Evaluating the ability of the surface renewal-energy balance method to determine coffee water use and its components in a traditional agroforestry system

Little is known about the water use of crops in agroforestry systems, particularly in the tropics where field data collection and monitoring is often challenging due to socio-economic circumstances. In addition, the presence of a tree canopy leads to a more complex water balance situation than in the case of a crop grown in the open field. For example, the tree canopy affects the evapotranspiration of the crop by changing the microclimate, such as reducing the irradiance and wind speed and possibly altering the air temperature and humidity around the crop. In this study, the ability of the surface renewal-energy balance method to determine the water use of coffee in a traditional agroforestry system is investigated. The potential advantages of this method are the low costs, low maintenance requirements, and the representativeness of the measurements over a relatively large area. To partition evapotranspiration into contributions from the crop and the ground layer, the surface renewal sensible heat fluxes were measured both above and below the coffee shrubs. Due to organic management practices, the soil in this coffee plantation was covered with a dense layer of herbaceous vegetation. The surface renewal sensible heat fluxes were validated and calibrated against sensible heat fluxes measured at the same levels using the eddy covariance method. Evapotranspiration (latent heat flux) was calculated as the residual from the energy balance. The net radiation incident on the coffee crop and the ground vegetation was calculated using net radiation observed above the tree canopy and a light transmission model, which was calibrated with radiation measurements at both levels. The soil heat flux was measured with heat flux plates buried 0.08 m below the surface and a thermistor between each plate and the surface. The latent heat fluxes derived with the surface renewal-energy balance method were compared with those measured using the eddy covariance method.