Assessment of river-groundwater interactions in the Barotse floodplain, Western Zambia.
Loading...
Date
2022
Authors
Mataa, Mulema
Journal Title
Journal ISSN
Volume Title
Publisher
The University of Zambia
Abstract
Floodplains are among the most dynamic, productive, diverse, and threatened ecosystems in the world. Their interactions between ground and surface water is a key element in understanding the resilience of floodplains to hydrological changes, especially with the advent of climate change and variability that has affected many aquatic ecosystems. Conjunctive evaluation of ground and surface water in floodplains is critical for integrated water resources management. The main objective of this study was to evaluate groundwater-surface water interactions in the Barotse Floodplain. To achieve this objective, remote sensing was employed to identify perennial green vegetated areas with a shallow depth to groundwater table in the floodplain. Normalised Difference Vegetation Index (NDVI) and Soil Adjusted Vegetation Index (SAVI) using Sentinel-2 imagery from 2015 to 2020 using a cloud computing approach in Google Earth Engine (GEE), were used to identify riverine perrennially green vegetation. Digital filtering using “EcoHydRology Library” in R statistical package was used to determine the Base Flow Index (BFI) from the hydrograph discharge data collected from the three main gauging stations (Lukulu, Matongo and Senanga) along the Zambezi River. Hydrochemistry was employed to determine the composition of the different water sources and identify the genetic link between the local and regional flow of ground water and interactions with the Zambezi River. In addition, water samples collected from various water sources were analysed for stable isotope [deuterium (2H) and oxygen (18O)] to determine the origin and identify the relationships between ground and surface water. Statistical analysis was applied to determine the significance of the correlation between remote sensed data and ground based measurements
including, pH, electrical conductivity (EC) and dissolved oxygen (DO).The results of the NDVI map shows a high occurrence of groundwater dependent vegetation downstream of the floodplain. These locations have a shallow depth to groundwater table and other locations had occurances of springs. The average baseflow index (BFI) in five hydrological years from October 2004 to September 2009 shows an average of 46.5 percent baseflow to the river flow across the wet and dry season. Furthermore, baseflow averages up to 90 percent of the flow
during the dry season between August to October when the flood water has receeded. The hydrochemistry shows a high concentration in calcium and the bicarbonate ion in both river and groundwater. The water was concluded to be that of a CaHCO3 type. From the stable isotopes the results show recharge from precipitation as values plotted on the meteoric line.
Both river and groundwater stable isotopes (δ18O: -57.4 ‰ to 1.4 ‰ and δ2H :-7.5 ‰ to 3.3 ‰ ) had a similar signature indicating that groundwater was discharging into the river channel.
Results of statistics between base flow as an index and ground based measurements show a strong correlation between baseflow index (BFI) and dissolved oxygen (DO) indicating that an
increase in baseflow increases the dissolved oxygen concentration in the river system. Overall, results show that the lower reach of the Zambezi River channel was a ‘gaining stream’ as
groundwater is discharging into the river, whereas the upper reach was a ‘losing stream’ as the river channel tends to discharge into the groundwater. This study demonstrates that the Barotse
Floodplain is a proportionally groundwater dependant ecosystem as its functionality in provision of ecosystem products and services requires groundwater. Conservation of this wetland requires prudent protection of groundwater aquifer systems.
Description
Thesis of Master of Science in Integrated Water Resources Management.