Tracing water paths through small catchments under a tropical montane rain forest in south Ecuador by an oxygen isotope approach

In three steep microcatchments under tropical montane forest, samples of rainfall, throughfall, organic layer solution (lateral flow), mineral soil solution, and stream water were collected between 23 August 2000 and 15 August 2001. Water samples were analysed for O and partly H isotopes to elucidate the preferential directions - vertical versus lateral - of water flow paths in soils and how they are linked to the precipitation and soil water regime. Additional soil moisture measurements were conducted to support the isotope study. The delta18O of rainfall shows large variations (-12.6 to +2.1 per thousand) related to different air-masses. There is no correlation between delta18O values in rainfall, temperature, and rainfall amount. Local meteoric water lines for rainfall and throughfall suggested that evaporation was minimal. The delta18O values of throughfall and lateral flow are similar to those in rainfall. Variations in delta18O values of the soil solution and the stream water are smaller (-9.1 to -3.0 and -8.7 to -5.8 per thousand) than those of rainfall, throughfall, and lateral flow. The delta18O values in stream water increased immediately after an intense rainstorm event to isotope values similar to those of rainfall and lateral flow indicating that during elevated rainfall the water flows rapidly in the organic layers to the stream channel paralleling the surface. This finding was confirmed by the higher volume of water in the organic layer than in the upper mineral soil during the rainstorm event. Our findings suggest that water flow paths through the ecosystem are dominated by vertical directions through the soil profile to the stream channels during normal wet conditions, interrupted by short-term flow direction changes to lateral pathways mainly in the organic layers during rainstorm events. Results from an isotope two-component hydrograph separation for the three microcatchments showed that new rain water ('event water') contributed 81, 44, and 78% to the total stormflow runoff during peak discharge of a studied rainstorm event, respectively.