Irrigation is the controlled supply of water to plants. Irrigation keeps plants healthy and productive during periods of low rainfall, but has other benefits too - controlling the timing of flowering and fruit ripening, potentially protecting soil from erosion or compaction, or suppressing weed growth.
Only 10% of the farmland used for growing coffee in Brazil is irrigated, however, this 10% provides 22% of the total coffee production each year. (Saturnino, 2007 and Gleice A. de Assis et al., 2014) What this statistic suggests is that irrigation has a dramatic effect on increasing yields.
Increasing the supply of water and nutrients through irrigation combined with fertilisation also allows plants to be grown at much higher densities, making land several times more productive. Irrigation also allows farming on land that might otherwise not have been suitable for coffee growing. However, as with fertilisation there is no one-size-fits-all approach: the ideal amount and timing of irrigation depends on local conditions including latitude, rainfall distribution, the timing and severity of the dry season, and soil type and depth.