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Enhancement of SAC-HT Evapotranspiration Component through Improved Water Exchange Mechanics

This document outlines the ongoing modifications to the SAC-HT model, which incorporates heat transfer effects to enhance evapotranspiration assessments. Recent evaluations highlighted significant biases in runoff volume and soil moisture predictions, particularly in arid regions, due to shortcomings in the evapotranspiration component. Key improvement strategies include adopting Noah's evapotranspiration parameterization, refining canopy resistance estimations, and exploring simpler potential evaporation models. These changes aim to optimize water exchange mechanisms and improve model accuracy for soil moisture and runoff simulations.

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Enhancement of SAC-HT Evapotranspiration Component through Improved Water Exchange Mechanics

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  1. Progress on the Modification of SAC-HT Evapotranspiration Component Project Victor Koren

  2. Problem Description 1. Recently, SAC-SMA was enhanced by incorporating heat transfer component, SAC-HT version. It accounts for the frozen ground effects and allows much better evaluation of the model by comparing, e.g., soil moisture and temperature at different soil layers 2. Evaluation of SAC-HT over Oklahoma region showed significant runoff volume bias and soil moisture underestimation for dry basins. The main reason is deficiency in SAC-HT evapotranspiration component that leads to disproportional removal of soil moisture from upper and lower zones

  3. Problem Description – cont’d 3

  4. Problem Description – cont’d Comparing Daily Runoff and Soil Moisture Simulated Using a Priori (yellow) and Climate Adjusted Parameters (purple) , #07316500, Gavg =0. 30 4

  5. Three steps are considered for the improvement of the water exchange mechanism of SAC-HT: Formulation of SAC-HT water exchange mechanism based on the Noah evapotranspiration parameterization Implementation of Noah-type canopy resistance parameterization Investigation of the potential for the use of simpler approaches of potential evaporation estimation Ongoing Modification to SAC-HT

  6. Soil Moisture Redistribution Etsp=f(Ep,σ,Si,Fr,Ft,Fq) Elo=f(ΔEp,Slo) Eup=f(Ep,Sup) Ebare=f(Ep,σ,S1) Ecan=f(Ep,σ,Sc) Etsp1 Etsp2 Etsp3 • SAC-SMA free water redistribution • (percolation based) • Noah tension water redistribution • (diffusion equation with only layer evaporation source) Etsp4

  7. Four canopy stress components: Solar radiation Soil Moisture Air humidity Air temperature Total canopy resistance combines all stress factors Actual evapotranspiration is a potential evaporation reduced by a plant coefficient estimated following Monteith approach Ch is the surface exchange coefficient, Rr represents the upward long wave radiation, Δ is a humidity/temperature gradient ratio Canopy Resistance Parameterization

  8. Changes to Noah Canopy Resistance Estimation Solar radiation is estimated from air temperature (Bristow & Campbell, 1984) Empirical relationship (Popov, 1948) is used in estimation of air humidity Logistic dose-response curve (Schenk & Jackson, 2002) is used for root distribution calculation Minimal stomatal resistance parameter (Rsmin) depends on climate not just vegetation type Rsmin = f(veg_type, Gind)

  9. Estimated half-hour Solar Radiation (left ) and Plant Coefficient (right) for three Locations: Alptal (Switzerland), Berm (Canada), and Fraser (USA) Correlation (R2): Solar Radiation Plant Coefficient Alptal 0.81 0.95 Berm 0.78 0.89 Fraser 0.77 0.95

  10. Observed and Simulated 1-hr Solar Radiation at Alptal (Switzerland), March 2003

  11. Observed (red) and Simulated Soil Moisture at 5cm (4), 0-25 (5), 25-75 (6), and Upper (2) & Lower (3) SAC Storages: ARNE site (Climate Index = 0.306).Lines: white – SAC, purple – Mod_SAC, yellow - Noah

  12. Soil Moisture Climatology from SAC-HT and Modified SAC-HT vs Measurements

  13. Overall statistics for 10 Oklahoma basins

  14. Soil Moisture Simulations using Climate (purple) and Penman estimated (yellow) PET, WTTO2

  15. OHD Climatological and Penman-based PETBasin WTTO2, G=0.63 Statistics using uniformly adjusted Penman PET: Runoff: RMSE=15.4(21.2), Bias=7.5(14.9), NS=0.71(0.45) Upper: RMSE=0.12(0.12), Bias=0.02(0.05), NS=0.43(0.44) Lower: RMSE=0.08(0.07), Bias=0.004(0.03), NS=0.51(0.60)

  16. Soil Moisture Simulations using Climate (purple) and Penman estimated (yellow) PET, #7300500

  17. OHD Climatological and Penman-based PETBasin 7300500, G=0.27 Statistics using uniformly adjusted Penman PET: Runoff: RMSE=1.41(1.56), Bias=-0.93(-1.02), NS=-0.30(-0.59) Upper: RMSE=0.10(0.11), Bias=-0.03(-0.06), NS=0.03(-0.18) Lower: RMSE=0.12(0.14), Bias=-0.09(-0.12), NS=-0.76(-1.44)

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