Impact of sum-of-hourly and daily timesteps in the computations of reference evapotranspiration across the Brazilian territory

Abstract

Estimating reference evapotranspiration (ETo) in 24 h timesteps has been considered sufficiently accurate for a long time. However, recent advances in weather data acquisition has made it feasible to apply the hourly procedures in ETo computation. Hourly timesteps can improve ETo estimates accuracy, for data averaged daily may misrepresent evaporative power during parts of the day. Only a few studies have assessed vast databases, yet, studies considering tropical regions are basically inexistent. The objective of the present study was to assess the differences between daily ETo computations performed on 24 h (ETod) and hourly (EToh) timesteps for the Brazilian territory. Daily and hourly ETo computations were performed according to the standardized ASCE (American Society of Civil Engineers) Penman-Monteith equation. Large daily weather variations of meteorological parameters resulted in ETod generally underestimating EToh, while lower variations resulted in ETod overestimating. Overall, ETod overestimated EToh in 0.7%, with monthly percentage bias ranging from -3.9% to 2.9%. This study considered one year of data from 567 automatic weather stations and, despite acknowledging the existence of interannual climate variability, findings strongly agreed with relevant literature. ETod predominantly overestimated EToh during wet periods in warm regions. Thus, this behavior is observed almost year-round for the tropical monsoon and rainforest climate, in the Amazon and Pantanal biome. For the tropical savannah and semi-arid climate, ETod overestimated EToh during wetter periods (main crop harvest) but underestimated along the dry season (second harvest). On the other hand, ETod predominately underestimated EToh for colder regions, such as the Pampa (humid sub-tropical climate), regardless of rainfall. The Cerrado and Caatinga are likely to be affected most, for EToh is underestimated in periods of lower water availability, making adequate irrigation techniques fundamental.