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

2. NWS Calibration Workshop, LMRFC March, 2009 Slide 2 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

3. NWS Calibration Workshop, LMRFC March, 2009 Slide 3 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

4. 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

5. 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

6. NWS Calibration Workshop, LMRFC March, 2009 Slide 6 0 1.5 Jan Dec June PE Adjustment Factor Month 1.0 Seasonal PE Adjustments

7. NWS Calibration Workshop, LMRFC March, 2009 Slide 7 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.

8. NWS Calibration Workshop, LMRFC March, 2009 Slide 8 Annual FWS Evaporation Illinois River above Watts, OK

9. NWS Calibration Workshop, LMRFC March, 2009 Slide 9

10. NWS Calibration Workshop, LMRFC March, 2009 Slide 10 Area = 46.37 in. Adjust to 41.34 in. Map 3 Tech Report 3

11. NWS Calibration Workshop, LMRFC March, 2009 Slide 11 Steps for Computing Daily MAPE Time Series 1.Select station 2.Compute point PE time series using Syntran or Browser 3.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 Mean MAPE = 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.

12. 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.

13. 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’