Papers by Olkeba Tolessa Leta
Integrating hydrology with climate is essential for a better understanding of the impact of prese... more Integrating hydrology with climate is essential for a better understanding of the impact of present and future climate on hydrological extremes, which may cause frequent flooding, drought, and shortage of water supply. This study assessed the impact of future climate change on the hydrological extremes (peak and low flows) of the Zenne river basin (Belgium). The objectives were to assess how climate change impacts basin-wide extreme flows and to provide a detailed overview of the impacts of four future climate change scenarios compared to the control (baseline) values. The scenarios are high (wet) summer (projects a future with high storm rain in summer), high (wet) winter (predicts a future with high rainfall in winter), mean (considers a future with intermediate climate conditions), and low (dry) (projects a future with low rainfall during winter and summer). These scenarios were projected by using the Climate Change Impact on HYDRological extremes perturbation tool (CCI-HYDR), which was (primarily) developed for Belgium to study climate change. We used the Soil and Water Assessment Tool (SWAT) model to predict the impact of climate change on hydrological extremes by the 2050s (2036–2065) and the 2080s (2066–2095) by perturbing the historical daily data of 1961–1990. We found that the four climate change scenarios show quite different impacts on extreme peak and low flows. The extreme peak flows are expected to increase by as much as 109% under the wet summer scenario, which could increase adverse effects, such as flooding and disturbance of the riverine ecosystem functioning of the river. On the other hand, the low (dry) scenario is projected to cause a significant decrease in both daily extreme peak and low flows, by as much as 169% when compared to the control values, which would cause problems, such as droughts, reduction in agricultural crop productivity, and increase in drinking water and other water use demands. More importantly, larger negative changes in low flows are predicted in the downstream part of the basin where a higher groundwater contribution is expected, indicating the sensitivity of a basin to the impact of climate change may vary spatially and depend on basin characteristic. Overall, an amplified, as well as an earlier, occurrence of hydrological droughts is expected towards the end of this century, suggesting that water resources managers, planners, and decision makers should prepare appropriate mitigation measures for climate change for the Zenne and similar basins.
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The integration of hydrology and climate is important for understanding the present and future im... more The integration of hydrology and climate is important for understanding the present and future impact of climate on streamflow, which may cause frequent flooding, droughts, and shortage of water supply. In view of this, we assessed the impact of climate change on daily streamflow duration curves as well as extreme peak and low flow values. The objectives were to assess how climate change impacts watershed-wide streamflow and its extreme values and to provide an overview of the impacts of different climate change scenarios (Representative Concentration Pathways (RCP) 4.5 and 8.5) on streamflow and hydrological extremes when compared with the baseline values. We used the Soil and Water Assessment Tool (SWAT) model for daily streamflow and its extreme value modeling of two watersheds located on the Island of Oahu (Hawaii). Following successful calibration and validation of SWAT at three USGS flow gauging stations, we simulated the impact of climate change by the 2050s (2041–2070) and the 2080s (2071–2100). We used climate change perturbation factors and applied the factors to the historical time series data of 1980–2014. SWAT adequately reproduced observed daily streamflow with Nash-Sutcliffe Efficiency (NSE) values of greater than 0.5 and bracketed >80% of observed streamflow data at 95% model prediction uncertainty at all flow gauging stations, indicating the applicability of the model for future daily streamflow prediction. We found that while the considered climate change scenarios generally show considerable negative impacts on daily streamflow and its extreme values, the extreme peak flows are expected to increase by as much as 22% especially under the RCP 8.5 scenario. However, a consistent decrease in extreme low flows by as much as 60% compared to the baseline values is projected. Larger negative changes of low flows are expected in the upstream part of the watersheds where higher groundwater contributions are expected. Consequently, severe problems, such as frequent hydrological droughts (groundwater scarcity), reduction in agricultural crop productivity, and increase in drinking water demand, are significantly expected on Oahu. Furthermore, the extreme values are more sensitive to rainfall change in comparison to temperature and solar radiation changes. Overall, findings generally indicated that climate change impacts will be amplified by the end of this century and may cause earlier occurrence of hydrological droughts when compared to the current hydrological regime, suggesting water resources managers, ecosystem conservationists, and ecologists to implement mitigation measures to climate change in Hawaii and similar Islands.
