthesis and carbon dioxide exchange
  6. 6. 1. SPECIAL FEATURES Effects of stand architecture  Foliage density and Canopy  Foliage density: Concentrated in the overall height of the stand, near the top of the stand, and near to the base of the stand Typical wind profile measured above a vegetation stand of height h, illustrating the concept of a zero plane displacement at the height d  Zero plane displacement, 𝑑 = 2 3 ℎ, for closely- spaced stands  𝑑 also depend on the drag elements and wind speed  𝑢 𝑧 = 𝑢∗ 𝓀 𝑙𝑛 𝑧−𝑑 𝑧0 , where 𝑢 𝑧 is mean wind speed (m s-1) at the height 𝑧, 𝑢∗ is friction velocity (m s-1),𝓀 is von Karman’s constant (≈ 0.40), 𝑧0 is roughness length (m)
  7. 7. Idealized relation between wavelength and the reflectivity (α), transmissivity (Ψ) and absorptivity (ζ) of a green leaf 2. LEAVES Radiation budget  The deposition of incident radiation is given by Ψ + 𝛼 + ζ = 1  Photosynthetically active radiation (PAR) [0.4 -0.7 μm] and Photosynthesis photon flux density (PPFD)  Leaves are full radiators if 𝜀 = 0.94 to 0.99  𝑄𝑙𝑒𝑎𝑓 ∗ = 𝐾𝑙𝑒𝑎𝑓 ∗ + 𝐿𝑙𝑒𝑎𝑓 ∗ = 𝐾(𝑡) ∗ + 𝐾(𝑏) ∗ + 𝐿(𝑡) ∗ + 𝐿(𝑏) ∗ = ζ 𝐾𝑖𝑛 𝑡 + 𝐾𝑖𝑛 𝑏 + 𝐿𝑖𝑛 𝑡 − 𝐿 𝑜𝑢𝑡(𝑡) + 𝐿𝑖𝑛(𝑏) − 𝐿 𝑜𝑢𝑡 𝑏 Where 𝛹 𝑎𝑛𝑑 𝛼 are assumed equal for the top and bottom sides of the leaf Schematic depiction of the fluxes involved in the radiation budget of an isolated leaf
  8. 8. 2. LEAVES Energy balance  𝑄𝑙𝑒𝑎𝑓 ∗ = 𝑄 𝐻(𝑙𝑒𝑎𝑓) + 𝑄 𝐸(𝑙𝑒𝑎𝑓) = 𝑄 𝐻(𝑡) + 𝑄 𝐻(𝑏) + 𝑄 𝐸(𝑡) + 𝑄 𝐻(𝑏) , Where both the physical and biochemical heat storage has been neglected.  Sensible heat transfer between the leaf surface and the air, 𝑄 𝐻 = 𝑄 𝐻(𝑙𝑒𝑎𝑓) = 𝐶 𝑎 𝑇0−𝑇 𝑎 𝑟 𝑏 , where 𝑇0 is the leaf surface temperature, & 𝑟𝑏 is the diffusive resistance of the laminar sub-layer adhering to the leaf.  By solving above two equations, 𝑇0 = 𝑇𝑎 + 𝑟 𝑏 𝐶 𝑎 𝑄(𝑙𝑒𝑎𝑓) ∗ − Schematic depiction of the fluxes involved in the energy balance of an isolated leaf Schematic cross-section through a portion of a leaf illustrating the exchanges of water vapour and CO2 through a stomate, and of heat from the leaf
  9. 9. 2. LEAVES Climate  A large sunlit leaf is 5 to 10°C warmer than the surrounding air  The leaf temperature excess increases from the windward to the leeward side  In the leading edge: Thinnest insulation, Greatest heat loss and Lowest temperature  In the trailing edge: Thickest insulation, Least heat loss and Highest temperature  This influences patterns of leaf ‘burn’, fungal growth and insect activity on the leaf  Desert plants have large leaves to maintain a thick insulating layer (high 𝑟𝑏) Variation of temperature over the surface of a bean leaf with a wind speed 0.7 ms-1. Values are the amount by which leaf exceeds the air temperature (°C). [𝑇𝑎= 25.6°C, 𝑄∗ = 150 W m-2, 𝑟𝑏=400 to 1300 s m-1.
