Middle Zambezi Research Articles

The ecology of Nkolola girls’ initiation rites in the Middle Zambezi Valley, northwestern Zimbabwe

ABSTRACT Drawing on five years of intermittent ethnographic fieldwork, this article examines the ritual ecology of the Nkolola girls’ initiation practice of the Tonga-speaking people of Mola, Zimbabwe. It focuses on the materiality framework to demonstrate the environmental contingencies of this ritual and how the breakdown of Nkolola indicates an environmental crisis. There are specific materials required for this ritual, but extreme weather patterns such as continuous droughts and heatwaves have made it challenging to gather essential items like red ochre, traditional herbs, firewood, and tree branches, as well as access to perennial rivers and streams needed to properly perform the ritual. The article discusses the importance of these materials in each stage of the ritual, as well as the significant consequences of their absence, including avoidance and delays in conducting the ceremony. It also explores an alternative ritual that is less material-intensive and demanding of natural resources.

Complex selection processes on invasive crayfish phenotype at the invasion front of the Zambezi floodplains ecoregion

Abstract Understanding the eco‐evolutionary processes which drive phenotypic variability in non‐native species is crucial to predicting impact and informing spatially explicit management models. Invasive redclaw crayfish, Cherax quadricarinatus, are spreading rapidly through the ecologically and economically important Upper and Middle Zambezi system in south‐central Africa. We tested whether there was an invasion front effect in the Barotse floodplain and disentangle whether these effects were related to environmental filtering, spatial sorting, or density‐dependent processes along a nonlinear 200 km invasion gradient over seasonal hydrological periods. Crayfish spread up‐ and down‐stream indicated an expanding population compared to the former invasion front in 2019, facilitated by both sexes. Wet season detection probability and relative abundance were lower than in the dry season. No signals of environmental filtering were detected as a result of high connectivity of habitats in the wet season. The invasion front population was characterised by lower relative abundance, lower body condition, larger overall body and chelae size, and longer front leg length. There was no effect on sex ratio. This indicated that spatial sorting, density‐dependent processes and predation pressure are acting as selective forces on phenotypic variability across the invasion gradient. Better understanding of the effects of flood regime and trophic interactions on phenotypic variability and dispersal is urgently needed. Population suppression to reduce spatial sorting and conspecific competition could be a viable management option at the invasion front.

Hidden in the riffles: A new suckermouth catfish (Mochokidae, Chiloglanis) from the middle Zambezi River system, Zimbabwe.

The recent surge in the discovery of hidden diversity within rheophilic taxa, particularly in West and East Africa, prompted a closer examination of the extent to which the current taxonomy may obscure the diversity of riffle-dwelling suckermouth catfishes in the genus Chiloglanis in southern Africa. Currently, the region comprises eight valid species within this genus. Seven of them have relatively narrow geographic distribution ranges except for C.neumanni, which is considered to be widely distributed, occurring from the Buzi River system in the south, and its northern limit being the eastward draining river systems in Tanzania. Recent surveys of the middle Zambezi River system revealed Chiloglanis specimens that were distinguishable from the known species of the genus from southern Africa. Integration of molecular and morphological data indicated that these specimens from the Mukwadzi River represent a new species to science, herein described as Chiloglaniscarnatus Mutizwa, Bragança & Chakona, sp. nov. This species is readily distinguished from its southern African congeners by the possession of a distinctive extended dermal tissue covering the base of the dorsal fin and the possession of ten mandibular teeth (vs 8, 12, or 14 in the other taxa). Results from this study add to the growing evidence of a high level of undocumented diversity within riffle-dwelling taxa in southern Africa.

Another potential disaster? On a proposal to introduce Nile perch, Lates niloticus , into Lake Kariba (Zambia/Zimbabwe)

The introduction of Nile perch, Lates niloticus, into Lake Victoria resulted in a massive increase in fish catches, but with significant ecological costs. These included the near extirpation of the 500+ endemic haplochromine species and the eutrophication of the lake. A proposal to introduce Nile perch into the artificial Lake Kariba (Zambia/Zimbabwe) is of concern, therefore, because of potentially adverse impacts on its fisheries and biodiversity. It is very unlikely that Nile perch would improve the fisheries in Lake Kariba, as it did in lakes Victoria and Kyoga, because this lake is oligotrophic and much less productive. The principal fishery in Lake Kariba is already in difficulty, owing to overfishing, and Nile perch could reduce catches further. The biomass of other native fish species is very low and would be unlikely to support a significant Nile perch population. Lake Kariba has already experienced a major loss of biodiversity as river-adapted fish species have not been able to adapt to lacustrine conditions. The non-native Nile tilapia, Oreochromis niloticus, has also replaced the endemic tilapia species in the middle Zambezi system and the Nile perch could bring about a further loss of biodiversity. The ecological costs of its introduction into Lake Kariba would not be justified by an increase in fishery productivity, and the possible introduction of Nile perch should be forcefully rejected.

