Determination of Evapotranspiration and Annual Biomass Productivity of a Cactus Pear [Opuntia ficus-indicaL. (Mill.)] Orchard in a Semiarid Environment

A micrometeorological approach based on the surface energy balance was adopted to estimate evapotranspiration fluxes and crop coefficient data from an irrigated cactus pear [Opuntia ficus-indica L. (Mill.)] orchard under Mediterranean climatic conditions. High-frequency temperature readings were taken above the canopy top to get sensible heat flux values ($H_{\mathrm{SR}}$) using the surface renewal technique. These values were compared against eddy covariance sensible heat fluxes ($H_{\mathrm{EC}}$) for calibration. Latent heat flux (or evapotranspiration, ET) was obtained by solving the daily energy balance equation. Measurements of soil hydraulic components were integrated with the analysis of the surface energy fluxes and crop development in terms of phenology and aboveground biomass accumulation. Microlysimeters were used to compute evaporation rates, allowing the separation of daily transpiration from ET data. Ecophysiological measurements were carried to estimate dry weight accumulation and partitioning. Cactus pear evapotranspired a total of approximately 286 and 252 mm of water during the two monitored growing seasons, respectively. Average daily values of crop (${\mathrm{ET}}_c$) and reference (${\mathrm{ET}}_0$) evapotranspiration were in the order of 2.5 and 5.0 mm, respectively, with a corresponding value of the mean crop coefficient of approximately 0.40. The annual dry mass fixed per single tree was $38.8\pm 1.3\mathrm{kg}$, with a total production of $12.9\mathrm{t}{\mathrm{ha}}^{-1}$, which is comparable to many $C_3$ and $C_4$ plants and resulted in a water use efficiency (WUE) of 4.6 and $4.4\mathrm{g}\mathrm{DM}\mathrm{kg}{\mathrm{H}}_2{\mathrm{O}}^{-1}$ in 2009 and 2010, respectively. The stem area index (SAI) was 3.5.