Using on-the-ground surface energy balance to monitor vine water status and evapotranspiration under deficit irrigation and rainfed conditions

Vineyards occupy large areas in many Mediterranean agroecosystems, in which their evapotranspiration is a major component of the water cycle. The correct determination of the actual crop evapotranspiration (ETa) is critical for accurate irrigation scheduling, allowing to match irrigation applications to crop water needs. This study was conducted to monitor the vineyard ETa and its partitioning into soil evaporation (E) and plant tran- spiration (T), under deficit irrigation and rainfed conditions, and to obtain the relationship between the esti- mated relative water stress coefficient (Ks,r) and measured midday stem water potential (psi stem). A 2-year field experiment was carried out in a 'Monastrell' commercial vineyard located in Fuente ' Alamo (Albacete, SE Spain). Thermal infrared radiometers were deployed in both treatments, together with a weather station, to gather all the inputs required in the simplified two-source energy balance (STSEB) approach. Results showed that the accumulated ETa ranged between 183 and 253 mm for the irrigated treatment, and between 121 and 158 mm for the rainfed conditions. Canopy transpiration in the irrigation treatment almost double the value obtained in rainfed conditions for both years, and the ratio T/ETa increases around 8-9% in the irrigation treatment. The E component ranged from 21% for the irrigated treatment up to 50% for the rainfed treatment, confirming its importance in the vineyard water balance. A good linear relationship (r2=0.650) was obtained between Ks,r and psi stem, indicating the potential of using a surface energy balance approach in conjunction with plant radiometric temperature measurements to monitor the vines water status. Moreover, the rainfed treatment reduced the values of stomatal conductance (gs), transpiration (El) and net photosynthesis (AN) compared to the irrigated treatment, whereas no increase in the intrinsic water-use efficiency (WUEi) nor the instantaneous water-use efficiency (WUEinst) was observed. The reported results will be useful for improving deficit irrigation sched- uling techniques for 'Monastrell' red grapes.