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

Abstract

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 km 2 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 m 2 plot scale during a storm. The composition of particulate organic matter (total organic carbon and total nitrogen concentrations, δ 13 C and δ 15 N) 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 δ 18 O 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 C 3 vegetation cover (upland rice, fallow vegetation and teak plantations). In contrast, channel banks characterized by C 4 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-andburn operations in the catchment.

From shifting cultivation to teak plantation: effect on overland flow and sediment yield in a montane tropical catchment

“…This process can lead to the more frequent collapse of channel banks and the remobilisation of material deposited in the channel 33 . Groundwater outflow can also decrease the concentration of suspended sediments in the river 16, 29 .…”

“…Typically, water mixing models using geochemical properties (e.g. electric conductivity, δ 18 O) are applied to quantify these contributions 15, 16 .…”

Molecular fingerprinting of particulate organic matter as a new tool for its source apportionment: changes along a headwater drainage in coarse, medium and fine particles as a function of rainfalls

“…Contrary to the previously cited investigations (Adams et al, 2015;Nosrati et al, 2011;Tamooh et al, 2012), this present study focused on a headwater catchment (0.79 km 2 ). In these small catchments, POM mainly comes from the erosion of surrounding soils, as ob- served for monsoon floods in Laos (Gourdin et al, 2015;Huon et al, 2017) or from a combination of bedrock and surface erosion in an Alpine catchment with relative proportions controlled by the types of precipitation (Smith et al, 2013). However, in this catchment, the mobilization of stream banks has been shown to be effective in winter due to the freezethaw process .…”

An approach to analysing plot scale infiltration and runoff responses to rainfall of fluctuating intensity

“…Overland flow resulting from rainfall is involved in many hydrological and geomorphic processes, including soil erosion (Holz et al ., ); agrochemical, nutrient, and pesticide leaching in croplands (Sigua et al ., , ; Ryan et al ., ; Srinivasan et al ., ; Bramorski et al ., ); and the translocation of seeds and organic matter (Bochet, ; Gourdin et al ., ). The infiltrability of soils is an especially important parameter for understanding infiltration excess (Hortonian) overland flow and may partially account for annual net primary productivity in some important ecosystems, including croplands (Stroosnijder, ; Shaver et al ., ; Reinhart et al ., ).…”