Distribution and establishment of the alien Australian redclaw crayfish, Cherax quadricarinatus, in the Zambezi Basin

Abstract Crayfish are invasive polytrophic keystone species, which are phylogenetically unique on the African continent. The Australian redclaw crayfish Cherax quadricarinatus is invasive in southern African freshwater systems including the Zambezi River Basin. Surveys conducted across the Zambezi Basin (Botswana, Zimbabwe, Namibia and Zambia) between 2017 and 2019 showed that C. quadricarinatus is broadly distributed across the Upper and Middle Zambezi and is rapidly spreading through natural and human‐mediated means across several ecoregions. The probability of capture (Pcapture), catch per unit effort (CPUE) and population characteristics of C. quadricarinatus from the recent Barotse floodplain invasion were compared with older invasions from Lake Kariba and Kafue River. The Pcapture and CPUE of C. quadricarinatus in the recently invaded region of the Barotse floodplain were similar to those of the older invasions. Mass and carapace length of C. quadricarinatus from the Barotse floodplain were significantly lower than those of C. quadricarinatus from the older invasions. Sex ratios differed significantly between the three invasive populations. The Barotse floodplain population had a disproportionate investment in females (65.3%) and intersex individuals (8.4%). No crayfish were detected in the Zambezian Headwaters or the Okavango Floodplains ecoregions, but current spread rates are 49 km yr−1 downstream and 12 km yr−1 upstream. Investment in population management and the prevention of spread will have high conservation value across the invaded regions in order to restrict crayfish ecological impacts via direct predation and competition. In areas where crayfish are in high abundance, existing fisheries are affected through damage to nets, leading to increased net abandonment and ghost gear pollution in invaded regions. The emphasis should be on developing cohesive transboundary biosecurity policies in southern Africa to limit further spread that will threaten the integrity of freshwater ecosystems. However, long‐term monitoring is needed to gauge invasion risk to sensitive areas such as the Okavango Floodplains ecoregion and determine field‐based ecological impacts.

Decline of zooplankton food resources of Limnothrissa miodon fishery in Lake Kariba: Global warming‐induced ecosystem disruption by Cylindrospermopsis raciborskii

AbstractThe present study was conducted on the Lake Kariba portion of the Middle Zambezi Biosphere Reserve. Magadza (2011) previously reported the pelagic Lake Tanganyika sardine (Limnothrissa miodon) fishery introduced into Lake Kariba was declining. This phenomenon was attributed to observed climate‐change‐related warming of the lake, facilitating establishment of the invasive Cylindrospermopsis raciborskii, currently the dominant alga in the lake. Chlorophyceae contribute to less than 10% of the phytoplankton biomass. Accordingly, it was postulated that its dominance in the lake resulted in a food chain disruption because this cyanobacterium was not preferred by zooplankton grazers. Accordingly, the present study examined the feeding behaviour of L miodon with reference to available food items in the Sanyati Basin of Lake Kariba. The results highlighted a virtual absence of entomostraca zooplankton in the Sanyati Basin of Lake Kariba, in contrast to previous findings of previous studies, the avoidance of C. raciborskii in the diet of L. miodon, as measured by the Ivlev electivity index, a low stomach fullness index, the movement of L. miodon to macroinvertebrates associated with the littoral zone as a food source, and unfavourable dietary indicators of L. miodon. These results collectively suggest a food deficit for L. miodon in Lake Kariba, leading to the previously observed decline in the lake fishery.

