Research Repository :: Browsing by Author "Ndomba, Preksedis M."

Browsing by Author "Ndomba, Preksedis M."

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Analysis of Flow Estimation Methods for Small Hydropower Schemes in Bua River
(Scientific Research publishing, 2015-02) Kasamba, Chisomo; Ndomba, Preksedis M.; Kucel, Samuel Baker; Uamusse, Miguel M.
Any hydropower project requires an ample availability of stream flow data. Unfortunately, most of the hydropower projects especially small hydropower projects are conducted on ungauged river and consequently hydrologists have for a longtime used stream flow estimation methods using the mean annual flows to gauge rivers. Unfortunately flow estimation methods which include the ru- noff data method, area ratio method and the correlation flow methods employ a lot of assumptions which affect their uncertainty. This study was conducted on Bua River in Malawi to unveil the un- certainties of these flow estimation methods. The study was done on a well gauged catchment in order to highlight the variations between the observed, true stream flows and the estimated stream flows for uncertainty analysis. After regionalizing the homogenous sites, catchments using L-moments, an uncertainty analysis was done which showed that the area method is better fol- lowed by the correlating flow method and lastly the runoff data method in terms of bias, accuracy and uncertainty.
Application of SWAT in Natural Wetland Catchments. A Case of Rugezi Catchment in Rwanda
(College of Engineering and Technology, University of Dar es Salaam, 2010) Ndomba, Preksedis M.; Magoma, D.; Mtalo, Felix W.; Nobert, Joel
Hydrologic models are increasingly used to support decisions at various levels and guide water resources policy formulation, management and regulations. In this study the Soil and Water Assessment Tool (SWAT) was examined for its applicability in wetland catchment of Rugezi in Rwanda covering an area of, approximately to, 197 Km2 . The study adopted SWAT model as recommended by others workers in the region in order to extend its testing. The application of SWAT model entailed setting up the model using readily available data, sensitivity analysis and calibration. In this study wetland model physical parameters such as width, depth, length, slope and area were generated by GIS based SWAT interface. The ground photos available from literature supplemented and validated the GIS data. An independent simulation was also conducted without implementing the wetlands in order to study its impacts on the hydrology. Sensitivity analysis indicated that Channel effective hydraulic conductivity (CH_K2), Surface runoff lag time (Surlag), SCS runoff curve number under moderate moisture condition (CN2), and Manning’s n value for main channel (Ch_n) are the most sensitive parameters. Comparable results between simulated and observed streamflows at the catchment outlet, Rusumo gauging station, were obtained. The Nash and Sutcliffe coefficient values (CE) and Index of Volumetric Fit (IVF) were 49.15% and 95.6% for calibration and 51.4% and 98.6% for validation periods, respectively, indicating a satisfactory simulation. It was evident from this study that the wetland was a major hydrological controlling feature in the basin and should be safeguarded. These results suggest that SWAT model is potentially useful in studying the hydrology of natural wetland catchments with limited data. Besides, the authors would like to recommend extensive testing of the model in other wetland catchments in the region.
Application of SWAT Model for Mountainous Catchment
(FWU Water Resources Publications, 2007) Birhanu, B. Z.; Ndomba, Preksedis M.; Mtalo, Felix W.
A GIS based hydrologic model, SWAT (Soil and Water Assessment Tool) was applied for modeling the WeruWeru catchment at the foot slopes of Mt. Kilimanjaro in Northern Tanzania. The catchment has an approximate drainage area of 101 km2 and a mean annual precipitation between 1500mm and 3000mm. The water balance modeling was performed on annual and monthly bases using spatial and temporal data. A statistical weather generator file WXGEN was prepared for ten years to generate climatic data and fill in gaps in the measured records of climatic data. Various GIS data preprocessor modules involving watershed delineation, input map characterization and processing, stream and outlet definition, the computation of the geomorphic parameters, and characterization of the landuse/land cover and soil were developed in the course of modeling. Surface runoff computation was done using Soil Conservation Service-Curve Number (SCS-CN) method; and Muskingum routing method was used for flow routing. The Rainfall-Runoff modeling was based on a long term global water balance simulation for 15 years (1972-1986) and temporal calibration technique. The Nash and Sutcliff efficiency criterion (R2 ) and the Index of Volumetric Fit (IVF) were adopted for the measure of efficiency of the performance of the model. An R2 of 82% and 59% was obtained during calibration and verification periods respectively. The predicted mean daily stream flow was found to be 1.92m3 /s exactly as observed during the water balance simulation. Besides, modeling result gave a total annual water yield of 597.1mm, from which the annual surface water component was 155.8mm and that of the base flow component was 441.4mm in the long term simulation period with IVF unity. While demonstrating the catchment is rich in ground water sources as a result of high magnitude of precipitation and good water retention capacity, this study shows that SWAT model can be a potential monitoring tool for watersheds in mountainous catchments.
