Urbanization has rapidly grown across all regions over the past century. According to the United Nations, the percentage of people living in urban areas was 53% in 2018, with an urban growth rate of 2% in the last 20 years [1
]. Increasing urbanization has a consequence of expediting the fulfillment of facilities to meet the needs of residents. Previous studies have shown that population growth also raises the water adequacy standard [2
], affecting the urban water balance. This also takes place in big cities in Indonesia, including Semarang City, a city with a significant increase in population. According to Semarang City’s Central Statistics Agency, the average migration rate was 24 people per 1000 population until 2017 [6
], thereby giving rise to overexploitation of groundwater resources so that the groundwater level tends to decrease every year.
Groundwater is the largest freshwater source that is relatively easy to acquire [7
]. In 2010, total groundwater abstraction is estimated to be 982 km3
/year worldwide. The country having the largest abstraction is India, at 251 km3
/year, followed by China and the US, with groundwater abstraction at 112 km3
/year. Among them, most of the use of groundwater is for irrigation [10
]. Most of the developing countries in Asia have a large amount of groundwater abstraction. Indonesia is a country with a groundwater abstraction of 15 km3
/year, the largest in Southeast Asia, with 93% for domestic use, 5% for industry, and 2% for irrigation [10
]. In the future, groundwater is expected to be increasingly utilized, since groundwater is a cheap and practical water source as compared to seawater desalination or wastewater treatment to make freshwater [10
The use of groundwater has prevailed for hundreds of years. However, groundwater users often ignore the sustainability of groundwater supplies. This has long led to groundwater over-pumping, which in turn causes a lowering of the water table, a drying-up of riverbeds, along with especially in coastal areas, seawater intrusion and land subsidence [13
]. Groundwater management is necessary to ensure groundwater sustainability. However, unlike surface water management, groundwater management is inherently more intricate. Groundwater management requires the delineation of aquifer boundary for analyzing, which is more difficult to implement than surface water boundary since the former is not easily accessible due to its location below land surface. In addition, there are many uncertainties in subsurface parameters, including groundwater hydraulic and elasticity properties as well as human exploiting conditions. Numerical groundwater modeling is an important tool for effective groundwater management. Based on performing a computer-aided groundwater modeling, the managers can understand the regional flow and hydrogeological condition of the groundwater system, observe and analyze the potentiality of groundwater recharges, and then select optimization scenarios for groundwater abstraction, so that fluctuations in groundwater levels caused by background pumping and climate changes can be better appreciated [10
]. Groundwater modeling mainly concerns the flow, head, and mass transport in aquifer systems, but instead does not emphasize the effective control measure for human exploitation of groundwater. Apparently, the latter is the most important element in groundwater resources management to meet long-term environmental needs [10
A number of studies have attempted to utilize numerical models to evaluate the potentiality of groundwater resources in order to develop the optimum groundwater management policy. Groundwater numerical modeling generally combines different tools to gain better results. Geographic Information System (GIS) has been extensively used in recent years for both the management and exploration of groundwater resources. Using the spatial analysis, the groundwater researcher or manager can create a comprehensive database of hydrogeological parameters, population, facilities related to groundwater resources, and so on. Accordingly, the researcher or manager can integrate different data into one project and then create new information, in turn, making the implementation of groundwater management more feasible. The use of GIS is often combined with the use of hydrodynamic groundwater models, such as MODFLOW, MIKE, etc. A GIS-based hydrogeological database model in Wallon, Belgium, was performed and processed to obtain hydraulic head maps, pump distribution maps, and statistical data maps to make it easier to be treated as data entry in GMS (Groundwater Modeling System) [18
]. Using GIS, aquifer modeling has been developed in the western Nile Delta in Egypt to provide analysis tools and visual images so that the result of groundwater modeling is visible in pre-modeling and post-modeling [19
]. In Haryana State, India, [20
] groundwater modeling was performed in the period of 1997–2010 using SGMP (Standard Groundwater Model Package) with irrigation, soil, harvest, and climate parameters, as well as applied to assess the salt shallow water based on geographical information [20
]. Various models were also simultaneously employed. The combination of Bayesian Model Averaging (BMA) with Generalized Likelihood Uncertainty Estimation (GLUE) was carried out to take into account the predictive uncertainty of parameters, input data, and conceptual model definition. Rojas et al. 2010 [21
] adopted MODFLOW to establish the conceptual model for groundwater simulation. A numerical analysis has been undertaken to examine the effect of land use change on groundwater runoff at the regional level [22
] in Grote-Nete Basin, Belgium, with incorporation of the vegetation of the delimited alkalinity of phreatophyte plants based on a trace of particles.
