SEBAL - Dca.ufcg.edu.br

More documents

Recommendations

Info

elevation data is provided by the DEM. This “artificial” lapse-adjusted surface temperatureis referred to as a DEM corrected surface temperature (T s-dem ).The DEM corrected surface temperature is calculated by the following equation:Ts_ dem= Ts+ 0. 0065∆z(12-7)where; ∆z is the elevation of each pixel minus the elevation of a datum (meters). The term∆z is positive if the elevation of a pixel is higher than the datum. The representativeelevation for the image, usually at the weather station, is used for the datum elevation.I. Outgoing Longwave RadiationThe outgoing longwave radiation (R L↑ ) is computed as in the Flat Model using theuncorrected surface temperature (T s ).J. “Cold” and “Hot” Pixel SelectionThe corrected surface temperature (T s-dem ) is used for the selection of the two “anchor”pixels. The T s image shows high and low pixels rather than wet or dry pixels because of theelevation effects. For higher than datum pixels, the lapse correction is added and T s-dem ishigher than T s . Using the T s-dem data allows us to find truly wet and dry pixels. The methodsdescribed in Appendix 7 are followed except that the T s-dem image is used rather than the T simage.K. Incoming Longwave RadiationThe incoming longwave radiation (R L↓ ) is computed since the transmissivity of air is nolonger a constant for all pixels. The reference temperature (T cold ) is adjusted for the lapseeffect in a similar way as in equation (12.7):T cold(each pixel) = T cold + 0.0065∆z (12-8)where; ∆z is the elevation of the “cold” pixel minus the elevation of each pixel. Theelevation of the “cold” pixel is taken from the DEM subset image by entering itscoordinates.L. Net Surface RadiationThe net surface radiation (R n ) is computed in model_14 similar to the Flat Model exceptthat incoming longwave radiation (R L↓ ) and incoming shortwave radiation (R S↓ ) are nowfiles rather than constants.M. Soil Heat FluxThe soil heat flux (G) and G/R n are computed as in the Flat Model using the uncorrectedsurface temperature (T s ).96
N. Momentum Roughness LengthThe momentum roughness length (z om ) is initially computed as in the Flat Model using aland use map and LAI as input. It is then adjusted for the slope using the followingequation:z om(mnt) = z om × [1 + (slope - 5)/20] (12-9)O. Initial Friction Velocity and Aerodynamic Resistance to Heat TransportThe wind velocity at 200 meters (u 200 ) is adjusted for elevation effects before computing thefriction velocity (u * ) and the resistance to heat transport (r ah ). The elevation effects on windspeed are complicated and difficult to quantify. A mountain wind speed weightingcoefficient (ϖ) is used to adjust u 200 as follows:ϖ = 1 + 0.1[ ({Elevation – Elevation station }/1000)] (12-10)where; Elevation is the elevation of each pixel and Elevation station is the elevation wherethe wind speed is measured. u 200 for each pixel is then adjusted by multiplying it by ϖ.P. Sensible Heat FluxThe sensible heat flux (H) is computed as in the Flat Model. The uncorrected surfacetemperature (T s ) is used to compute a new air density. The corrected surface temperature(T s-dem ) is used for the computation of dT.Q. Atmospheric Stability CorrectionThe iterative process for the atmospheric stability correction is similar to the Flat Model anduses the same spreadsheet.R. Applications and Precautions For The SEBAL Mountain ModelThe Mountain Model is intended to improve the computation of components of the energybalance for sloping areas. However, some uncertainties remain, so that while the MountainModel does provide improved estimates of ET in mountainous areas, the ET estimates inmountains have higher uncertainty than those for flatter areas. When integrated over asub-basin scale, the ET from the SEBAL mountain model is probably reasonably accurate.This is because some of the uncertainties (due to unknown wind speed and direction forexample) tend to cancel.At the present time, the “SEBAL mountain model” is offered by the University of Idaho for apurchase fee. The fee is intended to recoup development costs for the model and tosupport user inquiries and needs, which may be extensive, due to the higher level ofsophistication in the mountain model.97
Page 1 and 2:
SEBALSurface Energy Balance Algorit
Page 3 and 4:
6. Tables for surface albedo comput
Page 5 and 6:
δ declination of the earth radians
Page 7 and 8:
INTRODUCTION1. BACKGROUNDLand manag
Page 9 and 10:
THE THEORETICAL BASIS OF SEBAL1. OV
Page 11 and 12:
The surface emissivity is the ratio
Page 13 and 14:
• Reference evapotranspiration, E
Page 15 and 16:
Figure 3. Flow Chart of the Net Sur
Page 17 and 18:
2. The reflectivity for each band (
Page 19 and 20:
and ET are specified for the two ex
Page 21 and 22:
To complete step 2, model F06_surfa
Page 23 and 24:
D. Choosing the “Hot” and “Co
Page 25 and 26:
irrigated crops near Kimberly, Idah
Page 27 and 28:
where; u * is the friction velocity
Page 29 and 30:
Weather stationu, z x, z om, u *H f
Page 31 and 32:
Note: An alternative method used by
Page 33 and 34:
where;⎛ 2 ⎞⎜1+ x(0.1m)Ψ = 2
Page 35 and 36:
where; ET inst is the instantaneous
Page 37 and 38:
Figure 6 shows an example of a dail
Page 39 and 40:
1. If there is some cloud cover in
Page 41 and 42:
• For all other formats (includin
Page 43 and 44:
On the main page, select Type in Pa
Page 45 and 46: These values for z om can vary by r
Page 47 and 48: Appendix 3Reference Evapotranspirat
Page 49 and 50: and click on “Open”3. Provide t
Page 51 and 52: 5. Describe weather station and wea
Page 53 and 54: 8. Save the output file using a nam
Page 55 and 56: 4. Go to the Viewer tool and view t
Page 57 and 58: Appendix 5Time Issues Around Weathe
Page 59 and 60: 2. What time interval does the data
Page 61 and 62: The wind speed is 3.4 m/s for the 1
Page 63 and 64: Table 6.3. ESUN λ for Landsat 5 TM
Page 65 and 66: Table 6.6. Typical albedo values.Fr
Page 67 and 68: 4. Make a first guess for T cold by
Page 69 and 70: Fig.3. P40/R30, Landsat 7 ETM+ 4/8/
Page 71 and 72: Fig.5. P40/R30, Landsat 7 ETM+ 4/8/
Page 73 and 74: We open the image or subset image a
Page 75 and 76: Now, we look at the northwest area
Page 77 and 78: (2) The regional weather or soil co
Page 79 and 80: We conclude that this “heat islan
Page 81 and 82: Part 1: using the manual iteration
Page 83 and 84: Part 2: Using the automatic iterati
Page 85 and 86: Appendix 9SAVI and LAI for Southern
Page 87 and 88: equation for G. The following are e
Page 89 and 90: and for 24-hours:Gwater = 0.9 Rn -
Page 91 and 92: The Stability of the AtmosphereAppe
Page 93 and 94: Appendix 12The SEBAL Mountain Model
Page 95: where; t is the standard clock time
show all

SEBAL - Dca.ufcg.edu.br

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?