Comparing Flow Regime, Channel Hydraulics and Biological Communities to Infer Flow-Ecology Relationships in the Mara River of Kenya and Tanzania
(2014) McClain, Michael E.; Subalusky, Amanda L.; Anderson, E.; Dessu, Shimelis B.; Melesse, Assefa M.; Ndomba, Preksedis M.; Mtamba, Joseph O. D.; Tamatamah, Rashid A.; Mligo, Cosmas
Equatorial rivers of East Africa exhibit unusually complex seasonal and inter-annual flow regimes, and aquatic and adjacent terrestrial organisms have adapted to cope with this flow variability. This study examined the annual flow regime over the past 40 years for three gauging stations on the Mara River in Kenya and Tanzania, which is of international importance because it is the only perennial river traversing the MaraSerengeti ecoregion. Select environmental flow components were quantified and converted to ecologically relevant hydraulic variables. Vegetation, macroinvertebrates, and fish were collected and identified at target study sites during low and high flows. The results were compared with available knowledge of the life histories and flow sensitivities of the riverine communities to infer flow–ecology relationships. Management implications are discussed, including the need to preserve a dynamic environmental flow regime to protect ecosystems in the region. The results for the Mara may serve as a useful model for river basins of the wider equatorial East Africa region.
Developing an Excellent Sediment Rating Curve from one Hydrological Year Sampling Programme Data Approach
(Universidade Federal da Paraíba, 2008-05-16) Ndomba, Preksedis M.; Killingtveit, A.
This paper presents preliminary findings on the adequacy of one hydrological year sampling programme data in developing an excellent sediment rating curve. The study case is a 1DD1 subcatchment in the upstream of Pangani River Basin (PRB), located in the North Eastern part of Tanzania. 1DD1 is the major runoff-sediment contributing tributary to the downstream hydropower reservoir, the Nyumba Ya Mungu (NYM). In literature sediment rating curve method is known to underestimate the actual sediment load. In the case of developing countries long-term sediment sampling monitoring or conservation campaigns have been reported as unworkable options. Besides, to the best knowledge of the authors, to date there is no consensus on how to develop an excellent rating curve. Daily-midway and intermittent-cross section sediment samples from Depth Integrating sampler (D-74) were used to calibrate the subdaily automatic sediment pumping sampler (ISCO 6712) near bank point samples for developing the rating curve. Sediment load correction factors were derived from both statistical bias estimators and actual sediment load approaches. It should be noted that the ongoing study is guided by findings of other studies in the same catchment. For instance, long term sediment yield rate estimated based on reservoir survey validated the performance of the developed rating curve. The result suggests that excellent rating curve could be developed from one hydrological year sediment sampling programme data. This study has also found that uncorrected rating curve underestimates sediment load. The degree of underestimation depends on the type of rating curve developed and data used.
The development of micro hydro for rural energy supply in Tanzania
(Aqua-Media International, 2012) Kaunda, C. S.; Kimambo, C. Z. M.; Ndomba, Preksedis M.
Estimating Gully Erosion Contribution to Large Catchment Sediment Yield Arte in Tanzania
(Elsevier, 2009-12) Ndomba, Preksedis M.; Mtalo, Felix W.; Killingtveit, A.
