Study of Groundwater Availability as Natural Recharge for Hydrothermal Reservoir in Semi-Arid Region

Abstract

According to Koppen, the climate in Indonesia is divided into tropical, arid/semi-arid, mild temperate, continental and polar climate. Tropical climate covers Sumatra, Jawa, Kalimantan, and Papua. Arid/semi arid climate extends over East Nusa Tenggara region and its vicinity. Mild temperate climate covers almost the entire Indonesian archipelago, especially Sumatra, Kalimantan, and Papua, while polar climate covers Jaya Wijaya Mountains. Most of the geothermal fields in Indonesia are hydrothermal system and have similar tropical climate. Tropical climate area like Jawa preserves precipitation values of at least 60 mm/month, while in arid/semi-arid climate area like East Nusa Tenggara has an average rainfall of 20-40 mm/month. Groundwater potential from unconfined aquifer in Jawa was recorded 38.8 billion m3/year meanwhile calculation from confined aquifer resulted 10.1 billion m3/year. Groundwater potential from unconfined aquifer in East Nusa Tenggara was recorded 2 billion m3/year whereas confined aquifer supplied 0.3 billion m3/year. From those data, annual groundwater in arid/semi-arid area is less than tropical climate area, and it tends to remain in a deficit. These conditions will affect the supply of groundwater to the geothermal reservoir. One of the main strategies to sustain reservoir is by estimating the availability of groundwater supply based on the calculation of limited precipitation and evapotranspiration value in water-balance method from Mock. This study can be used as the starting point in order to support the recommendations to maintain the reservoir on a long term economical periods.