Integrated groundwater resource management in the peri-urban area of Banguntapan, Bantul

Banguntapan District in Bantul Regency is one of the peri-urban areas in the Special Region of Yogyakarta. The peri-urban area is an area located between the urban (Jogja City) and the rural (Bantul Regency). As a result of this strategic location, the development of this district is very rapidly marked by the growth of settlements, which in turn further requires water for housing purposes.The purpose of this research is to formulate an integrated water resources management model in the study area. To achieve this goal dynamic modeling is used. The model obtained was then simulated using the Powersim 2.5c computer program. Management efforts that can be carried out in this district are reducing groundwater use through saving water use and increasing water use from local water companies. Simulation results show that the combination of these two methods will slow down the time the safe yield will be exceeded, from 2039 to 2049.


Introduction
Water is one of the important natural resources for humans and other living things on earth. Without water, there might not be life on this earth. Problems arise when precisely with the advancement of civilization problems relating to water are also increasing. Flooding, drought and water pollution are some of the problems that often occur. It is expected that the availability of water resources in accordance with the demand and requirements is sufficient in quantity, both in quality and always available at all times.
Groundwater is one of the most important forms of water resources [1]. Even according to [2], groundwater is also one of the natural resources that plays a very important role in supporting human health, economic growth and biodiversity. However, the availability of groundwater is unlimited, because it is controlled by the geological conditions of an area [3,4].
According to [5], the increase in groundwater demand is in line with population growth, expansion of agricultural land and industrial development. On the other hand, according to [6], an increase in groundwater extraction to meet the increasing demand of residents in urban areas causes a decrease in groundwater levels in big cities. In fact, according to [7], increasing population, increasing water use, water transfer between watersheds in one groundwater basin and the impact of global climate change can cause changes in groundwater level which in the long run will cause land subsidence [8,9].
Because located in the peri-urban area of Yogyakarta City, Banguntapan District is one of the districts in * Corresponding author: Setyapurna@geo.ugm.ac.id Bantul Regency that is experiencing rapid development, as indicated by the increasing population and settlement development. This district has an area of 28.48 km 2 and is administratively divided into 8 villages namely Banguntapan, Baturetno, Singosaren, Jagalan, Tamanan, Wirokerten, Potorono and Jambidan. With a population of 145 956 people in 2018, the population density is 5 124 people km 2 , with a population growth rate of around 2.44 %. The impact of this population increase is the increasing number of water demand, while on the other hand the water recharge is reduced due to the increasingly narrow catchment area.
One effort that can be done to address these problems early is to develop a management model. One model that can be used is a dynamic model system [10], while the simulation can be done using Powersim. In the simulation, Powersim can be considered as a mini laboratory to experiment several alternative water management policies in an integrated manner before they are applied to the real world.

Method of research
The formulation of integrated water resources management in the study area uses dynamic modeling. The initial stage of dynamic modeling is to process primary and secondary data that are related and considered important in influencing groundwater availability, such as population data, population growth rate, rainfall, air temperature, runoff coefficient, aquifer thickness, specific yield and groundwater fluctuations [11]. Population and population growth affect the amount of groundwater used, while rainfall, air temperature and runoff coefficient, play a role in determining groundwater supply. Groundwater fluctuations, aquifer thickness and specific yields will determine groundwater volume and the safe yield of its use. The secondary data is used to determine the rate of change in groundwater quantity from year to year. The model obtained was then simulated using the Powersim 2.5c computer program [12].

Determination of rain volume
The amount of rain in Banguntapan District was determined based on the recording of rain at Gandok Station. Based on the recording at the station, it is known that the amount of annual rainfall in the District of Banguntapan is 2,461 mm/year. With an area of 28.48 km 2 , the rain volume is 70,089,280 m 3 /year.

Evapotranspiration calculation
Potential evapotranspiration in the study area is calculated by the Tornthwaite Method which requires air temperature data. Based on data from the Central Statistics Agency of Bantul Regency, the average temperature in the Banguntapan District is 26.7 °C, so the calculation is as follows:

Calculation of groundwater volume
Groundwater volume is calculated based on the assumption of static groundwater flow, which is the multiplication between aquifer thickness, wide area and specific yield. With a 35 m thick aquifer, a wide area of 28.48 km 2 and a specific yield of 0.28, the groundwater volume in the Banguntapan District is 279,004,000 m 3 /year.

