ارزيابي تأثير تنش‏هاي خشكي بر سيستم آب زيرزميني دشت باغملك با تغيير در الگوي بهره‌برداري

Evaluating the Impact of Drought Stresses on Groundwater System in Bagh- Malek Plain by Discharge Pattern Changes

آب زیرزمینی بزرگ‌ترین منبع تأمین آب محسوب می‏گردد و كمبود منابع آب سطحی در شرایط خشكسالی موجب توسعه برداشت از آب زیرزمینی شده است. در تحقیق حاضر، مدل مفهومی آبخوان باغملك در چارچوب روش عددی تفاضل محدود برای شبیه‏سازی رفتار جریان آب زیرزمینی استفاده شده است. داده‏های سطح ایستابی 8 پیزومتر در طول 10 سال از 1381 تا 1391 بدین منظور جمع‏آوری شد. سال‏های مورد مطالعه به 40 دوره تنش تقسیم و هدایت هیدرولیكی، آبدهی ویژه و تغذیه در این دوره‏ها واسنجی شد. صحت‏سنجی رقوم هیدرولیكی شبیه‏سازی نسبت به اندازه‏گیری برای سال بعد از واسنجی انجام گرفت. نتایج شبیه‌سازی جریان نشان می‌دهد واریانس اختلاف سطح ایستابی مشاهداتی و محاسباتی در فرایندهای واسنجی و صحت‏سنجی به ترتیب 1/3 و 84/3 محاسبه شد. پهنه‏بندی هدایت هیدرولیكی واسنجی‏شده نشان می‏دهد شمال شرقی آبخوان قابلیت انتقال و پتانسیل برداشت بیشتری در مقایسه با بخش‏های جنوبی دارد. حساسیت مدل جریان به كاهش هدایت هیدرولیكی بیش از افزایش آن است. 50 درصد كاهش در هدایت هیدرولیكی موجب افزایش واریانس اولیه از 1/3 به 44 می‏گردد. هم‌چنین نتایج نشان داد مدل آبخوان باغملك به ترتیب نسبت به تغذیه، هدایت هیدرولیكی و آبدهی ویژه حساسیت بالاتری دارد.

Introduction: Groundwater is the largest resource of water supplement and shortages of surface water supplies in drought conditions that requires an increase in groundwater discharge. Groundwater flow dependson the subsurface properties such as hydraulic gradient (water table gradient or head loss in artesian condition) and hydrodynamic coefficients. The flow treatment is analyzed with an accurate estimation of effective parameters in groundwater equation. This function is obtained using the continuous equation. Inlet and outlet flows of a cell are equal to storage amounts in the continuous equation. Analytical solution of this equation is complex, so numerical methods are developed including finite element and finite difference methods. For example, Feflow is a groundwater modeling tool that makesuse of finite element method (Reynolds and Marimuthu, 2007). Modflow as a finite difference three-dimensional model simulated underground flow under steady and unsteady conditions in anisotropic and non-homogeneous porous media. Modflow is designed to simulate aquifer systems in which saturated-flow conditions exist, Darcy’s Law applies, the density of groundwater is constant, and the principal directions of horizontal hydraulic conductivity or transmissivity do not vary within the system. In Modflow, an aquifer system is replaced by a discretized domain consisting of an array of nodes and the associated finite difference blocks. Groundwater modeling and water table prediction by this model have the acceptable results, because many different informations of water resource system are applied. Many people and organizations have contributed to the development of an effective groundwater monitoring system, as well as experimental and modeling studies (Lalehzari et al., 2013). The objective of this paper is investigation of hydraulic and physical conditions. So, a numerical model has to be developed by PMWIN software for Bagh-i Malek aquifer to calculate hydrodynamic coefficients and predict water table in the future. Materials and Methods: Bagh-i Malek aquifer located in Khuzestan province is mainly recharged by inflow at the boundaries, precipitation, local rivers and return flows from domestic, industrial and agricultural sectors. The discharge from the aquifer is through water extraction from wells, springs, and qanats as well as groundwater outflow and evapotranspiration. In this study, conceptual model of Bagh-i Malek aquifer on the framework of finite difference numerical approach has been used in simulating groundwater flow treatment. Water table data of 8 piezometers was collected for the 10 year duration from 2002 to 2012. The study years are divided into 40 seasonal stress periods with daily time step. Hydraulic conductivity, specific yield and recharge were calibrated in these periods. Verification was made between the simulated and measured hydraulic heads in the next calibration year. To simulate the groundwater table elevation in this study area, the PMWIN model is used. Bagh-i Malek aquifer is considered as a single layered aquifer, and therefore only the horizontal hydraulic conductivity is estimated. Modflow was used to simulate both steady state and transient flow systems. In steady conditions it is assumed that the total of time simulation is a time period and it does not change inlet data in the modeling duration. In unsteady conditions,the duration of study is divided into some stress periods that data is changed in every period. Results and Discussion: Estimation of hydraulic conductivity is the first step of calibration process at steady state conditions. The correct assignment of hydraulic conductivity has a main effect on other parameters accuracy. Hydraulic conductivity mapping indicated that the maximum values are in the Eastern North (6-7 m/day) of the aquifer. The twice calibrated parameter is specific yield in unsteady conditions. Specific yield mapping indicated that the values vary from 0.03 to 0.08 and are maximum in the Southern regions of the plain similar to hydraulic conductivity. The results confirm that the flow model has the tolerable simulation accuracy by variances of 3.1 and 3.84 in calibration and verification processes, respectively. The sensitivity of the flow model to decreasing the hydraulic conductivity is more than increasing it. 50 percentage declined into the hydraulic conductivity causes the increase of the variance from 3.1 of initial value to 44. Conclusions: Mapping of calibrated hydraulic conductivity showed that the Eastern North of aquifer has the higher transmissivity and discharge capability in comparison to Southern parts. At last, the result show that the Bagh-i Malek aquifer model is sensitive to recharge, hydraulic conductivity and specific yield, respectively.