Simulation of Delfan- Lorestan Aquifer and Investigation of Management Scenarios by Using MODFLOW Model

Introduction : Sustainable development of groundwater resourcesrequires a proper assessment of available resources, understanding of system behavior and interaction between groundwater and surface water.In recent years, a Delfan plain (in Iran) is facing a sharp decline in groundwater levels due to increasing in population and exploitation of groundwater resources.In this study, using modflow model effect of drought and wet conditions on water table fluctuations of Delfan plain aquifer was evaluated. Materials and Methods: Delfan plain is one of the Lorestan Plains (in Iran Country) and located in the north of the Lorestan Province, around the city ofNurabad (Delfan).Precipitation survey of the region shows that the average annual rainfall in the plains is 480 mm and aquifers of the region has 10 piezometric wells. Drawing of the groundwater hydrograph from 2004 to 2013 shows that the general trend of the groundwater level is downward, which represent decreasing in groundwater resources of the region. At the beginning of the modeling process using Modflow model, after gathering all the required information, conceptual model of the plain was generated. To preparing this model, various data such as topographic maps, geophysical data, logs of wells, pumping tests and observation wells data and flow data taken from exploitation wells was used. Water level data of October 2007 which has the lowest fluctuation was used for the calibration of steady state.In this step with model successive run, hydraulic conductivity is optimized. After model calibration in the steady state, do same in the unsteady state.Specific discharge was optimized at this step.After calibration in the unsteady state, model needs verification to be trusted.For this purpose, verification was done in November 2012 to November 2014.After calibration and validation of the model, the model was carried out under drought and wet conditions.Drought is one of the environmental disasters that its occurrence could bring the water challenges in the field of quality and quantity. Because of drought and lack of rainfall affect groundwater resources, soil moisture and river flow, used index called Standard Precipitation Index (SPI) to quantify the impact of rainfall in of 3, 6, 12, 24-month period.This index is calculated based on long-term statistics. Results Discussion :In steady state, the model's sensitivity is studied according to changes in hydraulic conductivity value and discharge of pumping wells and in the unsteady state according to specific yield and other parameters was investigated. Based on this analysis in steady state, generally it can be said that the model is more sensitive to the exploitation wells. In unsteady state, the model is more sensitive to specific yield and hydraulic conductivity and other parameters are in the next level.With SPI reviewing of 120-months, it was seen that the plain in 1984 and 1993 has the lowest 120-month SPI with the value of -1.08 (with average precipitation value of 423 mm).For applying virtual wet period with 30-years precipitation reviewing, it was observed that years of 2001 and 2010 have the most 120-month SPI value with value of 1.86 (with average precipitation value of 587 mm).For applying the virtual wet conditions in the next step, the model was simulated with the rainfall data of 2001 and 2010.To decrease the water table drop, considering the amount of drop and water needs of the region, several runs were performed which ultimately results showed to offset the drop in these three exploitation areas, the discharge of exploitation wells must be reduce 20% that This strategy is able to reduce the average annual rate of water table drop for the next 10 years. Finally, after model’s run and piezometers drop, plain model was used to obtain groundwater balance. Conclusion: The model implementation in drought and wet conditions shows that in these conditions the groundwater level decreases with the average of (-7.80m) and (-5.83m), respectively. which with the 20 % decrease of the discharge of the exploitation wells in these conditions, the level groundwater and aquifer balance improves.For the next ten years in the normal condition or present situation of exploitation, plain balance is -83.20 million cubic meters which by 20% reduction in wells exploitation, this water balance is predicted -41.20 million cubic meters for next 10 years.In the drought conditions Delfan aquifer water balance is predicted as -91.20 million cubic meters during ten years which by 20% reduction of wells exploitation this water balance increases to -49.20 million cubic meters.