Groundwater and Surface Water Interaction in the Downstream Part of Luk Ulo River, Kebumen Regency, Central Java']Java Province, Indonesia

Water resources are an important factor in the lives of living organisms. Water resources include groundwater and surface water. The interaction between surface water and groundwater is an important component of groundwater cycle research (Winter et al., 2003; Meng et al., 2019). The LukUlo Watershed has an area of 675,53 km(2). The groundwater and LukUlo River conditions differed between the upstream and downstream areas. These different conditions can potentially affect the physical quality of groundwater and surface water, particularly in the downstream area of the Luk Ulo River. This study aimed to determine the interaction between groundwater and surface water in the lower reaches of the Luk Ulo River. Understanding the interaction between surface water and groundwater in an area is required to better manage watersheds. (Khan and Khan, 2019). The Global Information System (GIS) method was used in this study. GIS applications interpolate groundwater conditions and Total Dissolved Solids (TDS) content to determine the direction of groundwater flow and the distribution of TDS content. In addition, water discharge measurements were carried out in several rivers to ascertain the interaction between groundwater and surface water in the downstream portion of the LukUlo River. Based on the results of data analysis, it was concluded that the nature of groundwater to surface water in the lower reaches of the LukUlo River is effluent. From the condition of the direction of groundwater flow towards the LukUlo River, the distribution of TDS values for most groundwater is higher than surface water. Meanwhile, in measuring the surface water discharge, the average discharge is 26.38 m(3)/s, higher than groundwater. The effluent's nature can cause the cliffs on the river walls downstream of the LukUlo River to become prone to landslides when conditions are saturated or during the rainy season. Therefore, spatial planning is needed to sterilize the area around the riverbanks from buildings to minimize casualties in a landslide disaster.