Eric Harmsen, Antonio González Pérez Amos Winter

1. Estimating Long-Term Average Monthly Evapotranspiration from Pan Evaporation Data at Seven Locations in Puerto Rico Eric Harmsen, Antonio González Pérez Amos Winter

2. Department of Agricultural and Biosystems Engineering, University of Puerto Rico, Mayagüez-Campus This Material Is Based on Research Supported by University of Puerto Rico Agricultural Experiment Station

3. SITUATION The pan evaporation method is a widely used method for scheduling irrigation because it is simple and inexpensive The University of Puerto Rico Agricultural Experiment Station is promoting its use with vegetable crops.

5. Crop Water Requirement (CWR) = Crop Water Use (CWU) = Potential Evapotranspiration (ET) = Consumptive Use (CU)

9. Why is Knowing the Correct Consumptive Use Important?

10. Inaccurate consumptive use information may leads to: • Loss of water by deep percolation and surface runoff • Waste of energy • Reduction in Yield ($$) • Leaching of chemicals to groundwater

12. Procedure for Estimating Evapotranspiration from Pan Data

13. ET = Kc ETo ET= Crop evapotranspiration Kc = Crop coefficient ETo = Reference evapotranspiration

14. ET = KcKp Epan ET= Crop evapotranspiration Kc = Crop coefficient Kp = Pan coefficient Epan = Pan evaporation

15. A monthly average pan coefficient (Kp) can be estimated from the monthly average values of Epan and ETo: Kp = Epan / ETo

16. In 1989 the UPR Ag. Experiment Station published average monthly values of Kp for seven Experimental Substations

17. UPR AGRICULTURAL EXPERIMENT SUBSTATION NOAA CLIMATE DIVISIONS OF PUERTO RICO: 1, NORTH COASTAL; 2, SOUTH COASTAL; 3, NORTHERN SLOPES; 4, SOUTHERN SLOPES; 5, EASTERN INTERIOR; AND 6, WESTERN INTERIOR.

18. The Kp data were based on: • Average monthly pan data from 1960 to 1980. • Average monthly ETo data based on the SCS Blaney-Criddle method.

19. The Problem • Pan evaporation may have changed over the last 20 years. • The SCS Blaney-Criddle method produces estimates of ETo of low accuracy.

20. OBJECTIVE Evaluate historical pan evaporation data from seven experimental substations in Puerto Rico. Update pan evaporation coefficients (Kp) values for the seven University of Puerto Rico Experimental Substations, based on the Penman-Monteith reference evapotranspiration.

21. ETo from the Penman Monteith Method • Food and Agr. Organization (FAO) of the United Nations has recommended use of the Penman-Monteith (PM) method for estimating reference evapotranspiration(ETo) . • The PM method is the preferred method based on an ASCE study which compared twenty (20) ET estimation methods with weighing lysimeters. • The PM method performed better than all other methods.

22. Δ = slope of the vapor pressure curve Rn = net radiation G = soil heat flux density γ = psychrometric constant T = mean daily air temperature u2 = wind speed at 2 m height es = the saturated vapor pressure and ea = the actual vapor pressure.

23. FAO States: • “If some of the required weather data are missing or cannot be calculated, it is strongly recommended that the user estimate the missing climate data with one of the following procedures and use the FAO Penman-Monteith method for the calculation of ETo.” • “The use of an alternative ETo calculation procedure, requiring limited meteorological parameters, is less recommended."

25. RESULTS Historical Pan Data

26. Average Monthly Pan Evaporation with Time At Lajas and Río Piedras, Puerto Rico

27. AVERAGE MONTHLY PAN EVAPORATION WITH TIME AT ADJUNTAS AND GURABO, PUERTO RICO

28. Average Monthly Pan Evaporation with Time at Corozal, Isabela and Fortuna, Puerto Rico

29. Results of Pan Evaporation Trend Analysis

30. Reevaluation of the Pan Coefficient Kp = Epan / ETo

31. LONG-TERM AVERAGE MONTHLY PAN EVAPORATION FOR THE SEVEN EXPERIMENTAL SUBSTATIONS IN PUERTO RICO

32. LONG-TERM AVERAGE MONTHLY REFERENCE EVAPOTRANSPIRATION FOR THE SEVEN EXPERIMENTAL SUBSTATIONS. REFERENCE EVAPOTRANSPIRATION WAS ESTIMATED USING THE COMPUTER PROGRAM PR-ET (HARMSEN AND GONZÁLEZ PÉREZ, 2002).

33. Pan Coefficients (Kp) based on 1981 through 2000 pan evaporation data, for seven experimental substations.

34. METHOD LIMITATIONS

35. ET  Epan

36. Possible differences in loss of water from a water surface and from a cropped surface: • Reflection of solar radiation from the water surface might be different than the assumed 23% for the grass reference surface. • Storage of heat within the pan can be appreciable and may cause significant evaporation during the night while most crops transpire only during the daytime. • There are differences in turbulence, temperature and humidity of the air immediately above the respective surfaces; and • Heat transfer occurring through the sides of the pan can affect the energy balance.

37. Sweet Pepper Crop – Isabela, PR

38. Seasonal ET Penman-Monteith 403 mm (Preferred Method) Pan 365 mm (Difference = -10%) Long-term Average Penman-Monteith 451 mm (Difference = +12%)

39. CONCLUSION • Historical pan evaporation data were evaluated to determine if increasing or decrease trends exist for data from the seven UPR Experimental Substations. • Significant decreasing pan evaporation was observed at Lajas and Río Piedras. • Significant increasing pan evaporation was observed at Gurabo and Adjuntas. • No significant trends were observed at Fortuna, Isabela and Corozal.

40. CONCLUSIONS – CONT. • A significant difference was found to exist between the mean Kp calculated with pan evaporation data from 1960-1980 and 1981-2000. • The mean of Kp values based on 1981-2000 data was approximately 5% higher than the mean of the Kp values based on 1960-1980 data.

41. CONCLUSIONS – CONT. • Errors in the estimated seasonal ET of -10% and +12% were obtained for a sweet pepper crop when using the pan-based and long term Penman-Monteith ET, Respectively. • The magnitude of these errors could lead to substantial reductions in yield and, in the latter case, leaching of chemicals to the groundwater.