Comparison of hourly and daily reference crop evapotranspiration equations across seasons and climate zones in Australia

Abstract

Estimates from the FAO Penman–Monteith (FAO-PM) and the standardized ASCE Penman–Monteith(ASCE-PM) hourly and daily reference evapotranspiration (ET0) equations were compared at daily scale,based on the hourly climate data collected from forty (40) geographically and climatologically diverseAutomatic Weather Stations (AWS) across the Australian continent. These locations represent 23 agri-cultural irrigation areas in tropical, arid and temperate climates. The aims of this paper are to: comparethe effects of different methods of estimating Clear-sky-radiation—(Rso); compare sum-of-hourly anddaily ET0; compare the results of aggregation of hourly ET0over 24 h compared with daylight hours;and examine the impact of seasonality and climate type. At selected AWS locations, the hourly ET0wascalculated using the hourly FAO-PM and the ASCE-PM equations and then summed to derive daily ET0(reported as ET0,soh). This was compared against the daily ET0values, calculated using the correspondingdaily equation (reported as ET0,daily). Using Rsocalculated following the “complex” approach improves theagreement between ET0,sohand ET0,dailyof both hourly equations, compared with the “simple” approach.Better agreement between ET0,sohand ET0,dailyestimates for the FAO-PM and ASCE-PM were found, whenthe hourly values are aggregated over 24 h rather than over daylight hours. The average ratio betweenET0,sohand ET0,dailyfor the FAO-PM and ASCE-PM equations is 0.95 and 1.00, respectively. The range ofthe former is 0.90–0.98 and that of the latter is 0.96–1.04. There was very strong correlation between thetwo hourly equations at the daily time step: on average 0.997, with a range of 0.993–0.998. The resultsimply that the ASCE-PM hourly equation’s daily ET0values are higher than those of FAO-PM, which canbe explained by the difference in the treatment of surface resistances. Better agreements between ET0,sohand ET0,dailyvalues for winter, spring and autumn were found for the FAO-PM version, while during sum-mer, the ASCE-PM version showed better agreement. The best agreement between the hourly and dailyresults for the FAO-PM version was found in temperate climates and the ASCE-PM version showed bestagreement in the tropical and arid climates.