The objective of this paper is to report on the issues and proposed approaches in estimating the contribution of gully erosion to sediment yield at large catchment. The case study is the upstream of Pangani River Basin (PRB) located in the North Eastern part of Tanzania. Little has been done by other researchers to study and/or extrapolate gully erosion results from plot or field scale to large catchment. In this study multi-temporal aerial photos at selected sampling sites were used to estimate gully size and morphology changes over time. The laboratory aerial photo interpretation results were groundtruthed. A data mining tool, Cubist, was used to develop predictive gully density stepwise regression models using aerial photos and environment variables. The delivery ratio was applied to estimate the sediment yield rate. The spatial variations of gully density were mapped under Arc View GIS Environment. Gully erosion sediment yield contribution was estimated as a ratio between gully erosion sediment yield and total sediment yield at the catchment outlet. The general observation is that gullies are localized features and not continuous spatially and mostly located on some mountains’ foot slopes. The estimated sediment yield rate from gullies erosion is 6800 t/year, which is about 1.6% of the long-term total catchment sediment yield rate. The result is comparable to other study findings in the same catchment. In order to improve the result larger scale aerial photos and high resolution spatial data on soil-textural class and saturated hydraulic conductivity – are recommended.
Estimation of Soil Erosion in the Pangani Basin Upstream of Nyumba ya Mungu Reservoir
(2002) Mtalo, Felix W.; Ndomba, Preksedis M.
A Guided SWAT Model Application on Sediment Yield Modeling in Pangani River Basin: Lessons Learnt
(Universidade Federal da Paraíba, 2008-12-10) Ndomba, Preksedis M.; Mtalo, Felix W.; Killingtveit, A.
The overall objective of this paper is to report on the lessons learnt from applying Soil and Water Assessment Tool (SWAT) in a well guided sediment yield modelling study. The study area is the upstream of Pangani River Basin (PRB), the Nyumba Ya Mungu (NYM) reservoir catchment, located in the North Eastern part of Tanzania. It should be noted that, previous modeling exercises in the region applied SWAT with preassumption that inter-rill or sheet erosion was the dominant erosion type. In contrast, in this study SWAT model application was guided by results of analysis of high temporal resolution of sediment flow data and hydro-meteorological data. The runoff component of the SWAT model was calibrated from six-years (i.e. 1977–1982) of historical daily streamflow data. The sediment component of the model was calibrated using one-year (1977–1988) daily sediment loads estimated from one hydrological year sampling programme (between March and November, 2005) rating curve. A long-term period over 37 years (i.e. 1969–2005) simulation results of the SWAT model was validated to downstream NYM reservoir sediment accumulation information. The SWAT model captured 56 percent of the variance (CE) and underestimated the observed daily sediment loads by 0.9 percent according to Total Mass Control (TMC) performance indices during a normal wet hydrological year, i.e., between November 1, 1977 and October 31, 1978, as the calibration period. SWAT model predicted satisfactorily the long-term sediment catchment yield with a relative error of 2.6 percent. Also, the model has identified erosion sources spatially and has replicated some erosion processes as determined in other studies and field observations in the PRB. This result suggests that for catchments where sheet erosion is dominant SWAT model may substitute the sediment-rating curve. However, the SWAT model could not capture the dynamics of sediment load delivery in some seasons to the catchment outlet.
A Proposed Approach of Sediment Sources and Erosion Processes Identifiacation at Large Catchments
(Universidade Federal da Paraíba Brasil, 2007) Ndomba, Preksedis M.; Mtalo, Felix W.; Killingtveit, A.
In the subject of identifying sediment sources and erosion processes at catchment level researchers have proposed various methods. Most of the techniques have been applied in isolation. A few workers have combined some methods but still they could not ascertain their findings. As a result they recommended more sophisticated methods in order to compare the results. Little however has been done to correlate suspended sediment concentrations using spatial and temporal hydrological variables like rainfall and surface runoff at reasonable time step such as daily time series. In this study selected methods by previous workers are used and compared. The hydrological variables mapping technique has complemented the results of various renowned sediment sources identification techniques. The introduced method gives not only probable sources and processes but also it additionally identifies location based sediment sources using rainfall stations as pointers. The combined results from both methods indicate that either clay soil land plots or agricultural areas are potential sediment source areas. The result is comparable to previous researchers’ findings in the Pangani River basin that mapped the erosion zones using simple empirical and complex physics-based mathematical models. Although, the methods adopted in this study lacked high-resolution data, the authors believe that the methods and modifications applied give a quick, reliable and more insight to future sediment yield modelling efforts at a catchment level. For instance, a distributed watershed sediment yield model would be appropriate based on high spatial and temporal variation of the hydrological variables as reported in this study. Also, the results suggest that Sediment yield model that simulates sheet erosion might be an ideal tool since the major source areas of the transported sediment are topsoils or sheet erosion.