  10. 10. 3. PLANT COVERS AND CROPS Mass balances (Water)  The resistance offered to moisture extraction: by the soil rsoil, by extension of root development rroot, by the vascular system of the xylem rxylem, by the diffusion within the leaf rleaf, by the laminar boundary layer rb𝑜𝑢𝑛.𝑙𝑎𝑦𝑒𝑟 or rb, and by the turbulent layers (aerodynamic) rair or r 𝑎  Canopy resistance, 𝑟𝑐 = 𝜌 𝑣(𝑇 𝑐) ∗ −𝜌 𝑣0 𝐸 ≈ 𝑟𝑠𝑡 𝐴1 , where 𝜌 𝑣(𝑇𝑐) ∗ is the saturation vapour density at the canopy surface temperature (𝑇𝑐), and 𝐴1 is the leaf area index of the canopy. [In the case of a wetted leaf 𝑟𝑐= 0 because the stomata play no regulatory role] The water balance and internal flows of water in a soil-plant-atmosphere system. At the right is an electrical analogue of the flow of water from the soil moisture store to the atmospheric sink via the plant system  p = E + ∆r + ∆S + ∆A, where ∆S = net water storage in the air and soil, and ∆A = net horizontal moisture exchange due to advection
  11. 11. 3. PLANT COVERS AND CROPS Mass balances (Carbon dioxide)  During night: 𝐹𝐶 is directed away from the vegetation into the atmosphere (loss of CO2 from the system by respiration from plant top) Diurnal variation of carbon dioxide concentration in the air at a rural site in Ohio for different months Diurnal variation of the vertical flux of carbon dioxide (𝐹𝐶 ) over a prairie grassland at monthly intervals during the growing season (Data are 10-day averages)  The seasonal decrease in 𝐹𝐶: due to soil moisture depletion  The midday minimum: due to increase in canopy resistance (the effects of temperature and stomatal closure)  Growing seasons (May – July): Highest 𝐹𝐶 in the plant system and Highest C02 concentration in the air
  12. 12. 4. ORCHARDS AND FORESTS Mass balances  Forest can retain a larger proportion of the precipitation as interception storage Deciduous forest: 10% – 25% of annual precipitation Coniferous forest: 15% - 40% of annual precipitation The relation between the rainfall interception efficiency of tropical and temperature forests and the amount of rain precipitated by a storm  Interception depend on the nature of rainstorm  Maximum storage capacity For rain: 0.5 – 2 mm for all forest types For snow: 2 to 6 mm for all forest types  Evapotranspiration (Evaporation > Transpiration)  The principal features of CO2 balance of forests are same as for other vegetation
  13. 13. 3. PLANT COVERS AND CROPS Radiation budget  Beer’s Law: 𝐾 ↓(𝑧)= 𝐾 ↓0 𝑒−𝑎𝐴1(𝑧), where 𝑎 is extinction coefficient of plant leaves and 𝐴1(𝑧) is the leaf area accumulated from the top of the canopy down to the level 𝑧. Relation between the albedo of vegetation and its height. Vertical lines: two standard deviations, & horizontal lines: seasonal range of canopy height. Measured profiles of (a) incoming solar (𝐾 ↓), and (b) net all-wave radiation (𝑄∗ ) in a 0.2 m stand of native grass at Matador, Sask. Relation between the albedo of vegetation and solar altitude on sunny days.  Albedo and vegetation height relation is for: 1) Green vegetation surface 2) Midday period  Albedo depends on the radiative properties of the surfaces, stand architecture of the plant, and the angle of solar incidence
  14. 14. 4. ORCHARDS AND FORESTS Radiation budget  The principal radiative exchanges occur at the canopy layer (upper and lower boundary)  Approximate attenuation of SW with height is given Beer’s law  Amount of SW transmission depend on the height, density and species of the stand, the angle of solar incidence (generally 5% - 20% of flux 1 reaches floor of a stand) Schematic model of radiation exchanges above and within a forest. (𝐊 ↓) (𝐊 ↑) (𝐋 ↑)(𝐋 ↓) (𝐊 ↓) (𝐊 ↑) (𝐋 ↓) (𝐋 ↑) SW radiation budget of (a) an orange orchard, and (b) a single-layer mosaic of fresh orange leaves. All values expressed as percentages of the incident radiation
