Evapotranspiração no Brasil: cenário atual e projeções climáticas

Abstract

Reference evapotranspiration (ETo) has spatial and temporal variation, due to the performance of meteorological systems, vegetation type and soil properties. It is an essential parameter for planning irrigation systems and water use efficiency through the optimization of water resources, make it necessary to quantify their spatial distribution and magnitude. However, ETo is a variable that is difficult to measure in meteorological stations, in addition to not being directly present in climate projection models, making the use of estimation methods an attractive tool. However, information on the analysis of the behavior of this variable is relatively restricted in Brazil, probably due to the limited availability of data and the long time required to process this information. ETo information in Brazil is restricted to a regional scale, in addition to being produced by complex estimation methods or incomplete determination tools. Additionally, Brazil is one of the largest producers of agricultural commodities in the world, and a large part of its territory is vulnerable to climate changes projected to occur during the 21st century, directly reflecting changes in ETo. In this sense, it is essential to check the ETo space-time pattern, accurately estimate its response to climate change and identify the main meteorological variables that cause variations in evapotranspiration. Therefore, the present work aims to analyze the influence of climate projections for the end of the 21st century (2071-2100), using two scenarios of radiative forcing (Representative Concentration Pathways - RCPs 4.5 and 8.5), on the behavior of daily evapotranspiration in the Brazil, based on the ETo estimation method that presents the best representativeness for the Brazilian climatic conditions. For the analysis of the current climatic conditions, daily data of air temperature (maximum and minimum), relative humidity, wind intensity at 2 m, global solar radiation, ETo and atmospheric pressure for the period from 1980 to 2017 were used. For climate change, daily data on air temperature, global solar radiation and relative humidity on the surface of six climate system models from simulations and projections of the Coupled Model Intercomparasion Project Phase 5 (CMIP5) were used. The behavior of ETo was mainly accompanied by the variables of air temperature, global solar radiation and relative humidity. For this reason, the Turc method was superior to the other in estimating ETo, with root mean square error values between 0.3 and 0.5 mm day-1, mean absolute percentage error between 8 and 10 % and performance index c between 0.65 and 0.93. The climatic projections of the six climate models indicated a diversified pattern in the ETo estimate. Considering RCP 8.5, despite the distribution of seasonal and annual differences in ETo present a spatial structure similar to RCP 4.5, there is an intensification of the reduction (~ 0.30 mm day-1), opposite to what was expected. One justification is the uncertainties about future changes in ETo, both related to the interactions of meteorological factors, as well as the perform