Using stable hydrogen and oxygen isotopes to reveal monsoonal and related hydrological effects on meteoric water in the Western Pacific monsoon region: A case study of the Ilan region, northeastern Taiwan

This study analyzes the isotopic compositions (δ2H and δ18O) of meteoric waters, including precipitation and stream water, to reveal what major hydrological processes affect the hydrological regime of the Ilan region, northeastern Taiwan. The isotopic results indicate monsoonal flows as the fundamental factors affecting studied precipitation and stream water. Summer precipitation sourced from southerly air mass exhibits lower δ and deuterium-excess (dE) values than winter precipitation sourced from northerly air masses. The δ18O and dE values are respectively −7.7‰ and 8‰ for summer precipitation and −3.3‰ and 24‰ for winter precipitation. Furthermore, semi-quantitative estimations using dE evidence indicate that summertime southerly air masses generally contribute more to stream water than wintertime northerly air-mass flows (approximately 54% vs. 46%). However, the contribution fractions are controlled by the orientation of catchments to the windward side of respective monsoonal flows. Northern catchments, located on the windward side of southerly air masses, receive about 60% of their water from precipitation condensed from the southerly air masses, and 40% from the northerly air masses. By comparison, southern catchments, located on the windward side of northerly air masses, receive about 59% of their water from northerly air masses and 41% from southerly air masses. Additionally, catchment effect, induced from δ value, is notable in stream basins with high elevations but this is not related to catchment sizes. Besides this, altitude effect, which is determined in terms of δ18O values, was derived using data from four precipitation stations of northern Taiwan. It ranges from −2.5 to −3.0‰ per 100 m depending on the season; moreover, based on the dE evidence, secondary evaporation effects are apparent with moisture recycling influencing inland summer precipitation and raindrop evaporation influencing inland winter precipitation.