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 diverse Automatic Weather Stations (AWS) across the Australian continent. These locations represent 23 agricultural irrigation areas in tropical, arid and temperate climates. The aims of this paper are to: compare the effects of different methods of estimating Clear-sky-radiation-(Rso); compare sum-of-hourly and daily ET0; compare the results of aggregation of hourly ET0over 24h compared with daylight hours; and examine the impact of seasonality and climate type. At selected AWS locations, the hourly ET0was calculated 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 corresponding daily equation (reported as ET0,daily). Using Rsocalculated following the "complex" approach improves the agreement 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, when the hourly values are aggregated over 24h rather than over daylight hours. The average ratio between ET0,sohand ET0,dailyfor the FAO-PM and ASCE-PM equations is 0.95 and 1.00, respectively. The range of the former is 0.90-0.98 and that of the latter is 0.96-1.04. There was very strong correlation between the two hourly equations at the daily time step: on average 0.997, with a range of 0.993-0.998. The results imply that the ASCE-PM hourly equation's daily ET0values are higher than those of FAO-PM, which can be 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 summer, the ASCE-PM version showed better agreement. The best agreement between the hourly and daily results for the FAO-PM version was found in temperate climates and the ASCE-PM version showed best agreement in the tropical and arid climates.
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