A modelling study on the effect of groundwater on vegetation and the effect of vegetation on groundwater in the Pantanal of Mato Grosso, Brazil

Abstract

This report focuses on the interaction of vegetation and groundwater in a study area in the Pantanal of Mato Grosso, Brazil. This area is a mosaic of patches of different vegetation types. Many studies have focused on the effect of the inundation that occurs every summer and depending on the elevation inundates the vegetation zero to nine months. From these studies it is apparent that the vegetation distribution is influenced by inundation. However, during the annual dry season the surface water recedes almost entirely and the groundwater can drop up to 5.6 m below soil surface. The effect of this drought has not been investigated further. To study the effects of groundwater on the vegetation distribution a groundwater model is created using PCRasterMODFLOW. Using the vegetation distribution and a digi-tal elevation map created from 230 elevation points, three scenarios are modeled in which only the evaporation varies per scenario. In the first scenario the evaporation does not decrease when the groundwater drops and the evaporation is uniform over space. In the second scenario the evaporation decreases linearly with falling groundwa-ter levels. In this scenario all the vegetation has a root depth of 3 m . This indicates that the vegetation has no influence on the evaporation. In the third scenario the evaporation again decreases linearly with falling groundwater levels, however the root depth de-pends on the vegetation. This indicates that the evaporation is dependent on both root depth (thus vegetation) and groundwater depth. The results show that the vegetation distribution is not affected by the groundwa-ter. However, the vegetation has a strong influence on the groundwater distribution. Deep rooted vegetation decrease the groundwater level up to 2 m lower than shallow rooted vegetation does. This causes groundwater to flow in the direction of the deep rooted vegetation. Deep rooted vegetation immediately adjacent to shallow rooted vege-tation benefits most, for the shallow rooted vegetation immediately adjacent to deep rooted vegetation it is most disadvantageous. Further, it can be shown that deep rooted vegetation is least stressed by the drought and therefore the deep rooted vegetation is not limited by groundwater. A different environmental variable will more likely be re-sponsible for the patchiness of this area.