The reference evapotranspiration (ETo) is a complex measure essential to water resources planning and management, irrigation scheduling and rational agricultural water use. This research aimed to evaluate the performance of the empirical methods in estimating reference evapotranspiration based on daily meteorological data of the surface and comparing them with the standard method of Penman-Monteith (PM-FAO 56), during the period of January/2004 to December/2013 for the climatic conditions of the semiarid region of Brazil. Five empirical methods were evaluated: Hargreaves-Samani (HS), Camargo (C), Thornthwaite (Th), Priestley-Taylor (PT) and Jensen-Haise (JH). For this purpose, maximum air temperature (Tmax, ºC) and minimum (Tmin, ºC); maximum air relative humidity (RHmax, %) and minimum (RHmin, %); maximum air atmospheric pressure (Pmax, kPa) and minimum (Pmin, kPa); average wind speed at a height of 2 meters (u2m, m s-1); global solar radiation (MJ m-2); and precipitation (mm) were used at this study. Analysis data were executed using descriptive statistics, correlation, dispersion, and statistical accuracy indexes to compare empirical models with the PM-FAO 56. The JH method overestimated the standard Penman-Monteith method, and underestimated by the HS, C, Th and PT methods, on a monthly average scale. In general, HS and JH methods had better adjustments with PM-FAO 56, on a daily scale. In conclusion, empirical methods, on a daily scale, presented good correlations and satisfactory statistical indices, being important in agricultural planning and irrigation management, becoming a good alternative for semiarid conditions of Petrolina-PE in San Francisco mesoregion. Comparação da Evapotranspiração de Referência entre Métodos Empíricos e Penman-Monteith em Região Semiárida do Nordeste Brasileiro R E S U M OA evapotranspiração de referência (ETo) é uma medida complexa essencial ao planejamento e manejo dos recursos hídricos, manejo de irrigação e uso racional da água na agricultura. O objetivo deste trabalho foi avaliar a precisão de métodos empíricos na estimativa da evapotranspiração de referência baseados em dados meteorológicos diários de superfície, comparando-os com o método padrão de Penman-Monteith da FAO-56 (PM-FAO 56), durante o período de janeiro/2004 a dezembro/2013, para as condições climáticas da região semiárida do Brasil. Foram avaliados cinco métodos empíricos: Hargreaves-Samani (HS), Camargo (C), Thornthwaite (Th), Priestley-Taylor (PT) e Jensen-Haise (JH). Para tanto, foram utilizados as seguintes variáveis: temperatura máxima do ar (Tmax, ºC) e mínima (Tmin, ºC); umidade relativa máxima do ar (RHmax, %) e mínima (RHmin, %); pressão atmosférica máxima do ar (Pmax, kPa) e mínima (Pmin, kPa); velocidade média do vento a uma altura de 2 metros (u2m, m s-1); radiação solar global (MJ m-2); e precipitação (mm). A análise dos dados foi realizada por meio da estatística descritiva, correlação, dispersão e índices estatísticos para comparar os modelos empíricos com o PM-FAO 56. O método de JH superestimou o método padrão de Penman-Monteith, enquanto que ocorreu a subestimativa pelos métodos de HS, C, Th e PT, na escala média mensal. Em geral, os métodos de HS e JH tiveram melhores ajustes com o PM-FAO 56, na escala diária. Em conclusão, os métodos empíricos, na escala diária, apresentaram boas correlações e índices estatísticos satisfatórios, sendo importantes no planejamento agrícola e manejo da irrigação, tornando-se uma boa alternativa para condições semiáridas de Petrolina-PE na Mesorregião do São Francisco.Palavras-chave: Vale do São Francisco, manejo da irrigação, seca, dados meteorológicos, recursos hídricos.

In irrigated areas around the world, the recommendation for the use of subsurface drainage is also associated with controlling salinity problems. Due to the high implementation cost, the search for solutions that make this requirement more flexible is necessary. Among the options to be investigated is the hypothesis that the height and salinity of the water table in plots located at the highest points of a toposequence are lower and do not compromise plant development, even without underground drainage systems. In this context, the present work was developed to monitor and evaluate the variation in water level or mottling over twelve months, as well as to measure and analyze the electrical conductivity and average pH of the water table during this period and its possible impact on plants. For this purpose, three lots in toposequence were selected in the Senador Nilo Coelho Public Irrigation Project, Petrolina—PE, with previously defined characteristics: soil classification (Plinthic Yellow—Ultisol), crop planted (Mangifera indica L.) and irrigation system used (micro-sprinkler). Precipitation, reference evapotranspiration and volume of water applied via irrigation were monitored by an automatic weather station and hydrometers in each lot. In each plot, nine observation wells were installed, distributed in a grid, with the aim of make monthly measurements of the water table level or mottling. The electrical conductivity and pH of the groundwater were also measured to obtain the average monthly value for each lot. Illustrative 3D maps of the water table level in relation to the ground surface were created using the simple kriging method, in the UTM SIRGAS 2000 24S projection system. The absence and presence of groundwater in the upper and lower hillslope lots, respectively, were favored by the toposequence. The decision to install underground drainage or not can be made on a case-by-case basis; this must take into account, among other aspects, changes in physical characteristics along the soil profile, possible occurrence of mottling, the quality of water for irrigation, the irrigation management adopted and the position of the lot in the toposequence.