Minimum water flow required for Zayandehroud to maintain hydrological balance in the city of Isfahan

As urban agglomerations grow, they face management challenges in terms of providing water and sanitation services to an ever-growing population, the impact of urban development on natural resources and the effects of extreme weather events. Simulation scenarios of future climate change using Global Circulation Models showed that the surface water supplies in Central Asia will be reduced due to reduced thickness of glaciers. Iran is also one of those arid regions. Therefore, the performance of management operation on water resources is mandatory. Coordinated management of the surface and groundwater resources guarantee the water supply in face of water stress situations. One of the most important instances of management plans is to estimate the relationship between these two sources of water. The groundwater recharge methods vary from small-field scale to large irrigation schemes and from simple water balance estimations to complex recharge models. In this research, for the first time, the isopotential map and Lohman method have been used for calculating the recharge and discharge between Zayandehroud River and the Isfahan aquifer. The semi-arid Zayandehroud River Basin is one of the most strategic river basins of Iran. The Zayandehroud River, as one of main river in central Iran is selected as the domain for the present study. Originated from the Zagrous Mountains, the Zayandehroud River with an average flow of 1400 million cubic meters (MCM) per year is the most important available water resource for domestic, industrial and agriculture consumptions in the basin. The study area includes an unconfined aquifer which its slope is from the South to the North and West to East. The elevation of Isfahan is about 1570 meters above sea level and its annual rainfall is about 160 mm. Zayandehroud River enters in the west area of the city, with water level of height around 1587 meters and exits in the East of Isfahan with water level of height 1560 meters. Over recent years in Zayandehroud basin, excessive amounts of surface water and groundwater have been used for different consumptions. Therefore, Zayandehroud River is alternately dry and wet periods. This phenomenon has been significantly affected the hydrologic balance of the basin and resulted in marked decrease in groundwater level. In this study, in both dry river and wet conditions of the Zayandehroud River in the Isfahan reach, the groundwater table was measured at 23 points. Then, using the geophysical and geotechnical data sets, the hydraulic conductivity of the aquifer was determined. Finally, using the Lohman equation, the amount of water transported from each aquifer section was calculated. The difference between flow rate for mentioned periods (i.e. River water less and River wet periods) is the role of Zayandehroud River in recharging the Isfahan aquifer. The results show, when the river was dry, the recharge point almost is relevant to southern part of Isfahan and the discharging point is related to the north western and north eastern parts of the city. In this situation, the river plays no role on Isfahan aquifer. In the river wet period, the river recharges Isfahan aquifer in both sides, from upstream to Shahrestan Bridge area. After Shahrestan Bridge, the recharging behavior of the river is the same as dry period. According to the obtained results in the dry period of the river, the groundwater flow rate in Isfahan aquifer in the area of Zayandehroud from southern part of the aquifer to its northern part is about 7356 m3/day. In contrast, when water flows in the river (i.e. river wet period), the above-mentioned groundwater flow rate has been increased to about 62384 m3/day. Therefore, the recharging rate of the river to the aquifer is about 55028 m3 /day. In other words, the recharging flow rate from the river to the aquifer in the region of the Isfahan city is about 0.64 m3/sec. Therefore, a perennial flow rate equal to 0.64 m3/sec in the Zayandehroud River can prevent hydrological imbalance and its impacts such as land subsidence, water shortage and salinization of groundwater. These results represent a strong and effective relationship between groundwater and the Zayandehroud River, which more emphasis on the role of recharging by the River to the groundwater, and wet period of the River plays a significant role on the aquifer of Isfahan city.