Evaluation of groundwater resources in the Geba basin, Ethiopia

This article presents an assessment of the groundwater resources in the Geba basin, Ethiopia. Hydrogeological characteristics are derived from a combination of GIS and field survey data. MODFLOW groundwater model in a PMWIN environment is used to simulate the movement and distribution of groundwater in the basin. Despite the limited data available, by simplifying the model as a single layered semi-confined groundwater system and by optimising the transmissivity of the different lithological units, a realistic description of the groundwater flow is obtained. It is concluded that 30,000 m(3)/day of groundwater can be abstracted from the Geba basin for irrigation in a sustainable way, in locations characterised by shallow groundwater in combination with aquitard-type lithological units

Analytical Solution for Fractional Well Flow in a Double-Porosity Aquifer with Fractional Transient Exchange between Matrix and Fractures

An analytical solution is presented for groundwater flow to a well in an aquifer with double-porosity behavior and transient transfer between fractures and matrix. The solution is valid for fractional flow dimensions including linear, cylindrical or spherical flow to the well and for fractional inter-porosity diffusive transfer including release from storage in infinite slabs, infinite cylinders or spherical matrix blocks. Approximations are also presented for small and large times that are easy to evaluate in practice. The solution can be used to analyze pumping tests via coupling with a parameter estimation code. The utility of the method is demonstrated by a practical example using data from a pumping test performed in a fractured chalk aquifer. The analytical solution allows the accurate modeling of pumping tests and the estimation of aquifer parameters that are statistically significant and physically relevant

A Combined Hydrological and Hydraulic Model for Flood Prediction in Vietnam Applied to the Huong River Basin as a Test Case Study

A combined hydrological and hydraulic model is presented for flood prediction in Vietnam. This model is applied to the Huong river basin as a test case study. Observed flood flows and water surface levels of the 2002–2005 flood seasons are used for model calibration, and those of the 2006–2007 flood seasons are used for validation of the model. The physically based distributed hydrologic model WetSpa is used for predicting the generation and propagation of flood flows in the mountainous upper sub-basins, and proves to predict flood flows accurately. The Hydrologic Engineering Center River Analysis System (HEC-RAS) hydraulic model is applied to simulate flood flows and inundation levels in the downstream floodplain, and also proves to predict water levels accurately. The predicted water profiles are used for mapping of inundations in the floodplain. The model may be useful in developing flood forecasting and early warning systems to mitigate losses due to flooding in Vietnam

Analysis and Mapping of Rainfall-Induced Landslide Susceptibility in A Luoi District, Thua Thien Hue Province, Vietnam

Rainfall-induced landslides form an important natural threat in Vietnam. The purpose of this study is to explore regional landslide susceptibility mapping in the mountainous district of A Luoi in Thua Thien Hue Province, where data on the occurrence and causes of landslides are very limited. Three methods are applied to examine landslide susceptibility: statistical index, logistic regression and certainty factor. Nine causative factors are considered: elevation, slope, geological strata, fault density, geomorphic landforms, weathering crust, land use, distance to rivers and annual precipitation. The reliability of the landslide susceptibility maps is evaluated by a receiver operating characteristic curve and the area under the curve is used to quantify and compare the prediction accuracy of the models. The certainty factor model performs best. This model is optimized by maximizing the difference between the true positive rate and the false positive rate. The optimal model correctly identifies 84% of the observed landslides. The results are verified with a validation test, whereby the model is calibrated with 75% randomly selected observed landslides, while the remaining 25% of the observed landslides are used for validation. The validation test correctly identifies 81% of the observed landslides in the training set and 73% of the observed landslides in the validation set

Improved calibration of groundwater models using baseflow estimates for sub-catchments. Case-study: Upper Alzette, Luxembourg

Using MODFLOW the groundwater system of the Upper Alzette catchment was modelled. In general, calibration of groundwater models involves an inverse modelling strategy, using the measured groundwater heads in observation wells. An additional calibration strategy was employed considering the water balances of a number of subcatchments. Baseflow for each of the subcatchments was estimated and used as a criterion for an additional calibration. This analysis clearly showed that considering sub-water balances in the calibration process greatly improves the model results. The uncertainty of the model results was assessed, defining a spatially distributed ‘range’ for the simulated groundwater levels.status: publishe

Distributed recharged estimation for groundwater modeling using WetSpass model, case study - Gaza Strip, Palestine

Regional groundwater models used for analyzing groundwater systems (infiltration–discharge relations) are often steady state and, therefore, need long-term average recharge input. On the other hand, the spatial variation in the recharge due to distributed land-use, soil type or texture, slope, groundwater level, meteorological conditions, etc., can be significant and should, therefore, be taken into account. Hence, WetSpass was developed as a physicallybased methodology for estimation of long-term average spatial patterns of surface runoff, actual evapotranspiration, and groundwater recharge. The model is especially suitable for studying effects of land-use changes on the water regime in a basin. The computer model was integrated in the GIS ArcView. Its set-up is extremely flexible; it allows easy new definition of natural or man-made land-use types. This paper describes the concept of the model and gives an example of a developed WetSpass recharge map for the Gaza Strip, Palestine.status: publishe

Lineament extraction and analysis, comparison of LANDSAT ETM and ASTER imagery. Case study: Suoimuoi tropical karst catchment, Vietnam

Vast areas of the world consist of hard rocks (basement complexes), where water is restricted to secondary permeability, and thus to the fractures and the weathered zones. As the success ratio of drilling in hard rock terrain may be low, and the use of geophysics is often judged as too expensive, the study of lineaments from remote sensed imagery offers an attractive alternative analysis technique. High production areas in hard-rock aquifers are generally associated with conductive fracture zones. An effective approach for delineation of fracture zones is based on lineament indices extracted from satellite imagery. Together with a detailed structural analysis and understanding of the tectonic evolution of a given area it provides useful information for geological mapping and understanding of groundwater flow and occurrence in fractured rocks. The accuracy of extracted lineaments depends strongly on the spatial resolution of the imagery, higher resolution imagery result in a higher quality of lineament map. The ASTER sensor provides imagery with a higher resolution (15m) than the LANDSAT sensor (30m). It is tested and shown here that extracted lineaments from the VNIR ASTER imagery are considerably less noisy and show a higher accuracy than lineaments extracted from other imagery.status: publishe

Estimation of underground rivers in a tropical karst area by way of a multi-thematic study

This paper presents results of the estimation of underground rivers connecting the sinkholes and the resurgence of the Nam La blind river in the tropical, highly karstified limestone area of NW Vietnam. In the Nam La catchment, the Nam La River disappears underground in several sinkholes near the outlet of the catchment. A hypothesis is that the Nam La river resurges at a large cavern spring 4.5 km east of the catchment outlet. A multithematic study is carried out to prove possible connection between the sinkholes and the resurgence, including geostructural - tectonic analysis, cave structure analysis, hydrograph analyses and tracer experiments. Those underground rivers are used to better understand flooding records of the study area. The maximum water ponding volume in nearby dolines was estimated and shown to play an important role in the flooding mitigation of the catchment outlet area.status: publishe