VADOCHARGE: Groundwater Recharge Model for an Uplifted Island Karst Aquifer, Guam, USA

The Northern Guam Lens Aquifer (NGLA) presents unique challenges to modelers due to the complex hydrologic properties of its thick karst vadose zone. To meet these challenges, we developed VADOCHARGE, an analogue model based on the US Army Corps of Engineers’ Streamflow Synthesis and Reservoir Regulation (SSARR) model, to produce the effect of attenuated and lagged recharge through the vadose zone to the water table. It incorporates a soil moisture model to account for soil moisture storage, evapotranspiration, and recharge determined by a soil moisture curve relationship. To simulate flow through limestone bedrock, the model applies time storage routing technique in a series of reservoirs, to simulate the time arrival of both fast and slow recharge past the soil to the water table reach. The model is implemented by assigning recharge from the portions of the surface watershed to each node of an underlying coupled phreatic model. Several simulations were performed and adjustments to hydraulic parameters were made until a close match with observation well data was achieved. The results improved upon all former models of the NGLA, and provide the tool for sophisticated and accurate coupled vadose-phreatic models of the NGLA and similar karst aquifers. Copyright © IJESTR, all rights reserved.