Water balance and variations of nutrients and major solutes along a river transect through the Okavango Delta, Botswana (original) (raw)
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Water SA
Water chemistry is important for the maintenance of wetland structure and function. Interpreting ecological patterns in a wetland system therefore requires an in-depth understanding of the water chemistry of that system. We investigated the spatial distribution of chemical solutes both in soil pore water and surface water, along island-floodplain-channel hydrological gradients in seasonally and permanently inundated habitats between major regions in the Okavango Delta, Botswana. Our results show that major cations (Ca, Na, Mg, and K), dissolved silica (DSi), dissolved boron (B), dissolved organic matter (DOC) and electrical conductivity increased significantly, at p ≤ 0.05, from the inlet of the Delta (the Panhandle) to the distal downstream reaches, suggesting the influence of evapoconcentration. Concentrations of dissolved Fe, Al, Zn, Cu, and Mn significantly decreased, at p ≤ 0.05, from the inflow of the Delta to the distal reaches. Only Na, Mn, Fe, Al, and DOC showed significant differences, at p ≤ 0.05, along the local floodplain-channel hydrological gradients, with higher solute concentrations in the floodplains than the channels. Solute concentrations in soil water exhibited similar distribution patterns to those in surface water, but concentrations were higher in soil water. Based on the results, we hypothesise that floodplain emergent vegetation and the channel-fringing vegetation in the Panhandle (a faultbounded entry trough to the Delta) and the permanently inundated eco-region together influence the cycling of solutes that enter the Delta through uptake.
Wetlands, 2011
The annual flood pulse in the Okavango Delta (Botswana), has a major influence on water chemistry and habitat. We explore spatial and temporal patterns in a suite of chemical variables, analysed from 98 sample points, across four regions, taken at different stages of the flood cycle. The major pattern in water chemistry is characterised by an increasing gradient in ionic concentration from deep-water sites in the Panhandle to more shallow, distal regions to the south. Concentrations of cations, anions, dissolved organic carbon, and SiO2 are significantly higher in the seasonally inundated floodplains than in permanently flooded regions. Several variables (including Na and total nitrogen) significantly increase from low flood to high flood, while others (including HCO3, SiO2, and Cl) increase in concentration, initially between low flood and flood expansion, before declining at maximum flood extent. Redundancy analysis (RDA) revealed that hydrological variables (water depth, flow velocity, flood frequency, and hydroperiod class) significantly explain 17% variation in surface water chemistry. Predictions of increasing flood volume in the near future may result in a decline in alkalinity and dilution of DOC. Our study provides an important baseline from which to monitor future change in the Delta.
Abstract In East Africa where wetlands are becoming essential systems for waste water treatment and potable water supply, hydrological characterisation is crucial to understand the dynamics of their functioning on a spatial and temporal scale. Beside ecosystem services, basic ecological properties such as vegetation structure and nutrient cycling of wetland ecosystems depend on hydrology. Nabajjuzi wetland, in central Uganda, East Africa is internationally recognised as a Ramsar site and currently used for potable water supply in addition to other essential ecological and social economic services to the riparian communities. To contribute to the limited and scanty information of this ecosystem, the study focussed on hydrological and water quality characterisation using a water-nutrient mass balance as the major output. Also, the flow dynamics of Fe and allochthonous suspended solids loading into the wetland were investigated. Results show that the water balance of Nabajjuzi wetland is dominated by surface flow, which is influenced by rainy and dry periods. During peak flow, the estimated hydraulic retention time was found to be 1 month compared to 1.5 months during the low flow period. Also, water loss flux due to evapotranspiration in this region is high and in Nabajjuzi wetland, it is twice the daily rate of abstraction. Analysis of long term flow data also revealed that the current water abstraction rate can potentially result in hydrological stress to the system during extremely low flow periods. However, the wetland N and P loading is very low compared to other urban disturbed wetlands. Consequently, high concentration of Fe and suspended solids loading are the major constraints to the surface water quality hence potable water supply. Generally, there is need for a more comprehensive hydrological-water quality study and careful planning of the future catchment land use strategies. This can enhance management and conservation of the wetland to guarantee the crucial ecosystem services it provides. Keywords: Hydrology, Riverine wetland, Hydrological gradient, Surface-flow, Nutrients, Iron, Mass-balance, Lake Victoria basin.