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Episodic Combined Sewer Overflow (CSO) discharges effectively control the ecological status of re... more Episodic Combined Sewer Overflow (CSO) discharges effectively control the ecological status of receiving water bodies. Hydrodynamic models like the Storm Water Management Model (SWMM) are often used to model the CSO events. However , such detailed models are computationally demanding especially when a long-term simulation of a complex system is required. Considering this, we developed an alternative simple continuous simulation model (COSIMAT) using the SIMULINK TM module in MATLAB TM as a means of solving the issue of computational time associated with the detailed models. The COSIMAT model was tested against a detailed model set up on the SWMM. The Paruck collector – one of the major Collector of the Brussels' sewer system was used as an example case. Results showed that the accuracy of the simplified COSIMAT model was comparable to that of the detailed hydrodynamic model (SWMM) with a significant reduction of computational time by a factor of 8. We believe such alternative approaches would be useful to replace a computationally demanding model component of an integrated modelling system of a complex sewer system.
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In the past, the QUAL principles have been widely used to describe the processes that define the ... more In the past, the QUAL principles have been widely used to describe the processes that define the oxygen levels in rivers. The more recently developed IWA river water quality model No.1 (RWQM1) has however a more sound theoretical background. Moreover, as the state variables of RWQM1 are similar to those of the activated sludge model for waste water treatment plants, RWQM1 better suits the needs in the framework of integrated river basin management. Accordingly, we used the RWQM1 principles to model the in-stream physico-chemical processes that affect the oxygen concentration. Hereby, the RWQM1 simulator was used as a model component in an integrated model that also contains three other models: a Soil and Water Assessment Tool (SWAT) model to simulate the hydrologic processes, the Storm Water Management Model (SWMM) to simulate the hydraulic processes and a stream water temperature model. We linked the models dynamically, using the Open Modelling Interface (OpenMI). The integrated model is applied to simulate the water quality of the river Zenne (Belgium). The results show that the integrated model simulates various components with a quality that ranges from 'Unsatisfactory' to 'Very Good'.
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Despite large investments made for the management of the Brussels' sewer systems, the river Zenne... more Despite large investments made for the management of the Brussels' sewer systems, the river Zenne still receives high loads of pollutants, especially considering the small size of this receiving water. An interuniversity, multidisciplinary research project 'Good Ecological Status of the river Zenne (GESZ)' was therefore launched to evaluate the effects of the wastewater management plans in the river basin on the ecological functioning of the river. Hereto, different water quantity and quality processes were considered: the hydrology in the river basin, the hydraulics in the river and in the sewers, erosion and sediment transport and faecal bacteria transport and decay. To this purpose, existing models are being used (the SWAT for the hydrology and the erosion, the SWMM for the hydraulics) and new simulators have been developed for the sediment transport and for the faecal bacteria. For the integration of these models and to allow a flexible selection of the different models, it was decided to opt for the OpenMI platform. This paper discusses the early results of this integrated modelling of the river Zenne using OpenMI, considering the hydrologic model, the hydraulic model, the sediment transport model and a model describing the faecal bacteria dynamics. Results are presented for Escherichia coli (E.coli), taken as an indicator bacteria. The results indicate that the interactions between the sediments and the bacteria should be taken into account by the modelling.
