Abstract
The Brazilian savanna (Cerrado) has been heavily impacted by agricultural activities over the last four to five decades, and reliable estimates of reference evapotranspiration (ETo) are needed for water resource management and 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 which variables are critically important for reliable estimates of ETo and how climatic variables are related to water requirements and atmospheric demands. In this study, ETo was computed for a grass-dominated part of the Cerrado from April 2010 to August 2019. We tested 12 different scenarios considering radiation, relative humidity, and/or wind speed as missing climatic data using guidelines given by the FAO. Our results presented that wind speed and actual vapor pressure do not affect ETo estimates as much as the other climatic variables; therefore, in the Cerrado’s conditions, wind speed and relative humidity measurements are less required than temperature and radiation data. When radiation data were missing, the 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 because the method could not estimate negative Rn. Our results indicate that radiation data have the highest impact on ETo for our study area and presumably for regions with similar climatic conditions. In addition, those FAO procedures for estimating radiation are not suitable when radiation data are missing.
Highlights
Introduction conditions of the Creative CommonsOver the last few decades, the hydrological cycle and climate of the Brazilian savanna have been heavily affected by human activities, especially by the expansion of irrigation and the replacement of native vegetation by crops [1,2,3,4,5,6]
Djaman et al [59] presented unsuitable Food and Agriculture Organization (FAO)-PM performances in dry conditions when wind speed was considered as 2 m.s−1 ; using daily average wind speed in the same conditions, the results presented mean bias error (MBE) values between −0.05 to 0.04
Our results indicate that wind speed and relative humidity and their variations throughout the year have a small effect on ETo estimates in the Cerrado region studied here
Summary
Over the last few decades, the hydrological cycle and climate of the Brazilian savanna (locally known as Cerrado) have been heavily affected by human activities, especially by the expansion of irrigation and the replacement of native vegetation by crops [1,2,3,4,5,6]. This irrigated agricultural expansion, it is important to have good management of available water resources. To handle issues involving water requirements and atmospheric demand, the United Nations Food and Agriculture Organization (FAO) recommended calculating crop evapotranspiration (ETc ) from reference evapotranspiration (ETo ) [7]. A “real” ETo value can only be obtained using lysimeters or other precision-measuring devices, which require time and are expensive [10,15,16], ETo can be computed from weather data, and climatic parameters are the only factors that affect ETo estimates [17,18]
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