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Ungauged Catchment Hydrology: The case of Lake Tana Basin

Janaka Perera, B.Upul (2009) Ungauged Catchment Hydrology: The case of Lake Tana Basin.

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Abstract:Estimating the water balance of a hydrologic basin often requires an effective procedure to quantify the contribution of the ungauged catchments. In this study a regionalizing based procedure is followed to quantify the flows of the ungauged catchments in the Lake Tana basin. Linking model parameters to physical catchment characteristics is a popular approach that enables the application of a conceptual model to an ungauged site. The HBV semi-distributed conceptual rainfall-runoff model is selected to simulate runoff of nine gauged catchments on a daily basis in the period 1994-2003. Some eight model parameters are selected for model calibration that also are used in the regionalisation procedure to simulate runoff from ungauged catchments. For nine gauged catchments the optimum parameter sets are derived through an automatic optimisation procedure based on Monte Carlo Simulation. By solving the multi-objective calibration problem that measures the different aspect of the hydrograph: (1) overall water balance (relative volume error), (2) overall shape of the hydrograph (Nash-Sutcliffe value). In the calibration procedure, for each catchment and for each single objective function best and worst values are used for rescaling to allow comparison. Thus, model performance is assessed through the use of rescaled objective functions that serve as criteria for selection of a best parameter set. In regionalisation, optimal model parameters are related to selected physical catchment characteristics (PCCs) that are used to estimate parameter values for ungauged catchments. Since PCCs from ungauged catchments generally differ from those of gauged catchments, also model parameter values will changed. By establishing the relations between values of HBV model parameters and PCCs, information was transferred from the gauged catchments to ungauged catchments. The transferred parameter sets were used to simulate the runoff from the ungauged catchments of the Lake Tana basin. Four parameter regionalization methods (multiple regression, spatial proximity, similarity approach and sub-basin mean) were tested in this study to transfer model parameter values to the ungauged catchments. To evaluate the reliability of the simulations from the ungauged catchments, a water balance model of Lake Tana is developed at daily time step in the period 1994-2003. This model used area-volume and elevation-depth relations based on a bathymetric survey to simulate lake level fluctuations by calculation of the net inflow by estimating lake areal precipitation, lake areal evapotranspiration, inflow from gauged catchment, inflow from ungauged catchment and lake outflow. The water balance model of Lake Tana is evaluated by selected objective functions that are the Relative Volume Error and Nash-Sutcliffe objective functions as well. Among nine gauged catchments in Lake Tana basin six of them are satisfactory calibrated under the criteria of relative volume error between ±5% and Nash-Sutcliffe value greater than 0.6 for the period of 1994-2000. Daily lake level simulation with inflow from ungauged catchment estimated from regression method shows the best performance with a relative volume error of -2.17% and a Nash- Sutcliffe coefficient of 0.92. Key words: Regionalization, Water balance, HBV, Monte Carlo Simulation
Item Type:Essay (Master)
Faculty:ITC: Faculty of Geo-information Science and Earth Observation
Programme:Geoinformation Science and Earth Observation MSc (75014)
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