Phreatology: characterizing groundwater recharge and discharge using remote sensing, GIS, ecology, hydrochemistry and groundwater modelling
Batelaan, O. (2006). Phreatology: characterizing groundwater recharge and discharge using remote sensing, GIS, ecology, hydrochemistry and groundwater modelling. Vrije Universiteit Brussel: Brussel. xviii, 333 pp.
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Document type: Dissertation
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| Abstract |
Phreatology is a new field of research that is defined as ‘The science of the geological, hydrological, biological and chemical processes at the interface of the phreatic water with the vadose or surface waters, it focuses on the part of the surface and subsurface in which over time the groundwater table and capillary zone reside’. The concept of phreatology is given and its interdisciplinary character is stressed. Methodologies supporting phreatology are developed and applied to analyze groundwater discharge areas, and recharge areas. Tools and techniques from groundwater modelling, hydrochemistry, vegetation ecology and remote sensing are used and integrated; GIS technology combines the approaches. Applications from regional to site level are given for the Dijle, Demer and Nete catchment, Belgium.
A review of literature shows that groundwater discharge is not a well established research theme. An overview is given of the development and state-of-the-art of the scientific knowledge, measurement techniques, and understanding of groundwater discharge. The need for more attention is argued. The spatially distributed water balance model ‘WetSpass’ is developed; it simulates recharge in dependence of land cover, soil texture, topography and hydrometeorological parameters. Groundwater discharge areas can be simulated with the groundwater flow model MODFLOW, however, conceptual and practical problems arise. A new ‘SEEPAGE’ package is developed to resolve these problems.
Imaging spectroscopy is applied to characterize phreatophytic vegetation and to estimate evapotranspiration in a groundwater dependent wetland. The influence of soil moisture condition is investigated. A methodology is presented to assess regional ecohydrological differences in recharge-discharge systems. Vegetation of groundwater dependent ecosystems are mapped and analyzed, groundwater levels and seepage zones measured and modelled, hydrochemical patterns measured and analyzed, and groundwater ages simulated. The effects on the groundwater system and discharge, due to anthropogenic impacts on the land-use, are studied by simulation of the present, pre-development, and future situation. It is concluded that the regional context is often not explicitly taken into account in the ecohydrological analysis of wetlands, which hampers the identification and quantification of regional abiotic factors for ecohydrological site modelling. Recharge is shown to have a complex spatial pattern, depending to a large extend on the soil texture and land cover. The identification and delineation of regional groundwater discharge areas is proven more accurate by using the SEEPAGE package. The occurrence of phreatophytic vegetation clearly corresponds to groundwater discharge locations. However, the relationship is complex and has to be handled with care. The impact of the changes in recharge for both the pre-development and the future situation differs from large decrease to large increase in total groundwater discharge. Some areas show an opposite behaviour regarding the changes in groundwater discharge area and fluxes. The delicate shifts in the groundwater systems, which cause the changes in the recharge and discharge, clearly urge for hydrological modelling.
Shallow groundwater levels in valleys cause negative recharge conditions as a result of evapotranspiration by abundant phreatophytic vegetation. The red-edge index is indicative for the soil moisture regime of vegetation types. Estimated evapotranspiration and evaporative fraction show a non-linear relationship with soil moisture.
It is concluded that the evaporative fraction is dependent on the general wetness of the area. For the Doode Bemde study area it is feasible to map the average soil moisture regime by vegetation characteristics, and spatially detailed soil moisture on basis of estimation of the evaporative fraction.
The spatial distribution of the recharge has no important influence on simulated groundwater ages in groundwater dependent wetlands. The convergence of flow paths and the groundwater ages have a high spatial variability within the studied wetlands. Important consequences for vegetation patterns and groundwater sampling networks in wetlands are that very different groundwater fluxes and qualities can be expected at very short distances. Geochemical processes in the feeding aquifer and the convergence of flow paths strongly determine the hydrochemistry of discharging groundwater. It is the most important factor in the explanation of the occurrence and spatial distribution pattern of vegetation types.
The synergy of hydrological modelling, vegetation mapping, hydrochemistry and remote sensing proves advantageous and reveals ecological differences in the catchment. The results contribute to an increased understanding of the phreatological functioning of the studied areas. |
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