Spatial modelling of the contributions from surface and subsurface water to river flow in catchments
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Date
2007
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Abstract
The groundwater and surface water resources were historically modelled separately because of laws of the governing bodies. Movement towards equity and sustainable development demands the integration of groundwater and surface water in decision making and modelling of these water resources.
This research attempts to simulate the contributions in river runoff from surface and groundwater resources, by conceptualizing the flow pathways of the different resources present in a river's catchment. It utilizes the spatial information of the catchment, along with the observed flow hydrograph characteristics, to create a model of the flow components in the river runoff sequence.
The model conceptualizes the observed flow hydrograph from a rainfall event as a combination of flow from three different pathways. Excess rainfall (the part of measured rain that causes the storm hydrograph) is separated into the surface runoff; the throughflow (through the unsaturated soil structures and macropores); as well as baseflow (through the deeper saturated soil structures of the catchment): All of these components contribute to the measured flow at the catchment outlet.
Analysis of observed flow hydrographs (i.e., the separation of the observed flow into different flow components); indicates constant recession rates for each flow component present in the hydrograph. Information derived from observed flow hydrograph analysis includes the recession rate of each flow component, the percentage of water that is allocated to each flow component for a particular storm event, and the times to peak and recede. This information is used along with the spatial information of the catchment, to derive a simulated flow hydrograph for a rainfall event, for each flow path.
The Digital Elevation Model (DEM) of the catchment and geological features are used to determine the pathways and distances that water travels to the outlet. Flow velocities, along these pathways, are influenced by the slopes and the roughness of the medium over/through which the water travels. The flow velocities are estimated from adaptations of recognized hill slope and channels flow velocity equations. The channel geometry, that determines the flow rate through each catchment segment in the DEM, is derived from the contributing area and scaled by the total catchment size.
Cumulative flow times along each pathway are used to derive a flow response function for each flow component. These response functions are unique to each catchment and represent the equivalent of a unit hydrograph for each flow component. These response functions are scaled and superimposed to simulate the observed storm hydrograph of a rain event.
Storm events are divided into four scenarios representing a combination of high and low intensity rainfall events, as well as events of long and short duration.
The model is applied to a rainfall series of five months in the Ntuze research catchments, during which various rain storm types occurred.
Model parameters are applied to the much larger Goedertrouw Dam catchment to evaluate the transferability of the model.
Description
A dissertation submitted to the faculty of Science at the University of Zululand, in partial fulfilment of the requirements for the Doctor of Philosophy in the Department of Hydrology, South Africa, 2007.
Keywords
Hydrology--Data processing, Ground water flow--South Africa--Ntuze River--Mathematical models, Subsurface drainage--South Africa--Ntuze River--Mathematical models, Streamflow--South Africa--Ntuze River--Mathematical models