PENMAN-MONTEITH, FAO-24 REFERENCE CROP EVAPOTRANSPIRATION AND CLASS-A PAN DATA IN AUSTRALIA

The Food and Agriculture Organisation (FAG) methods are recognised as the international standard for estimating reference crop evapotranspiration (ET(o)). The Penman-Monteith method is currently favoured by the FAO over the FAO-24 methods. The FAO also recommends alternative methods which may be used where there are limited data. In this paper, ET(o) estimated using the Penman-Monteith and FAO-24 methods and class-A pan data for 16 Australian locations with a wide range of climate conditions are compared. The analyses indicate that the FAO-24 Penman ET(o) estimates are generally 20 to 40% higher than the Penman-Monteith estimates. However, the FAO-24 Radiation and Penman-Monteith methods give similar daily ET(o) estimates, Unlike Penman-Monteith, which also requires windspeed data, the FAO-24 Radiation method estimates ET(o) from temperature and sunshine hours, climate variables which are relatively conservative in space. The FAO-24 Radiation method can thus be used as a surrogate for Penman-Monteith to estimate daily ET(o) for areas where windspeed data are not available. The FAO-24 Blaney-Criddle method, which uses only temperature data, gives similar monthly ET(o) estimates as Penman-Monteith, and is therefore adequate for applications where only long-term ET(o) estimates are required. The comparisons also show that there is a satisfactory correlation between class-A pan data and Penman-Monteith ET(o) for evaporation totals over 3 or more days. However, the pan coefficient is very dependent on local climate and physical conditions, and it should be determined by comparing the pan data with either the Penman-Monteith or FAO-24 Radiation ET(o) estimates.