Biogeochemistry, 2013
The Okavango River, in semi-arid northwestern Botswana, flows for over 400 km in a pristine wetland developed on a large ([22,000 km 2) alluvial fan (Okavango Delta). An annual flood pulse inundates the floodplains of the wetlands and travels across the Delta in 4-6 months. In this study, we assess the effects of long hydraulic residence time, variable hydrologic interaction between river-floodplain-wetland and evapotranspiration on carbon cycling. We measured dissolved inorganic carbon (DIC) concentrations and stable carbon isotopes of DIC (d 13 C DIC) from river water when the Delta was not flooded (low water) and during flooding (high water). During low water, the average DIC concentration was 31 % higher and the d 13 C DIC 2.1 % more enriched compared to high water. In the lower Delta with seasonally flooded wetlands, the average DIC concentration increased by 70 % during low water and by 331 % during high water compared to the Panhandle with permanently flooded wetlands. The increasing DIC concentration downriver is mostly due to evapoconcentration from transpiration and evaporation with increased transit time. The average d 13 C DIC between low and high water decreased by 3.7 % in the permanently flooded reaches compared to an increase of 1.6 % in the seasonally flooded reaches. The lower d 13 C DIC during high water in the permanently flooded reaches suggest that DIC influx from the floodplain-wetland affects river's DIC cycling. In contrast, higher river channel elevations relative to the wetlands along seasonal flooded reaches limit hydrologic interaction and DIC cycling occurs mostly by water column processes and river-atmospheric exchange. We conclude that riverwetlands interaction and evapoconcentration are important factors controlling carbon cycling in the Okavango Delta. Keywords Carbon cycle Á Dissolved inorganic carbon Á Okavango river Á Flood pulse Á River-wetland interaction Á Evapoconcentration
Journal of Rural and Development, 2017
Wetland hydrology is important in understanding wetland systems, evaluating wetland functions and processes and assessing wetland conditions. Wetlands assimilate and transform pollutants and nutrients ensuring that quality water is discharged from the wetland into streams. The objective of this study was to characterise wetland hydrology and evaluate the water quality so as to determine the ecological functioning of the Khalong-la-Lithunya wetland. Wetland hydrology and water quality of the three sub-catchments were monitored from October 2015 to March 2016. Water levels in piezometers were recorded once a month and monthly water levels data for the years 2010, 2011, 2012 and 2013 previously recorded by the Millennium Challenge Account-Lesotho (MCA-L) project were integrated to this study’s data. Rainfall, piezometer and stream water were similarly obtained once every month. These were analysed for δ2H and δ18O water stable isotopes and water quality parameters determined. The estim...
WATER BALANCE AND INFILTRATION IN A SEASONAL FLOODPLAIN IN THE OKAVANGO DELTA, BOTSWANA
2006
Water balance in a seasonal floodplain in the Okavango Delta, Botswana was determined for three years (1997)(1998)(1999). There was no surface outflow, and infiltration to ground water was very large (4.7-9.7 m during 90-175 days of flooding, or on average 4.6-5.4 cm?d 21 ), amounting to 90% of total annual loss of water from the floodplain. At the arrival of the flood, when floodplain ground water was 3-5 m below ground, infiltration was controlled by vertical percolation through the aeration zone and was taking place with rates as high as 1.11-1.74 m during 10 days, or on average 11.1-17.4 cm?d 21 . Lateral ground-water flow from the floodplain toward surrounding dryland became the dominant process after the first days of flooding, when the floodplain ground-water table rose to the surface. Lateral ground-water drainage accounted for at least 80% of total infiltration. Direct measurements of infiltration confirmed high rates obtained from the water balance and revealed that the majority of infiltration occurred within a 10-m belt along the shore of the inundated area, with point infiltration rates as high as 42 cm?d 21 . The infiltration values are high compared to other large recharge wetlands (e.g., the Everglades, the Hadejia-Nguru) and result from a combination of lack of a low permeability surface layer in the floodplain and strong drainage of floodplain ground water driven by evaporation from the surrounding drylands. High infiltration and lateral ground-water flows have major implications for the Okavango Delta ecology, as they provide water to riparian vegetation, affect floodplain nutrient balance, and are part of the process responsible for immobilization of dissolved minerals.
