The role of water in the development and yield of the coffee crop (Coffea arabica L.) is reviewed. A period of water stress, induced either by dry soil or dry air, is needed to prepare flower buds for blossoming that is then stimulated by rain or irrigation. Although attempts have been made to quantify the intensity and duration of stress required, these have not yet been specified in ways that are commercially useful. Water must be freely available during the period of rapid fruit expansion to ensure large, high-quality seed yields. Depending on the time and uniformity of flowering this can occur at times when rainfall is unreliable, particularly in equatorial areas.
Although there are differences in their responses to drought, commercial cultivars have retained many of the characteristics adapted to the shady environment of the forests in the Ethiopian highlands in which C. arabica is believed to have originated. These include partial closure of the stomata when evaporation rates are high as a result of large leaf-to-air saturation deficits (>1.6 kPa), even if the soil is at field capacity. This is thought to be an adaptive mechanism that minimizes transpiration at high irradiances when the leaves are light-saturated.
Our understanding of the actual water use of coffee crops grown in diverse ways is imperfect. For mature crops, well supplied with water, the crop coefficient (Kc) appears to have a value in the range equivalent to 0.7–0.8 times the evaporation from a US Weather Bureau Class A pan. There is some evidence that Kc values are less than this on days when evaporation rates are high (>7 mm d−1). For immature crops allowance has to be made for the proportion of the ground area shaded by the leaf canopy, but this alone may underestimate rates of water use. Present methods of calculating crop water requirements for the purposes of irrigation scheme design and management are imprecise and, probably, subject to large errors depending on local circumstances.
The need for irrigation, and its role in controlling the timing of flowering, varies depending on the rainfall distribution, the severity of the dry season, and soil type and depth. Two geographic areas need to be distinguished in particular; those close to the equator with a bi-modal rainfall pattern and those at higher latitudes with a single rainy season and an extended dry season. Despite the international importance of irrigation in coffee crop production, the benefits to be derived from irrigation, in yield and in financial terms, have not been adequately quantified in either location. Allowable soil-water deficits have been specified for deep-rooting crops (2–3 m) on water retentive soils, usually linked to conventional over-tree sprinkler irrigation systems. Other, potentially more efficient, methods of irrigation are now available for coffee grower use, in particular, micro-jet- and drip-irrigation systems. However, there appears to be little advice, based on sound experimental work, on how to design and operate these to best advantage.
There is a need to interpret and apply the scientific understanding of the role that water plays in the growth and development of the coffee plant into practical advice that can assist the grower to plan and to use water efficiently, whether rainfall or irrigation, for the production of reliable, high-quality crops. Future research opportunities are identified.