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In environmental modelling, sensitivity analysis (SA) is very often applied to allow a dimensiona... more In environmental modelling, sensitivity analysis (SA) is very often applied to allow a dimensionality reduction of the parameter estimation problem. Due to the complexity and high computational demand of many environmental models, it is mostly impossible to use variance based methods. As a consequence, it is more common to apply screening methods for this task, like the Morris method [1] or the Latin-Hypercube – One-factor-at-A-Time (LH-OAT) technique [2]. In general, these screening methods are conceptually simple and can yield qualitative SA results (e.g. parameter rankings) with only a limited number of model evaluations. They are also particularly suited to identify model parameters for Factor Fixing (FF), i.e. putting those parameters that have no influence at all on the model output (elementary effect equal to 0) to a certain value [3]. Besides the non-influential factors, modellers also regularly fix a number of parameters that do have a (major or minor) influence on the model output to a chosen value, in order to additionally simplify the parameter estimation problem of an over-parameterized model. Hence, only the parameters that have the highest influence according to the ranking inferred with the screening method are considered in the optimization, leading to an (important) loss of model output variability. Since it has been shown [4] that for a Morris-like screening SA with a standard number of 5 to 10 " trajectories " [5] the parameter rankings do not converge, any selection of the parameters included in the estimation process becomes questionable. In addition, a drawback of screening methods is that they can be prone to type II errors (i.e. failing to identify a factor with considerable influence on the model) [6]. Therefore, the more the parameter rankings are mixed up and facing type II errors, the higher the chance to exclude an influential parameter from the optimization and the lower the model output variance might become. This study provides a methodology to assess the convergence of the SA results of a Morris-like screening method and shows that the number of trajectories needed to screen the parameter hy-perspace of a complex environmental model should be higher than 100 to achieve converged results and parameter rankings. Moreover, it is shown that with this higher number of trajectories, the screening methods become more resilient to type II errors. To achieve these results, the parameter sensitivities of the Soil and Water Assessment Tool (SWAT) [7] have been investigated for 2 case studies, by performing a screening with the LH-OAT technique. For both applications of this complex, computationally expensive and over-parameterized environmental model, a large set of model parameters has been included in the SA (resp. 40 and 26) and different variables have been considered as model output (discharge, suspended sediment, nitrogen concentrations, etc.). The convergence of the SA results has been tested by examining the influence of an increasing base sample size (i.e. an increasing number of trajectories), on the evolution of the mean and the variance of the elementary effects. The latter statistics are the principal sensitivity measures of Morris-like screening methods. To enhance the reliability of the convergence results, 95% confidence intervals (CIs) are assessed for the values of the mean and the variance, by performing bootstrapping with resampling [8]. The evolution of these CIs provides an additional measure to evaluate the convergence of the SA results.
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The European Union Water Framework Directive (EU-WFD) called its member countries to achieve a go... more The European Union Water Framework Directive (EU-WFD) called its member countries to achieve a good ecological status for all inland and coastal water bodies by 2015. According to recent studies, the river Zenne (Belgium) is far from this objective. Therefore, an interuniversity and multidisciplinary project " Towards a Good Ecological Status in the river Zenne (GESZ) " was launched to evaluate the effects of wastewater management plans on the river. In this project, different models have been developed and integrated using the Open Modelling Interface (OpenMI). The hydrologic, semi-distributed Soil and Water Assessment Tool (SWAT) is hereby used as one of the model components in the integrated modelling chain in order to model the upland catchment processes. The assessment of the uncertainty of SWAT is an essential aspect