Sediment Yield Modelling using SWAT model at Larger and Complex Catchments: Issues and Approaches. A Case of Pangani River Catchment, Tanzania
(2007) Ndomba, Preksedis M.; Mtalo, Felix W.; Killingtveit, A.
SWAT model is a semi-distributed, physics based water shed model. The model is now being applied/customized in Tanzania, the successful stories on SWAT applications motivated the study, unfortunately, and the model is developed from multitudes of parameters, hence complex. It’s also data expensive. Modeling uncertainty is high if not applied with caution. Unfortunately, SWAT model applications techniques have Not been adequately documented. Little has been done by other works to compare SWAT stimulations performance with data from intensive sediment sampling programme. Therefore this study used SWAT model in larger and complex catchment in order to estimate sediment yield and document application techniques and give insight to possible model customization opportunities.
Spatial Uncertainty Handling in Lake Extent Trend Analysis Using Remote Sensing and GIS Tools: The Case of Lake Naivasha
(Scientific Research, 2012-06) Ijumulana, Julian; Ndomba, Preksedis M.
SWAT Model Application in a Data Scarce Tropical Complex Catchment in Tanzania
(Elsevier, 2008) Ndomba, Preksedis M.; Mtalo, Felix W.; Killingtveit, A.
This study intended to validate the Soil and Water Assessment Tool (SWAT) model in data scarce environment in a complex tropical catchment in the Pangani River Basin located in northeast Tanzania. The validation process involved the model initialization, calibration, verification and sensitivity analysis. Both manual and auto-calibration procedures were used to facilitate the comparison of the results with past studies in the same catchment. For this study, some model parameters including Soil depth (SOL_Z) and Saturated hydraulic conductivity (SOL_K) were assumed uniform within the study catchment and were therefore lumped comprising the huge computation resource requirement of the SWAT model. Results indicated that the same set of important parameters was identified with or without the use of observed flows data. Some of the parameters had physical interpretation and could therefore relate directly to hydrological controlling factors within the catchment. Despite swapping ranking importance of parameters, these results suggest the suitability of the SWAT model for identifying hydrological controlling factors/parameters in ungauged catchments. Results of calibration and validation at the daily timescale gave moderately satisfactory Nash–Sutcliffe Coefficient of Efficiency (CE) of 54.6% for calibration and 68% for validation while simulated and observed mean annual flow discharges gave an Index of Volumetric Fit (IVF) of 100%. The study further indicated the improvement of model estimation when more reliable spatial representation of rainfall was used. Although in this study SWAT model has performed satisfactorily in data poor and complex catchment, the authors recommend a wider validation effort of the model before it is adopted for operational purpose.
Use Of Earth Observation Data For Hydrodynamic Modelling In The Mara Wetlands
(ESA Living Planet Symposium, 2013-09-13) Mtamba, Joseph; Van der Velde, Rogier; Ndomba, Preksedis M.; Zoltan, Verkedy; Mtalo, Felix W.; Crosato, Alessandra
Vegetation characteristics can not only be used to derive spatial hydrodynamic roughness parameters but also to correct vegetation artefacts in freely downloadable Digital Elevation Models for hydrodynamic modelling. An exponential backscattering model for vegetation canopy height model was developed using standard deviation of cross polarization backscatter coefficient of Radarsat-2 SAR wide swath mode and in situ vegetation height data. The retrieved spatial vegetation height was used to correct vegetation artefacts in freely downloadable Advanced Spaceborne Thermal Emission and Reflectance Radiometer Global Digital Elevation Model (ASTERDEM) released in 2011. The relative spatial hydrodynamic roughness within a vegetation class was derived from cross polarization ratio. Preliminary results show that the accuracy of ASTERDEM improved the elevation estimates by root mean square error from 5.1 m to 3.0m. Simulation results using Earth Observation (EO) data for calibration and validation using an internal gauging station yielded promising Nash - Sutcliffe efficiency criterion of 0.38 and 0.45. The results shows that if high resolution DEM is available, spatial roughness parametrization using cross-polarization ratio of Synthetic Aperture Radar (SAR) imagery may be useful in modelling extensive floodplains where optimization of roughness parameter is not necessary due to computational limitations.
