Hillslope Sediment Trapping by Natural or Cultivated Riparian Vegetation in Northern Laos

In recent years growing human pressure on agricultural land in the north of the Lao PDR has led to accelerated degradation processes along hillslopes, and higher sediment delivery rates to streams. Furthermore, high demand from urban centres is attracting farmers to cultivate vegetables in riparian areas. These land-use changes may have negative affects on stream water quality. The aims of this study were to assess i) the water and sediment trapping efficiency of riparian vegetation in northern Laos, and ii) the potential effect of cultivating riparian land on these efficiencies. In May-October 2006, flows of surface runoff and sediment were monitored across three riparian sites. Sites were arranged into pairs of adjacent riparian land plots; one plot was covered with riparian vegetation (native grass, banana or bamboo) and one plot cultivated with upland rice, which was used to test the effect of annual cropping and also acted as the control. Plots were equipped with three Gerlach troughs placed at the upper rim of the riparian zone and three troughs at the lower rim to measure event inflow and outflow (runoff volumes and sediment loads). Event trapping efficiency was calculated as the portion of inflowing water and sediment retained in the plot. Results showed that banana and native grass plots were active sinks, retaining sediment and water (median runoff trapping efficiency =0.10, median sediment load trapping efficiency = 0.40). In contrast, the trapping efficiencies of runoff and sediment load by bamboo and upland rice were mostly negative, suggesting that these areas not only fail to retain inflows of water and sediment but also act as active sources, delivering more runoff and solids to the stream. We measured sediment concentrations in the runoff exported from upland rice plots and found that these were three-and-a-half times higher than the sediment concentrations in the runoff entering the plots, and nine to seven times the runoff sediment concentration exported from the banana and native grass plots. Thus, the cultivation of annual crops (Le. upland rice) leads to the delivery of turbid runoff from the riparian areas directly into the stream, and therefore has a considerable effect on downstream water quality.