Abstract
Groundwater sustainability initiatives, including sustainable yield and watershed policy protection policies, are growing globally in response to increasing demand for groundwater, coupled with concerns about the effects of climate and land-cover change on groundwater supply. Improved understanding of the impacts of watershed management on groundwater yields and management costs—particularly in the broader context of climate and land-cover change—is critical to inform these initiatives and facilitate integrated land and water management. This study develops a novel, spatially explicit groundwater hydrologic ecosystem services framework, which combines stakeholder-defined land-cover scenarios, sustainable yield estimation using a groundwater simulation optimization approach, and economic valuation, and applies it in the most heavily utilized aquifer Hawai‘i (USA). Sustainable yield estimates and resulting differences in replacement costs are estimated for six land-cover scenarios (with varying levels of urban development and watershed management) crossed with two water demand scenarios in a context of a dry future climate (Representative Concentration Pathway [RCP] 8.5 mid-century). Land-cover change is found to be an important, though less significant drive of changes in groundwater recharge than climate change. The degree of watershed protection, through preventing the spread of high-water-use, invasive plant species, is projected to be a much stronger land-cover signal than urban development. Specifically, full forest protection increases sustainable yield by 7–11% (30–45 million liters per day) and substantially decreases treatment costs compared with no forest protection. Collectively, this study demonstrates the hydrologic and economic value of watershed protection in a context of a dry future climate, providing insights for integrated land and water policy and management in Hawai‘i and other regions, particularly where species invasions threaten source watersheds.
Highlights
Planning for water security in a context of global environmental change is one of the most pressing challenges worldwide (Taylor et al 2013; Wheater and Gober 2015; Elshall et al 2020; Gleeson et al 2020)
In order to evaluate the potential effect of a range of future land-cover possiblities, including changes in watershed management and urban development, this study developed stakeholder-informed, spatially-explicit scenarios of future land cover over 50 years (2021–2070)
Groundwater recharge is projected to decline by ~16% under representative concentration pathway (RCP) 8.5 midcentury rainfall compared with baseline rainfall conditions
Summary
Planning for water security in a context of global environmental change is one of the most pressing challenges worldwide (Taylor et al 2013; Wheater and Gober 2015; Elshall et al 2020; Gleeson et al 2020). A range of groundwater sustainability policies have emerged in response to current and projected water resource degradation (Gleeson et al 2012; Elshall et al 2020). This includes sustainable yield and related policies, which limit groundwater pumping to levels that can be sustained over the long-term without compromising societally defined human and ecological uses of the resource (Hansen 2012; Pierce et al 2013; Rudestam and Langridge 2014; Elshall et al 2020). Translating changes in groundwater recharge and supply into metrics directly related to groundwater management (e.g. change in sustainable yields, supply costs, number of households serviced, etc.) is critical to support integrated land and water management and funding decisions (Guswa et al 2014; Kroeger et al 2019; Wada et al 2020)
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