Coffee crop coefficient for precision irrigation based on leaf area index

Crop coefficient (Kc) for coffee plantations was found to be linearly related to the leaf area index (L) up to 3, i.e., Kc = b L. The basic assumption is that for irrigated trees the water use per unit leaf area (ETLA) is equal to the reference evapotranspiration (ETo) expressed also on a unit leaf area basis of the reference surface (ETLA = ETo/Lo). As recommended by FAO-56 the leaf area index (Lo) for the hypothetical reference surface (grass) is equal to 2.88, then the most likely value is b = Lo(-1) = 2.88(-1) = 0.347. However, for L > 3 (completely covered ground surface) Kc decreased from a peak value (similar to 1.05) tending to an asymptotic low value around 0.7 for L > 6, but the linear model gives unrealistic Kc estimates; tentatively the empirical function Kc = 1.8 L-0.5 is offered here as an initial guess due to the lack of experimental results for the interval 3.5 < L < 5.5. To become operational under commercial fields it is necessary to estimate the leaf area per coffee tree (LA, m(2) tree(-1)), and based on a very limited set of data, LA was estimated as a function of planting density (PD, trees ha(-1)), i.e., LA = 88.38 - 8.63 Ln (PD). Alternatively, L (< 3.4) can be computed directly as a function of canopy volume (for V < 7.2 m(3)).