Analysis of Evaporation

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

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

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

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

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

6. Seasonal PE Adjustments 1.5 1.0 PE Adjustment Factor 0 Jan June Dec Month

7. Estimation of Mean ET-Demand • Determine mean annual PE or FWS evap. using Tech. Report #33 (in CAP) • Break down mean annual PE into ave. daily values for each month • Estimate seasonal PE adjustment curve • Personal observations • Irrigation and Drainage paper #24 • NDVI greenness data • Nearby watersheds. • For each month, multiply daily average PE by the PE adjustment for the middle of the month.

8. Annual FWS Evaporation Illinois River above Watts, OK

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

11. Steps for Computing Daily MAPE Time Series • Select station • Compute point PE time series using Syntran or Browser • Select weighting scheme • Grid point • Pre-determined • Check consistency • Generate MAP time series using MAPE program • 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 • 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.

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.

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’