بررسی تغییرات زمانی و مکانی سطح آب زیرزمینی نسبت به فاصله از رودخانه زاینده‌رود (بررسی موردی: دشت کوهپایه- سگزی اصفهان)

As an indispensable part of water resources, groundwater plays an important role in the ecological environment. This is especially true in arid and semi-arid regions, where changes in groundwater level and hydrochemistry have significant impacts on ecological processes. The interactions between groundwater and surface water are complex. Groundwater and surface water are not isolated components of the hydrologic system, but instead interact with a variety of physiographic and climatic landscapes. River flow changes can influence recharge of regional groundwater aquifers, causing shifts in groundwater levels. Actually, The river plays an important role in recharging groundwater. Streams interact with ground water in all types of landscapes. The interaction takes place in three basic ways: streams gain water from inflow of ground water through the streambed (gaining stream), they lose water to ground water by outflow through the streambed (losing stream), or they do both, gaining in some reaches and losing in other reaches. Therefore, an understanding of the basic principles of interactions between groundwater and surface water is needed for effective management of water resources. Also, climate fluctuations are probably one of the most challenging pressures facing hydrological systems and water resources. Moreover, population growth, expanding areas of irrigated agriculture and economic development are drivers for an ever-increasing demand for water worldwide. Because of these reasons, most plains of Iran have been confronted with declining groundwater levels. Koohpayeh- Segzi Plain is the largest plain in Zayandehrud basin and groundwater is the main resource for supplying the water demand in this plain. Then the aim of this study was to evaluate groundwater resources and relationship with surface water and climate variables in the Koohpayeh- Segzi Plain in the time period of 1995-2009.
In this study, the data of 59 groundwater observation wells along the river, 2 hydrometric stations at the beginning and ending of the plain and 14 meteorological stations in and around of the plain have been used. Parameters of stream flow to the region, precipitation, evaporation, mean temperature and deflation from the water table have been considered and assessed the water table reaction rather to different parameter variations for years of 1995 and 2009 as well as in the different months of these two years. Accordingly, using interpolation methods such as Kriging, Co– Kriging, Inverse Distance Weighting (IDW) and Radial Basis Functions (RBF) as a technique to convert point data into the area, amount of input climate variables (precipitation, evaporation and mean temperature) has been obtained in the well place. Also, because of distribution wells around of the river, the wells were divided into 8 classes based on distance from the river (0-2, 2-4, 4-6, 8-10, 10-15, 15-20 and >20).
The results showed that groundwater level has been declined 3.5 m on average during the considered period. This declination rate has increased around the Zayandehrud River, especially in 30 kilometers of the first part of the river (such as Sharq Gavdari Foka, Shydan Baraan, Amin Abad Qahab and Hasan Abad) and have reached to over 14 meters. Also assessing of climate variables trend by Mann-Kendall tests showed no significant trend in the most of meteorological stations. Therefore, changes of climate variables cannot explain the sharp drop of water table in this plain. Furthermore, the results showed that the water level of the wells located within the 6 km strip width of the river are strongly influenced by river discharge fluctuations during the year, and with increasing the distance from the river is reduced. Actually the results indicated that there was a significant relationship between the groundwater level and river discharge (surface water), as the river discharge is the main factor of supplying groundwater in this plain. A drastic reduction in input flow at the beginning of the plain has caused increasingly pressure on the groundwater resources (near the Choom bridge) for agricultural activities. At the end part of the river (near the Varzaneh bridge) by decreasing stream flow and consequently declining of water table and quality of groundwater resources, agricultural activities have been disrupted that makes less declining rather than the initial region and causing many socio-economic problems in the Koohpayeh- Segzi Plain. Finally, increasing of groundwater level and improvement of groundwater quality have aided greatly in restoring the environmental ecology and improving the quality of life that have been destroyed in recent years.