of the decision making process, in order to design robust management strategies that take the predicted uncertainties into account. Model uncertainty stems from the uncertainties on the model parameters, the input data (e.g, rainfall), the calibration data (e.g., stream flows) and on the model structure itself. The objective of this paper is to assess the first three sources of uncertainty in a SWAT model of the river Zenne basin. For the assessment of rainfall measurement uncertainty, first, we identified independent rainfall periods, based on the daily precipitation and stream flow observations and using the Water Engineering Time Series PROcessing tool (WETSPRO). Secondly, we assigned a rainfall multiplier parameter for each of the independent rainfall periods, which serves as a multiplicative input error corruption. Finally, we treated these multipliers as latent parameters in the model optimization and uncertainty analysis (UA). For parameter uncertainty assessment, due to the high number of parameters of the SWAT model, first, we screened out its most sensitive parameters using the Latin Hypercube One-factor-At-a-Time (LH-OAT) technique. Subsequently, we only considered the most sensitive parameters for parameter optimization and UA. To explicitly account for the stream flow uncertainty, we assumed that the stream flow measurement error increases linearly with the stream flow value. To assess the uncertainty and infer posterior distributions of the parameters, we used a Markov Chain Monte Carlo (MCMC) sampler – differential evolution adaptive metropolis (DREAM) that uses sampling from an archive of past states to generate candidate points in each individual chain. It is shown that the marginal posterior distributions of the rainfall multipliers vary widely between individual events, as a consequence of rainfall measurement errors and the spatial variability of the rain. Only few of the rainfall events are well defined. The marginal posterior distributions of the SWAT model parameter values are well defined and identified by DREAM, within their prior ranges. The posterior distributions of output uncertainty parameter values also show that the stream flow data is highly uncertain. The approach of using rainfall multipliers to treat rainfall uncertainty for a complex model has an impact on the model parameter marginal posterior distributions and on the model results
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Hydrological models can be used for assessing the impact of climate change, but their applicabili... more Hydrological models can be used for assessing the impact of climate change, but their applicability needs to be evaluated before they are used for scenario analysis. This study evaluates the applicability of SWAT model for hydrological modeling of Heeia watershed and assesses the impact of climate change on water budget. We developed and calibrated the SWAT based on the available geo-spatial and hydro-meteorological data within Heeia and neighboring watersheds. The simulated and observed streamflow showed an agreement with a satisfactory model performance, indicating the applicability of the model with a modification to reflect typical watershed characteristics. Overall, SWAT is suitable for modeling the Heeia Watershed. However, the model did not show sensitivity to the surface runoff lag parameter, which is commonly identified as a sensitive parameter, while it showed high sensitivity to curve number, channel hydraulic conductivity and baseflow alpha parameters. Furthermore, the derived curve number values with the streamflow data are relatively low compared to reported values. The calibrated model was used to assess the impact of rainfall and temperature changes on the water balance of the watershed. Findings revealed that the decrease in rainfall during wet season and marginal increase in dry season will generally cause a decrease in water balance components. More importantly, the groundwater flow component will be adversely affected by the combined change in rainfall and temperature in comparison to the other components. For example, the expected change of monthly streamflow ranges from-15% to 6% while a maximum decrease of-19% in groundwater recharge is predicted, compared to baseline. Our results suggest that climate change will negatively impact streamflow and groundwater reservoirs, affecting the groundwater sustainability and the riparian ecological functioning of the watershed in the future. The developed SWAT model, together with the applied climate change scenarios, can provide a useful information for evaluating the future freshwater availability and designing mitigation measures.
