Soil's Role in Restoring Ecosystem Services

March 06 - 09, 2014

In West Maui, Wahikuli and Honokowai are watersheds in transition. Many hectares of former sugarcane and pineapple land is currently fallow, and previous management practices for irrigation and erosion control have not been maintained.  The forest ecosystem of the West Maui Mountain conservation land is also under pressure from introduced invasive species, illegal dirt bike activities and increased feral pig activity. The activities in the watershed directly impact the coastal ecosystem, where water quality correlates negatively to the health of the coral reef and the aesthetic experience for recreational users. Watershed managers are currently considering different options for land use and development, yet with limited physical data available to make decisions, leaders struggle to incorporate sediment retention as an ecosystem service. This study compared two different models, InVEST and N-SPECT, for their use as decision support tools. While InVEST only considers annual sediment retention, N-SPECT can model storm runoff events, but does not record sediment retention, only yield. Results indicate that InVEST overestimates sediment yield. Lacking data, parameters were calibrated using data from a watershed with similar soil and climate types in Hawaii.  A parameter sensitivity analysis showed that the density of the stream network was a significant factor in the results of the model, indicating that hydrologic connectivity is important and should be investigated further. The steep terrain and intermittent storm events that categorize the region suggest that either model, based on sheet erosion as empirically modeled using the USLE equation, might be only part of a sediment retention framework that also includes sediment contributions from gullies and landslides.