Hydrogeological Assessment of the Gonabad Plain Qanats

Qanats are the most important device for the exploitation of groundwater in arid and semi-arid regions. The hydrogeology of Gonabad Plain Qanats was studied in this research. For this purpose, catchment area of Gonabad Plain Qanats was determined primarily. Water budget calculation in the catchment area shows that 3.7 MCM of groundwater more than catchment recharge and from aquifer storage was extracted annually which led to 0.06 meter per year groundwater level drawdown in catchment aquifer and decrease in qanats water bearing zone. To identify the effect of precipitation decrease on qanats discharge, Variations in catchment precipitation during 1997-2017 along changes in mean precipitation in four periods (five years period) were perused. Six major qanats of Gonabad Plain namely, Qasabeh, Bahabad, Baidokht, Rahn, Dizagh and Khesuie were selected, hydrographs of them during 1997-2017 were drawn and changes in hydrograph slopes in two specific periods were considered.The hydrograph slope of Qasabeh and Bahabad qanats become five and two times steeper during 2006-2017 compare to same in 1997-2006 periods respectively. Lack of dredging, dam construction and drilling of deep well in the catchment of these qanats has reduced the discharge of them. The hydrograph slope of Dizagh Qanat from the beginning has been constant almost 0.3 meter per year and this qanat has the lowest discharge decrease. High specific yield of aquifer in the qanat water bearing zone, absence of well in the qanat catchment and qanat recharge through groundwater infiltration in tunnels of upstream qanats have caused this qanat not to have high discharge decrease. The discharge of Gonabad Plain qanats has dropped from 6 to 20 liters per second during 1997-2017.