Transcript Analysis of Evaporation Basic Calibration Workshop March 10-13, 2009 LMRFC

Analysis of Evaporation
Basic Calibration Workshop
March 10-13, 2009
LMRFC
NWS Calibration Workshop, LMRFC March, 2009
Slide 1
Analysis of Evaporation
Definition of Terms
• Potential Evaporation (PE):
– The evaporation from a well wetted (i.e., moisture supply is not limiting
the evaporation), actively growing grass surface.
– PE rate is calculated from current met. conditions: temp, humidity, wind,
radiation, etc.
• Free Water Surface Evaporaton (FWS)
– Evaporation from a water surface with no heat storage
– Lake evaporation involves some energy transfer in warming,cooling
during, thus is different than FWS.
– FWS evaporation is equivalent to PE from a grass surface
• Evapotranspiration (ET) Demand
– SAC-SMA term for evaporation that occurs when moisture is not limiting
– Considers type and activity of vegetation
NWS Calibration Workshop, LMRFC March, 2009
Slide 2
Analysis of Evaporation
Definition of Terms, cont’d.
• Seasonal PE Adjustment Curve
– Modifies PE values on a seasonal basis for
• Type of vegetation
• Activity level of vegetation
• ET Demand = PE X PE adjustment curve
• Actual ET
– Amt. of evaporation loss given
• ET Demand
• current moisture conditions and snow cover
• Pan Evaporation
– Amt. of evaporation measured by an evaporation pan
– Differs from FWS evap. due to change in heat storage
– Class A pan most common
• Pan Coefficient
– Average ratio of FWS evaporation to that measured by a pan
– < 1.0
– Range: usually 0.66 to 0.88
NWS Calibration Workshop, LMRFC March, 2009
Slide 3
Determination of PE
• Monthly Average PE
– Computed from met. Factors
– Computed from average monthly pan evaporation.
• Daily PE
– Computed from met. factors using a Penman type
equation
• Factors: air temp, dew point, wind speed, solar radiation.
– Computed from daily pan measurements
• Adjust by pan coefficient
NWS Calibration Workshop, LMRFC March, 2009
Slide 4
Seasonal PE Adjustment Curves
• Defined at the mid-point of each month
• Linear interpolation between points.
• Evap. rates for actively growing vegetation
differ from grass (basis for PE).
• Seasonal differences
NWS Calibration Workshop, LMRFC March, 2009
Slide 5
Seasonal PE Adjustments
PE Adjustment Factor
1.5
1.0
0
Jan
June
Dec
Month
NWS Calibration Workshop, LMRFC March, 2009
Slide 6
Estimation of Mean ET-Demand
1. Determine mean annual PE or FWS evap.
using Tech. Report #33 (in CAP)
2. Break down mean annual PE into ave. daily
values for each month
3. Estimate seasonal PE adjustment curve
•
•
•
Personal observations
Irrigation and Drainage paper #24
NDVI greenness data
•
Nearby watersheds.
4. For each month, multiply daily average PE by
the PE adjustment for the middle of the month.
NWS Calibration Workshop, LMRFC March, 2009
Slide 7
Annual FWS
Evaporation
Illinois River above
Watts, OK
NWS Calibration Workshop, LMRFC March, 2009
Slide 8
Seasonal Variation in FWS
Evaporation
Ave. Daily FWS Evap.
(mm)
Illinois River At W atts, OK.
7
6
5
4
3
2
1
0
1 2 3 4 5 6 7 8 9 10 11 12
month
Fort Smith FWS
Fort Gibson FWS
average
NWS Calibration Workshop, LMRFC March, 2009
Slide 9
Derivation of ET Demand Curve
Area = 46.37 in.
Adjust to 41.34 in. Map 3 Tech Report 3
6
1.2
5
1
4
0.8
3
0.6
2
0.4
1
0.2
0
0
PE Adjustment
Ave. Daily FWS Evap.
(mm)
Illinois River At Watts, OK.
1 2 3 4 5 6 7 8 9 10 11 12
month
average
pe-adj
ET demand
NWS Calibration Workshop, LMRFC March, 2009
Slide 10
Steps for Computing Daily MAPE Time Series
1.
2.
3.
Select station
Compute point PE time series using Syntran or Browser
Select weighting scheme
1. Grid point
2. Pre-determined
4.
Check consistency
5.
Generate MAP time series using MAPE program
6.
Adjust long term mean from MAPE time series to agree with basin
mean measured from NWS 33 Map no. 3:
PEADJ x MeanMAPE = Mean Map No. 3
7.
8.
PEADJ is a parameter for SAC-SMA. It multiplies all values in
MAPE time series
Derive PE Adjustment curve to modify daily MAPE time series for
the effects of vegetation. PE-ADJ curve in this case is explicitly
entered on line 5 of SAC-SMA input.
NWS Calibration Workshop, LMRFC March, 2009
Slide 11
Problems
• ASOS sky cover data are not the same as
the original manual sky cover
observations.
• Thompson (1976) used these manual
observations of sky cover as a
measurement of net solar radiation.
NWS Calibration Workshop, LMRFC March, 2009
Slide 12
Current Research
• NASA Marshall SFC R&D:
– Use MODIS sensor cloud mask
– ASOS cloud cover to 12,600 ft
Combine to create
‘sky cover’
• Input ‘sky cover’ into existing equations to
compute daily PE
• Simulation results reasonable, but need
further analysis
• ABRFC using ASOS only
NWS Calibration Workshop, LMRFC March, 2009
Slide 13