Estimating Aquifer Transmissivity based on Streamflow Records using an Analytical Approach in Kilombero Valley, Tanzania
Streamflow recession techniques have been useful in estimating aquifer hydraulic propertiesthat are not easily measured, but are important for the hydrologic responses of catchments.Compared to other hydraulic properties, aquifer transmissivity (T) is a key parameterproviding insight on groundwater development at a local and regional scale. However, fieldbasedmeasurements of T, are often scarce and uncertain. This study aims to use an analyticalapproach adopted by Huang et al., (2011) to quantify aquifer T based on streamflow recordsin Tanzania‘s Kilombero Valley. The study also considers T derived from groundwaterpumping test data from selected wells located within the Kilombero Valley for comparison.The programs RECESS and RORA were applied in this study to simplify and minimizesubjectivity inherent in using manual methods associated with recession analysis. Based onthe analysis presented, seasonal variability of T indicated dry season recession eventsgenerated higher T values than wet season events. This variability could be accounted forwhen considering the distribution of soil properties relative to water table positions duringwet and dry seasons. Furthermore, the average basin slope contributed to the spatialvariability of T estimates in the valley, as the 1KB14 catchment with steep slopes showedhigher T values compared to the gentle slopes 1KB4 catchment. Interestingly, no significantrelation was found between the elevations of individual well locations and T derived frompump test data.By comparison, the T values derived from large-scale streamflow records underestimatethose derived from small-scale field-based measurements. It is however necessary to notethat, thicker aquifers (greater or equal to 30m) gave reasonable T estimates that moderatelyagree with streamflow derived T. The differences observed in T estimates suggestsdifficulties when comparing the analytical approach with field-based data in the KilomberoValley. From a hydrological modeling perspective, the reasons for disparities between twomethods could come from: high heterogeneity of aquifer within the valley, difficulty andlimited number of small-scale measurements, limitation inherent in the modeling approach inthe valley, improper quality of data, seasonality and scale of measurements. In general, as theanalytical approach applied was typically inconsistent with field-based measurements ofaquifer T, this study can be considered as a basis for future work(s). Further, the studyrecommends multiple approaches should be considered to provide reliable theoretical Tvalues in comparison with field-based measurements data.
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