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GEOFRAMECafe Giacomo Bertoldi
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GEOFRAMECafe Giacomo Bertoldi

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  • 1. GEOtop vegetation description. Current parameterization and proposed improvements. Geotop café 18.12.2008 Giacomo Bertoldi stb_stn
    • Review of the current Evapotranspiration description in GEOtop
    • (examples referred to Stubai Valley case study)
    • Proposed improvements
    • (some of them already implemented in Meckanzie, but to test)
    • Stubai simulations:
    • We choose here an inclined or flat plane domain to perform plot scale simulations, to check the sensitivity of the energy balance partition to slope, vegetation properties, initial conditions.
    • Simulation assumptions:
    • - Saturated initial conditions, 10 days spin up.
    • Soil parameters as in Neustift station soil (very organic soil at the surface).
    • - Flat (or inclined) 11x11 dtm
    • - Forcing as stb_tot simulations (Clear sky days in September 1999)
    • Reference simulation with intermediate parameters (stb_stn004f)
    • Canopy properties zo=0.1 m; do=0.66 m; fc=0.5
    • Rs minimum stomata resistance Rs=60 s m^-1
    • Number of soil layers: 4 SOIL LAYER THICKNESSES d[mm] float vector d {50,50,250,650}
    • Residual water content[-] of the layer 1: 0.283000
    • Saturated water content[-] of the layer 1: 0.525000
    • Water content of wilting point  wp [-]: 0.231000; Water content of field capacity  fc [-]: 0.333000
    • Kv_sat of layer 1 [mm/s]: 0.009033
    • The root fraction [-] of layer 1: 0.360000
    • (for all parameters see 'sensitivity' in Inpts_stb_stn004.xls)
  • 2. Equations used in GEOTOP to calculate vegetation ET stb_stn004f
    • ET = (1-fc) Eg + fc (Etc + Evc)
    • Eg ground bare soil evaporation; Etc canopy transpiration; Evc wet canopy leaves evaporation (only during rainfall )
    • Eg= (r_h/(r_h+r_gr))*Epot;
    • r_h atmospheric resistance
    • r_gr ground soil moisture resistance
    • r_gr = f (  1 ,  r ,  s )
    •  1 ,first layer water content
    •  r ,residual water content
    •  s ,saturated water content
    • Etc= ∑ i (r_h/(r_h+ r_veg ))*Epot * Rf ii ;
    • Rf ii root fraction layer i
    • r_veg vegetation resistance
    • r_veg=Rs /(LAI * (fS*fee*fT*fM)); (Best, 1998)
    • Rs minimum stomata resistance (species dependent, but constant over time)
    • fee water vapor deficit controlling factor
    • fS solar radiation controlling factor
    • fT temperature controlling factor
    • fM soil moisture controlling factor
    • [Wigmosta et al., (1994); Feddes et al.(1978)]
    • fM = f (  i ,  wp ,  fc )
    •  i , layer i water content
    •  wp , wilting point water content
    •  fc , water content at which the plant is beginning to feel water stress
  • 3. Fig 1 Environmental dependencies of stomata conductance For daytime conditions of simulation stb_stn004f Stomata close for high vapor pressure deficit Transpiration stop for too high and low Ta Transpiration is decreased below a critical water content Photosynthesis increases with PaR
    •  fc vs.  fc
    •  wp vs.  fc
     wp  fc
  • 4. Ground evaporation vs. Transpiration For daytime conditions of simulation stb_stn004f  wp  fc Etc > Eg Eg > Etc Rs min/LAI
    • Eg = f(  ) linear vs. not linear? ()
    • Eg = f(psi) ?
  • 5. Drainage experiment: sensitivity of E TC /E G to soil moisture stb_stn005
  • 6. Effect of a different vegetation fraction in GEOtop (I) Simulation on Little Washita with GEOtop 0.875 (Rigno et al., JHM, 2006)
  • 7. Effect of a different vegetation fraction in GEOtop (II) Simulation on Little Washita with GEOtop 0.875 (Rigno et al., JHM, 2006)
  • 8. Vegetation energy balance in GEOtop. What are the problems ? Stubai Neustift data - Snow-free daily averaged energy and Ts cycle - simulation stb_stn028 G daily cycle over-estimated Ts daily cycle over-estimated It behaves as bare soil (See Best, 1998)
  • 9. Vegetation energy balance in GEOtop. What is missing ? 2. Radiation adsorption in the canopy (shadow of leaves) Increase Rlw, Decrease Rsw 3. Reduction wind speed in the canopy. Little H, ET from below the canopy. Decrease G. What we can to do to improve vegetation description ?
    • Work with parameters: increase z 0 (roughness): more turbulent exchange, increase H and LE (from above the canopy)
    • Explicit model T c
    • H and LE from the canopy function of T c ≠T a and e as (T c );
    • Now T c = T a ; e as (Tc) = e as (Ta)
  • 10. Bare soil evaporation. Some improvements.
    • Eg = f(  ) linear vs. not linear? ()
    • Eg = f(psi) ?
  • 11. 2. Proposed vegetation layer energy balance scheme