Predicting Sediment Export into Tropical Coastal Ecosystems to Support Ridge to Reef Management

Abstract

Sediment, a principal land-based source pollutant, can negatively affect coral reef ecosystems and impact the services they provide to society, such as recreation and fishing. Watershed managers interested in evaluating options and trade-offs require decision support tools that predict sediment export and retention. However, sediment load has previously been difficult to model in Hawaii because 1) observed annual sediment load data is not widely available and 2) Hawaii’s volcanic hydrology makes it difficult to use models designed on the continent. One method to estimate annual sediment loads is by measuring deposition rates in reservoirs. As of 2013, Hawaii had 140 regulated water storage reservoirs that perform as retention basins, yet the extent of their functionality had not been analyzed. I conducted a state-wide survey of reservoir capacity. Results demonstrated that only 15% of the total water storage capacity of Hawaii’s reservoirs is currently used, and 40% of all reservoirs are affected by sedimentation, representing a significant maintenance cost and potential risk. I also investigated spatial patterns of a broader array of ecosystem services including sediment retention, nitrogen retention, water yield, carbon sequestration, and agricultural production in West Maui. Between 1778 and 1920, I determined that sediment export increased by 18 times, while nitrogen export increased by 11 times over the same period. I demonstrated that past impacts from agriculture were more severe than predicted future development and climate change. Lastly, to calculate the first state-wide estimates of annual sediment export, I used a RUSLE-based model coupled with an estimate of the sediment delivery ratio (SDR), and adapted input datasets, including the erodibility and cover factors. To calibrate the model, I analyzed 60 watersheds with observed data for annual sediment load. I calculated that only 42% of the total land area contributes to sediment export, and that Hawaii and Maui export 57% of the total sediment load. Annual specific sediment yields ranged from 26 to 273 tons km-2 yr-1 across all islands, with Kahoolawe and Maui having the highest rates. These projects combined provide a base for watershed managers to identify management strategies to mitigate sediment export within an ecosystem context.