Elsevier

Journal of Hydrology

Volume 320, Issues 3–4, 15 April 2006, Pages 283-301
Journal of Hydrology

Dynamics of floodplain-island groundwater flow in the Okavango Delta, Botswana

https://doi.org/10.1016/j.jhydrol.2005.07.027Get rights and content

Abstract

Surface water-groundwater interactions play a crucial role in the hydrology and ecology of the Okavango Delta. The hydrology of the Delta is dominated by the annual arrival of a flood wave which is distributed over an number of branches. Subsequently, the flood water feeds the phreatic aquifers underlying the Delta islands. In order to evaluate the seasonal and long-term dynamics of the surface water-groundwater interactions between the floodplains and the islands, a network of piezometers located in various locations of the Delta was monitored. Groundwater table fluctuations observed for up to 6 years were analysed and modelled using groundwater flow models. The floodplain-island groundwater flow is in general very dynamic and driven by island evaporation and transpiration. A typical small to medium sized island (width <500 m), appear not to be influenced by long-term antecedent conditions. Only on large islands (width >500 m) and at the perimeter of the flooded area is the influence of long-term antecedent conditions apparent. The knowledge gained during this study will be used for the improvement of the hydrological and hydro-ecological model of the Delta, and can be useful for the description of floodplain dynamics in semi-arid regions in general.

Introduction

Surface water–groundwater (SW–GW) interactions, or infiltration and exfiltration processes, are important processes in the hydrology and ecology of wetlands. Wetlands are generally linked to groundwater (Mitsch and Gosselink, 2000); one can distinguish wetlands dominated by groundwater discharge, by groundwater recharge or flow-through wetlands. The nature of interactions between surface water and groundwater, i.e. direction and magnitude of the water flux, is a result of a complex interplay of climate, morphology, soils, geology, vegetation and hydrology of a system (Sophocleous, 2002, Winter, 1999, Woessner, 2000). Work of Meyboom, 1967, Winter, 1999 shows that subtle changes in groundwater table driven by recharge, evaporation from groundwater and surface water level fluctuations can cause considerable seasonal variation in magnitude and direction of SW–GW fluxes. The direction and magnitude and direction of the SW–GW flux is important for bio-chemistry of the wetland (LaBaugh et al., 1987) and particularly for biological and biochemical processes occurring at the water-soil interface, or hyporheic zone (Brunke and Gonser, 1997). SW–GW interactions are also one of the pathways of exposure of wetlands to pollution and degradation by groundwater exploitation (Suso and Llamas, 1993).

The dynamics of the interactions between surface water and groundwater often determine the wetland's hydroperiod or the duration, extent and depth of inundation (Mitsch and Gosselink, 2000). A striking example is that of prairie pothole wetlands in US and Canada, where SW–GW interaction, as determined by geology and topography, is one of the main factors causing differences in duration of inundation between various potholes ranging from episodic to permanent (Winter and Rosenberry, 1995). In seasonally inundated alluvial plain wetlands, SW–GW interaction causes modification of the wetland's hydroperiod through recharge of shallow groundwater during flood propagation, and, after flood recession, release of water from bank storage (e.g. Weng et al., 2003).

SW–GW interaction causes the groundwater in the vicinity of a wetland to be functionally related to the wetland itself, leading to an ecological framework where the riparian vegetation is considered part of the wetland (Tiner, 1999). In this setting, the dynamics of SW–GW fluxes can affect the ecology of the riparian vegetation (Hughes, 1990), and on the other hand, the riparian vegetation can influence wetland's hydrology through transpirative uptake of groundwater (Doss, 1993, Sacks et al., 1992, Winter and Rosenberry, 1995).

In this paper we focus on the seasonal dynamics of hydrological interactions between seasonal floodplains and islands in the Okavango Delta. We do it on the basis of observations of groundwater table fluctuations and modelling of floodplain-island groundwater flow at two sites, representing typical Okavango Delta islands. Additionally, by simulating hypothetical conditions of prolonged flooding and no-flood situations, we assess the inter-annual dynamics of surface water-groundwater flows at these sites. By this work we contribute to the quantitative understanding of ‘losses’ of floodwater and of factors affecting the spatial and temporal variability of the hydrological system in the Okavango Delta. In this way we make the contribution to the improvement of the hydrological model of the Delta, which in the past did not account for the lateral groundwater fluxes and their role on the system's water balance. Also, we assess the impact of floodplain dynamics on the riparian vegetation. In a broader sense, we provide a quantitative description of surface water-groundwater interactions in a floodplain system dominated by groundwater recharge, examples of which are few in the literature.