Use of Radarsat-2 and Landsat TM Images for Spatial Parameterization of Manning’s Roughness Coefficient in Hydraulic Modeling
(remote sensing, 2015-01-14) Mtamba, Joseph; Van der Velde, Rogier; Ndomba, Preksedis M.; Zoltán, Vekerdy; Mtalo, Felix W.
Vegetation resistance influences water flow in floodplains. Characterization of vegetation for hydraulic modeling includes the description of the spatial variability of vegetation type, height and density. In this research, we explored the use of dual polarized Radarsat-2 wide swath mode backscatter coefficients (σ°) and Landsat 5 TM to derive spatial hydraulic roughness. The spatial roughness parameterization included four steps: (i) land use classification from Landsat 5 TM; (ii) establishing a relationship between σ° statistics and vegetation parameters; (iii) relative surface roughness (Ks) determination from Synthetic Aperture Radar (SAR) backscatter temporal variability; (iv) derivation of the spatial distribution of the spatial hydraulic roughness both from Manning’s roughness coefficient look up table (LUT) and relative surface roughness. Hydraulic simulations were performed using the FLO-2D hydrodynamic model to evaluate model performance under three different hydraulic modeling simulations results with different Manning’s coefficient parameterizations, which includes SWL1, SWL2 and SWL3. SWL1 is simulated water levels with optimum floodplain roughness (np) with channel roughness nc = 0.03 m−1/3/s; SWL2 is simulated water levels with calibrated values for both floodplain roughness np = 0.65 m−1/3/s and channel roughness nc = 0.021 m−1/3/s; and SWL3 is simulated water levels with calibrated channel roughness nc and spatial Manning’s coefficients as derived with aid of relative surface roughness. The model performance was evaluated using Nash-Sutcliffe model efficiency coefficient (E) and coefficient of determination (R2), based on water levels measured at a gauging station in the wetland. The overall performance of scenario SWL1 was characterized with E = 0.75 and R2 = 0.95, which was improved in SWL2 to E = 0.95 and R2 = 0.99. When spatially distributed Manning values derived from SAR relative surface values were parameterized in the model, the model also performed well and yielding E = 0.97 and R2 = 0.98. Improved model performance using spatial roughness shows that spatial roughness parameterization can support flood modeling and provide better flood wave simulation over the inundated riparian areas equally as calibrated models.
Validation of PSIAC Model for Sediment Yields Estimation in Ungauged Catchments of Tanzania
(Scientific Research, 2013-09) Ndomba, Preksedis M.
The main objective of this paper is to report on preliminary validation results of the newly applied sediment yields estimation model in Tanzania, the Pacific Southwest Inter-Agency Committee (PSIAC). This is a follow-up research on the call to customize simple and/or multi-processes sediment yields estimation models such as PSIAC in the region. The PSIAC approach is based on a sediment yield classification scheme employing individual drainage basin characteristics: surface geology, soils, climate, runoff, topography, ground cover, land use, upland erosion, channel erosion, and sediment transport. In this study, PSIAC model is built from readily available environmental variables sourced from Government ministries/agencies and public domain global spatial data. The sediment classification exercise was verified with field observations. The set up model was then validated by 31 small dams’ siltation surveys and previous sedimentation study findings. PSIAC model performance for major part of central Tanzania was good during calibration (BIAS = 7.88%) and validation (BIAS = 18.12%). Another observation was that uncalibrated model performs fairly well, though performance improves with calibration. The extension of the uncalibrated PSIAC model to 3 selected large basins of Tanzania, with drainage areas size up to 223,000 km2, registered a satisfactory performance in one of them with fair performance in the rest. For large basins, the performance seems to correlate with general ground slope. The higher the slope, the better the performance. It is, however, not apparent from this study on the threshold drainage area and slope requirements for better performance of the model. Notwithstanding, the PSIAC model has improved previous sediment yields estimates based on simple regressive models. Finally, the paper proposes two main further research works: use of high resolution geospatial data and additional validation dams siltation data even beyond the central part of Tanzania, and carries out rigorous study on spatial scale model application limitations.

Browsing by Author "Ndomba, Preksedis M."

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