Capturing People on the Move: Spatial Analysis and Remote Sensing in the Bantu Mobility Project, Basanga, Zambia

From its inception in 2014, the interdisciplinary Bantu Mobility Project has sought to refocus research on the Bantu Expansions away from the macroscale towards a “writ small” approach within a well-defined region with well-understood episodes of language expansion, namely, the middle Kafue and middle Zambezi catchments of southern Zambia. This tighter focus enables the project to capture the human agency shaping movements of people, animals, material goods, and languages, and to consider the productive tension between mobility and rootedness as Bantu-speaking populations became settled in particular regions between the sixth and sixteenth centuries AD. From an archaeological standpoint, careful study of the spatial contexts of recovered artifacts—and of the various human activities that left them behind—captures different forms and scales of mobility that existed alongside the rootedness of mounded settlements occupied over generations. This paper shows how a better understanding of those spatial contexts, and the settlement patterns and land use they encode, is being achieved around Basanga, Zambia, by combining systematic archaeological survey with data derived from satellite imagery using analytical techniques available through GIS, such as spatial interpolation and linear regression modeling. Ultimately, the project will aim to integrate the insights of that geospatial analysis with other archaeological, linguistic, historical, and environmental datasets to capture the stories of the people whose ideas, practices, and forms of mobility and rootedness constituted the local experience of the Bantu Expansions.

Mapping floods in the middle Zambezi Basin using earth observation and hydrological modeling techniques

The Lower Middle Zambezi catchment, on the Zimbabwean side, is vulnerable to floods almost every year. In this work, the causes and impacts of floods in the Mbire District of the Middle Zambezi Basin are investigated. An algorithm based on the binary logistic regression is used together with MODIS NDVI images to determine the spatial and temporal variation of flood inundation. The HEC-HMS model is used to simulate rainfall time series at daily time step. The quantified peak flow is given as an input to a mono-dimensional hydraulic model, HEC-RAS. Results from the mapped inundated areas for the period 2005 to 2015 showed that 16 January 2006 had the highest flooded area of 1934 km2. Factors explaining causes of flooding were distance from surface water bodies. The simulated flooded areas obtained are used to deepen our understanding of the contribution of geomorphic features towards flooding as well as the extent of flooding. Results obtained contribute towards defining new strategies for prompt flood risk management in the District.

Incorporating Geomorphic Knowledge in the Management of Wetlands in Africa’s Drylands: a Rapid Assessment of the Kafue Wetland

Limited knowledge of wetland geomorphic processes often results in poor wetland management. This study aims to illustrate the importance of incorporating geomorphic knowledge of wetland origin in their management. The geomorphic origin and dynamics of the Kafue wetland were determined from the analysis of remotely sensed and geological data. The wetland is a remnant of a paleo-lake that was captured by the tributary of the middle Zambezi River. At the point of capture, resistant Muva Group metavolcanic rocks dominate narrow valleys characterised by straight channels. The resistant lithology prevents the lower Kafue River to cut into the wetland thus maintaining wetland conditions upstream. Sedimentation regime through overbank and bed deposits shaped the wetland’s structure, ecological diversity, and hydrological functioning. The operation of the Itezhi-tezhi dam has negatively impacted the wetland’s hydrological and sedimentological regime. The dam starves the system of sediment needed for promoting levee and channel bed aggradation. Regulated discharge with reduced sediment load can lead to channel incision and therefore reduce flood frequency, which may ultimately lead to wetland desiccation. The study recommends the incorporation of geomorphic knowledge into wetland management since the understanding of wetlands from a geomorphic perspective can complement natural processes occurring within wetlands.

Local Ecological Knowledge on Climate Change and Ecosystem-Based Adaptation Strategies Promote Resilience in the Middle Zambezi Biosphere Reserve, Zimbabwe.

Understanding local community perceptions on impacts, causes, and responses to climate change is vital for promotion of community resilience towards climate change. This study explored local ecological knowledge (LEK) held by local communities on climate change trends and impacts in the Middle Zambezi Biosphere Reserve (MZBR), Zimbabwe. The objectives of the study were to (i) investigate local community perceptions on trends and causes of climate change, (ii) identify biophysical impacts of climate change at the local level, and (iii) explore the ecosystem-based adaptation strategies towards climate change. The study used a mixed methods approach where a household questionnaire survey (n=320), key informant interviews (n=12), and focus group discussions (n=8) were used to collect data between April 2015 and October 2016. Results from the study show that local communities have observed decreasing rainfall and increasing temperatures as key indicators of climate change. Local communities observed water scarcity, changes in vegetation phenology, livestock and wildlife mortalities, and food shortages due to drought as the major impacts on their livelihoods. LEK can contribute to adaptive management strategies that enhance resilience of socioecological systems (SES) in the face of climate change by providing information on the status and use of biophysical components of the environment and by highlighting potential local adaptation strategies that can sustain key livelihood practices.

