Oxygen isotope analysis
Root crown and leaf water oxygen isotope ratios (δ18OLW) were analyzed by isotope ratio mass spectrometer (IRMS). A 0.3% CO2/He gas mixture was flushed through the vials and equilibrated with water for 48 h on a ThermoFinnigan GasBench II (Bremen). The equilibrated CO2 gas was separated at 40 °C with a 25 m x 0.32 mm ID GC column (Varian, poraplot Q) and analyzed with a continuous flow isotope ratio mass spectrometer (Delta PlusXP, Thermofinnigan, Bremen) (Brenna et al. 1997; Qi et al. 2003) to derive the oxygen isotope ratios of the water. Standards (SLAP and Puerto Rico water) were analyzed alongside the samples. Final delta values were the mean of 5 sample peaks reported relative to Vienna Standard Mean Ocean Water (V-SMOW).
Irrigation water and water vapor were measured by isotope ratio-cavity ringdown spectroscopy using a water analyzer (model L1102-i, Picarro, Sunnydale, CA, USA). Each sample was analyzed six consecutive times, and the mean of the last three analyses was used. Three laboratory standards, calibrated to the V-SMOW scale, were interspersed among the samples and used to correct the sample δ18O values to the V-SMOW scale. Water vapor was introduced into the water analyzer through the inlet valve and analyzed for at least 15 minutes, and the δ18O of water vapor (δ18Ov) was calculated as the mean δ18O over the last five minutes. The δ18Ov was corrected to the V-SMOW scale in the same way that irrigated water samples were corrected. Further δ18Ov was corrected for water concentration dependency. The stable isotope composition of oxygen (δ18O) was reported in δ notation (Eqn. 12),