The Positive Effect of Micro-Dams for Groundwater Enhancement: a Case Study around Tsinkanet and Rubafeleg Area, Tigray, Northern Ethiopia

The government of Tigray Regional State, Northern Ethiopia has been conducting a massive construction of micro-dams (small reservoirs) in order to decrease the rainfall dependency and alleviate food insecurity in drought prone areas of the region. Tsenkanet and Rubafeleg reservoirs are examples of this endeavor. The purpose of this investigation is to evaluate the role of these micro-dams in enhancing the surrounding groundwater by artificial recharge. In this study the physical characteristics of the reservoir are discussed including the groundwater surface water relationship with respect to water level elevation and also hydro-chemical composition. Six water samples have been analyzed. Samples are taken after rinsing the plastic bottles with distilled water and the sample to be taken. One surface water sample from each reservoir and one groundwater samples from each well next to each reservoir has been collected. Moreover, one sample from spring and one from river has been analyzed. The chemical composition of the reservoir water and the surrounding groundwater seem to suggest a similar origin and to be resulting from similar hydro-chemical processes. The chemical composition of all samples is found to be of the CaHCO3 type, which could be explained as resulting from precipitation water in which the mineral calcite has been dissolved. Both the topographic evidence and the water level monitoring data has confirmed the feeding of the reservoir to the near by shallow groundwater system. Quantifying the amount of groundwater accretion by the use of modeling and water balance method is recommended. Keywords: Groundwater, Surface water, Topography, Water Level, Type, Hardness, Class

Advances in understanding natural groundwater quality controls in coastal aquifers

Groundwater quality in coastal aquifers is largely influenced by the interaction between the sea and the bordering aquifer systems. This interaction can result in freshening of saline aquifers or salinization of fresh water bodies. In complex cases even both situations can be found in the same aquifer system. While the main mechanism for salinization or freshening is hydrodynamically driven (groundwater flow), also physical and chemical processes within the aquifer will alter groundwater composition. Cation exchange is in many cases an important process to consider; it results in a hydrochemical spectrum of groundwater types reflecting both the hydrodynamical and hydrochemical characteristics of the aquifer.In order to understand the natural groundwater quality and the controlling processes in a coastal aquifer, it is crucial to known both the hydrodynamical and hydrochemical behaviour and the way these are linked together, because usually only the interaction between them can explain the observed quality istributions.Where mixing of fresh and salt water occurs, density-driven flow may become important and change quality distribution, while the groundwater composition itself influences hydrodynamics. An integrated approach of both aspects is indispensable.Modelling is an important tool in understanding how aquifer systems work. Until recently, hydrodynamical and hydrochemical aspects were tackled separately, with different models. Hydrodynamical aspects have been studied with flow models (such as MODFLOW) or a combination of a flow and a solute transport model (such as MT3D) for simulating salt transport, coupled together for incorporating density-driven flow (such as in SEAWAT). On the other hand, hydrochemical aspects have been investigated with hydrogeochemical models based on speciation and a reaction scheme. In hydrogeochemical models, groundwater flow was either not taken into account, or, at best, it was considered in a very simplified approach (along a path-line). Recently, codes have been developed combining three-dimensional flow models fully with the flexibility of an extensible hydrochemical model based on thermodynamic databases, such as PHAST, which combines the well-known PHREEQC model with the HST3D model for flow and transport. Simulations with the PHAST model have shown that it is capable of simulating the whole hydrochemical spectrum of groundwater types in coastal aquifers under freshening and salinizing conditions incorporating cation exchange processes and mineral equilibria such as calcite dissolution. The capabilities of this new generation of models will also allow for including redox components (iron content of the water) or processes such as surface complexation (e.g. sorption onto hydroxides). The future for such models looks bright, because for the first time they will provide the complete groundwater composition as their output.However, models need to be based on good and sufficient field data! Without them the reliability of the models is unknown and their value for predictive purposes is hypothetical. Therefore the advances in modelling should go together with new techniques of sampling, measuring and monitoring and with improved analytical methods. Advances have been made also in these fields. Yet, the most important advance in understanding natural groundwater quality is not related to a specific technical innovation but rests in the minds of hydrogeologists. Indeed, only a profound insight in the combined hydrodynamical and hydrochemical aspects by a refined system analysis can provide the key to really understand what controls groundwater quality, also in coastal aquifers