Impacts of landcover changes on streamflows in the Middle Zambezi Catchment within Zimbabwe

Abstract. We investigate the impacts of land cover changes on the river flows of the Middle Zambezi tributary catchments in Zimbabwe. Trend analysis on rainfall and streamflow was carried out using the Mann-Kendall test at monthly and annual time steps. Rainfall analysis indicated an increasing trend which was not statistically significant (p<0.05) for all stations. Annual streamflow time series indicated negative decreasing trends which were not statistically significant (p<0.05) except for the rainfall months of November and December. The study deduced that the changes in rainfall did not affect hydrological catchment behaviour and changes in streamflow were thus caused by anthropogenic factors such as land cover changes. Statistical tests indicated a weak but significant correlation between rainfall and streamflow which also supports the fact that changes in streamflow are mainly driven by land cover changes. Land cover change assessments were done through supervised classification of Landsat images for the years 1989, 1998, 2008 and 2014. All catchments exhibited increases in cultivation area and decreases in forest and grassland. The semi-distributed HBV-Light model was applied for change detection modelling of the gauged Musengezi catchment. We conclude that the HBV Light model can be successfully used to simulate flows for the catchment.

Evaluation of sub daily satellite rainfall estimates through flash flood modelling in the Lower Middle Zambezi Basin

Abstract. Flash floods are experienced almost annually in the ungauged Mbire District of the Middle Zambezi Basin. Studies related to hydrological modelling (rainfall-runoff) and flood forecasting require major inputs such as precipitation which, due to shortage of observed data, are increasingly using indirect methods for estimating precipitation. This study therefore evaluated performance of CMORPH and TRMM satellite rainfall estimates (SREs) for 30 min, 1 h, 3 h and daily intensities through hydrologic and flash flood modelling in the Lower Middle Zambezi Basin for the period 2013–2016. On a daily timestep, uncorrected CMORPH and TRMM show Probability of Detection (POD) of 61 and 59 %, respectively, when compared to rain gauge observations. The best performance using Correlation Coefficient (CC) was 70 and 60 % on daily timesteps for CMORPH and TRMM, respectively. The best RMSE for CMORPH was 0.81 % for 30 min timestep and for TRMM was 2, 11 % on 3 h timestep. For the year 2014 to 2015, the HEC-HMS (Hydrological Engineering Centre-Hydrological Modelling System) daily model calibration Nash Sutcliffe efficiency (NSE) for Musengezi sub catchment was 59 % whilst for Angwa it was 55 %. Angwa sub-catchment daily NSE results for the period 2015–2016 was 61 %. HEC-RAS flash flood modeling at 100, 50 and 25 year return periods for Angwa sub catchment, inundated 811 and 867 ha for TRMM rainfall simulated discharge at 3 h and daily timesteps, respectively. For CMORPH generated rainfall, the inundation was 818, 876, 890 and 891 ha at daily, 3 h, 1 h and 30 min timesteps. The 30 min time step for CMORPH effectively captures flash floods with the measure of agreement between simulated flood extent and ground control points of 69 %. For TRMM, the 3 h timestep effectively captures flash floods with coefficient of 67 %. The study therefore concludes that satellite products are most effective in capturing localized hydrological processes such as flash floods for sub-daily rainfall, because of improved spatial and temporal resolution.

Impacts of hydropower dam operations in the Mana Pools National Park floodplains

The Mana Pools flood plain along the Middle Zambezi River and part of the popular Mana Pools National Park of Zimbabwe has been experiencing both ecological and morphological changes since the construction of upstream hydropower reservoirs. This study investigated the hydro-dynamics from natural and human influenced flows, particularly at the Mana Pools floodplains which are downstream of Kariba and Kafue hydropower dams. Descriptive statistics were used to analyse the differences in flow regime upstream and downstream of the hydropower reservoirs. A standard t-test was done to compare the natural and modified mean annual flows. Results show that the presence of upstream reservoirs has reduced the high peak average monthly flows by 17% and increased the average monthly low flows by 5% at Mana Pools. However, the mean annual runoff has not significantly changed. Results also established that. In line with recent calls for ecological considerations in reservoir management, the study recommends that the reservoir operation in the Middle Zambezi River should mimic the pre-impoundment Middle Zambezi River flow regime at Mana Pools.