Section snippets

The Okavango Delta

The Okavango Delta (Fig. 1) is a large inland wetland created by the Okavango river. Morphologically, the Okavango Delta is a mosaic of flat broad floodplains and round to shapeless islands ranging in size from several square meters to 500 km2. The principal hydrological feature of the Okavango Delta is the seasonal flood stemming from the Okavango river catchment in Angola, and to a lesser extent local rainfall. The area covered by water expands from its annual low of 2500–4000 km2 in

Camp Island

Camp Island is a relatively large (approximately 400 by 1500 m) and elongated island, located in the regularly flooded zone adjacent to the main channel of Boro river (Fig. 2). Annual surface water level fluctuations vary between 0.8 and 1.5 m. Camp Island has a relatively wide (100 m) vegetated fringe composed of riparian woodland. The island's centre is covered by grassland. The island is underlain by a relatively thick (more that 7 m) lens of deposits of predominantly clayey texture.

Phelo's floodplain and Pan floodplain

Phelo's

Piezometric network

Groundwater table fluctuations were observed through a network of piezometers made of 25.4 mm PVC pipes installed in hand-augered holes, 2–11 m deep. The bottom part of the pipes was slotted and covered with several layers of filter material (vylene non-woven and agritex) over a length of 1 m. The pipes were cut at ground level to prevent destruction by elephants and baboons and covered with a cap. The cap was not hermetically closed to prevent build up of pressure inside the piezometer during

Hydraulic properties of the groundwater aquifers

There is a considerable difference in hydraulic conductivity between the island lens material and the sands beneath the lens and in the floodplains (Table 1). The sands are characterized by high total porosity (in order of 25–38%), and contain very little clay. Not surprisingly, they display high hydraulic conductivities-up to 35 m d−1. Lens material samples taken from the top 3 m of Camp Island are characterized by a total porosity in the order of 21–59%, but due to the high content of clay

Model configuration

A transient groundwater model was prepared for the transect C0–C8 located in the SW part of the island, adjacent to the main Boro channel (Fig. 2). Boundaries of the modelled domain were selected at the depression in the island centre, along the flow lines derived from the groundwater table map (obtained from survey of 45 auger holes uniformly distributed over the island) and in the middle of the Boro channel, approximately 300 m from the island's fringe. These were taken as no-flow conditions.

Seasonal dynamics of floodplain-island groundwater flow

Flow between floodplain and island (i.e. across the island boundary) calculated from the model, expressed in m3 d−1 per unit length of shoreline, is presented in Fig. 9. A regular pattern is observed of increase at the arrival of the flood with a subsequent slow decrease throughout the advanced flood and flood subsidence periods. Peak flow rates during the rise of the flood are in the order of 0.55–0.6 m3 d−1 m−1. It appears, however, that flow rates in the dry period before the flood arrival are

Dynamics of floodplain-island groundwater flow

Floodplain-island groundwater flow displays a certain degree of similarity for the two sites analysed, and, based on that, a general understanding of the dynamics of this interaction in the Okavango Delta can be obtained. The seasonal variation of floodplain-island groundwater flow is the effect of the superposition of a relatively constant evaporation and transpiration demand from island groundwater on the flood-induced replenishment of groundwater storage under the island. On the arrival of

Summary

Surface water-groundwater interactions play a crucial role in the hydrology and ecology of the Okavango Delta. In order to evaluate seasonal and long-term dynamics of one of the processes influencing surface water-groundwater interactions, namely the floodplain-island groundwater flow, a network of piezometers located in various settings of the floodplain-island system was monitored. Groundwater table fluctuations observed for up to 6 years were analysed and modelled using groundwater flow

Acknowledgements

Work presented here was funded by the University of Botswana as a project named ‘Surface water-groundwater interactions in the Okavango Delta’, partly financed by the EU in the framework of ICA-4-CT-2001-10040 project. P. Wolski would like to express his gratitude to all who helped with fieldwork during the three years of the project. The valuable suggestions by anonymous reviewers have been highly appreciated.

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