  15. 15. 4. ORCHARDS AND FORESTS Radiation budget  The canopy also affects diffuse radiation (𝐷) and its spectral composition  Net all-wave radiation (𝑄∗ ) is similar to other vegetation Component fluxes of the radiation budget of a 28 m stand of Douglas fir (coniferous forest) at Cedar River, Washington (47°N) on the 10 August 1972 SW radiation measured above the canopy, and at one point on the floor of a 23 m stand of Loblolly pine near Durham, N.C. (36°N) on October 1965 Low albedo (0.09)  𝐿 ↑ varies with the canopy surface temperature  𝑄∗ varies with stand height, density, species & solar altitude
  16. 16. 3. PLANT COVERS AND CROPS Energy balance  Net long-wave radiation budget (𝐿∗ ) of vegetation is almost always negative, as with most other surfaces. However, the reduction of the sky view factor is reason behind the net loss.  Day time energy dissipating was dominated by 𝑄 𝐸 and some occasions 𝑄 𝐸 > 𝑄∗ due to the flow of heat from the atmosphere to the crop in the morning and afternoon  𝑟𝑐 is very small in the morning due to dew, is almost constant for most of the day and is high in the late afternoon due to decreasing light intensity and increasing water stress (a) The radiation budget, (b) energy balance and (c) canopy resistance of a barley field at Rothamsted, England (52°N) on 23 July 1963. [On the day with clear skies, mostly sunny day, wind < 2.5 ms-1 & light rain at night]
  17. 17. 3. PLANT COVERS AND CROPS Energy balance Water relations of a field of alfalfa at Phoenix, Arizona (33°N)  The crop field was flooded by irrigation on 27 May.  Soil water potential (ψ) increased with time  After 20 June, the role of canopy resistance (𝑟𝑐) began  On 28 June, near the limit for moisture availability for plants (ψ = −1.1 𝑀𝑃𝑎)  𝑟𝑐 became very large during the middle of the day  In day time: 𝑟𝑐(31 𝑑𝑎𝑦𝑠) > 𝑟𝑐(23 𝑑𝑎𝑦𝑠) whereas 𝐸(31 𝑑𝑎𝑦𝑠) < 𝐸(23 𝑑𝑎𝑦𝑠) (a) Daily average soil moisture potential and canopy resistance, and (b) diurnal variation of the evaporation rate and canopy resistance at period of 23 and 31 days after irrigation. Water stress and high leaf temperature
  18. 18. 4. ORCHARDS AND FORESTS Energy balance  The principal energy budget is similar to other vegetation  Some nocturnal cloud at Thetford (00 to 05 h) and some daytime cloud at Haney (11 to 20 h)  Albedo, day length & cloud → Net radiative surplus for the day Diurnal energy balance of (a) a Scots and Corsican pine forest at Thetford, England (52°N) on 7 July 1971, and (b) a Douglas fir forest at Haney, BC (49°N) on 10 July 1970, including (c) the atmospheric vapour pressure deficit Albedo (0.08) Albedo (0.09)
  19. 19. 4. ORCHARDS AND FORESTS Energy balance  The higher 𝑟𝑎 at Haney in the evening due to local wind stagnation  Generally, 𝑟𝑎 values are low in forests due to roughness (more turbulent atmosphere than the other natural surfaces, excluding topographic effects)  𝑟𝑐 at Thetford >> 𝑟𝑐 at Haney  𝑟𝑐 >>> 𝑟𝑎  𝑟𝑐 are larger in the afternoon Diurnal variation of (a) the canopy and (b) aerodynamic resistance of coniferous forests
  20. 20. CLIMATE Orchards and forests Profiles of (a) wind speed, (b) temperature, (c) vapour pressure and (d) carbon dioxide concentration in and above a barley field at Rothamsted, England on 23 July 1963 Typical mean hourly profiles of climatological properties in a Sitka spruce forest at Fetteresso near Aberdeen (57°N) on a sunny day in July 1970 at midday. Also included is the profile of the leaf area showing the vertical distribution of foliage density. The characterize conditions above the canopy at a reference height of 13 m above the ground: 𝐾 ↓ = 605 W m-2, 𝑄∗ = 524 W m-2, 𝑇 = 11.8°C, 𝑒 = 1,110 Pa, 𝜌 𝑐 = 315.4 ppm, 𝑢= 3.9 m s-1 Plant covers and crops
  21. 21. 5. COMPARISON OF LOW PLANT & FOREST WATER USE  𝐸𝑡(𝑐𝑟𝑜𝑝) = 𝜌 𝑣(𝑇 𝑐) ∗ −𝜌 𝑣𝑎 𝑟 𝑐+𝑟 𝑎𝑉  𝐸𝑡(𝑓𝑜𝑟𝑒𝑠𝑡) = 𝜌 𝑣𝑎 ∗ −𝜌 𝑣𝑎 𝑟 𝑐 = 𝑣𝑑𝑑 𝑎 𝑟 𝑐  𝐸𝑡(𝑐𝑟𝑜𝑝) > 𝐸𝑡(𝑓𝑜𝑟𝑒𝑠𝑡)  𝐸𝑖(𝑐𝑟𝑜𝑝 𝑜𝑟 𝑓𝑜𝑟𝑒𝑠𝑡) = 𝜌 𝑣(𝑇 𝑐) ∗ −𝜌 𝑣𝑎 𝑟 𝑎𝑉  𝐸𝑖(𝑐𝑟𝑜𝑝) < 𝐸𝑖(𝑓𝑜𝑟𝑒𝑠𝑡)  Evapotranspiration, 𝐸 = 𝐸𝑡 + 𝐸𝑖  In temperate climates it is common to have wet; Windy winters when 𝐸𝑐𝑟𝑜𝑝 < 𝐸𝑓𝑜𝑟𝑒𝑠𝑡 Less rainy summer when 𝐸𝑐𝑟𝑜𝑝 > 𝐸𝑓𝑜𝑟𝑒𝑠𝑡 Observed mean annual water and energy balances for two catchments in Wales, UK: (a) Wye catchment (grassed) ∆𝑟 𝑝 = 0.83 𝑎𝑛𝑑 𝑄 𝐸 𝑄∗ = 0.58 (b) Severn catchment (forested) ∆𝑟 𝑝 = 0.61 𝑎𝑛𝑑 𝑄 𝐸 𝑄∗ = 1.15 . Grass Forest
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