Nathan C. Habana, Leroy F. Heitz, Dannika Kate U. Valerio
March 21, 2022
Acknowledgements
- Guam Hydrologic Survey (GHS) Program, DOI USGS
- WERI-RCUOG, Kumisión I Na’an Lugåt Guahan
- GHS interagency partners: GWA, GBSP, GDLM,
and Maria Kottermair
Abstract
The mapping of the surface hydrology of the Northern Guam Lens Aquifer (NGLA) is integral to our efforts of protecting our most valuable freshwater source. The NGLA is an uplifted island karst aquifer with a plateau terrain of well-developed stormwater transfer and infiltration system that effectively recharges the freshwater lens. However, as the island continues to develop the land atop its designated sole source aquifer, there is an increased risk of loading stormwater runoff with anthropogenic contaminants that may transfer, accumulate, and infiltrate surface depressions of hydrologically significant sinkholes, and transport into our utility water source. That is if we are not careful and strategic with planning and development. And that is if we are not informed with a good set of surface hydrologic maps of the NGLA that can assist with strategic design. And it is at the surface where we may only have the chance to do so. Now, with an online map application as WERI Hydrologic Web MApps, Guam Environmental Protection Agency, Guam Waterworks Authority, and developers can work towards determining strategic storm drain systems, mitigation, management, and protection of our precious designated sole source.
Introduction
The goal was to map the fundamental hydrologic areas, boundaries, and runoff paths on the NGLA plateau. The map products would then be made available to interagency partners, with recommended training, to have an improved reference for identifying threats to the aquifer and for developers to determine strategic design to manage stormwater runoff. Specific objectives were to produce the hydrologic map features of fill areas, fill area depth, watersheds, runoff paths, and plateau basins. The basis for delineation of these boundaries and runoff paths is the 2012 LiDAR (bare-earth) based digital elevation model (DEM), the highest precision elevation map available to date. These computational processes were applied to produce features and rasters that could be organized to be presentable and useful for determining aquifer protective strategies, right from the surface.
Background
The NGLA has an effective and organized recharging system that infiltrate stormwater into the aquifer at closed surface depressions. In recent WERI-RCUOG scientific investigation reports addressing surface depressions and groundwater protection, surface hydrology analyses were applied that discovered an organized collection of tributary watersheds and focal watershed(s) in plateau basins. Of such great value in a new perspective and insight to the reports, the obvious thought was to map the entire surface hydrology of the NGLA.
The surface of the NGLA has a plateau basin system that effectively transfers stormwater runoff and infiltration, recharging the aquifer’s freshwater lens. The plateau exhibits an organized stormwater tributary watershed that cascades overflow runoff into doline infiltration system. A plateau basin at minimum has a focal watershed, but can be a larger rain catchment area system of tributary watersheds that during intense rainfall may fill shallow depression and overflow runoff in a cascading fashion to the focal watershed. The focal watershed has a well-developed deep doline that does not overflow into the next watershed, even during the most intense storms.
The focal watershed is of major concern since it is the deepest surface depression in a plateau basin that translates to a major recharge area. These surface depressions in karst terrain, especially relatively deep ones, are likely sinkholes. If verified to be so, then that depression is highly likely a fast recharge route to the freshwater lens. The common surface depressions found on Guam are solution dolines, collapsed doline, polygonal karst, uvalas, karrenfelds, poljes, cenotes, and excavations (ponding basin, landscape, drainage ditch, channel, and quarry). The natural closed surface depressions are usually dolines (sinkholes), and some of these low areas have been the obvious choice for placing urban stormwater ponding basins, on or nearby. Natural depressions are a result of karstification processes: cockpit karst and dolines, speleogenesis, and subterranean cavity collapse. A terrain analysis observes that most of the surface depressions occur along strikes and lineaments. Guam EPA regulates development on and near sinkholes. The map they currently use are extracted areas of closed contour depressions, based on old topographic maps. New LiDAR based DEM are of high precision that results in refined delineation of hydrologic boundaries.
Methods
Again, the goal of this project was to produce a fundamental surface hydrologic map of the NGLA plateau. This was done with the application of GIS hydrologic spatial analysis to the 2012 bare-earth DEM of northern Guam to create fundamental hydrologic features and rasters: fill areas, fill depth, fill area watershed, runoff paths, and plateau basins. Prior to hydrologic analysis, null values (“holes”) in the DEM were patched smoothly using a technique of elevation points extracted from DEM raster cells surrounding the holes and natural neighbor interpolation. These holes appear at some of the large buildings and corners of a DEM block sets that emerged in the process of piecing the entire DEM of Guam.
