Groundwater is one of the main sources to supply drinking water, especially in arid and semi-arid areas. The interaction of surface water and groundwater is one of the important issues in groundwater management and modeling, and it is essential to understand. This research aims at investigating river and groundwater interactions in the Dezful-Andimesh plain, which is about 2073 Km2 as the largest agricultural plain in the northern part of Khuzestan province, southwest of Iran. This plain contains five smaller plains: western Dez, eastern Dez, Sabili, Deymche and Lor. The Dez, Karkhe, Balaroud, Kohnak and Shavoor rivers are located in the study area. Thus this area is very important for supplying water needs and requires an efficient water resource management, a correct planning and reconsideration of water policy. Only 4% of the region is classified as the urban area and the remaining are agricultural plains. All the plains have irrigation networks except the Lor plain. Although, the irrigation networks supply a great part of the crops demand, in some regions groundwater use is also significant. Wheat, sugarcane and maize (32%, 21% and 16%) are the most common crops in this region. The cultivation period of wheat is usually from November to May, so the maximum recharge of groundwater by irrigation networks occurs in this period. The study area has a semi-arid climate with the mean monthly temperature of 36.5°C in July and 11.8°C in January. The precipitation varies from 296 mm in eastern Dez to 394 mm in Lor. The Dezful aquifer with an average thickness about 100 m is an unconfined aquifer system. There are over 2700 wells which pump about 500 million m3 per year, in this area. The general direction of groundwater flow in the area is from the north to the south. Recharging the aquifer is via direct rainfall infiltration and return water from irrigation networks. Hydraulic conductivity range is from 14 m/day for clayey sediments in Sabili to 49 m/day for sandy deposits, particularly in Lor. In order to model the groundwater, a MODFLOW programming code was implemented in a GMS environment. MODFLOW is a numerical model that is one of the best methods for assessing the quantity and quality of groundwater. The numerical models are difficult and time-consuming. However, in the recent decades' research using simulation models have been developed due to the improvement of high-speed computers. Information related to steady-state flow along river information was used in the model. Water level and bed river data were measured in river-cross sections and hydrometric stations in different months. The model was configured to a steady-state condition in October (2009) and was calibrated using 62 groundwater wells. Also, the model was calibrated in an unsteady-state condition for five years (October 2006 to September 2011). Then, the model was validated for the period of 2011-2012.
According to the results, the regulated rivers such as Karkhe and Dez were the major source of recharging aquifers; however, there was a decrease in their contribution to aquifers in wet seasons. There was an increase in groundwater level during wet seasons due to the groundwater recharge from rainfall and irrigation systems. However, there were no significant changes in the water level of the regulated rivers, and the minimum water level was observed in November. Therefore, an increase in groundwater level could not be related to rivers when the river levels were constant, during the wet season. The Balaroud non-regulated rivers generally drained the aquifers during dry seasons, but not during wet seasons (February), when recharge occurred from these rivers to aquifers. The rivers such as Shavoor and Kohnak and their tributaries drained aquifers in every month. There were not increasing or decreasing patterns in groundwater discharge when there was a change in both rivers and groundwater levels with different patterns in different months of a year.
According to the simulation results over six years, the most river- groundwater interaction was belong to Dez and Karkhe rivers with the highest amount of recharge to aquifers. The Shavoor and Kohnak rivers and their tributaries generally drained groundwater. However, there was no specific pattern of river-groundwater interaction for the Balaroud seasonal rivers. There was more groundwater recharge through regulated rivers in dry seasons compared to wet seasons which was due to the high amount of rainfall (along with the amount of water from irrigation systems) in wet seasons.