Prepared in cooperation with the County of Maui Department of Water Supply and the State of Hawai‘i Commission on Water Resource Management Spatially Distributed Groundwater Recharge Estimated Using a Water-Budget Model for the Island of Maui, Hawai‘i, 1978–2007 Scientific Investigations Report 2014–5168 Version 2.0, February 2018 U.S. Department of the Interior U.S. Geological Survey COVER Top left: View toward summit of West Maui Mountain from a forested ridge near Honokōhau Valley on northern West Maui Mountain, Island of Maui, Hawai‘i. Photograph by Sarah N. Rosa, U.S. Geological Survey, 2013. Top center: View of Waihe‘e River on northeastern West Maui Mountain, Island of Maui, Hawai‘i, 2009. Top right: View of Ke‘anae Valley, on eastern Haleakalā, Island of Maui, Hawai‘i. Photograph by Adam G. Johnson, U.S. Geological Survey, 2014. Top right inset: Taro fields near Ke‘anae, eastern coast of Haleakalā, Island of Maui, Hawai‘i. Photograph by Adam G. Johnson, U.S. Geological Survey, 2014. Bottom right: View toward southern flank of Haleakalā from the coastline near Kaupō, Island of Maui, Hawai‘i. Photograph byAdam G. Johnson, U.S. Geological Survey, 2013. Bottom left: View of shrubs, grass, and barren rock on a steep valley wall near Ukumehame Gulch on southern West Maui Mountain, Island of Maui, Hawai‘i. Photograph by Adam G. Johnson, U.S. Geological Survey, 2010. Center: Map of the Island of Maui, Hawai‘i, showing the relative distribution of estimated mean annual recharge for average climate conditions. Estimated recharge is shown using a spectral color scale with red representing very low recharge and dark blue representing very high recharge. Spatially Distributed Groundwater Recharge Estimated Using a Water-Budget Model for the Island of Maui, Hawai‘i, 1978–2007 By Adam G. Johnson, John A. Engott, Maoya Bassiouni, and Kolja Rotzoll Prepared in cooperation with the County of Maui Department of Water Supply and the State of Hawai‘i Commission on Water Resource Management Scientific Investigations Report 2014–5168 Version 2.0, February 2018 U.S. Department of the Interior U.S. Geological Survey ii U.S. Department of the Interior RYAN K. ZINKE, Secretary U.S. Geological Survey William H. Werkheiser, Deputy Director exercising the authority of the Director U.S. Geological Survey, Reston, Virginia: 2018 First release: 2014, online Revised: February 2018 (ver. 2.0), online For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov/ or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Johnson, A.G., Engott, J.A., Bassiouni, Maoya, and Rotzoll, Kolja, 2018, Spatially distributed groundwater recharge estimated using a water-budget model for the Island of Maui, Hawai`i, 1978–2007 (ver. 2.0, February 2018): U.S. Geological Survey Scientific Investigations Report 2014–5168, 53 p., https://doi.org/10.3133/sir20145168. ISSN 2328-0328 (online) iii Contents Abstract ........................................................................................................................................................1 Introduction ...................................................................................................................................................1 Purpose and Scope .............................................................................................................................4 Previous Studies ..................................................................................................................................4 Description of Maui ......................................................................................................................................4 Physical Setting ...................................................................................................................................4 Climate ................................................................................................................................................5 Hydrogeology .......................................................................................................................................5 Surface Water ......................................................................................................................................7 Soils ................................................................................................................................................7 Land Cover ..........................................................................................................................................7 Water-Budget Model .....................................................................................................................................7 Conceptual Model .............................................................................................................................10 Model Calculations ...........................................................................................................................10 Model Input .................................................................................................................................................14 Land Cover ........................................................................................................................................14 Impervious Surfaces .................................................................................................................15 Rainfall ..............................................................................................................................................15 Monthly Rainfall ........................................................................................................................15 Daily Rainfall .............................................................................................................................15 Fog Interception .................................................................................................................................16 Irrigation .............................................................................................................................................17 Irrigation Estimates ...................................................................................................................17 Irrigation Calibrations ................................................................................................................18 Septic-System Leaching ....................................................................................................................18 Storm-Drain Systems .........................................................................................................................18 Direct Runoff ......................................................................................................................................19 Computation of Seasonal Runoff-to-Rainfall Ratios..................................................................19 Runoff-to-Rainfall Ratios Assigned to Ungaged Catchment Zones .........................................24 Runoff-to-Rainfall Ratios Assigned to Gaged Basins within a Single Catchment Zone ............25 Runoff-to-Rainfall Ratios Assigned to Gaged Basins with Multiple Catchment Zones..............25 Evapotranspiration .............................................................................................................................25 Forest-Canopy Evaporation and Net Precipitation ....................................................................26 Potential Evapotranspiration .....................................................................................................27 Reference Evapotranspiration .........................................................................................27 Crop Coefficients .............................................................................................................27 Moisture-Storage Capacity of the Plant-Root Zone ..........................................................................30 Direct Recharge .................................................................................................................................30 Other Input .........................................................................................................................................30 Model Exclusions and Limitations ..............................................................................................................32 Model Randomness ....................................................................................................................................33 Water-Budget and Groundwater-Recharge Estimates ...............................................................................33 iv Water-Budget and Recharge Estimates for Average Climate Conditions ..........................................33 Comparison with Water Resources Protection Plan .................................................................42