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TECHNICAL REPORTS

Surface Water Quality

Trapping Efficiencies of Cultivated and Natural Riparian Vegetation of Northern Laos

^a International Water Management Inst., IWMI-Laos, P.O. Box 811 Vientiane, Lao PDR (current address: DPI Rutherglen Centre, RMB 1145 Chiltern Valley Road, Rutherglen, VIC 3685, Australia)
^b Institut de Recherche pour le Développement, IRD, P.O. Box 5992, Vientiane, Lao PDR, seconded to IWMI
^c National Agriculture and Forestry Research Inst., P.O. Box 811, Vientiane, Lao PDR
^d Insitut de Recherche pour le Développement, IRD, 32, av H. Varagnat, 93143 Bondy cedex, France, seconded to IWMI

^* Corresponding author (olga.vigiak{at}dpi.vic.gov.au).

Received for publication May 21, 2007. In northern Laos, intensification of cultivation on sloping land leads to accelerated erosion processes. Management of riparian land may counteract the negative impacts of higher sediment delivery rates on water quality. This study assessed water and sediment concentration trapping efficiencies of riparian vegetation in northern Laos and the effect of cultivation of riparian land on water quality. Runoff flowing in and out of selected riparian sites was monitored by means of open troughs. In 2005, two native grass, two bamboo, and two banana sites were monitored. In 2006, adjacent to steep banana, bamboo, and native grass sites, three upland rice sites were established and monitored. Water trapping efficiency (WTE) and sediment concentration trapping efficiency (SCTE) were calculated on an event basis; means and 95% confidence intervals (CIs) were estimated with a bootstrapping approach. Confidence intervals were large and overlapping among sites. Seepage conditions severely limited trapping efficiency. Native grass resulted in the highest WTE (95% CI, –0.10 to 0.23), which was not significantly different from zero. Banana resulted in the highest SCTE (95% CI, 0.06–0.40). Bamboo had negative WTE and SCTE. Median outflow runoff from rice sites was nine times the inflow. Median outflow sediment concentration from rice sites was two to five times that of their adjacent sites and two to five times the inflow sediment concentration. Although low-tillage banana plantation may reduce sediment concentration of runoff, cultivation of annual crops in riparian land leads to delivery of turbid runoff into the stream, thus severely affecting stream water quality.

Abbreviations: SCTE, sediment concentration trapping efficiency • WTE, water trapping efficiency