THE CHARACTERISTICS OF INFILTRATION ON THE SOUTHERN FLANK OF MERAPI VOLCANIC PLAIN, YOGYAKARTA, INDONESIA

The broad development of built areas has decreased infiltration areas, which in turn gives impacts on water table subsidence. The infiltration capacity and distribution are factors that need to consider in preserving the groundwater. This research aims to examine the infiltration rate, infiltration capacity, and distribution of potential infiltration on the southern flank of the volcanic foot plain and the fluvial-volcanic plain of Merapi Volcano. The method employed in this research is a direct measurement using a double-ring infiltro-meter. The constant infiltration rate was analyzed using the Horton Method. The results show that the high infiltration rate spreads over an area with an altitude of less than 200 Meter Above Mean Sea Level (MAMSL) which has very deep groundwater. Meanwhile, the infiltration rates above 200 MAMSL tend to be smaller in shallower groundwater depths. The infiltration capacity in the research area tends to be high. The highest infiltration rate is 1.2 cm/minute with the infiltration equation of ft = 1.20 + (1.967-1.20)e 0.0492t, while the lowest infiltration rate is 0.1 cm/minute with the infiltration equation of ft = 0.10 + (0.149-0.10)e 0.0396t. Most of the research areas have moderate or fairly rapid infiltration capacity that spreads from areas below 200 MAMSL to the top altitude at 400 MAMSL. The potential for water infiltration in the research area mostly belongs to the medium category. Overall, this paper presents new insights to understand the characteristics of infiltration in volcanic plain related to the development of land use.