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University of Venda (2019)

Development of risk-based groundwater operating rules : a case study of Siloam Village, South Africa

Makungo, Rachel

Titre : Development of risk-based groundwater operating rules : a case study of Siloam Village, South Africa

Auteur : Makungo, Rachel

Université de soutenance : University of Venda

Grade : Doctor of Philosophy (PhD) in Environmental Siences (Hydrology) 2019

Résumé partiel
This study developed operating rules for groundwater supply from a probabilistic (risk-based) approach. Groundwater supply systems are often operated without relating groundwater yield/availability to demand which makes groundwater resource planning and management challenging and unpredictable. Risk-based approaches for developing groundwater operating rules comprehensively incorporate assurance of supply and also account for uncertainty due to model inputs, model structure and climate variability. A groundwater resource unit (GRU) was delineated and its hydrogeological conceptual model developed. Automatic curve matching was used to identify appropriate aquifer models and test solutions for estimating hydraulic characteristics (storativity, transmissivity and hydraulic conductivity) based on Aquifer Test Solver (AQTESOLV) Pro version 4.5. Limited groundwater levels and rainfall data were infilled and/or extended using Output Error-Nonlinear Hammerstein Weiner (OE-NLHW) and non-parametric regression (NPR), respectively. Performances of these models were based on relative error (RE), correlation coefficient (COR), root mean square error (RMSE), coefficient of determination (R2) and Nash Sutcliffe coefficient of efficiency (NSE). A program for generation of monthly groundwater levels for the GRU was coded in FORTRAN based on the revised version of the Pitman model (referred to as GW-PITMAN model). The model was calibrated using groundwater levels from a neighbouring borehole due to lack of observed representative data for the GRU. Validation was done by establishing the realistic nature of simulated runoff, recharge and groundwater levels. A Variable Length Block (VLB) bootstrapping model was used for simultaneous generation of stochastic inputs (rainfall, evaporation and groundwater levels) of the groundwater operating rules model. Operating rules were developed from statistical analysis of 100 base yields for the GRU simulated from 5-year long stochastically generated sequences (with length of 34 years) of rainfall, evaporation and groundwater levels. The hydrogeological conceptual model indicated presence of faults and diabase dykes which influence preferential flow paths and storage of water in the aquifer. Identified aquifer test solutions were found to be suitable for estimation of hydraulic characteristics, since they had generally good model fits and low mean residual errors. Heterogeneous aquifer types were identified though leaky aquifer dominated.

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