All rivers flow to the sea, they say, but what flows into the rivers? When it rains, fertile top soil is lost, and everything from sediment to fertilizers and pesticides to soil pathogens get washed in. Then the streams silt up; pesticides and suspended sediments threaten stream micro-organisms and aquatic life. These negative impacts could extend into the receiving ocean.
To stop or at least minimize pollution of water resources, riparian buffer zones are used. These are areas of special plantings that capture the soil and nutrients and make use of them before they wash into the water. Constant-width riparian buffers (CWRBs) are effective in reducing sediment, pathogen, and nutrient loads from agricultural catchments into surface and groundwater resources. However, CWRBs use more valuable agricultural land than needed. Variable riparian buffers, known also as precision riparian buffers (PRBs), can be as effective as CWRBs but only use around 20% of the land along the stream banks. PRBs have not been evaluated in Hawai‘i, though, until now. Researchers Ali Fares, Carl Evensen, Aly El-Kadi, and Catherine Chan-Halbrendt teamed up with other researchers and graduate students to evaluate the effectiveness of PRBs as soilconservation and pollution-prevention management practices in Hawai‘i’s environment and to optimize their configuration. They tested the erosion-control ability of two native plants, Pili grass and Ahuawa sedge, as well as invasive Guinea grass, at research sites on the Leeward (Pioneer Hybrid, Wailua) and Windward (Kualoa Ranch) sides of O‘ahu. They tested surface runoff entering and leaving these buffers for major water-quality indicators.
Preliminary results show that Pili grass and Ahuawa significantly reduced the sediment transported to streams by more than 50% at Pioneer Hybrid, Waialua site. The native and invasive plant species were equally efficient in removing sediment, but PRBs using native species will help biodiversity. A cost-benefit economical analysis found that PRBs are economically viable: The largest cost is the agricultural revenue farmers won’t get from the land used for the buffers, but the reduced erosion and improved water quality make up for that. The PIs will be conducting several outreach activities to convince more farmers to consider adopting these PRBs—and to protect surface water flowing to the sea.
