Energy Balance and Evapotranspiration Measurement Yu-Jun Cui Ecole Nationale des Ponts et Chaussée, Paris, France Jorge G. Zornberg The University of Texas at Austin, USA
ABOUT THIS TOPIC - Multi-disciplinary Unsaturated soil mechanics, meteorology, hydrogeology, agronomy etc. - Broad application field Analysis of drought effect, slope stability, agricultural soil compaction ; design and monitoring of evapotranpirative cover, etc - Application to be enhanced in geotechnical and environmental engineering Only ONE paper submitted to this Symposium
OUTLINE Evapotranspiration background - Solar Radiation - Evaporation and Transpiration Energy Balance Approach - Background - Case history 1: Boissy-le-Châtel (France) - Case history 2: OII Superfund site (USA) Direct measurement - Background - Case history: Monticello (USA) Water balance approach - Background - Case history: Rocky Mountain Arsenal (USA) Final remarks
Energy balance
Solar radiation Emission rate of the sun : 73 million W/m 2 Portion on the top of the earth atmosphere : 1380 W/m 2 30% scattered back to space 70% transmitted through the atmosphere - 19% absorbed by gazes - 51% transmitted down to the earth surface Radiation balance : R n = [( S + D) ( S + D) a] + ( L L ) down up S : direct shortwave radiation D: diffuse shortwave radiation L: Longwave radiation a : earth albeto reflected radiation
Soil water balance P ( I + R ) = ET + R S + nt off wt Water infiltration to the soil ground : ( R + S ) = P ( I + ET R ) I = + wt nt off
Evapotranspiration Evaporation : change in state of water from liquid to water vapor when the transfer of energy towards water increases the kinetic energy Transpiration : evaporation from the vascular system of plants Estimating evapotranspiration (Penman 1948): PET 1000 R /( ρ L n w v a = + γ ) + γe = 4099P vs E = 0.165( P P )( 0.8 + u /100) ( T + 237.3) 2 a vs v 2 u 2 = u z ln 4.87 ( 67.8z 5.42)
Evapotranspiration measurement Direct measurement Energy balance approach Soil water balance approach
Energy balance approach R n = Le + H + G Sensitive heat transfer H = ρ C a p k H T z Latent heat transfer (evaporation) Le = L ρεk v P v P v z Soil heat transfer G T = λ y soil
Energy balance approach H PCp T T β = = = γ Le L ε P P v v v Le R G n = 1 + β
Bowen ratio β measurement Water vapor perssure Hygrometers ( dew point hygrometer in the system of Campbell Scientific) Air temperature Thermocouples (chrome constantan thermocouples in the Campbell Scientific) Net radiation flux Net radioameter
Bowen ratio β measurement Campbell Scientific B023 S = TC t s d C s = d ( C + wc ) = ρ C θρ C d w d d w w ρ + G(z = 0) = G(z = d) + S
Bowen ratio system (after Blight 1997)
Case History: Boissy-le-Chatel 0.05 Lodge Drilling hole 1.00 0.05 Temperature monitoring zone Precipitation J monitoring zone (n 28) B H G E F TDR Probes D C A Water collection zone Working house TDR Probes Piezometer Drainage network Legendes: Boundary of experimental 2 drainage zone (615m ) Station of meteorology Measuring zone of water (Flux, Quantity) Garage Laboratory Office Living room 0 5 10 15m After Cui et al. 2005 (in print)
Field monitoring during the year 2003 50 40 30 20 10 0-10 1/1 1/2 1/3 1/4 1/5 1/6 1/7 1/8 1/9 1/10 1/111/12 Date (day/month) RH (%) 100 90 80 70 60 50 40 30 20 10 0 1/1 1/2 1/3 1/4 1/5 1/6 1/7 1/8 1/9 1/10 1/11 1/12 date (day/month) 250 200 150 100 50 Solar radiation (J/m²day) 1500 1000 500 1/1 0 1/2 1/3 1/4 1/5 1/6 1/7 1/8 1/9 Date (day/month) 1/10 1/11 1/12 0 1/1 1/2 1/3 1/4 1/5 1/6 1/7 1/8 1/9 Date (day/month) 1/10 1/11 1/12
Field monitoring during the year 2003 20 (continued) Wind speed (m/s) 15 10 5 0 1/1 1/2 1/3 1/4 1/5 1/6 1/7 1/8 1/9 1/10 1/11 1/12 Date (day/month) 25 20 15 10 5 0 1/1 50 cm 100 cm 1/2 1/3 1/4 1/5 1/6 1/7 1/8 1/9 1/10 1/11 1/12 Date (day/month) ETP (mm/day) 80 70 60 50 40 30 20 10 0 1/1 1/2 1/3 1/4 1/5 1/6 1/7 1/8 1/9 1/10 1/11 1/12 Date (day/month)
-Darcy s law for liquid phase : -Fick s law for vapor phase : coupled through P v to z h k q w w = 1 z P D q V v = 2 + = z P D z C z h k z C t h v v v w w w + = z P D z P P P L t T z t T C v v v e h λ Determination of changes in T soil and θ (after Wilson et al 1994)
Resolution method (Wilson et al s equations) Attribute an initial value to β and determine the corresponding values of H, G, PET and L e Calculate s and T by using H and L e definitions Lower boundary condition (T and h w ) known Resolution of Wilson et al s equations repeat the calculations with another β value until an admissible difference between calculated L e and PET
