Using δ2H and δ18O to Identify the Effect of Land Use and Season on Water Fluxes and Soil-Plant-Water Relations in a Panamanian Lower Montane Watershed

Monday, 6 June 2016
Luitgard Schwendenmann, The University of Auckland, Auckland, New Zealand, Francois Jost, Dresden University of Technology, Dresden, Germany and Eric E Flores, Sistema Nacional de Investigación de Panamá (SNI) & Panama Wildlife Conservation, Panama City, Panama
Abstract:
Understanding the role of vegetation and land use in the hydrological cycle is crucial for the effective management of water resources. The aim of our study was to assess the effect of land use and season on water fluxes and soil-plant-water relations in a lower montane watershed in the central Cordillera region, Veraguas province, Panama, using water stable isotopes (2H, 18O). Precipitation, throughfall, soil water, stream and river water samples were collected over 14 months in four different land use types (primary forest, secondary forest, agriculture and pasture). Water uptake in secondary forests (five tree species, three sites) was quantified using 2H and 18O and multiple source modelling.

The volume weighted δ2H and δ18O values ranged from -110 to 10 ‰ and from -16 to 0 ‰, respectively. Across all land use types, δ2H and δ18O in precipitation, throughfall and shallow soil water showed a strong seasonal pattern with water being isotopically heavier during the dry season (January to April). The isotopic composition of soil water collected from 60 cm depth was more depleted than shallow soil water and did not show a pronounced seasonal pattern. A similar temporal pattern was observed for stream and river water indicating that a substantial amount of stream and river flow originates from deeper soil water.

The δ2H and δ18O values in precipitation and throughfall did not differ significantly among land use types. In contrast, soil water δ2H and δ18O values under agricultural sites tended to be more enriched compared to secondary forests and pastures especially during the dry season. This suggests that evaporation was highest in agricultural sites, due to the lower leaf area index in this land use type providing less shade. Water uptake varied considerably among species and sites. However, the majority of trees obtained most of their water from layers below 30 cm depth.

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