Groundwater modeling was also performed for various sites, such as for a coastal region in the Jijel Plain, Algeria [23
]. In this study, a groundwater analysis was conducted for a transient flow so that the effect of groundwater pumping on subsidence in the Jijel plain in 2042 can be quantitatively projected with the integration of MODFLOW. Conversely, for an arid region, MODFLOW was used to assess the effectiveness of groundwater management for the next 30 years in the Nubia aquifer, Egypt [24
]. The result of the study can be treated as a guideline to design the operation of future groundwater pumping for areas with wider hydraulic fractures.
The purpose of this study was thus to assess, in a systematical manner, the impact of rapid urban development in Semarang City on changes in groundwater level due to excessive and unplanned withdrawal caused by the use of industry, private households, and farms. Next, the temporal and spatial evolution of its groundwater level in the future will be predicted, and then formulate the proper management strategies for the sustainability of groundwater. The groundwater modeling in this study uses GIS to build a database for analyzing groundwater level alteration. The database from GIS is then used both to create a conceptual model with GMS (Groundwater Modeling System) MODFLOW and to support data in decision-making [18
]. Semarang City was chosen as an illustrative area because it has similar features to other cities in Indonesia that have excessive groundwater removal with enough data to analyze, but, to date, not too much research on groundwater modeling has been done in Indonesia. The groundwater modeling is undertaken based on two time steps. The first one is the steady-state flow simulation to obtain the accurate groundwater level that satisfies the regional flow condition and serves as the initial condition for the next phase of the transient flow simulation. The latter is then performed for the calibration and validation processes, and provides the predicted groundwater level. Lastly, four scenarios of groundwater control measures for the next 30 years (up to 2050) are carried out and investigated.
Overexploiting freshwater aquifers has caused a gradual decline in the groundwater level. In the current study, Semarang City was investigated for the abstraction of groundwater in deep wells due to the expansion of industrial facilities. A numerical MODFLOW model for Semarang City was developed to examine the physics and dynamics of groundwater flow and to evaluate the impact of groundwater extraction. The calibration is delicately performed under two very different flow conditions of the steady and transient states at six observation wells. The mean square error and the coefficient of determination are used as quantitative indicators to assess the reliability of the model.
Although there are only the limited number of observation wells that have brought the complexity in calibrating and validating the hydrological parameters and recharge rates, especially in the southern part of Semarang City, MODFLOW has been demonstrated as a powerful numerical model. Combination of MODFLOW and spatial analysis indeed provides valuable and robust reference for groundwater resources management. However, to aid the formulation of better groundwater management plans, it should increase the number of new observation wells or reactivate the number of existing observation wells, especially in the western and southern parts of Semarang City.
Four management strategies are then presented in the current study in an attempt to achieve sustainability of the groundwater resources in relation to their excessive extraction caused by rapid urbanization in Semarang City. The result shows that even though 10% reduction in the development of the deep wells and their production capacity is implemented from 2035 to 2050 (TS4), only the groundwater level in Prawiro Jaya Baru (P1) can restore back to that recorded in 2025 among these 6 observation wells. The declines in groundwater level can be essentially controlled in Wot Gandul (P4) and LIK Kaligawe (P6). However, in PRPP (P2), SMKN 10 (P3), and Santika Hotel (P5), the groundwater level still continues to drop and is even lower by 20 m than that in 2010 in SMKN 10 (P3). Therefore, to fulfill sustainable groundwater management that meets future beneficial uses without leading to unacceptable environmental consequences, such as land subsidence and, in turn, susceptibility to flooding, in Semarang City, it should strengthen administrative regulation with a systematic registration of pumping wells, implement abstraction control policy, and even place restrictions on future groundwater use, in quantity, by more than 10%, as indicated in the current study. Reduction of the grid size together with consideration of land use change and human behavior towards groundwater use are highly recommended for further studies. Moreover, due to the excessive exploitation of groundwater, it is strongly recommended to conduct further research as to the effect of groundwater resources overexploitation on land subsidence.