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Sorghum is of significant economic importance for Northeastern Brazil, since it exhibits high growth rates in regions with irregular rainfall distribution and high temperatures, and is an alternative to corn, which has greater water requirements. Despite being a traditional crop in the region, there are few studies on irrigation management in the Apodi plateau. The aim of this study was to determine the evapotranspiration of the crop and the crop coefficient (Kc) for the different stages of sorghum growth in two cycles, and establish the relationship between the Kc and the normalized difference vegetation index (NDVI) obtained by radiometry. Two weighing lysimeters were used to estimate crop evapotranspiration (ETc). Reference evapotranspiration (ETo) was estimated by the Penman-Monteith method (FAO) and the crop coefficient determined using two methodologies: simple Kc and dual Kc. Total crop evapotranspiration in the two cycles was 452 and 557 mm. The ETc value was 23% higher in the second cycle compared to the first. The maximum Kc values for the first and second cycles were 1.21 and 1.35, respectively, using the dual Kc methodology. The linear relationship found between the Kc values and the NDVI allows monitoring and estimating the water requirements of the crop.

The objective of this study was to evaluate the models proposed by manufacturers and in the literature with respect to soil moisture measurement and to evaluate the performance of the CS616 sensor in the calibration of disturbed and undisturbed soil samples. These calibrations were performed using linear and quadratic models. Disturbed samples were collected in São Gabriel/BA, six samples placed in pots, whereas undisturbed samples were collected in Cruz das Almas/BA, three samples directly collected in the area and placed in a container. A calibration was performed between 21/12/2016 and 08/01/2017. The models proposed in the literature and by manufacturers differed in the estimation of volumetric soil moisture. Disturbed soil samples had higher data dispersion than undisturbed samples, due to factors such as grain size and bulk density, which influence the calibration data. The CS616 sensor had satisfactory performance in the calibration of disturbed and undisturbed samples, with excellent fit of the soil moisture data. Using soil moisture contents obtained by the CS616 sensor, without a previous calibration, may lead to errors in the results, confirming the need for a specific calibration for each type of soil.

The estimate of the actual surface evapotranspiration (ET) contributes to quantifying the water needs of crops. An alternative to the use of lysimeter for an accurate estimation of water needs, which has proved to be of great value in recent years, is the use of remote sensing combined with models based on surface energy balance. There is wide variety of models that can be classified into two types: one-source models, such as the Mapping EvapoTranspiration at high Resolution with Internalized Calibration (METRIC) algorithm, or two-source models, such as the Simplified Two-Source Energy Balance (STSEB). The objective of this study was to analyze how METRIC and STSEB can be used to estimate ET, in comparison with the lysimeter data, for the different stages of development of the sorghum crop in Apodi, RN, Brazil. The accuracy of both models in the daily ET estimation for the semi-arid conditions of the experiment, with RMSE values of 0.8 and of 0.7 mm d-1 through METRIC and STSEB, respectively, is considered acceptable for irrigation management purposes. The errors obtained with METRIC at an instantaneous scale were 60, 50, 130 and 5 W m-2 for Rn, LE, H and G, respectively, on the other hand, using STSEB these errors were of 40, 70, 120 and 21 W m-2 for Rn, LE, H and G, respectively. The METRIC and STSEB models are very similar when it comes to providing information on water needs of the sorghum.

COMPARAÇÃO DE MÉTODOS DE ESTIMATIVA DE ETO E ANÁLISE DE SENSIBILIDADE PARA DIFERENTES CLIMAS BRASILEIROS     JOÃO GUILHERME ARAÚJO LIMA1; PAULA CARNEIRO VIANA1; JOSÉ ESPÍNOLA SOBRINHO2 E JOÃO PAULO CHAVES COUTO3   1Departamento de Engenharia Civil, UNINASSAU, BR 104, KM 68, N° 1215, Agamenon Magalhães, 55000-000, Caruaru, Pernambuco, Brasil. joaopibe@gmail.com; pcvengenharia@gmail.com; 2Departamento de Ciências Ambientais e Tecnológicas, UFERSA, Rua Francisco Mota, N° 572, Presidente Costa e Silva, 59625-900 Mossoró, Rio Grande do Norte, Brasil. jespinola@ufersa.edu.br; 3Programa de Pós-Graduação em Engenharia Agrícola, Núcleo de Engenharia de Água e Solo, UFRB, Rua Rui Barbosa, N° 710, Centro, 44380-000, Cruz das Almas, Bahia, Brasil. E-mail: jpauloengagro@gmail.com.     1 RESUMO    A estimativa da evapotranspiração de referência (ETo) tem grande importância para a agricultura e manejo da irrigação. O método Penman-Monteith é considerado padrão para estimativa da ETo. No entanto, por ser completo, o método padrão apresenta como desvantagem a necessidade de uma gama de variáveis meteorológicas. O objetivo dessa pesquisa foi, em escala diária, avaliar o desempenho dos métodos de Hargreaves-Samani, Makkink, Priestley-Taylor, Turc, Radiação FAO-24 e Blaney-Criddle, para as condições climáticas das seis regiões do Brasil. A verificação do desempenho desses modelos foi por meio da comparação ao método de Penman-Monteith. Para avaliar o desempenho dos métodos foi utilizada a raiz quadrada do quadrado médio do erro (RQME), erro absoluto médio (EAM), erro de estimativa (PE) e coeficiente de determinação (R2). Entre os métodos estudados, o de Turc foi o que apresentou melhores resultados para todos os climas do Brasil, exceto para o clima Tropical litorâneo. O método de Makkink foi o que apresentou melhor resultado para o clima Tropical litorâneo. A análise de sensibilidade revelou que a temperatura do ar e a radiação global são as variáveis mais importantes para o método do método Penman-Monteith, exceto para o município BL, em que a variável umidade relativa do ar foi a mais importante.   Palavras-Chave: irrigação, consumo de água, evapotranspiração.     LIMA, J. G. A.; VIANA, P. C.; SOBRINHO, J. E.; COUTO, J. P. C. COMPARISON OF ETO ESTIMATION METHODS AND SENSITIVITY ANALYSIS FOR DIFFERENT BRAZILIAN CLIMATES     2 ABSTRACT   Estimation of reference evapotranspiration (ETo) is of great importance for agriculture and irrigation management. The Penman-Monteith method is considered standard for estimating ETo. However, because it is complete, the standard method presents as a disadvantage the need for a range of meteorological variables. The objective of this research was to evaluate the performance of Hargreaves-Samani, Makkink, Priestley-Taylor, Turc, FAO-24 and Blaney-Criddle methods for the climatic conditions of the six Brazilian regions. The verification of the performance of these models was made by comparison to the Penman-Monteith method. To evaluate the performance of the methods, the square root of mean-square error (MSE), mean absolute error (MAE), error of estimate (EE) and coefficient of determination (R2) were used. Among the methods studied, that of Turc was the one that presented the best results for all the climates of Brazil, except for the tropical coastal climate. The Makkink method was the one that presented the best result for the coastal tropical climate. Sensitivity analysis revealed that air temperature and global radiation are the most important variables for the Penman-Monteith method, except for BL municipality, where the variable relative humidity was the most important.   Keywords: irrigation, water consumption, evapotranspiration.