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Fig. 3: Flow chart of SWAT model set up, calibration and validation processes, and scenarios anal... more Fig. 3: Flow chart of SWAT model set up, calibration and validation processes, and scenarios analyses. HRUs = Hydrological Response Units; LH-OAT = Latin Hypercube-One-factor-At-a-Time; SWAT-CUP = SWAT Calibration and Uncertainty Program Fig.2: DEM with hydro-meteorological stations (A), land use (B), soil type (C), and delineated sub-basins (D) of the study area
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Fig.3: Land use (A), soil type (B), DEM with hydro-meteorological stations (C), and delineated su... more Fig.3: Land use (A), soil type (B), DEM with hydro-meteorological stations (C), and delineated sub-basins (D) of the Heeia watershed Fig. 4: Flow chart of SWAT model set up, calibration and validation processes. HRUs = Hydrological Response Units; LH-OAT = Latin Hypercube-One-factor-At-a-Time; SWAT-CUP = SWAT Calibration and Uncertainty Program Fig.2: The DEM with hydro-meteorological stations (A), land use (B), soil type (C), and delineated sub-basins (D) of the Nuuanu area watersheds
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The modelling of the different catchment processes is key for integrated water resources manageme... more The modelling of the different catchment processes is key for integrated water resources management. Constructing a single model for all the catchment processes may not always be a feasible option and it does not make appropriate use of existing models. The Open Modelling Interface (OpenMI), which allows time-dependent models to exchange data at run-time, might just be useful for such proposes. We used the Soil and Water Assessment Tool (SWAT) and the Storm Water Management Model (SWMM) models for simulating rural and urban catchment processes respectively. SWMM was also used to model the river processes. To link these models in OpenMI, both models were migrated to the OpenMI platform. As the water quality processes in the SWAT model are based on the QUAL2E process description, a new OpenMI compliant water quality module that is based on the same principles was developed to simulate the water quality processes in the river. The latter model, which uses a river network that is similar to that of the SWMM river model, is then also linked to the SWMM model using OpenMI. We tested this integrated model for the river Zenne in Belgium. The integrated model results show that such integration can be very useful as a decision support tools for integrated river basin management approach.
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Spatially distributed hydrologic simulators, such as the Soil and Water Assessment Tool (SWAT), a... more Spatially distributed hydrologic simulators, such as the Soil and Water Assessment Tool (SWAT), are important tool for integrated river basin management, as they represent different spatial processes-like evapotranspiration (ET), interception, infiltration, groundwater-surface water interaction (GWSW)-at river reach or basin scale. SWAT calculates evaporation from interception, based on the amount of water stored on the canopy. A close look at the estimated ET for forested areas in the Zenne basin indicates that the actual implementation of this process in SWAT potentially underestimates the forest ET, namely when the maximum canopy storage is activated by the CANMX parameter and increased from zero. Consequently, we corrected the SWAT codes, in order to improve the intercept ET estimation. Another issue in SWAT is related to the GWSW interaction. In the actual version of SWAT, either seepage from the river bed or groundwater flow to the river take place. It is hence not possible to simulate both gaining and losing processes, e.g. when rivers seasonally alternate from losing streams to gaining streams and back. Therefore, we also modified the codes to allow either gaining or losing, depending on the groundwater discharge from the sub-basin. The objective of this study is to evaluate how the modified SWAT simulates the intercept ET and the GWSW interaction. It is shown that the
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ET is an important component of the hydrological cycle and it completes the energy balance of the... more ET is an important component of the hydrological cycle and it completes the energy balance of the cycle in the vegetation-soil and atmosphere interface. ET is a combination of evaporation from land surface, open water, soil zone and transpiration from vegetation. The proportion of the ET flux in the water balance varies and it could be as high as 90% in some river basins. The representation of the ground water component in hydrological models influences the water distribution in the basin, affecting the runoff and evapotranspiration components. In this study we investigate how the representation of groundwater in the hydrological model influences the ET component by comparing different model structures. We made the comparison for monthly ET estimations at pixel and at a catchment scale. Monthly average ET from SWAT, Water and Energy Transfer between Soil, Plants and Atmosphere (WetSpass), Precipitation and runoff simulation model (PRMS) and MODIS were compared. The ET results from the different models were validated using values derived from MODIS16. The monthly average ET from these models is different, moreover as expected the spatial variation is better represented in semi distributed models. ET estimation of the SWAT model is the lowest compared to estimation from the other models. When we look at the ground water contribution for ET the variation of the result from the models is more significant, time series plot of monthly average ground water ET contribution results from SWAT and PRMS have different shape. Result from WetSpass models, in which spatial variation of ground water depth is represented, show very sensitive ET to groundwater level representation.
