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Phreatology. Characterizing groundwater recharge and discharge using remote sensing, GIS, ecology, hydrochemistry and groundwater modelling

vrijdag, 14 april, 2006 - 17:00
Campus: Brussels Humanities, Sciences & Engineering campus
Okke Batelaan

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.