File Name: hydrological modelling in arid and semi-arid areas .zip
Rehana, G. Sireesha Naidu, N.
UC Santa Barbara
This dissertation employs hydrologic modeling to assess probable impacts of changes in vegetation cover and in the channel network on streamflow and floodplain groundwater levels in the Baviaanskloof catchment, South Africa. The Baviaanskloof serves as a case study of a semi-arid, mountainous, meso-scale catchment that has been subject to agricultural land use and is regionally important for water supply. In this catchment livestock grazing has resulted in a loss of subtropical thicket cover on hillslopes and the channel network in the central valley has become increasingly connected and incised.
In order to build an appropriate model of the Baviaanskloof, streamflow, groundwater, surface runoff, and soil moisture data were analyzed for diagnostic patterns that revealed information about hydrologic connectivity at different spatial and temporal scales. Critical results of these analyses were that: a the central valley alluvial aquifer is recharged by subsurface flows from surrounding mountain areas following two major pathways, a likely interflow contribution following large rainfall events and a more temporally consistent contribution from the bedrock aquifer, and b the dominant direction of exchange of water between the alluvial aquifer and the main floodplain channel regularly fluctuates between losing and gaining.
To capture the observed patterns, the numeric model structure consisted of a coarse-scale sub-model of the mountain tributary subcatchments surrounding the central valley alluvial fill, and a higher resolution, coupled hydraulic-hydrologic sub-model of the central valley alluvial fans and floodplain.
This model was calibrated in a multi-criteria calibration process using various observational datasets. It was found that including multiple streamflow-based criteria and including criteria based on additional data types improved model performance and better constrained the model parameter space.
Alternative scenarios of further degradation and of restoration of the hillslope vegetation, alluvial fan surfaces, and floodplain channel were modeled individually and in combination. Models were run using 38 years of local climate data and differences between model predictions for the current catchment state and each alternative scenario were assessed. Outputs suggested that, of the individual restoration intervention scenarios considered, hillslope thicket restoration would have the most significant impact on streamflow, driven by large reductions in storm event runoff and floodplain channel restoration would have the largest impact on the floodplain water table, driven by decreased drainage into the channel and increased recharge due to overbank flooding.
Greater modeled water retention and evapotranspiration on vegetated hillslopes reduced runoff to the floodplain, resulting in a deepened water table and decreased baseflow in the model.
Comparing the alluvial fan restoration scenario to a more extreme channelized case did show small, but detectable, increases in baseflow and floodplain groundwater levels. Modeled impacts on average yield and baseflow were not statistically detectable. These results highlight various potential tradeoffs that would need to be considered in restoration planning and catchment management. Predictions made here would need further integration into reservoir and water supply system models, as well as sediment transport models to consider reservoir sedimentation, in order to better understand implications for local and downstream water supply availability.
Nevertheless a net decrease in annual average available supply appears likely. The catchment and climatic contexts of these changes were shown to be important in determining the magnitude and direction of the predicted impacts, with dispersive flow paths through central valley alluvium dampening impacts of hillslope vegetation changes and with the frequency and magnitude of storm events determining how often different thresholds of flow path connectivity were reached.
This highlighted the importance of including best available understanding of a landscape's hydrologic connectivity in modeling, even when focusing on a local scale change, and of modeling impacts over a long time period to include long-term weather patterns.
Skip to main content. UC Santa Barbara. Email Facebook Twitter. Abstract This dissertation employs hydrologic modeling to assess probable impacts of changes in vegetation cover and in the channel network on streamflow and floodplain groundwater levels in the Baviaanskloof catchment, South Africa. Thumbnails Document Outline Attachments. Highlight all Match case. Whole words. Toggle Sidebar. Zoom Out. More Information Less Information.
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UC Santa Barbara
Despite the high accuracy of the model, the lack of rainfall data at short time scales hour and less than hour restricted implementation of the model especially for long time simulations. Some studies use simple division for daily rainfall disaggregation into the hourly values to provide data requirements of HSPF model. In simple division, each rainfall event is divided into 24 pulse stochastically and the peak flows may not properly being simulated due to the lower rainfall intensities. In this study, random parameter Bartlett—Lewis rectangular pulse BLRP model was implemented to disaggregate daily rainfall time series into the hourly values and the results compared with that of simple division. The calibrated model was then implemented to disaggregate daily rainfall data into the hourly values. To compare two disaggregation approaches, daily stream flow simulation by HSPF model is initialized in 2 scenarios by applying the hourly rainfall data resulted from two disaggregation methods. The results indicated that while using the simple division method leads to the underestimation of peak flows, using the BLRP model improved peak flow simulations.
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Yuan, Y. Nie, AND E. Given the growing demand for water due to urban growth and the likelihood of decreasing precipitation due to climate change, water sustainability has become a dominant issue in the arid and semi-arid regions such as the Southwestern USA.