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According to recent studies, the river Zenne (Belgium) is far from fulfilling the objectives of E... more According to recent studies, the river Zenne (Belgium) is far from fulfilling the objectives of European Water Framework Directives (EU, 2000). The collaborative and inter-university project towards the Good Ecological Status in the river Zenne (GESZ) was launched in 2009 to assess the ecological status of the river and to re-evaluate the Brussels' wastewater management. In this project, an integrated water quantity and water quality modeling approach have been done for sediment transport, nitrogen (N), phosphorus (P), biochemical oxygen demand (BOD), dissolved oxygen (DO), soil erosion, heavy metals and bacteria modeling to evaluate the ecological status of the river. The hydrological and water quality model which is physically-based and semi-distributed using Soil and Water Assessment Tool, SWAT (Arnold et al., 1998) is among the component models. (Leta et al., 2013) in the integrated modeling chain which is used to model river Zenne catchment processes. Assessment of the climate impacts on the flows, sediment transport and water quality of the basin, using the developed hydrologic model is the main aim of this research. It is an essential part for decision making, which allows designing a robust management strategies that takes into account the predicted impact and uncertainties of climate change. This study is limited to the upstream part of the basin (Figure 1)
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Although the soil and water assessment tool (SWAT) is a physically based hydrologic simulator, it... more Although the soil and water assessment tool (SWAT) is a physically based hydrologic simulator, it has many parameters that cannot be measured directly in the field, but must be obtained through a model calibration process. Model calibration is thus an essential task to obtain the optimal parameter values, which match simulations with observations as closely as possible. This study used the Zenne River Basin (Belgium) as a case study, which experiences high spatial heterogeneity in terms of geological formation, groundwater recharge, and rainfall-runoff responses. Therefore, the objectives of this paper were to calibrate the SWAT model on the basis of different calibration techniques and identify which technique is suitable for such a heterogeneous basin so that the calibrated SWAT can be used as a tool for integrated management of the Zenne River Basin. Prior to calibration, the sensitive parameters were identified on the basis of a detailed sensitivity analysis (SA) of the Latin hypercube one-factor-at-a-time (LH-OAT) technique and increased sampling intervals. Then, SWAT was calibrated by using single-site calibration (SSC) at the watershed outlet; sequential calibration (SC), calibration from upstream to downstream; and simultaneous multisite calibration (SMSC), where data of two flow gauging stations were simultaneously used in a single calibration. It was found that at least 200 sampling intervals should be considered for the LH-OAT SA method to obtain converged rankings of SWAT parameters. In addition, to well capture the spatial variability of heterogeneous catchment and achieve stable sensitivity ranking, simultaneous multisite SA technique is important. Streamflow findings suggested that the SC and the SMSC techniques provided very good results and significantly improved model performance, but the SSC results were merely satisfactory. Though the results of SC and SMSC techniques were similar, the SMSC was selected over SC because it simultaneously handles the entire catchment spatial variability by assigning different parameter values and allows data information communication among stations in a single calibration. It was thus concluded that simultaneous multisite calibration should be considered for catchments with a high spatial variability like the Zenne River Basin.
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Although rainfall input uncertainties are widely identified as being a key factor in hydrological... more Although rainfall input uncertainties are widely identified as being a key factor in hydrological models, the rainfall uncertainty is typically not included in the parameter identification and model output uncertainty analysis of complex distributed models such as SWAT and in maritime climate zones. This paper presents a methodology to assess the uncertainty of semi-distributed hydrological models by including, in addition to a list of model parameters, additional unknown factors in the calibration algorithm to account for the rainfall uncertainty (using multiplication factors for each separately identified rainfall event) and for the heteroscedastic nature of the errors of the stream flow. We used the Differential Evolution Adaptive Metropolis algorithm (DREAM (zs)) to infer the parameter posterior distributions and the output uncertainties of a SWAT model of the River Senne (Belgium). Explicitly considering hetero-scedasticity and rainfall uncertainty leads to more realistic parameter values, better representation of water balance components and prediction uncertainty intervals. Published by Elsevier Ltd.
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Papers by Olkeba Tolessa Leta