Bootsma, H.

MSc Thesis 

Groundwater is an important resource and an important factor to consider in pollution management. Accurate estimates of groundwater recharge are essential for effective management of groundwater. St. Eustatius is a small island of the Leeward Islands in the Caribbean Sea with a brackish groundwater lens with high nutrient concentra- tions at the inhabitated part of the island. In this study, groundwater recharge was estimated with a modelling approach, using the unsaturated zone model SWAP. A period of 30 years was simulated. Field work was performed on St. Eustatius to determine vegetation and soil parameters.

Model calculations show that for the inhabited part of the island, the Cultuurvlakte, long term average groundwater recharge ranges 75–210 mm year-1. Quantitative uncertainty bounds could not be determined. High groundwater nitrogen concentrations suggest that in some parts of the Cultuurvlakte a third to all of groundwater is infilrated sewage water.

A global sensitivity analysis was performed using the Morris method. The sensitivity analysis showed that rainfall is the most important factor in determining groundwater recharge, followed by refference evapotranspiration and vegetation characteristics that control evapotranspirative fluxes. Rainfall intensity and antecedent soil moisture play a key role in generating groundwater recharge; a simple conceptual reservoir model proved capable of reproducing the recharge simulated by SWAP. The results emphasize the importance of accurate rainfall measurements and the need to develop methods for estimating the evapotranspiration of natural vegetation.

Please contact Klaas Metselaar (klaas.metselaar@wur.nl)