Investigating the effects of groundwater level changes on GNSS observations in the Konya Closed Basin

Abstract

The Konya Closed Basin (KCB) that inhabited by approximately 4 million people has a semi-arid climate and has been an important agricultural and economic production area with its alluvial fertile soils. In recent years, the use of groundwater as a main source in the intense need for irrigation in the basin due to climate change, drought and wrong agricultural policies triggered land subsidence in the basin by causing the aquifer system compaction. In this study, the effects of groundwater level changes obtained from groundwater monitoring stations on the positions of continuously operating GNSS stations located in KCB were investigated by time series analysis. As a result of trend component analyses of time series processed based on the ITRF08 fixed frame, it was determined that the horizontal positions of stations in this region were moving in the northeast direction (18.88 mm/year). For the up coordinates, while stations ANRK, KLUU, and NIGD had movements (0.56 mm/year) in the up direction, the other stations moved in the down direction. The vertical movement of KNY1 station based on 560-day data was in the down (-) direction, with an annual movement of 70.96 mm, and this was the maximum movement or velocity compared to the other stations in this region. The trend analysis of 19 wells revealed linear groundwater level changes in the down direction with a mean value of -39.22 cm/year. The decrease in the water level of the wells in the Konya Closed Basin was statistically significant for the majority of the wells and at meter level in some wells. It was determined that the linear decrease was generally caused by the reduced level of groundwater in this region. The periodic component analysis showed that the movements of the GNSS stations changed from a day to a year due to several factors, such as atmospheric and hydrological loading and climatic effects.