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Studies of the dynamics of water in the Soil-Plant-Atmosphere system have been constantly carried out, as they directly contribute to the increase in production and the rational use of water for the most diverse cultures. In this approach, the aim of the study was to assess the water demand in forage Urochloa decumbens, with application of the SIMDualKC model. The meteorological variables used in the paper were obtained by an automatic INMET station. And to estimate forage production, cuts were made at a height of 15 cm, then removing it, simulating cattle grazing. In each cut, the accumulation of degree-days was determined and the base and cut temperatures were 10°C and 30°C, respectively. For the dynamics of crop water demand, the SIMDualKc model was used, in a daily time interval to estimate the basal crop coefficient (Kcb), evaporation coefficient (Ke), crop evapotranspiration (ET), in addition to Kc and the water available in the soil in the forage growing cycle. Kcb and Ke had an inverse relation, which means that when Kcb increased, Ke showed a decreasing trend. The Kc values did not change until the 3rd cut, but in the 4th cut there were changes with a reduction in rainfall and water availability in the system. It is observed that the model showed good ability to estimate the amount of water available in the soil, in addition to the physiological attributes of the forage, being a great option in the study of water dynamics for the forage.
