Citation: Indonesian Journal of Geography, Vol 39, December 2007, pp. 157-171 APPLICATION OF WATER TABLE FLUCTUATION METHOD TO QUANTIFY SPATIAL GROUNDWATER RECHARGE WITHIN THE SOUTHERN SLOPE OF MERAPI VOLCANO, INDONESIA by: Tjahyo Nugroho Adji (adji_tjahyo@ugm.ac.id ) Faculty of Geography, Gadjah Mada University, Jogjakarta ABSTRACT The southern slope of Merapi Volcano plays a significant role to recharge groundwater resources for Jogjakarta Municipality. Groundwater recharge, herein meant as the amount of effective precipitation that infiltrates into unsaturated zone and then percolates as a water yield to water table after exceeds the soil field capacity, is the main subject in this article. The objective of this research is to apply Water Table Fluctuation (WTF) method to spatially define the groundwater annual recharge. Four wells are selected to represent some geomorphologic units of southern Merapi Volcano slope. Then, four Automatic Water Level Recorders were installed in every well to record the yearly shallow groundwater table fluctuation with the intention of achieving the spatial rate of groundwater recharge. The magnitude of groundwater recharge is then defined by WTF method that requires groundwater fluctuation data and specific yield. The rate of groundwater recharge in every well confirms the varying value. However, it seems that geomorphologic unit may be principally control to the time series variation of shallow groundwater fluctuation that results in groundwater recharge characteristic. The volcanic slope unit (above 600 m asl) has the lowest water table fluctuation indicates the resistant comportment to the annual rainfall. This unit is characterized by the relatively high magnitude of recharge of approximately 4270 mm/year. RESEARCH BACKGROUND Groundwater recharge is the amount of effective precipitation that infiltrates into unsaturated zone and then percolates as a water yield to water table after exceed the soil field capacity (Simmers, 1988). Infiltration refers to water movement downward from the surface into the subsurface (Scanlon et al., 2002). In many unsaturated or soil zone studies, terminologies such as drainage, net infiltration, or percolation are often to describe water movement below the root zone, and these are often approached to groundwater recharge. In this paper, recharge is focused on downward water movement within unsaturated zone into a water table. Rushton (1997) distinguishes actual recharge, estimated from groundwater studies, and which reaches the water table, from potential