Groundwater recharge estimation using groundwater level fluctuation patterns in unconfined aquifer of Yogyakarta City, Indonesia

Water demand in the world increases rapidly every year, especially in urban areas due to population growth and improved economic conditions. The largest freshwater source on earth is groundwater that can be utilized and easily obtained. However, the number of groundwater resources is very dynamic, depending on the recharge and discharge, including groundwater extraction. Increasing groundwater abstraction will cause a decline in groundwater level and seawater intrusion in the coastal area. Groundwater level fluctuation trends can be used to estimate groundwater recharge for optimum utilization. Therefore, this study aims to find the trend of groundwater level fluctuation related to rainfall to estimate the groundwater recharge in Yogyakarta City, Indonesia. The study was carried out by measuring the groundwater level of four monitoring wells that spread evenly in Yogyakarta City from 2011 to 2017 every month and comparing the result to monthly rainfall and annual groundwater abstraction from hotels in Yogyakarta City. The annual trend of rainfall and groundwater level fluctuation analysis using the nonparametric Mann-Kendall test and analyzing the trend's magnitude using the nonparametric Sen's method. The groundwater recharge was estimated by using the water table fluctuation (WTF) method. The study results showed that the trend of rainfall and groundwater level fluctuation is positive in the city's western and eastern parts. However, the groundwater level fluctuation in the city's central and southern parts is a negative trend due to groundwater over-exploitation by hotels. Groundwater recharge is estimated at around 158 up to 538 mm/year based on the WTF method. Higher groundwater recharge was identified in the city's central and eastern parts due to the high contribution from urban wastewater recharge.