Evaluation of FAO-56 procedures for estimating reference
evapotranspiration using missing climatic data for a Brazilian tropical
savanna
Abstract
Since the Brazilian Cerrado has been heavily impacted by agricultural
activities over the last four to five decades, reference
evapotranspiration (ETo) plays a pivotal role in water resources
management for irrigation agriculture. The Penman-Monteith (PM) is one
of the most accepted models for ETo estimation, but it requires many
inputs that are not commonly available. Therefore, assessing the FAO
guidelines to compute ETo when meteorological data are missing could
lead to a better understanding of how climatic variables are related to
water requirements and atmospheric demands for a grass-mixed savanna
region and which variable impacts the estimates the most. In this study,
ETo was computed from April 2010 to August 2019. We tested twelve
different scenarios considering radiation, relative humidity, and/or
wind speed as missing climatic data using guidelines given by FAO. When
wind speed and/or relative humidity data were the only missing data, the
PM method showed the lowest errors in the ETo estimates and correlation
coefficient (r) and Willmott’s index of agreement (d) values close to
1.0. When radiation data were missing, computed ETo was overestimated
compared to the benchmark. FAO procedures to estimate the net radiation
presented good results during the wet season; however, during the dry
season, their results were overestimated, especially because the method
could not estimate negative Rn. Therefore, we can infer that radiation
data have the highest impact on ETo for our study area and also regions
with similar conditions and FAO guidelines are not suitable when
radiation data are missing.