Soil Evaporation in a Shaded Coffee Plantation Derived From Eddy Covariance Measurements

Little is known about soil evaporation (E_soil) in shaded coffee plantations. In this study, E_soil and its proportion to total evapotranspiration and rainfall (P) were quantified for a densely shaded Arabica coffee plantation (~80% tree cover) in central Veracruz, Mexico, using eddy covariance (EC) measurements made above the tree canopy and at the boundary between the stem space and canopy of the coffee crop. While measurements above the canopy were made continuously during the 1-year study period (October 2016 to September 2017), this was not possible for the above-soil level due to logistical considerations, and hence, annual E_soil was calculated from a relationship between the ratio of the actual to the equilibrium E_soil and surface soil water content. This empirical model was calibrated using 31 days of EC measurements, representing the range in surface soil water content observed during the study period. Examining the above-canopy and above-soil EC measurements for quality yielded similar results with regard to stationarity (99% and 85% of the time, respectively) and degree of energy balance closure (81% and 96%, respectively). The permanent and relatively dense vegetation cover provided by the shade trees and coffee plants reduced net radiation at the soil surface to only about 12% of the above-canopy value. Consequently, E_soil was a minor fraction of evapotranspiration (10–13%) and P (8–9%), with the spread reflecting uncertainties related to the failure to close the energy balance. In addition to the low-energy availability, E_soil may have been reduced by the presence of a litter layer.