Regional review: the hydrology of the Okavango Delta, Botswana—processes, data and modelling
Hydrogeology Journal, 2009
The wetlands of the Okavango Delta accommodate a multitude of ecosystems with a large diversity in fauna and flora. They not only provide the traditional livelihood of the local communities but are also the basis of a tourism industry that generates substantial revenue for the whole of Botswana. For the global community, the wetlands retain a tremendous pool of biodiversity. As the upstream states Angola and Namibia are developing, however, changes in the use of the water of the Okavango River and in the ecological status of the wetlands are to be expected. To predict these impacts, the hydrology of the Delta has to be understood. This article reviews scientific work done for that purpose, focussing on the hydrological modelling of surface water and groundwater. Research providing input data to hydrological models is also presented. It relies heavily on all types of remote sensing. The history of hydrologic models of the Delta is retraced from the early box models to state-of-the-art distributed hydrological models. The knowledge gained from hydrological models and its relevance for the management of the Delta are discussed.
2006
For the past eleven years, the Great Ruaha River which flows through the Ruaha National Park, has ceased flowing during the dry season, with extended periods of zero flow. The drying up has resulted in social conflicts between upstream and downstream users. It has also caused adverse impacts on the ecosystem of the Ruaha National Park, disrupting the lives of many animals and causing changes in their behavior. In this paper we present the findings of a hydrological study conducted to estimate environmental flow requirements. The desktop reserve model was used to determine maintenance high and low flows, and drought low flow requirements within the Ruaha National Park. The results indicate that to maintain the basic ecological functioning of the river requires an average annual allocation of 635.3 Mm 3 (equivalent to 21.6% of mean annual runoff). This is the average annual maintenance flow; comprising of maintenance low flows (i.e. 15.9 % MAR; 465.4 Mm 3 ) and maintenance high flows (i.e. 5.8% of MAR; 169.9 Mm 3 ). The absolute minimum water requirement was estimated to be 0.6 m 3 s -1 with the probability of exceedance of 0.99. The study confirms that in the absence of ecological information hydrological indices can be used to provide a first estimate of environmental water requirements. However, before being applied, greater understanding of the relationships between flow and the ecological condition of the riverine ecosystem is required.
Wetland water balance assessment: a case study of the Sosa wetland, Maseru, Lesotho
Water Practice and Technology
The Sosa wetland is a sensitive wetland, situated at the headwaters of the Jordan catchment in Maseru. Due to unregulated land use activities in the past decades (2010–2020), the Sosa wetland nearly dried up resulting in a water shortage. Therefore, this study performed a wetland water balance of the Sosa wetland in Lesotho for the period of 1975–2020 using GIS and remote sensing. Landsat imageries of 1975–2020 were used for land use and land cover while the Penman–Monteith and Thornthwaite methods were used to estimate evapotranspiration. Results show that water/marsh, cultivation, settlements and bare-land increased by 2.04, 4.1, 5.82 and 28.71%, respectively, from 1975 to 2020. Forest and shrubs as well as grasslands decreased by 38.83 and 1.76%, respectively, from 1975 to 2020. Evapotranspiration estimates for the period 1984–2020 were in the range of 900–1,071 mm/year which is substantially greater than the annual mean rainfall of the catchment which ranges from 550 to 850 mm/y...
Water S a, 2002
The Mkuze Wetland System in northern KwaZulu-Natal constitutes an important source of freshwater to Lake St Lucia. The St Lucia System, including both the Mkuze Wetland System and Lake St Lucia, is recognised as a Wetland of International Importance under the Ramsar Convention. The long-term survival of Lake St Lucia depends on an adequate supply of freshwater. Consequently, the role of the Mkuze Wetland System in water quality enhancement is essential.