Temperature change at 0.5 m depth 14.0 13.0 12.0 Temperature ( C) 11.0 10.0 9.0 8.0 Simulation Measurement Day(April, 1999) 0 5 10 15 20 25 30 (after Cui et al. 2005, in print)
Volumetric water content at 5 different depths Volumetric water content (%) 40 37 34 31 28 25 22 19 16 13 10 7 Simulation 15 cm 25 cm 35 cm 45 cm 55 cm 0 5 10 15 20 25 30 Day (April, 1999) (Cui et al. 2005, in print)
Energy balance approach See also Boulet G., Chehbouni A., Braud I., Vauclin M., Haverkamp R., Zammit C. 2000. A simple water and energy balance model designed for regionalization and remote sensing data utilization. Agriculture and forest meteorology 105, 117-132. Shen Y.J., Kondoh A., Tang C.Y., Zhang Y.Q., Chen J.Y., Li W.Q., Sakura Y., Liu C.M., Tanaka T, Shimada J. 2002. Measurement and analysis of evapotranspiration and surface conductance of a wheat canopy. Hydrological Processes 16, 2173 2187. Consideration of vegetation effect : LAI (Leaf Area Index)
Evapotranspirative Cover Systems Prescriptive Cover ( Barrier System) ET cover ( Reservoir System) Overland Flow Precipitation Overland Flow Precipitation Evapotranspiration Percolation Moisture Storage Percolation
Case History: OII Superfund Landfill
Energy Balance Approach 250 Liquid Quantity (mm) 200 150 100 50 0 Infiltration Evapotranspiration Moisture Storage Percolation (MS) ( I ) (E) Monocover (P) -50 Jan Feb Mar Apr May Jun Jul Aug Sep Time
Energy Balance Approach: Rooting Depth 1.60 Percolation (%). 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 Baseline Case 0 300 600 900 1200 1500 Rooting Depth (mm) Source: Zornberg et al. 2003
Energy Balance Approach: Irrigation 90 80 Percolation (%) 70 60 50 40 30 20 10 0 Recommended irrigation (MWD) Baseline Case 0 500 1000 1500 2000 Irrigation (mm/year) Source: Zornberg et al. 2003
Direct measurement using a weighing lysimeter Monitoring of: - Total weight, W - Precipitation, P - Percolation, G ET = P ( W + G) Sources: Benson et al. 2001, Waugh 2002
Case History: Monticello (USA) Sealing the Lysimeter drainage cap Weighing lysimeters for water storage
Case History: Monticello (USA) Source: Waugh (2002)
Monticello Vapor pressure deficit (VPD), photosynthetically active radiation (PAR), and transpiration rate of P. smithii measured July 6 on and adjacent to Lysimeters
ource: Khire et al. (1997) Water Balance Approach 20 Lysimeter SRO Diversion Berm 4 5 ET = P G S R off 20 All dimensions in meters Down Slope 30 0.6 0.6 10 5 SRO Pipe Perc. Pipe
ource: Khire et al. (1997) Typical Lysimeter Cross-Section 20 LLDPE Cutoff Root Barrier Cover Interim Cover Soil LLDPE Cutoff Earthen Berm Percolation Pipe LLDPE Geomembrane Earthen Berm Existing Slope (>2%) Geocomposite Drain
Case History: Rocky Mountain Arsenal, USA
Case History: Rocky Mountain Arsenal, USA 4 instrumented test covers WCR moisture sensors Lysimeter used to measure basal percolation Weather station Temperature Precipitation Solar radiation Wind speed Surface water runoff collection swales WCR sensors
Moisture Content and Percolation Monitoring Results Volumetric moisture content θ, % 30 25 20 15 10 5 0 θ(1080 mm) θ(678 mm) θ(76 mm) Percolation 0 200 400 600 800 1000 1200 1400 1600 1800 Time, days 0.020 0.018 0.016 0.014 0.012 0.010 0.008 0.006 0.004 0.002 0.000 Percolation, mm
Water Balance Approach 5 0 5 0 5 Calculated ET from HYDRUS Estimated ET from water balance Cumulative annual amount of water, mm 700 600 500 400 300 200 100 Calculated ET from HYDRUS Estimated ET from water balance 0 0 200 400 600 800 1000 1200 1400 1600 1800 Time, days 0 0 200 400 600 800 1000 1200 1400 1600 1800 Time, days Comparison between ET estimated from water balance and ET calculated using HYDRUS: - Significant daily discrepancies - Similar cumulative response
Summary
Summary Among the various components of the water balance, measurement of evapotranspiration has probably been the most difficult to quantify Direct measurement of evapotranspiration has been conducted using weighing lysimeters Quantification of evapotranspiration typically conducted using energy balance or water balance methods The current focus on evapotranspirative cover systems has brought renewed need for quantification of evapotranspiration
Final Remarks Significant improvements have been made regarding monitoring of evapotranspiration using direct methods (weighing lysimeter), energy balance methods, and water balance approaches. However, significant additional advances should be made towards integrating unsaturated soil mechanics with other areas such as meteorology,
Grazie Mille