High precipitation and evaporation rates in tropical climates result in highly dynamic moisture fluxes in the vadose zone. This shallow soil region provides a critical link between the land surface above and the groundwater aquifer below. Yet vadose zone hydrology in tropical regions has not been studied as thoroughly as in temperate, arid, or semi-arid regions. The lack of hydrologic investigations presents particular challenges in urban areas where subsurface aquifers are vulnerable to contamination from chemicals introduced at or near the ground surface. An improved understanding of the hydrologic processes in the vadose zone of sandy, humid, tropical environments can inform strategies to protect the water quality of such urban aquifers. This dissertation studies the moisture fluxes in the vadose zone at the Changi East Reclamation Area, a constructed fill site in Singapore. This site is under consideration for both urban development and use as a freshwater storage aquifer or a flow-through treatment facility. The findings presented here are relevant both for urban water resources management at this site and at locations with similar environments.

An understanding of fundamental hydrological processes specific to an area is needed prior to conducting groundwater vulnerability assessments on the scale needed for city-planning. In particular, vadose zone processes play a critical role in groundwater protection, but there is insufficient evidence from field or modeling studies to describe the relevant processes in detail for such tropical sandy environments. Field studies of these processes are challenging due to the difficult nature of vadose zone measurements in general and the limited accessibility at sites like the Changi East Reclamation Area. Mathematical studies of this environment are equally challenging. Most relevant analytic methods are devoted to steady-state results or involve simplified solutions that are not appropriate for describing these field conditions. In addition, numerical modeling of the vadose zone is computationally demanding, especially in this environment where precipitation rates change rapidly and high evaporative demand results in low near-surface soil moisture content between rain events. Despite these challenges, this study combines field observations at the Changi East Reclamation Area with one- and two-dimensional numerical modeling experiments to explore vadose zone moisture fluxes and their implications for urban groundwater quality.

This study provides insights into hydrologic processes in the vadose zone of a sandy soil in a tropical climate, a historically understudied environment. The knowledge gained is then synthesized to provide insight into groundwater vulnerability assessments and stormwater management practices useful for urban water resources planners. Some key outcomes from this study include: (1) Field observations showing that precipitation quantity and antecedent soil moisture are strongly correlated to wetting front depth, while maximum precipitation intensity plays a lesser role; (2) A data analysis method to correct tensiometer measurement errors introduced by daily temperature fluctuations at the ground surface; and (3) A comparative metric to evaluate the impact of different stormwater management scenarios on vadose zone travel times.

Additionally, the modeling results provide the following general observations about vadose zone moisture fluxes in tropical sandy soils: (1) When the water table is shallow (less than 2 m), upward fluxes from the water table are important to the overall water budget, so both recharge and net recharge (recharge minus upflux) quantities are relevant. (2) When the water table is deeper than 2 m, recharge and net recharge are directly proportional to the saturated hydraulic conductivity and nearly independent of water table depth. (3) One-dimensional modeling is appropriate in this environment when the vadose zone soil is homogeneous. This is independent of spatially-variable infiltration patterns due to impermeable ground covering from urban development because lateral fluxes do not significantly impact recharge and net recharge estimates. (4) Post-development net recharge drops proportionally with the introduction of impermeable surfaces when stormwater runoff is routed offsite. However, net recharge increases dramatically when runoff is infiltrated locally, exceeding the pre-development recharge.

These observations have implications for the water quality of the shallow groundwater aquifers found in this environment. Since these aquifers are extremely vulnerable to contamination introduced at the ground surface, the quality of runoff water should be ensured prior to infiltration when aquifer protection is part of the stormwater management plan. The results of this study are useful for developing urban water resources management strategies, particularly in sandy tropical environments.