Calculation of groundwater recharge
Land use in Banguntapan District is dominated by settlements and agricultural land ( Figure 1). Referring to the research of [13], the runoff coefficient in Banguntapan District is 0.648. With a rain volume of 70,089,280 m 3 /year, the runoff volume that occurs is 45,417,853 m 3

Determination of population increase
Determination of the percentage of population growth refers to the data from Bantul in Numbers (2019). Based on the data in the report, the percentage of population growth in Banguntapan District in the period 2010-2018 was 2.44%.

Determination of water demand
In this study, the calculated water demand are water demand for domestic demand which are determined according to the size of the population and the number of water demand per capita per day. Because Banguntapan District is located in the peri-urban area, referring to the results of a survey from the Directorate of Water Supply Development, the Directorate General of Human Settlements, the Ministry of Public Works and Public Housing, the water demand are determined to be 150 liters/person/day or 58.75 m 3 /year.

Determination of safe yield
Groundwater safe yield for use are the result of multiplication between groundwater fluctuations (F), area wide (A) and specific yield (Sy). With an average groundwater fluctuation of 1.64 m/year, an area wide of 28.48 km 2 and a specific yield of 0.28, the safe yield of groundwater use in the District of Banguntapan is 13,078,016 m 3 /year.

Groundwater management
Conceptually, the groundwater management unit should be the groundwater basin, in this case the Yogyakarta groundwater basin. For this reason, in the discussion of groundwater management in Banguntapan District, only consider aspects that can be done in this region as its contribution to overall groundwater management in groundwater basins. Based on this concept, the groundwater management model in Banguntapan District is shown in Figure 2, while the calculation results are visualized in graphical form as shown in Figure 3.  The safe yield value of groundwater use in this region is 13,078,016 m 3 /year, so that because the water demand is 13,199,658 m 3 /year, by the 2039 water demand will have exceeded the safe yield. As a result, there will be a decrease in groundwater stored from 283,057,772 m 3 to 264,375,082 m 3 (Table 1).

Groundwater management by water saving
What can be done to reduce the amount of groundwater utilization for domestic needs by reducing the use of water per person per day from 150 liters/day to 120 liters per day (medium city standard). Noting Table 1 and Figure 4, it is known that by reducing water use, there is a phenomenon of a very significant decrease in water demand. If previously in 2039 the population's water needs of 13,199,658 m 3 /year had exceeded the safe yield, with a reduction in water use would only reach 10,548,125 m 3 /year m 3 /year which had not exceeded the safe yield. With this water saving effort, the groundwater requirements will exceed safe yields in 2048.

Increasing Water Use from Local Water Companies
The basic concept of this conservation method is that if a large proportion of the population uses water from the Local Water Companies (LWC), groundwater exploitation will be reduced. For this reason, it is assumed that the use of water from the Local Water Company has doubled, from 55,007 m 3 /year to 110,014 m 3 /year. The simulation results show that the doubling of LWC water production does not significantly influence the availability of groundwater in the study area. Safe yield of groundwater use will still occur in 2039 (Table 1 and Figure 5).

Groundwater Management by Saving Water and Increasing LWC Production
In this alternative groundwater management model, the parameter for the amount of water used for domestic demand is reduced and the use of water from the Local Water Company is increased.
The simulation results show that there is indeed a decrease in water demand, but the decrease is almost similar to water saving. This combination of two methods will indeed slow down the time for exceeding safe results, from 2048 to 2049 (Table 1 and Figure 6).

Conclusion
Groundwater management in Banguntapan District cannot be separated from overall groundwater management in the Yogyakarta Groundwater Basin. Nevertheless, partial management in this district can also be done to contribute to the overall management of groundwater in this area. Based on this, the management effort that can be done in this district is to reduce the use of groundwater through saving water use and increasing water use from local water companies (LWC). Simulation results show that the combination of these two methods will slow down the time the safe yield will be exceeded, from 2039 to 2049.