Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
Journal topic
Volume 12, issue 4
Biogeosciences, 12, 1073–1089, 2015
https://doi.org/10.5194/bg-12-1073-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.
Biogeosciences, 12, 1073–1089, 2015
https://doi.org/10.5194/bg-12-1073-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 19 Feb 2015

Research article | 19 Feb 2015

Sources and export of particle-borne organic matter during a monsoon flood in a catchment of northern Laos

E. Gourdin1, S. Huon2, O. Evrard1, O. Ribolzi3, T. Bariac4, O. Sengtaheuanghoung5, and S. Ayrault1 E. Gourdin et al.
  • 1Laboratoire des Sciences du Climat et de l'Environnement (LSCE), UMR 8212 (CEA-CNRS-UVSQ-IPSL), Domaine du CNRS, avenue de la Terrasse, 91198 Gif-sur-Yvette CEDEX, France
  • 2Université Pierre et Marie Curie (UPMC), UMR 7618 iEES (UPMC-CNRS-IRD-INRA-Université Paris 7-UPEC), case 120, 4 place Jussieu, 75252 Paris CEDEX 05, France
  • 3IRD, Géosciences Environnement Toulouse (GET), UMR 5563 (CNRS-UPS-IRD), 14 avenue Edouard Belin, 31400 Toulouse, France
  • 4CNRS, UMR 7618 iEES (UPMC-CNRS-IRD-INRA-Université Paris 7-UPEC), campus INRA – AgroParisTech, Bâtiment EGER, 78550 Thiverval-Grignon, France
  • 5Department of Agriculture Land Management (DALam), P.O. Box 4199, Ban Nogviengkham, Xaythany District, Vientiane, Laos

Abstract. The yields of the tropical rivers of Southeast Asia supply large quantities of carbon to the ocean. The origin and dynamics of particulate organic matter were studied in the Houay Xon River catchment located in northern Laos during the first erosive flood of the rainy season in May 2012. This cultivated catchment is equipped with three successive gauging stations draining areas ranging between 0.2 and 11.6 km2 on the main stem of the permanent stream, and two additional stations draining 0.6 ha hillslopes. In addition, the sequential monitoring of rainwater, overland flow and suspended organic matter compositions was conducted at the 1 m2 plot scale during a storm. The composition of particulate organic matter (total organic carbon and total nitrogen concentrations, δ13C and δ15N) was determined for suspended sediment, soil surface (top 2 cm) and soil subsurface (gullies and riverbanks) samples collected in the catchment (n = 57, 65 and 11, respectively). Hydrograph separation of event water was achieved using water electric conductivity and δ18O measurements for rainfall, overland flow and river water base flow (n = 9, 30 and 57, respectively). The composition of particulate organic matter indicates that upstream suspended sediments mainly originated from cultivated soils labelled by their C3 vegetation cover (upland rice, fallow vegetation and teak plantations). In contrast, channel banks characterized by C4 vegetation (Napier grass) supplied significant quantities of sediment to the river during the flood rising stage at the upstream station as well as in downstream river sections. The highest runoff coefficient (11.7%), sediment specific yield (433 kg ha−1), total organic carbon specific yield (8.3 kg C ha−1) and overland flow contribution (78–100%) were found downstream of reforested areas planted with teaks. Swamps located along the main stream acted as sediment filters and controlled the composition of suspended organic matter. Total organic carbon specific yields were particularly high because they occurred during the first erosive storm of the rainy season, just after the period of slash-and-burn operations in the catchment.

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The origin and dynamics of particulate organic matter were studied in a 11.6km² catchment (northern Laos) during the first erosive flood of the 2012 rainy season. Upstream suspended sediments mainly originated from cultivated soils labelled by their C3 vegetation cover. In contrast, channel banks with C4 vegetation supplied significant quantities of sediment to the river downstream. Swamps located along the main stream acted as sediment filters and controlled the composition of suspended POM.
The origin and dynamics of particulate organic matter were studied in a 11.6km² catchment...
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