Watershed Management to Meet Water Quality Standards by Using the Vetiver System in Southern Guam

Sedimentation as the result of runoff is the principle human-caused threat to the environment in general and the water quality in particular in the Pacific island of Guam. Runoff water is characterized by flash floods of high velocity, but short duration. The rapid flow is attributed to low soil infiltration, a high proportion of rain converted to overland flow, and scanty or absent vegetation cover due to wildfires. In the areas where protective vegetation cover is lowest, the soil is subjected to the high shearing force by such an overland flow. Erosion damage is a serious problem to the environmental ecosystem of the island. Sediment lost to erosion clogs rivers, lakes, and waterways. Erosion and sedimentation loss are also a major source of water-quality problems in Guam. Sedimentation provides a vehicle for the transport of agricultural chemical residues into the canals, streams, rivers and eventually the near-shore ecosystems, where it damages coral reefs. The objective of the project reported here was to assess the sediment-loading rate to the near-shore coral reef originating from the upland watershed. The effectiveness of vetiver systems (VS) as a sediment trap and its effect on quality of the water leaving the upland watershed was evaluated. Four plots (22 x 1.5 m) were laid out on a uniformly sloped (12%) watershed for estimation of sedimentation rates. Each plot was equipped with 20 cm high flume wall, which separated its surface from those of the other plots and their surroundings. Flumes are equipped with cone-shaped weirs that directed the runoff and sediments into a collecting tank beneath the weirs. In order to evaluate the effect of different soil surface management on erosion and quantify the sedimentation and turbidity of the runoff water from each plot, the following treatments were examined at this particular watershed: (i) Natural vegetation ‘as it is’ treatment, (ii) ‘Vs’ treatment as a restoration technique, (iii) ‘Controlled burn’ treatment, (iv) ‘Exposed surface-no-cover’ treatment. The above-mentioned treatments represent a wide range of conditions that are present in a typical watershed area in southern Guam.