The map applications used were ESRI’s ArcGIS Pro and ArcGIS Online, and extension Spatial Analyst—Hydrology. Hydrology subcategory methods required Fill, Flow Direction, Accumulation, Basin, Watershed, and Strahler Stream Order. Other Spatial Analysis tools were Map Algebra and Extraction. The map feature and rasters produced were the following:
- Fill areas (raster and polygons) and fill depths (raster), delimited fill areas (polygons)
- Watersheds of delimited fill areas (polygons)
- Runoff paths – Watershed runoff paths and overflow runoff paths (polylines)
- Plateau basins (polygons)
- Basemaps: flow direction (raster), LiDAR first-return hillshade (raster), LiDAR based contours (polylines)
Fill areas represent the areal extent and fine elevation of a closed surface depression. Fill areas were then delimited to fill areas that were 300 sqm or greater, to produce a reasonable sized watershed. Fill depth is the difference from the fill elevation to the original DEM. Watersheds are the catchment area of the delimited fill areas. Runoff paths were generated using accumulation, conversions, and Strahler Stream Order (integer attributes). Stream order lines less than 5 are watershed runoff path, and greater than 4 are overflow runoff paths. Plateau basins were then generated, and boundaries reconstructed along tributary watershed boundaries that flow into a focal watershed. Focal watersheds were defined and delimited to fill depths of 20ft (expected to never overflow) or more. Another delimitation was the bounding of plateau basins (e.g., may have more than one focal watershed, such as the Mokfok Basins) with respect to area, as the basin were assigned CHamoru names, determined by the Kumisión I Na’an Lugåt Guahan.
The features and raster layers were organized, ordered, and uploaded into ArcGIS Online, as web maps, and then converted into a web map application. The final hydrologic product are made available as WERI Hydrologic Web MApps. These Web MApp products reside here on the Guam Hydrologic Survey website, in the Interagency Maps, under the Library menu. Four Web MApps are available:
- Surface Hydrology of the Northern Guam Lens Aquifer
- First-return hillshade, urban surface hydrology
- CHamoru names of Plateau Basins
- Terrain Analysis (high contrast) with surface hydrology
Results
The hydrologic map product is a set of plateau basins containing tributary watersheds, overflow runoff paths, and focal watershed(s). The background basemaps are hydrologic flow direction analysis and contours based on DEM derived from Guam 2012 LiDAR (bare-earth), and Guam 2007 LiDAR (first-return) hillshade. The following are sample figures/images of resulting products from the applied hydrologic spatial analyses.
The ultimate goal is the WERI Hydrologic Web MApp products that are now available online for anyone to access, worldwide. The following are sample images of the hydrologic Web MApp products.
The WERI hydrologic Web MApp products are available in our GHS Interagency Maps page.
Conclusion
The surface hydrology maps of the NGLA are GIS hydrologic spatial analysis of Guam’s 2012 LiDAR (bare-earth) based DEM, producing the highest precision hydrologic maps available to date. These computational processes produced features and rasters that are organized and made available for the purpose of having useful hydrologic map references for our interagency partners (GEPA, GWA, and contractors) who have a stake on developing on the aquifer. The hydrologic map products are advanced and goes beyond identifying suspect sinkholes from closed contour depressions. The maps from this project are a comprehensive hydrologic analysis of the plateau basin. A plateau basin is discovered in the NGLA as a large stormwater catchment area that contain tributary watersheds that may overflow and runoff stormwater in a cascade fashion into its focal watershed. The focal watershed has a deep doline and the lowest ground surface point in its plateau basin. It receives the terminal overflow runoff from tributary watersheds that infiltrates the surface through a fast flow route to the utility water source. To protect the water source, it is essential to map the surface hydrology and identify storm catchment areas and runoff that may transfer potential anthropogenic contaminants to dolines, fast recharge areas. Sixty plateau basins were named based on several historic maps of Guam by the Kumisión I Na’an Lugåt Guahan (Guam Place Name Commission). The hydrologic map products were then converted into an online map application, as WERI Hydrologic Web MApps. The hydrologic Web MApp products are available here in the Guam Hydrologic Survey website. To learn more about this project, see WERI Technical Report 175 in the GHS Library.
Phase II is underway to map tributary watershed divides and surface hydrologic details within the fill area.
