As Guam moves into the future, development demands will require increased sources of high quality water. A possible source of this increased water supply is the Northern Guam Lens Aquifer (NGLA). Eighty percent of the supply of water for the island’s residents and businesses depend on this water source for their daily water needs (Contractor et al., 1999). In 2005, Guam Visitor’s Bureau recorded a record high tourist arrival of over 1.2 million visitors (GVB, 2006). Guam is expecting more expansion in tourism in the future. In the next three years, the U.S. military plans to relocate approximately 8,000 Marines (Guam PDN, 2009) and 3,000 from the Air Force (Guam PDN, 2007) along with their dependents to Guam. These impacts along with other growth in the islands population will clearly have an effect on Guam’s demand for fresh water supply resources. Possible impacts could include exceeding pumping rate limits of sustainable yield to meet the increase population demands. This in turn could threaten the fresh water quality through salt water intrusion.

An accurate estimate of the NGLA’s sustainable yield is vital for water resource managers from Guam Waterworks Authority and Guam Environmental Protection Agency in order for them to plan and make confident decisions in managing and protecting the NGLA. Knowing the inflows or recharge to the aquifer is one important factor in regulating pumping so as not to exceed the limits of sustainable yield.

Attempts to estimate the NGLA recharge have been carried out since the late 1940s. Early estimates have been refined using increasingly sophisticated approaches. With modern computer technologies, more in-depth analyses are available to improve previous recharge estimates. These improvements are required as Guam’s water demands approaches the safe yield of the aquifer. The approach described in this thesis is an improvement over recent work accomplished at WERI (Contractor et al., 1999). In this new approach, we will be examining how soil moisture properties affect evapotranspiration (ET) and ultimately recharge estimates. Recharge from vadose flow synthesis is used as the water that arrives to the water table. The resulting recharge values are applied to a finite element ground water model. A computer program called AQUA CHARGE is developed to integrate the spatially distributed properties of the aquifer and the recharge model with the time variable inputs of rainfall and pan evaporation.

The Vadose Flow Synthesis Conceptual Model was designed for the NGLA to synthesize recharge to the aquifer. The model development begins with an area over a node and is the size of that node’s cell, of a two-dimensional groundwater model finite element mesh, from the water table, elevated to the soil surface as a vertical geologic column of shaft. This vertical shaft is termed node-shed and describes the vadose hydrologic watershed supplying each node. This node-shed is divided into two stages: the soil layer and the unsaturated limestone bedrock. The soil layer in the node-shed is composed of sub polygons with unique attributes we termed zones. These zones employ a mass balance equation referred to as the soil moisture model to account for the moisture input, the proportioning to recharge and evapotranspiration (ET), and the remaining soil moisture computed in a daily cycle. The proportioning of recharge and ET is determined by a soil moisture curve relationship for the two hydrologic processes. For a day’s computation of recharge for each zone in the node-shed, the soil stage ends with the calculation of the node-shed’s area weighted average recharge. This area weighted average recharge is split to account for the bedrock stage’s fast and slow flows to recharge. The time arrival of recharge to the lens, lagged and attenuated, is determined using a transfer function called the modified pulse routing technique. This conceptual model is derived from the US Army Corps of Engineer’sStreamflow Synthesis and Reservoir Regulation (SSARR) model (1987). It is an alternative route around the NGLA’s complex hydrogeology producing realistic estimates of the volume and time arrival of recharge to the lens. The project construction required ESRI® Geographic Information System Arc Editor and MICROSOFT® Excel 2003 and Visual Basic 6.0 Professional. The synthesized recharge serves as flux into a simple two dimensional, transient, saturated-flow, finite element groundwater model for history matching as a means of calibration. The ultimate purpose of AQUA CHARGE is to serve as a modeling tool to help water distribution and protection agencies on Guam to optimize the groundwater resource exploitation and development, to manage the population’s increased water needs and set limits to the aquifer’s yield capacity, and to maintain abundant quality water.