Identifying the relative importance of water-budget information needed to quantify how land-cover change affects recharge, Hawaiian Islands

Abstract

First posted September 28, 2023 For additional information, contact: Director,Pacific Islands Water Science CenterU.S. Geological SurveyInouye Regional Center1845 Wasp Blvd., B176Honolulu, HI 96818 Watershed management—the protection and restoration of native landscapes through a variety of actions—potentially can protect and improve groundwater availability by sustaining and enhancing groundwater recharge. The efficacy of watershed management for sustaining and enhancing groundwater recharge in the Hawaiian Islands, however, has not been quantified with certainty. A model that uses a water-budget approach—an accounting of the flow of water into and out of an area—is useful for assessing how regional-scale recharge for the Hawaiian Islands might be affected by land-cover changes associated with managed or unmanaged watersheds. However, the use of a water-budget model to confidently quantify how recharge might be affected by land-cover changes is impeded by uncertain values that model users assign to land-cover-dependent parameters. The parameter values, and thereby water-budget model recharge estimates, can likely be improved by the collection and analysis of additional hydrologic information.This report describes a sensitivity analysis of a water-budget model that was completed to identify the most important types of hydrologic information needed to reduce the uncertainty of model recharge estimates. The sensitivity of model recharge estimates for the Hawaiian Islands of Oʻahu and Maui was analyzed for seven model parameters potentially affected by land-cover changes within a watershed. The seven model parameters tested were canopy capacity, canopy-cover fraction, crop coefficient, fog-catch efficiency, root depth, stemflow, and trunk-storage capacity.Results of the sensitivity analysis were used to (1) quantify the relative importance of the seven model parameters to recharge assessments for three moisture zones (dry, mesic, and wet) on Oʻahu and Maui and (2) prepare a list of critical information needs for each moisture zone. The list of critical information needs was developed for three general types of land cover (forest, shrubland, and grassland) that are assumed to be affected by watershed management in the Hawaiian Islands. Identified information needs included estimates or measurements of (1) evapotranspiration processes needed to determine crop coefficients for land-cover types in all moisture zones, (2) rooting depths for land-cover types in the dry and mesic moisture zones, (3) canopy-cover fraction for forests in the wet and mesic moisture zones, (4) ratios of fog interception to rainfall for forests and shrublands in the wet moisture zone, and (5) canopy capacity for forests in the wet and mesic moisture zones. The list of information needs can guide data-collection strategies of future projects. Collection and analysis of the identified hydrologic information may help model users develop a better parameterization scheme, reduce uncertainty of values that model users assign to land-cover dependent parameters, and therefore allow future applications of the water-budget model to more accurately quantify how recharge in the Hawaiian Islands might be affected by future land-cover changes within a watershed.