J.J. de Vries

Groundwater recharge in the Kalahari, with reference to paleo-hydrologic conditions

The Kalahari is situated in the semi-arid center of southern Africa and can be characterized as a savannah with a sandy subsurface, deep groundwater tables and annual rainfall ranging from 250 mm in the southwest to 550 mm in the northeast. A high infiltration rate and high retention storage during the wet season and subsequent high transpiration by the dense vegetation during the dry season, make that very little water passes the root zone and contributes to aquifer recharge. A lively debate has continued for almost a century on the question whether the Kalahari aquifers are being replenished at all under present climatic conditions. The present paper reports on results of an extensive recharge research project at the eastern fringe of the Kalahari, which is the most favorable part for groundwater replenishment. Additional observations were made in the central Kalahari. Environmental tracer studies and groundwater flow modeling indicate that present-day recharge is in the order of 5 mm yr 21 at the eastern fringe of the Kalahari where annual rainfall exceeds 400 mm. Figures in the order of 1 mm were obtained from the central Kalahari with lower precipitation. A dry valley system refers to more humid paleo-climatic conditions with a higher groundwater recharge. A tentative reconstruction of the groundwater depletion history suggests a time lapse of several thousands of years since the end of the last wet period. q 2000 Elsevier Science B.V. All rights reserved.

Channel network morphology and sediment dynamics under alternating periglacial and temperate regimes: a numerical simulation study

The occurrence of permafrost in a highly permeable catchment has a profound effect on runoff generation. The presence of permafrost effectively makes the subsoil impermeable. Therefore, overland flow can be the dominant runoff-generating process during periglacial conditions. The absence of permafrost will promote subsurface drainage and, therefore, saturation excess overland flow can become the dominant runoff-generating process during temperate conditions. In this paper, we present a numerical modelling study in which the effect of alternating climate-related phases of permafrost and nonpermafrost on catchment hydrology and geomorphology is investigated. Special attention is given to the characteristics of the channel network being formed, and the sediment yield from these catchments. We find that channel networks expand under permafrost conditions and contract under nonpermafrost conditions. A change from permafrost to nonpermafrost conditions is characterised by a decrease in sediment yield, while a change towards permafrost conditions is marked by a peak in sediment yield. This peak is explained by the build-up of a reservoir of erodible sediment during the nonpermafrost phase. The driving force behind this reservoir build-up may be local base-level change due to tectonic uplift or eustacy. We present a number of experiments, which show the details of this process. The results are in line with existing reconstructions of climate and fluvial dynamics during the Pleistocene in Europe and offer a new explanation to these observations.

Seasonal expansion and contraction of stream networks in shallow groundwater systems

Abstract Surface water and groundwater are normally closely connected in areas with shallow aquifer systems. Stream systems can thus be considered as the outcrops of associated groundwater flows in areas with a shallow groundwater table and a previous subsurface. This situation prevails in sandy lowland areas where almost all rainfall percolates into the subsurface so that the surplus over evapotranspiration becomes part of a groundwater drainage system before it reappears at the surface in a stream. The stream network, being the interface with the groundwater system, must have the capacity to release the seasonally dependent precipitation surplus through the continuum of ground and surface waters. A river network therefore consists of a hierarchical system of different order and incision depth, of which the discharge-contributing component contracts and expands with the seasonal fluctuation in recharge and water table depth. Coupling the mathematical expressions for groundwater drainage and stream flow enables development of a conjunctive model which relates the properties of a seasonally contracting and expanding stream network and related groundwater level fluctuation to the seasonal rainfall character for given geological and geomorphological conditions. This model further allows for assessment of drainage network response to a changing environment.

The mechanism of soil water movement as inferred from 18O stable isotope studies

Abstract The seasonal movement of soil water through a deep vadose zone of a sandy hill was studied in the central part of The Netherlands. The behaviour of soil water was tracked by monitoring soil water fluctuations and changes in the environmental tracer 18O in vertical profiles at test sites with grassland, heathland and forest. In the temperate climate of the study area a soil moisture deficit develops during summer and a precipitation surplus prevails during winter. The 18O content in precipitation exhibits a distinct seasonal cycle with the highest (relatively enriched) values of c. −6% vs V-SMOW occurring in summer and the lowest (relatively depleted) values of c. −9% vs V-SMOW in winter. The influence of this cycle can be traced back in the subsurface to a depth of sometimes more than 6 m. This seasonal fluctuation moves around the average annual 18O content in rainfall, tending closer to the average as depth increases. In the saturated zone the same average value is observed. This suggests that,...

Dynamics of the interface between streams and groundwater systems in lowland areas, with reference to stream net evolution

Abstract Observations reveal a close relationship between groundwater depth and stream net characteristics in the permeable lowlands of the sandy Pleistocene area of the Netherlands. This applies for average conditions as well as for the seasonal expansion and contraction of the network of streams that participate in the drainage process. It is plausible that this observed relation is caused by the close connection between groundwater and surface water in an area where almost all precipitation surplus is discharged as groundwater. Under such conditions a stream system can be considered the outcrop of a groundwater flow system. The hypothesis is that the stream network, as the interface between both systems, must have adapted in response to the discharge capacity that is required to release the precipitation surplus through the continuum of ground waters and surface waters. A theoretical model of coupled groundwater and surface water drainage systems is proposed that gives the stream net density and channel dimensions as functions of the geological and climatic conditions. The theoretically derived relationships of increasing stream density and decreasing channel size with reducing depth to groundwater and reducing topgraphic slope reflect the observed situation. The model further allows for assessment of the response of a drainage network to a changing environment.

