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Alluvial aquifer systems are typically considered to provide significant amounts of groundwater in many parts of the world. Being mostly composed of sand, silt and gravel, these systems serve as ideal locations for drilling wells for domestic,

Department of Environmental Engineering, Dokuz Eylul University, Izmir, 35160 Turkey e-mail: [email protected]

C. Simsek

Department of Drilling, Torbali Technical Vocational School of Higher Education, Dokuz Eylul University, Izmir, 35860 Turkey agricultural and industrial water supply. Based on their lithological characteristics, alluvial systems are usually situated in lowland areas, particularly on plains and floodplains of river systems, which are also under the influence of anthropogenic stresses due to settlement, agricultural production and industrial development. Morphologically, these systems are generally surface aquifers that are characteristically recharged from surface infiltration. Consequently, water in surficial alluvial aquifers is typically short-circulated groundwater that has a fairly short recharge-discharge cycle. In addition, alluvial systems are not only the mostly used groundwater resources of the world but also rank very high in the list of water resources that is influenced from variations in precipitation patterns as a result of climate change.

The relationship between recharge and discharge in alluvial aquifers is quite dynamic. Many times, water level fluctuations in these systems demonstrate cycles not longer than one season. The decline and rise of water levels resemble a characteristic sinusoidal pattern. The slopes of the declining and rising curves as well as the difference between their crest and base are strongly related to aquifer characteristics (i.e., hydraulic conductivity, specific yield), recharge characteristics (i.e., precipitation-infiltration ratio, land use and land cover) and water extraction characteristics (i.e., number, capacity and configuration of wells, withdrawal rates). The changes in climatic pattern particularly influence the amount and extend of precipitation, which in turn affects the infiltration rates and ultimately the recharge amounts. Furthermore, the increasing dependency of communities on relatively more stable groundwater resources is also creating an extra pressure on groundwater thru ever-increasing extraction patterns. Hence, changes in climatic patterns and precipitation amounts influence alluvial groundwater systems not only from a decreased supply rate perspective, as occurs in many parts of the world, but also from an increased demand point of view.

Based on these fundamentals, this study focuses on the Torbali-Bayindir Plain that is situated in Kucuk Menderes River Basin to the south of Izmir-Turkey (Fig. 25.1) to analyze the trends in precipitation and groundwater levels. The plain is a part of a larger east-west directed graben system, on which intense agriculture and industrial development is currently present [1]. The water demand in this region is completely supplied from shallow groundwater through wells drilled in the alluvial aquifer, whose thickness could reach up to 100 m. A total of 20 groundwater monitoring wells with monthly level data extending back to 1970s are used to assess the fluctuations in groundwater levels. A more intensive monitoring activity is also conducted in one well equipped with automatic level recorder, which provided an inside look into more intermittent data from other wells. Moreover, daily precipitation data from Izmir and Adnan Menderes Airport meteorological stations are used to compute monthly and yearly precipitation totals that would mostly characterize the precipitation patterns in the study area. Groundwater level data is used to detect the general annual declining pattern in the plain. The relationships between precipitation and groundwater

» VLfelli Alluvial P13N-

Fig. 25.1 Location map of the study area

» VLfelli Alluvial P13N-

Fig. 25.1 Location map of the study area level are then used to evaluate the response of the alluvial aquifer system in Torbali-Bayindir plain.

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