NWS Calibration Workshop, LMRFC March, 2009 Slide 1

Analysis of Evaporation

Basic Calibration Workshop

March 10-13, 2009

LMRFC

NWS Calibration Workshop, LMRFC March, 2009 Slide 2

Analysis of EvaporationDefinition 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 3

Analysis of EvaporationDefinition 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 4

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 5

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 6

0

1.5

Jan DecJune

PE

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just

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nt F

act

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Month

1.0

Seasonal PE Adjustments

NWS Calibration Workshop, LMRFC March, 2009 Slide 7

Estimation of Mean ET-Demand1. 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 month3. 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 8

Annual FWSEvaporation Illinois River aboveWatts, OK

NWS Calibration Workshop, LMRFC March, 2009 Slide 9

Seasonal Variation in FWS Evaporation

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1 2 3 4 5 6 7 8 9 10 11 12

month

Ave

. D

aily

FW

S E

vap

.

(mm

)

Fort Smith FWS Fort Gibson FWS average

Illinois River At Watts, OK.

NWS Calibration Workshop, LMRFC March, 2009 Slide 10

Derivation of ET Demand Curve

0

1

2

3

4

5

6

1 2 3 4 5 6 7 8 9 10 11 12month

Ave.

Dai

ly F

WS

Eva

p.

(mm

)

0

0.2

0.4

0.6

0.8

1

1.2

PE

Adj

ustm

ent

average pe-adj ET demand

Illinois River At Watts, OK.

Area = 46.37 in.Adjust to 41.34 in. Map 3 Tech Report 3

NWS Calibration Workshop, LMRFC March, 2009 Slide 11

Steps for Computing Daily MAPE Time Series

1. Select station2. Compute point PE time series using Syntran or Browser3. Select weighting scheme

1. Grid point2. Pre-determined

4. Check consistency5. Generate MAP time series using MAPE program6. 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. PEADJ is a parameter for SAC-SMA. It multiplies all values in MAPE time series

8. 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 12

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 13

Current Research

• NASA Marshall SFC R&D:– Use MODIS sensor cloud mask– ASOS cloud cover to 12,600 ft

• Input ‘sky cover’ into existing equations to compute daily PE

• Simulation results reasonable, but need further analysis

• ABRFC using ASOS only

Combine to create‘sky cover’