The groundwater outcrop-erosion model; evolution of the stream network in The Netherlands

Abstract The stream network in the higher, pleistocene part of The Netherlands is genetically coupled with groundwater discharge systems of various extents. Streams of a given order can be explained as outcrops of groundwater flow systems of corresponding order. The drainage system is controlled by the precipitation surplus (climate), the resistance of the subsurface to groundwater flow (geology) as well as the previous relief and the incision depth (topography). This concept is defined as the groundwater outcrop-erosion model (GOEM). Stream nets can be synthesized theoretically from this model using geologic, geomorphologic and climatic data based on groundwater flow formulas and Hortons law on stream order vs. stream density.

Decomposition of groundwater level fluctuations using transfer modelling in an area with shallow to deep unsaturated zones

Abstract Time series analysis of the fluctuations in shallow groundwater levels in the Netherlands lowlands have revealed a large-scale decline in head during recent decades as a result of an increase in land drainage and groundwater withdrawal. The situation is more ambiguous in large groundwater bodies located in the eastern part of the country, where the unsaturated zone increases from near zero along the edges to about 40 m in the centre of the area. As depth of the unsaturated zone increases, groundwater level reacts with an increasing delay to fluctuations in climate and influences of human activities. The aim of the present paper is to model groundwater level fluctuations in these areas using a linear stochastic transfer function model, relating groundwater levels to estimated precipitation excess, and to separate artificial components from the natural groundwater regime. In this way, the impact of groundwater withdrawal and the reclamation of a 1000 km2 polder area on the groundwater levels in the adjoining higher ground could be assessed. It became evident that the linearity assumption of the transfer functions becomes a serious drawback in areas with the deepest groundwater levels, because of non-linear processes in the deep unsaturated zone and the non-synchronous arrival of recharge in the saturated zone. Comparison of the results from modelling the influence of reclamation with an analytical solution showed that the lowering of groundwater level is partly compensated by reduced discharge and therefore is less than expected.

Influence of grain fabric and lamination on the anisotropy of hydraulic conductivity in unconsolidated dune sands

In local-scale groundwater flow problems, like for instance in contaminated groundwater studies, it becomes increasingly important for proper prediction of contaminant migration to incorporate small-scale geological and associated parameter heterogeneity in groundwater flow models. Of the micro-scale geological heterogeneities, the effect of grain fabric and lamination in unconsolidated sediments on the vertical anisotropy of effective hydraulic conductivity has received relatively little attention. This is mainly due to the low conductivity contrast between laminae in combination with the friable nature of unconsolidated sediment which makes it sensitive to disturbances during experimental procedures. As an alternative, digital image analysis of sediment thin sections was applied to separate and quantify both causes of anisotropy. The effective value of vertical anisotropy ratio of unconsolidated eolian dune sand as determined experimentally on distributed, orientated core plugs ranges between 1.10 and 1.50. This wide range is partly caused by artificial noise introduced by distributed sampling in a heterogeneous medium and assuming thereby that the sampled material is homogeneous. By applying digital image analysis of thin sections, high-resolution logs of hydraulic conductivity were generated. Simple upscaling of these values to effective conductivity parallel and perpendicular to sedimentary laminae provided quantitative estimates of the vertical anisotropy caused by lamination. Anisotropy values range between 1.0 and 1.075. The anisotropy ratio due to grain fabric ranges in value between 1.05 and 1.45 assuming that both anisotropy due to grain fabric and lamination are orientated identically. If this is not the case the effective anisotropy is the weighted mean of anisotropies of the constituent layer types. This study shows that for eolian sands the anisotropy owing to grain fabric dominates the effective value of anisotropy. If conductivity contrast between laminae increases, the contribution of anisotropy caused by grain fabric to the effective value of anisotropy decreases.

Prediction in hydrogeology: two case histories

Abstract An inherent aspect of hydrogeology is the dynamic character of groundwater flow within the more passive lithospheric medium. Prediction therefore requires insight into the spatial pattern of aquifer properties as well as the dynamic character of groundwater flow systems. Both aspects together determine hydrogeological system behaviour in space and time. The spatial characteristics of an aquifer are mainly based on geological and geomorphological structure, whereas the dynamics of the system result predominantly from climate and topography; knowledge of paleoconditions are a prerequisite for sound understanding of present systems. The integration of exploration and forecast is illustrated by two case studies. The first study focuses on an assessment of groundwater resource and their sustainable development in semi-arid Botswana. The second study concerns the impact of land and water management in the Netherlands coastal area on the groundwater regime and its future development, including the redistribution of salt and fresh groundwater on a long time scale.

Barometric tides in partly-saturated confined aquifers in Botswana

Abstract Barometric fluctuations in Botswana are dominated by atmospheric tides with frequency components of 6, 8, 12, and 24 h, as well as quasi-periodic components with a periodicity of 4–7 days. These tides cause periodic water level fluctuations in the majority of the boreholes in fracture aquifers. Many of these aquifers are partly saturated and show free water table characteristics, but are separated from direct atmospheric influence by less-pervious beds near the surface. The barometric effect is caused by an imbalance between the water pressure change in the well and in the aquifer. A theoretical model is presented that includes the compression of the aquifer and the associated vertical flow through the aquifer to the water table. The model qualitatively explains the observed frequency-dependent phase and amplitude response. The generally observed strong barometric effects are explained by a combination of high porosity and low conductivity.