Ranking of water-table depths for purposes of ecosystem management in the coastal dunes of Belgium

Integrated monitoring was conducted to evaluate the result of nature development actions to increase the biodiversity. During the first year, the natural fluctuation of the piezometric level has been determined based on existing data and newly gathered data. From these data, the groundwater table classes following a classification, developed in The Netherlands, have been deduced. However, the result does not correspond with the observed vegetation. To predict the natural habitats and ecosystems in the coastal dunes of Belgium, a new classification adapted to natural conditions in the area is required. Advantages of existing approaches are integrated in the classification, combined with new elements, to represent the relationship between groundwater regimes and ecosystems. This new classification, consisting of 4 codes, provides information about the mean high water table depth, the mean low water table depth, mean spring water table depth and whether inundation can take place or not. It also gives an indication of the variability of the seasonal fluctuations of the water table and the importance of the fluctuation of the water table between years. With this new classification, a fine breakdown by ecotypes is possible. The new classification has been applied to the existing data in the study area

Performance analysis of priority queueing systems in discrete time

The integration of different types of traffic in packet-based networks spawns the need for traffic differentiation. In this tutorial paper, we present some analytical techniques to tackle discrete-time queueing systems with priority scheduling. We investigate both preemptive (resume and repeat) and non-preemptive priority scheduling disciplines. Two classes of traffic are considered, high-priority and low-priority traffic, which both generate variable-length packets. A probability generating functions approach leads to performance measures such as moments of system contents and packet delays of both classes

Inverse chemical modeling and radiocarbon dating of palaeogroundwaters: The Tertiairy Ledo-paniselian aquifer in Flanders, Belgium.

Groundwater samples from the Ledo-Paniselian aquifer have been interpreted for chemical reaction patterns

Spatial and temporal variability of groundwater recharge in Geba basin, Northern Ethiopia

This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ This author accepted manuscript is made available following 24 month embargo from date of publication (July 2017) in accordance with the publisher’s archiving policyWetSpa, a physically based, spatially distributed watershed model, has been used to study the spatial and temporal variation of recharge in the Geba basin, Northern Ethiopia. The model covers an area of about 4, 249 km2 and integrates elevation, soil and land-use data, hydrometeorological and river discharge data. The Geba basin has a highly variable topography ranging from 1000 to 3280 m with an average slope of 12.9%. The area is characterized by a distinct wet and long dry season with a mean annual precipitation of 681 mm and temperatures ranging between 6.5 °C and 32 °C. The model was simulated on daily basis for nearly four years (January 1, 2000 to December 18, 2003). It resulted in a good agreement between measured and simulated streamflow hydrographs with Nash-Sutcliffe efficiency of almost 70% and 85% for, respectively, the calibration and validation. The water balance terms show very strong spatial and temporal variability, about 3.8% of the total precipitation is intercepted by the plant canopy; 87.5% infiltrates into the soil (of which 13% percolates, 2.7% flows laterally off and 84.2% evapotranspired from the root zone), and 7.2% is surface runoff. The mean annual recharge varies from about 45 mm (2003) to 208 mm (2001), with average of 98.6 mm/yr. On monthly basis, August has the maximum (73 mm) and December the lowest (0.1 mm) recharge. The mean annual groundwater recharge spatially varies from 0 to 371 mm; mainly controlled by the distribution of rainfall amount, followed by soil and land-use, and to a certain extent, slope. About 21% of Geba has a recharge larger than 120 mm and 1% less than 5 mm

A discrete-time HOL priority queue with multiple traffic classes.

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