Evaporation from an Irrigated Terraced Rice Field Evaluated by Using Stable Isotopes: A Case Study in Batad, Philippines

Bench terracing is an agricultural technique done by creating a step-like feature, serving as paddies, along the contours of the mountain to make the land more arable. Inundation of the paddies is done in different ways and one way is through a gravity-driven flow mechanism which is observed in Batad. This mechanism involved the flow of water to the next lower terrace in a cascading manner by diverting the water to small canals called spillways. The study aims to detect evaporation of the irrigation water flowing across a continuous terrace using water stable isotopes as tracers. Sampling was done during the planting season to ensure that irrigation water is flowing across the terraces. Samples were taken along the spillways of each terrace. Thirty irrigation water samples from four terraces facing different directions: NE, E, SE and S, and one precipitation sample were analyzed for major ions concentrations and H and O isotope ratios. Stiff and Piper diagrams show that the water samples are of surface and Mg-HCO3 type, implying that the water has juvenile features and has experienced little or no chemical differentiation. Water chemistry does not show any significant trend as the water moves across a terrace. Isotopic compositions range from -51.8‰ to -14.6‰ and from -8.24‰ to -2.39‰ for δ2H and δ18O, respectively. The lowest isotopic compositions are observed mostly in the source waters of the terraces while the highest are at the lowest paddy of the NE-facing terrace. The data plotted in a δ2H-δ18O diagram for each terrace shows linear plots with slopes deviating from the Philippine Meteoric Water Line (PMWL). Highest deviation is observed for the NE-facing terrace with a slope of 6.22, while the lowest is 6.91 from the SE-facing terrace, all of which are significantly lower than the PMWL's slope of 8 (Gerardo-Abaya, 2005). In addition, enrichment in isotopic composition is observed as the water moves away from the source. These findings confirm that evaporation takes place as the water moves across the terrace. Using the Rayleigh distillation equation, the degree of evaporation loss ranges from 23.13% and 24.34% from the SE-facing terrace to 32.50% and 35.80% from the NE-facing terrace for the δ2H and δ18O, respectively. Variations in the results for each terraces can be attributed to the different ambient meteorological conditions during sampling. More sampling is needed to further investigate whether different terrace orientations with similar meteorological conditions can also have an effect with the evaporation.