Simulating Land Cover Change Impacts on Groundwater Recharge under Selected Climate Projections, Maui, Hawaiʻi

Abstract

This project developed an integrated land cover/hydrological modeling framework using remote sensing and geographic information systems (GIS) data, stakeholder input, climate information and projections, and empirical data to estimate future groundwater recharge on the Island of Maui, Hawaiʻi, USA. End-of-century mean annual groundwater recharge was estimated under four future land cover scenarios: Future 1 (conservation-focused), Future 2 (status-quo), Future 3 (development-focused), and Future 4 (balanced conservation and development), and two downscaled climate projections: a coupled model intercomparison project (CMIP) phase 5 (CMIP5) representative concentration pathway (RCP) 8.5 “dry climate” future and a CMIP3 A1B “wet climate” future. Results were compared to recharge estimated using the 2017 baseline land cover to understand how changing land management and climate could influence groundwater recharge. Estimated recharge increased island-wide under all future land cover and climate combinations and was dominated by specific land cover transitions. For the dry future climate, recharge for land cover Futures 1 to 4 increased by 12%, 0.7%, 0.01%, and 11% relative to 2017 land cover conditions, respectively. Corresponding increases under the wet future climate were 10%, 0.9%, 0.6%, and 9.3%. Conversion from fallow/grassland to diversified agriculture increased irrigation, and therefore recharge. Above the cloud zone (610 m), conversion from grassland to native or alien forest led to increased fog interception, which increased recharge. The greatest changes to recharge occurred in Futures 1 and 4 in areas where irrigation increased, and where forest expanded within the cloud zone. Furthermore, new future urban expansion is currently slated for coastal areas that are already water-stressed and had low recharge projections. This study demonstrated that a spatially-explicit scenario planning process and modeling framework can communicate the possible consequences and tradeoffs of land cover change under a changing climate, and the outputs from this study serve as relevant tools for landscape-level management and interventions.

Effects of land-cover and watershed protection futures on sustainable groundwater management in a heavily utilized aquifer in Hawai‘i (USA)

“…RCP 8.5 mid-century) is projected in this study to have approximately twice the impact of land-cover change, the results from this study demonstrate that informed land-cover planning, particularly watershed protection, can be an important climate change adaptation strategy. This supports general conclusions about the important, yet relatively smaller effect of land-cover versus climate change on groundwater that has been reported elsewhere (Candela et al 2009;Van Roosmalen et al 2009;Brewington et al 2019).…”

“…Based on input from the watershed management stakeholders, and following Brewington et al (2019), it was assumed that areas enclosed by fences are protected and do not convert to nonnative forest. To estimate the spread of nonnative forest outside of fenced areas in no-forestprotection and targeted-forest-protection scenarios, a spread model developed by Bremer et al (2019) was adapted to assess the trajectory of invasion of nonnative forest into native forest in East Maui.…”

“…Parameter values were derived from spatial datasets compiled for Hawai'i in Engott et al (2017) and Izuka et al (2018). Variations of this water balance approach have been used to generate recharge estimates across the Hawaiian Islands and are the most common source of groundwater recharge information utilized by resource managers (Johnson et al 2014;Engott et al 2017;Brewington et al 2019). Each land-cover scenario was run under the baseline climate (1978-2007Giambelluca et al 2013) and a dry future climate scenario RCP 8.5 mid-century (Elison Timm et al 2014) commonly used in management and research in Hawai'i (Wada et al 2017;Brewington et al 2019).…”

“…Variations of this water balance approach have been used to generate recharge estimates across the Hawaiian Islands and are the most common source of groundwater recharge information utilized by resource managers (Johnson et al 2014;Engott et al 2017;Brewington et al 2019). Each land-cover scenario was run under the baseline climate (1978-2007Giambelluca et al 2013) and a dry future climate scenario RCP 8.5 mid-century (Elison Timm et al 2014) commonly used in management and research in Hawai'i (Wada et al 2017;Brewington et al 2019). While RCP 8.5 was initially considered an extreme scenario, historical and current emissions are consistent with this scenario, and RCP 8.5 is currently seen as the most useful mid-century climate projection (Sanford et al 2014;Schwalm et al 2020).…”

“…While significant uncertainty remains on the impacts of species invasions on tropical ecohydrology (Hamel et al 2017;Wright et al 2018), a growing body of field studies suggest greater transpiration rates-and thereby lower groundwater recharge rates-in typical nonnative, invasive versus typical native forest stands (Giambelluca et al 2008;Kagawa et al 2009;Cavaleri et al 2014). Several studies in Hawai'i have utilized water balance and other hydrologic models parameterized by limited field data to assess the impact of land cover, including watershed protection on groundwater recharge (Engott 2011;Burnett et al 2017;Povak et al 2017;Brewington et al 2019), and linked economic metrics (Burnett and Wada 2014;Brauman et al 2015;Bremer et al 2019;Wada et al 2020). However, none of these studies link groundwater recharge to management implications through a spatially explicit groundwater model to evaluate the combined effect of watershed protection and climate change.…”