The effect of reservoir dam construction on groundwater fluctuations(Case study Reservoir Dam ShahreChay, Urmia Plain,IRAN)

The most distinguished effect of dams is change in the hydrologic regime of its downstream areas, which can be attributed to permanent supply flow and changes in the level of groundwater. The assessment of groundwater level changes is an important issue in analyzing hydrogeological time series of aquifers. Considering the different performance of effective factors in budget and hydrology in the catchment area of ShahreChay plain, it is necessary to review the trend analysis, which is complicated. Investigating temporal and spatial variations of groundwater levels in planning and managing sustainable water resources in arid and semi-arid regions is very importance. The purpose of this study were be to evaluation the changes in trend groundwater level of the aquifer piezometers in the ShahrChahi before and after the construction of the reservoir with the use of nonparametric methods of Mann-Kendal, the Sen's slope, Pattit, Boishand and comparind with the result of the MODFLOW software.

The methodology of this research was based on library, field, and statistical methods. In this study, 10 stations in ShahreChay aquifer were used to study the groundwater level changes from 15 years (1381-1396). Ground water level trend at all stations were evaluated annually and at minimum and maximum water levels of the aquifer. The groundwater model is provided using the Modflow code in the GMS software. In order to investigate the longitudinal groundwater slope changes, the Kriging method was used in GIS 10.5 software.

Research shows that the trend of ground water level changes in the 80% of piezometers was negative and a failure occurred in the time series in 1386. At the stations closer to ShahrChay Dam, it is usually the year of jumping from 1386 to 1387. It is usually the year of jumping after 1387, 89, or 90 at far and near the riverside and far off the rivers. In Modflow software, with using piezometric wells and other studies to predict the level of groundwater level in the stable state of the dam calibration and prediction was performed. Data from June 1381 were used to prepare the model in a steady state Modflow software was used to predict the level of groundwater level in the steady state of the dam calibration and prediction. For the years 1383 to 1385, groundwater level was predicted with less error. Continuing calculations, the groundwater level of observation by 1396 is less than predicted. Also, the study of changes in 15 years isopotential lines indicates a decrease in the slope of the basin due to the lack of nutrition due to the construction of the dam

The results show the effect of reducing the surface water level caused by the dam construction. One of the reasons is the water storage in the dam at flood time. How to use the stored water in the crop season conditions that are directly used before construction the dam the flood distributed in area and causes an increase in the level of ground water that construction dam haven't been caused the recharge in the groundwater.