Search

Copyright of Hydrogeology Journal is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Hydrogeology Journal is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Being located in the tectonically active Ethiopian Rift, the hydrogeology of the Gidabo River Basin is complex due to the disruption of lithologies by faults and the variability and lateral discontinuity of the aquifers. Hydrogeochemical and isotopic data suggest that the aquifers within the rift floor receive a relevant contribution of groundwater recharged in the highland. However, the incomplete knowledge about the aquifer properties, the hydraulic behavior of the faults, and the boundary conditions cause uncertainties in this conceptual hydrogeological model. To account for these uncertainties fourteen different numerical models with a stepwise increase from 7 to 40 adjustable parameters were developed, calibrated against the same hydraulic head observations, and ranked according to the Akaike information criteria (AIC and AICc) and Bayesian information criterion (BIC). Based on the information criteria five plausible models were identified, all of which were successfully verified against the river baseflow. The highest likelihood is attributed to a model with eleven adjustable parameters that does not explicitly account for the fault zones; other plausible models considering faults as semi-barriers achieve a slight improvement in model fit but have lower likelihood due to the increased number of calibration parameters. Thus, the effect of faults on groundwater flow needs further investigation, particularly at a local scale. On a regional scale, the hydraulic head distributions of the plausible models agree reasonably well with the equipotential map interpolated from well observations. The estimated transmissivity values range between 30 m² / day and 1350 m² / day and generally increase from the mountains towards the rift floor. The water budget shows that 75 % of the groundwater recharge supplies baseflow to the rivers. The remaining water infiltrates to the deeper aquifers where less than 1 % is abstracted by pumping wells and the rest flows towards Lake Abaya. Within the rift floor, the majority of inflow to the aquifers is from direct recharge; nevertheless, 35 % of the inflow is contributed by mountain block recharge (lateral groundwater flow from the escarpment and highland). The results of this study strongly advocate the idea to incorporate alternative plausible models instead of relying on single models in the practice of groundwater modeling especially in areas of complex hydrogeology and limited data availability. [ABSTRACT FROM AUTHOR]

In the years following a flood event in August 2005, the Hammerbach karst spring in Austria displayed hydraulic and physicochemical responses to recharge events that appear to be markedly different from those observed before. As likely cause of these changes the reduction of the conduit diameters resulting from redistribution of sediments during the flood event has been proposed (Birk et al., EGU2014-7550). According to Kovács et al. (J. Hydrol., 2005) the baseflow recession of mature karst aquifers generally is matrix restrained, i.e. controlled by the drainage from the low-permeability matrix but not by the conduit properties. It thus appears that the changed behavior of the Hammerbach karst spring corresponds to a shift from a matrix-restrained to a conduit-influenced flow regime. This contribution is aimed at understanding how such a shift of the flow regime may affect the recession behavior. On the one hand, conceptual recession models and the corresponding mathematical models are theoretically explored; on the other hand, the recession of the Hammerbach spring observed in the years after the flood event is analysed and compared to that before the flood event 2005. It is shown that – given some reasonable assumptions – a shift from an exponential recession under matrix-restrained conditions to a linear recession in a conduit-influenced flow regime may theoretically be expected and that the field data indeed appears to show a tendency towards such changes in the recession behaviour. [ABSTRACT FROM AUTHOR]