Runoff and erosion on the Cultuurvlakte of St. Eustatius
Erosion can cause large problems for ecosystems on and around tropical islands. On site, erosion leads to a decrease of available nutrients, and off site, accumulation of nutrients can occur or it can leave the system by entering the sea and thereby disturbing the marine ecosystem. This is particularly bad for coral reefs surrounding the island since they are very vulnerable to nutrient enrichment, sedimentation, and turbidity. Change in erosion rates is mainly caused due to anthropogenic influences such as urbanization, land use change, and overgrazing. St. Eustatius is a small island (21 km2) in the Caribbean Sea, where the Cultuurvlakte (urbanized area) is especially sensitive to runoff and erosion due to paved areas, overgrazing, and relatively little vegetation. This research focusses on runoff and erosion on the Cultuurvlakte and especially the effect of land use types and slope gradients. Runoff and erosion were measured in the field with runoff plots of 1 m2 and modelled with the soil erosion model LISEM. The runoff plots were installed on slopes of 5 and 10% and on three different land use types (grass, shrub, and bare soil). All rainfall events that caused erosion at the plots were also simulated with LISEM. The model was also used to analyze to which parameters erosion and runoff are most sensitive and to do scenario analyses. The results of the plot measurements clearly show that both runoff and erosion were highest for bare soil. The difference between grass and shrubs was less clear, but generally there was more runoff and erosion at the plots with grass. Effects of slopes were not visible in the plot measurements, most likely due to differences in hydraulic conductivity between the plots. The model simulations show the same effect of land use on runoff and erosion (most at bare soil and least at shrubs) but also show that the steepest parts of the Cultuurvlakte, with little vegetation, are most vulnerable. With the measured rainfall events, up to 15% of rainfall was runoff at the plots while the discharge to the sea was only up to 4% for the model, which is (mainly) caused by scale effects. The scenario analyses show that both overgrazing and removal of shrubs have a significant effect on runoff and erosion on the Cultuurvlakte, and bare soil (which often occurs for longer time during construction projects) has the most significant effect. To reduce erosion as much as possible, land should lie follow as little and short as possible and overgrazing and removal of shrubs should be limited, especially at steep slopes.
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