Scientists Develop First High-Resolution Water Budget Model for Tutuila Island in the American Samoa

  • 8 March 2021
Scientists Develop First High-Resolution Water Budget Model for Tutuila Island in the American Samoa

In a recent study, researchers from the Pacific Regional Integrated Sciences and Assessments, a CPO RISA team, developed the first island-wide high-resolution water budget model and groundwater recharge assessment for Tutuila, the main island in the American Samoa. Groundwater is Tutila’s primary source of drinking water and climate change has the potential to significantly impact water resource availability. Thus, accurate present and projected future recharge estimates are crucial for developing well-informed water management strategies for this limited resource. Using the water budget model, the study provides one of the most rigorous assessments of current and future water resource availability in the South Pacific Region to date.

The model results showed that of Tutuila’s annual average of 1,490 mL per day of precipitation, 23% was lost to evaporation, 21% was discharged as surface runoff, and 56% or 827 mL per day (plus another 57 mL per day of direct infiltration), became groundwater recharge. When comparing to water budget studies from similar islands, Tutuila had a higher rate of recharge relative to precipitation, which the authors attributed to higher rainfall amounts and intensities found in the South Pacific Conversion Zone (SPCZ), where Tutuila resides, relative to other regions. Assessing future recharge, the authors found that under RCP 8.5 and RCP 4.5 scenarios, annual average precipitation increases by 11% and 18%, respectively, by the end of the century. This translated to island-wide groundwater recharge increases of 8% and 14%, respectively. While increases in recharge roughly mirrored increases in precipitation, showing that precipitation is a primary driver of groundwater recharge, increases in surface runoff were significantly higher—on the order of 50 % more for the future scenarios. This suggests that while water resources availability may increase in the future, it could also cause increased flooding.

Because the model was developed with the goal of stakeholder engagement and use in mind, the researchers applied a participatory approach to develop future land-cover scenarios based on the stakeholders’ understanding of economic and environmental drivers. The authors combined the climate and land-cover scenarios to assess impacts on groundwater recharge. Using land-cover scenarios, the study showed that converting vegetated types to more urbanized types could cause up to 20% decreases in local recharge. However, on an island-wide scale, the authors found their stakeholder-informed land-cover change narratives caused only limited effects on groundwater recharge. This result could be partially due to offsetting effects from increased cultivated land and agroforest,  along with increased urbanization.

The authors maintained their work as an active open-source project on GitHub to increase transparency, reproducibility, and participation from American Samoa stakeholders and managers.

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