Assessing Sustainability of Groundwater Resources Under Future Climate Change Conditions
Ground water is the primary source of municipal water on most developed islands in the Pacific, and yet the reliability of water supplies on all islands will be challenged by reductions in rainfall and changes in its distribution owing to climate change. This project will use projections of climatic conditions together with stochastic hydrological models to assess the sustainability of ground water resources. Previous hydrological assessments have identified reliable potable water resources as a critical need throughout the Pacific Islands. With advance planning, the consequences of climate change on water systems can be mitigated. Most aquifers occur as a freshwater lens-shaped body that floats upon denser salt water. The freshwater lens usually stands no more than a few meters above sea level. The freshwater lens is dynamic: ground water flows constantly from inland areas of recharge to be discharged at the coast. Because of the underlying seawater, ground water pumping must be carefully managed to prevent salt water intrusion. Changes in recharge and pumping can alter the flow of freshwater through the aquifer, and therefore change the salinity of the pumped water. Of particular concern with regard to climate change is the possibility that decreases in precipitation or increases in evapotranspiration may reduce the amount of fresh water recharge and thus affect the sustainability of fresh ground water resources.
Recharge can be estimated from the water budget through assessing water volumes entering, leaving, and being stored within the plant-soil system. Recent studies on Kauai and Oahu assessed the water budget and expected load reductions for nutrients and sediments, based on suggested remediation strategies. On the island of Maui, the USGS has constructed a water budget that estimates the amount of ground water recharge for much of the island towards assessing resource sustainability.
Assessing Ground Water Sustainability
Ground water models are used in assessing ground water resources and defining appropriate management practices that set aquifer sustainable yield. Example model applications include studies by the UH Water Resources Research Center (WRRC) and Gingerich (USGS). However, available studies have not accounted for issues related to long-term recharge forecast or uncertainty. According to the State Water Code (State Legislature of Hawai‘i, Act 45, 1987; Hawai‘i Revised Statutes, chapter 174C), the sustainable yield is set by Hawai‘i Commission on Water Resource Management (CWRM) using the best available information and should be reviewed periodically. Due to difficulties involved in numerical modeling and an absence of detailed site-specific data, sustainable yield determination defaults to a simple analytical model. In addition, conventional approaches fail to account for uncertainties, especially related to recharge.
Approach and Utility
Working with Pacific RISA partners such as the USGS Pacific Islands Water Science Center, the WRRC will use forecast rainfall and other meteorological parameters generated by the International Pacific Research Center (IPRC) as input to models to calculate the water budget so that it reflects a range of future conditions. Uncertainties in climate predictions will be also considered as input to stochastic hydrological models. Recharge assessment will be done on two levels. The first is a simplified approach that develops regression equations relating rainfall and recharge. The second approach uses a more elaborate method based on completing a water budget of the system, including rainfall, surface runoff, evapotranspiration, infiltration, soil moisture storage, and fog drip. Once the model is calibrated, scenario runs are used to quantify recharge under various climate conditions, which will then be used as input to the ground water flow model under forecast conditions. The sustainability of water resources can be evaluated by seeing how the aquifer responds to both current and proposed rates and distributions of pumping. Wells that are forecast to pump water beyond an acceptable salinity will be judged not sustainable. It may also be possible to simulate where additional wells can be installed to mitigate any loss of capacity.We will focus first on the Iao-Waiheʻe Aquifer, Maui, where water budgets have been developed in the absence of future climate projections. Other potential aquifers are located at North Kohala, Southern Lihue, and Pearl Harbor. An additional direction for future work, if demanded by stakeholders, is to examine how climate change will affect rainfall and water resources on USAPIs such as Pohnpei, a high island like Maui, but with much lower GDP and consequently more limited resources. Partners, such as the University of Guam Water & Environmental Research Institute, working with USGS, have conducted hydro-meteorological research in Micronesia and would be able to later adapt these methods for use in Guam. The Hawai‘i CWRM currently assesses the sustainability of ground water resources using simplified models. The new water budget from USGS caused CWRM to revise its estimates of many aquifer sectors on Maui. The new research is expected to enhance decision processes by including climate uncertainty. The results will inform Pacific RISA education and outreach activities targeted towards water and other resource managers. Example management decisions include determining whether additional wells can be drilled in an area, estimating pumping rates, and redistributing pumpage.