Green economy initiatives in the face of climate change: experiences from the Middle Zambezi Biosphere Reserve, Zimbabwe

This study investigates climate change adaptation and mitigation interventions within the framework of green economy for sustainable development and poverty eradication in the Middle Zambezi Biosphere Reserve, Zimbabwe. The study adopted a mixed methods approach, mainly drawing data from field observations, focus group discussions (FGDs) drawing representatives from a household survey and key informant interviews. Primary data were collected in April and August 2015 from FGDs whose participants were derived from household heads who had previously participated in a broader climate change study. Key informant interviews were also held with traditional leaders, local experts and managers in the wildlife sectors. This was supplemented with data from secondary sources. Findings from the study indicate that stakeholders in the biosphere reserve implement green economy and climate change-related programmes and projects. Results also indicate that the biodiversity- and/or wildlife-related laws and policies developed prior to 2010 do not directly mention the term green economy and climate change yet these indirectly address the green economy agenda. However, recent soft law documents (post 2010) such as the Zimbabwe’s National Biodiversity Strategy and Action Plan (2013–2020) and local councils’ strategic plans prioritise climate change adaptation, mitigation and green economy-related issues. Although the wildlife sector has green economy-related initiatives in place, there exists a gap in terms of mainstreaming the green economy concept in biodiversity-related policies.

Re-dating the Ingombe Ilede burials

Several burials excavated during 1960 at Ingombe Ilede in southern Africa were accompanied by exceptional quantities of gold and glass beads, bronze trade wire and bangles. The burials were indirectly dated to the fourteenth to fifteenth centuries AD, prior to the arrival of the Portuguese on the East Coast of Africa. New AMS dates on cotton fabric from two of the burials now relocate them in the sixteenth century. This was a dynamic period when the Portuguese were establishing market settlements along the Zambezi, generating new demands for trade products from the interior, and establishing trade networks with the Mwene Mutapa confederacy. These new dates invite a reconsideration of Ingombe Ilede's relationship to Swahili and Portuguese trade in the middle Zambezi. This article is followed by four responses and a final comment by the authors.

Linking Social and Ecological Sustainability: An Analysis of Livelihoods and the Changing Natural Resources in the Middle Zambezi Biosphere Reserve

In this paper, we aim to explore community livelihoods and conservation issues surrounding natural resources that are utilised by resettled farmers within the Middle Zambezi Biosphere Reserve, Zimbabwe. Data collection was done in two phases. During the first phase undertaken in 2011, we administered household interviews, held focus group discussions (FGDs) and conducted in-depth interviews. The second phase, conducted in 2015, used FGDs and key informant interviews to gather data on changes in livelihoods and natural resources. Findings indicated limited options of social, human and financial capital. However, diverse livelihoods strategies are pursued in order to minimize the risk of biodiversity degradation. Structural factors that can contribute to cause biodiversity degradation include the failure of the CAMPFIRE Programme; natural resources governance flaws; contests over space and power, and weak policing by institutions devoted to the protection of natural resources. In conclusion, the authors observe how the institutions involved in the management of the protected natural area fail to promote the participation of the local community in the decision-making processes, thus limiting the potential benefits of the designation of the Middle Zambezi Biosphere Reserve.

Estimating runoff from ungauged catchments for reservoir water balance in the Lower Middle Zambezi Basin

The Lower Middle Zambezi Basin is sandwiched between three hydropower dams; Kariba, Kafue (Itezhi-tezhi) and Cahora Bassa. The operation of the upstream dams impacts on the inflows into the downstream Cahora Bassa Dam which, in turn, affects the area inundated upstream of the Cahora Bassa Dam. This study applied a rainfall-runoff model (HEC-HMS) and GIS techniques to estimate both the gauged and ungauged runoff contribution to the water balance of Cahora Bassa. The rivers considered in the study are the Zambezi, Kafue, Luangwa, Chongwe, Musengezi and Manyame. Missing data were generated using the mean value infilling method. The DEM hydro-processing technique was used to determine the spatial extent of the ungauged area. A hydrological model, HEC- HMS, was used to simulate runoff from the ungauged catchments. Results from the study show that the ungauged catchment contributes about 12% of the total estimated inflows into the Cahora Bassa Dam. Averaged results over 30 years show total inflows of 71.73 x 10 9 m 3 /yr, total outflows of 52.25 x 10 9 m 3 / yr and a residual storage of 20 x 10 9 m 3 /yr. The study successfully estimated the water balance of the Middle Zambezi Basin which, in turn, may help to inform the operation of the Cahora Bassa Dam and management of artificial floods in the basin. Keywords: Cahora Bassa, DEM hydroprocessing, HEC-HMS, remote sensing, reservoir operation, runoff simulation

