Assessing Hydrologic Processes to Mitigate Coral Reef Degradation in Hawaii
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Issue
Stormwater runoff is a major driving force of sediment and nutrient transport in watersheds, and land-based pollutants and sediments have been identified as the primary threats to coral reef ecosystems in Hawai‘i. Because of the distributed, or nonpoint source, nature of these threats, accurate estimates of runoff are an important aspect of watershed management. However, limited information exists that describes erosion, sedimentation, and water quality dynamics at watershed scales in Hawai‘i, including the Hanalei Bay watershed.
Process
To better understand hydrologic processes in the Hanalei Bay watershed, researchers at the University of Hawai‘i applied the Nonpoint-Source Pollution and Erosion Comparison Tool (N-SPECT) to simulate annual direct-surface runoff. N-SPECT is a GIS extension that uses Coastal Change Analysis Program (C-CAP) land cover, elevation, precipitation, and soils data to create quantitative predictions of nonpoint source pollution and erosion at watershed scales. The region’s high variability in precipitation, topography, and land cover made the hydrologic modeling quite challenging, but the team successfully predicted runoff to within 5 percent of measured values.
Impact
State and local land managers use the information generated by N‑SPECT to prioritize efforts and locations for runoff and erosion control, and to evaluate the impacts of alternative management scenarios on nonpoint source pollution and erosion. Several riparian and hillside restoration projects have been implemented to help reduce storm-generated sediment loads.
The image at left shows Coastal Change Analysis Program (C-CAP) land cover in the Hanalei Bay watershed on the Hawaiian island of Kaua‘i. The image at right displays sediment sources as predicted by N-SPECT.
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