Groundwater utilization as a source of fresh water supply for various purposes in discharge area shows an ever to increasing trend, while land use of recharges also changes as development progresses. To meet its needs, many people rely on land-based exploration and exploitation. Groundwater is one of nature’s most valuable and inseparable resources for life on earth. Groundwater exploration requires appropriate and efficient techniques. One commonly used method is the electrical resistivity method since, it is efficient and economical to determine groundwater. Vertical Electrical Sounding (VES) Method measurement of rock resistivity from the surface to determine the subsurface rocked layer [1]. Geoelectric resistivity is one of the variables which is the physical properties of the rock layers below the surface. Rock resistivity data can be used to develop a model of subsurface and stratigraphic structures in terms of electrical properties. Geoelectric resistivity depends on lithology, air content, porosity, and pore ions concentration [2]. Geoelectric method is divided into two parts: Mapping resistivity method as a resistivity method that aims to study horizontal variations of horizontal layer resistivity types and Resistivity sounding method or, the drilling resistivity method that studies the variety of rock resistivity beneath the surface of the earth vertically. Data processing of geoelectric measurement results can be done by standard curve method, 2-Dimensional method (Schlumberger and Wenner), 3-Dimensional Method (Configuration of dipole-dipole, Lee Partition Configuration, Rectangle Line Source, 3 Point Gradient System). Therefore, ambiguity in interpretation can occur and become indispensable for calibrating the geoelectric measurement data with available borehole data to establish the resistivity range of various lithology units. Resistivity data can be used to identify and describe subsurface features such as the presence and type of aquifers, contaminated groundwater, and sediment size distribution [3, 4, 5].

IJTech International Journal of Technology

Semarang City is one of the most influential coastal cities in Java Island. The city is facing increasingly-high water demand due to its development and water problems due to climate change. The spatial physiography and landscape of Semarang City are also exposed the city to water security problem. Hence, rainwater harvesting treatment is an urgent effort to meet the city's water needs. However, planning, implementation and management of rainwater harvesting are highly depended on multitemporal rainfall data. It has not yet been fully compiled due to limited rain stations. This study aims to examine the extent to which CHIRPS satellite data can be utilized in estimating volume of rainwater harvesting 16 sub-districts in Semarang and determine the water security status. This study uses descriptive statistical method based on spatial analyses. Such method was developed through spatial modeling for rainfall using isohyetal model. The parameters used are rainfall, residential rooftop area, administrative area, population, physiographic and altitude units. Validation is carried out by using monthly 10 rain stations data. The results show level of validity by utilizing CHIRPS Satellite data and mapping rainfall distribution. This study also produces a potential map of distribution rainfall volume that can be harvested in 16 sub-districts of Semarang.

More than 40% area in Jakarta is below mean sea level which it causes inundation to occur every year in Jakarta. In order to solve this problem, the government developed Jakarta flood control management. Polder system is one of drainage system for areas that cannot drain water by gravity. There is one polder system located at North Jakarta named North Sunter Polder System. However, the inundation still occurs every year in that area. Based on the problem, the aim of this study is to analyse the cause of flood inundation in the area, it can be known by simulation using the HEC-HMS and HEC-RAS applications. Based on the simulation results it is known that the cause of flood inundation in the North Sunter Polder system is due to the less optimum pump operation in the polder system. So that to overcome the flood inundation can be done by increasing pump capacity and changing the pump operating elevation.