Energy availability has direct influence over hydrological cycle, climate and preciptation. The determination of net radiation balance (Rn) and the spacial and temporal recognition of water availability allow establishing guidelines and planning the implementation of public resource management policies. The objective of this study was to analyze the water balance and net radiation balance of the year 2017 at two experimental sites in the state of Rondônia (RO), Brazil: an Amazon rainforest area located in the Jaru Biological Reserve (REBIO) and a pasture area (FNS) located in the municipality of Ouro Preto d’Oeste, near the forest área. The data were obtained from the REBIO climatological station, in the municipality of Ji-Paraná-RO, belonging to the experimental sites. Calculations for evapotranspiration potential were based on Thornthwaite and Mather’s method. The "Eddy Covariance" method was used to obtain the radiation balance. The forest system presented greater energy availability in the rainy season and higher evapotranspiration balance when compared to the pasture area. Water balance extract analysis pointed to water stress in pasture area, opposite to the forest area, which indicates that conversion of native forest to pasture is a process capable to compromise the availability of water resources and alter climatic aspects.