Community structure and leaf trait heterogeneity in a deciduous thicket community of the Middle Zambezi River Valley, Zambia

Deciduous Zambezi thicket forms a mixed-layered, dry woodland community on deep alluvial soils of the Middle Zambezi Valley. The dominant species are spiny, thicket-forming deciduous shrubs and small trees with no single dominant species and variable composition along the valley. A detailed community study of deciduous thicket along the Middle Zambezi River provided an indication of the wide variation of species-specific characteristics of growth form, phenology, and leaf traits that combine to produce a heterogeneous assemblage. Two families, the Capparaceae and Leguminosae, are particularly well represented. Leaf traits of nitrogen content, specific leaf area, δ13C, and δ15N are highly variable between species, with indications of some consistency in family traits within the Capparaceae and Leguminosae. A number of species exhibit a strong pattern of association with termite mounds, and termitaria are known to have significant potential impacts on spatial patterns of soil nutrient availability and water relations in savanna communities. There remains a poor understanding of the significance of symbiotic nitrogen fixation in deciduous African thicket, and savanna, and woodland communities and its importance in ecosystem fluxes of nitrogen, and these issues cannot be resolved by simple measurements of δ15N alone. Intraregional patterns of variation in leaf traits may impact the broad application of regional ecosystem models, as with those developed in the SAFARI 2000 program, when the variance between species is high, demonstrating that climatic controls on ecosystem δ15N abundance are scale‐dependent.

Clay mineralogy in southern Africa river muds

AbstractThe composition, morphology and crystal order of clay minerals in silt-sized sediments carried in suspensions from 25 major rivers across tropical southern Africa have been studied by X-ray diffractometry and scanning and transmission electron microscopy. Our goal was to determine the spatial variability of clay-mineral associations in diverse geological settings, and in climatic conditions ranging from humid Angola and Zambia to hyperarid Namibia and the Kalahari. Specific attention was paid to the micromorphology and chemical composition of smectite particles. The relative abundance of smectites, illite/mica, kaolinite and chlorite enabled identification of regions characterized by different physical and chemical processes: (1) negligible chemical weathering is documented in Namibia, where river muds mostly contain illite/mica or smectite derived from Damara metasedimentary or Etendeka volcanic rocks; (2) kaolinite documenting intense weathering, reaches a maximum in the Okavango, Kwando and Upper Zambezi, sourced in subequatorial Angola and Zambia; (3) suspended-load muds in the Limpopo and middle Zambezi catchments display intermediate features, with varied assemblages and smectite compositions reflecting diverse parent lithologies. Clay mineralogy and chemical composition are confirmed as a most effective tool to unravel present and past climatic conditions on a continental scale.

Effects of dam operation on downstream river morphology: the case of the middle Zambezi River

The sand-bed middle Zambezi River flows between two large dams: the Kariba in Zambia and Zimbabwe upstream and the Cahora Bassa in Mozambique downstream. Since the construction of Kariba for hydropower production in 1959, the river discharge has been regulated according to two different strategies. This makes the middle Zambezi a unique case for studying the effects of different dam operations on the long-term morphological trends of the river. To analyse these effects, this work focused on the interaction between the river morphological changes and the groundwater flow. The comprehensive approach, combining numerical modelling, analysis of field and historical data, as well as theoretical considerations, led to the conclusion that the morphological response of the middle Zambezi to damming has been mainly affected by the sudden closure of the dam flood-gates, originating seepage forces causing bank instability and river widening as a result. The local climatic oscillation, comprising dry and wet periods having a duration of tens of years, was found to have played a major role in the flow regulation, which resulted in a constant discharge in the period between 1982 and 2000.