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Mean annual water-budget components for Guam for historic (1990-2009) and future (2080-2099) climate conditions

October 9, 2019

This data release contains the shapefiles of mean annual water-budget components for Guam for historic (1990 - 2009) and future (2080 - 2099) climate conditions. Components estimated for the 1990 - 2009 scenario represent an update to the historic (1961 - 2005) components estimated by Johnson (2012), and serve as a historic baseline for the components estimated for the future (2080 - 2099) climate scenario. The recharge distributions were estimated as part of a larger effort (Gingerich and others, 2019) to evaluate how one set of climate projections may impact Guam's future groundwater resources. The recharge estimates may be used in numerical groundwater models that can evaluate the collective impacts of changes in recharge, changes in groundwater withdrawals, and sea-level rise on Guam's groundwater systems and wells.

The zipped folder (Mean_annual_water_budget_Guam_1990-2009_and_2080-2099.zip) for this data release contains two polygon shapefiles and associated metadata files. Both shapefiles contain spatially distributed estimates of mean annual groundwater recharge and other water-budget components, in inches, for Guam. "Mean_annual_water-budget_components_Guam_1990-2009.shp" contains mean annual water-budget components for a historic scenario (1990 - 2009), and "Mean_annual_water-budget_components_Guam_2080-2099.shp" contains mean annual water-budget components for a future climate scenario (2080 - 2099). Each shapefile contains 172,754 polygons, which represent the subareas of the Guam water-budget models described in the supplemental material of Gingerich and others (2019). For each of the 172,754 polygons, the Guam water-budget models computed daily water-budget components and then the daily water-budget components were summed and averaged into mean annual components for the shapefiles' attribute tables. The water-budget components associated with each subarea include rainfall, irrigation, septic leachate, water-main leakage, runoff, forest-canopy evaporation, actual evapotranspiration, total evapotranspiration, and recharge. The supplemental material of Gingerich and others (2019) contains explanations of the Guam water-budget models, subareas, and model inputs used to compute the water-budget components contained in the shapefiles. In summary, for both the 1990 - 2009 and 2080 - 2099 scenarios, model inputs used to simulate soil conditions, land-cover conditions, irrigation, septic leachate, and water-main leakage were the same as those used for a 1961 - 2005 water-budget scenario described in Johnson (2012). For the 1990 - 2009 scenario, model inputs used to simulate rainfall, runoff, and reference evapotranspiration (ET) were derived from historic records and estimates of rainfall, runoff, and reference ET for Guam. For the 2080 - 2099 scenario, model inputs used to simulate future mean monthly rainfall and mean monthly reference ET were derived from modified versions of historic rainfall records and reference-ET estimates used to create inputs for the 1990 - 2009 scenario. The historic records and estimates were modified with mean monthly change factors for rainfall and reference ET. The change factors were computed using dynamically downscaled climate variables that were produced by simulations of a high-spatial resolution (800 m) Weather Research and Forecast (WRF) model for Guam. The change factors represent ratios of the WRF-modeled climate variables for 2080-2099 to those for 1990-2009. The climate variables estimated by the WRF model for 2080-2099 represent a "business-as-usual" warming scenario denoted as representative concentration pathways 8.5 (RCP 8.5).

References Cited

Gingerich, S.B., Johnson, A.G., Rosa, S.N., Marineau, M.D., Wright, S.A., Hay, L.E., Widlansky, M.J., Jenson, J.W., Wong, C.I., Banner, J.L., Keener, V.W., and Finucane, M.L., 2019, Water resources on Guam - Potential impacts of and adaptive response to climate change: U.S. Geological Survey Scientific Investigations Report 2019 - 5095, 55 p., https://doi.org/10.3133/sir20195095.

Johnson A.G., 2012, A water-budget model and estimates of groundwater recharge for Guam: U.S. Geological Survey Scientific Investigations Report 2012-5028, https://pubs.usgs.gov/sir/2012/5028/.