Predicted and seasonal dynamics of intensive and hedge- row olive orchard water use in response to applied water, environment and drip emitter layout

Abstract

Olive trees are usually irrigated in southern Portugal and given the area devoted to this per- ennial crop it represents a large demand on regional water resources. Hundreds of intensive (≈300 trees per ha) and super-intensive hedgerow (≈2000 trees per ha) orchards have re- cently been established in the region to take advantage of the European Commission deci- sion of allowing the expansion of Portuguese olive tree planting quota. With enhanced olive production and yield depending on irrigation, a precise estimation of transpiration (Ep) under non-limiting conditions is required to set up the upper limit of irrigation requirements and wa- ter use. A distinct feature of Ep of olives is their tight coupling to the atmosphere, modulated by canopy conductance and vapour pressure deficit (Orgaz et al., 2007). For our intensive (cv. Cobrançosa) and hedgerow (cv. Arbequina) olive canopies this was evaluated in 2012 by predicting daily Ep with the Penman-Monteith "big leaf" equation coupled to a locally cali- brated Orgaz et al. (2007) model of bulk daily canopy conductance (gc) for unstressed cano- pies (PM model). For both orchards, dynamics of predicted Ep were compared to daily Ep field values obtained from sap flow velocity. Daily Ep at the stand scale (mm day-1) was ob- tained by dividing tree transpiration by the area of the planting pattern, 4.2 x 8 m (33.6 m2 per tree) and 3.75 x 1.35 m (5.06 m2 per tree) for the intensive and hedgerow orchard, respec- tively. Tree rows were supplied with water by single drip irrigation lines serviced by 1.6 (in- tensive) and 2.3 l h-1 emitters (intensive and hedgerow) spaced 1.0 m (intensive) and 0.75 m (hedgerow) apart throughout the entire length of the emitter row lines. The PM model effec- tively simulated and traced out the seasonal variability of Ep for the hedgerow olive orchard, cv. Arbequina, validating the model ́s applicability to hedgerow orchards in southern Portu- gal. On the contrary, concurrent weather and drip line emitter layout had meaningful and ad- verse effects on the intensive orchard daily Ep, with tree water use not enough to match pre- dicted Ep. For the intensive orchards in the reduced root zone moisture buffer capacity and low hydraulic conductivity soils of southern Portugal, servicing trees with single drip lines of 1.0 m spaced emitters (4 per tree in 33,6 m2 area) led to low transpiration and inadequate crop water use as well as low irrigation application efficiency. Considerable applied water is wasted due to the limited access of roots to water applied by emitters located away from the tree trunk.