Avaliação do atendimento da demanda hídrica da cultura do milho através da subirrigaçã

Crop rotation is a strategy for reducing soil deterioration, spread of diseases, pests and weeds, and to stabilize farm income. Including of an upland crop, such as maize, in rotation with flooded rice could become a common practice in Rio Grande do Sul, Brazil, as the existing infrastructure for rice cultivation allows a sub-irrigation system to provide water to the upland crop. In this study we evaluate the performance of a sub-irrigation system for maize cultivated on a lowland soil in the Central Lowland of Rio Grande do Sul, Brazil, where cultivation of flooded rice predominates. The approach combines Darcy's equation and the models of van Genuchten (1980) and Vogel et al. (2001) to represent the relationship between hydraulic conductivity and soil moisture. Results were shown to be very sensitive to the choice of model, and to the depth of the impermeable soil layer. Best results were obtained from the Vogel et al. (2001) model on a deep B horizon. In the present scenario, where drains are spaced at 50 m, the contribution of water through subirrigation reaches around 20% of the total maize water requirement when the impermeable soil layer is deep. When it is shallow, the contribution is insignificant. At 25-m spacing (i.e. twice the drainage density) the supplementary irrigation via subirrigation reaches 70% and 12% respectively, of the total water requirement in the more optimistic and less optimistic scenarios, respectively.