Analysis on unconfined groundwater availability during dry and rainy season using dynamic approach in Ngemplak, Sleman, Yogyakarta, Indonesia

Groundwater is a natural resource with disparity across the different regions throughout time. In comparison to surface water, the groundwater in unconfined aquifers is easily accessible and generally of good quality. Groundwater macro zoning is classified into three, namely water catchment areas, transition areas, and groundwater discharge areas. Based on the macro zoning, previous research has mostly been carried out in catchment areas and discharge areas with a focus on groundwater potential and temporary groundwater conditions. The dynamics of groundwater availability, especially in the water transition zone, has not been studied much, even though the dynamics of groundwater availability in the transition zone plays a vital role for areas in the discharge zone. This study identified the availability of groundwater during rainy and dry seasons in groundwater transition zone. Groundwater availability was assessed through a dynamic discharge approach. The study variables included the hydraulic conductivity of the aquifer, the hydraulic gradient, and the cross-sectional area of the aquifer. Dynamic discharges were analysed during the rainy and dry seasons. The results showed that the availability of groundwater during the rainy season was 4,333,906.1 liters/day and 3,898,850.4 liters/day during the dry season. Based on the calculation of the dynamic discharge (Q), the decrease in the quantity of groundwater is affected by the variable hydraulic gradient (I) and cross-sectional area of the aquifer (A). The numbers of these two variables are smaller during the dry season than the rainy season. The decrease in the quantity of groundwater during the dry season is of course closely related to reduced rainfall which is a source of infiltration and percolation. Reduced rainfall causes the groundwater level to decrease, then technically reduces the groundwater hydraulic gradient (I) and aquifer cross-sectional area variable (A). There was no indication of groundwater scarcity in the study area. This study can serve as a reference related to the application of dynamic discharge theory to assess groundwater availability. Periodic monitoring of groundwater quantity, rainwater harvesting, and intensification of water infiltration wells can be carried out as a recommendation to anticipate problems related to groundwater availability in the study area. This study can serve as a reference related to the application of dynamic discharge theory to assess groundwater availability.


Introduction
Groundwater is a natural resource with disparity across the different regions throughout time.Groundwater quantity is significantly impacted by geological features, changes in land use, and climate change (Santosa & Adji, 2014;Sejati & Saputra, 2022).People's dependency on groundwater to fulfilling their needs also affects the groundwater dynamic (Sejati, 2021).This study was carried out in Ngemplak sub-district due to it is located in the groundwater transition zone in the southern area of Merapi Volcano (Hendrayana & Vicente, 2013).Most of the population in Ngemplak sub-district use dug wells to meet their clean water needs, as shown in Figure 1.The transition zone carries a strategic function to link the recharge and discharge zones.The groundwater disruption in the transition zone also results in issues in the water discharge zone.Additionally, the problems in the discharge zone potentially carry negative effects on a number of sectors since the discharge zone is commonly utilized in the field of economy and service.Thus, as a prevention, the study on groundwater availability in the Ngemplak sub-district is essential.Studies on groundwater have been carried out using different methodologies.Among those methods, the geographic information system is the most sophisticated device to predict and inventory groundwater availability in different locations.The inventory of groundwater condition is generally carried out through remote sensing and field survey data integration, producing the geospatial thematic information on groundwater potential (Adji & Sejati, 2014;Arulbalaji et al., 2019;Lee et al., 2019;Lee et al., 2020;Manny et al., 2016;Meerkhan et al., 2016;Viossanges et al., 2018;Yeh et al., 2016).However, previous studies primarily only focus only on the environmental variables affecting groundwater existence.Previous research has also focused more on temporary groundwater conditions.The dynamics of groundwater availability which is affected by the change of seasons has not been studied much .In contrast, this study focuses on the estimation of groundwater availability during the rainy and dry seasons.
This study aims to identify groundwater availability of groundwater during the rainy and dry seasons.The quantity of groundwater in unconfined aquifers was calculated using the Darcy Law.Darcy Law states that the groundwater quantity or availability can be estimated using the variables of hydraulic conductivity, hydraulic slope, and aquifer cross-sectional area (Riasasi & Sejati, 2019;Sejati & Adji, 2013;Todd, 2005).The research results were expected to enrich the literature on geohydrology, especially in the implementation of geohydrology in identifying the water availability during the rainy and dry seasons.

Method
The groundwater quantity was estimated using the Darcy Law.We used the concept that water flows horizontally through the aquiver cross-section (Todd, 2005).The concept was later developed into a mathematical formula to find the groundwater quantity flowing through the aquiver cross-section per unit of time (Hendrayana & Vicente, 2013;Riasasi & Sejati, 2019;Santosa & Adji, 2014;Sejati & Adji, 2013) .The dynamic debit variable consisted of the hydraulic conductivity, hydraulic gradient, and cross-sectional area of the aquifer.Meanwhile, the groundwater dynamic (Q, unit m 3 /day) is the result of multiplication from the free aquiver attributes, consisting of hydraulic conductivity (K), hydraulic gradient (I), and cross-sectional area (A) (Riasasi & Sejati, 2019;Sejati & Adji, 2013;Todd, 2005).The mathematical formula for the dynamic debit is presented in Formula 1.
In which: Q = dynamic I = hydraulic gradient K = hydraulic conductivity A = aquifer cross-sectional area Hydraulic conductivity (K) was identified using the lithology profile data.Meanwhile, the hydraulic gradient (I) and cross-sectional area (A) were determined from the flow nets.Flow nets are the thematic spatial information that visualizes the prediction of groundwater flow.It consists of two primary components, namely the groundwater level contour line and flow direction.The flow nets were determined using the data of groundwater level gathered through the field survey.The groundwater level was measured in the dug well.Meanwhile, the well location was determined using a random sampling method.The measurement was carried out during the rainy and dry seasons to find various groundwater levels.During the rainy season, the measurement was carried out in March 2019, while in the dry season, it was carried out in August 2020.In March 2019, we collected data from 76 dug well, while in August 2020, we garnered data from 70 dug well.
The obtained data on groundwater level were analyzed using the spatial interpolation method to attain the groundwater counter line.The spatial interpolation was completed using the ordinary Kriging method through the Arc GIS Pro software.Further, the obtained groundwater counter line was used as the basis for examining the groundwater flow direction.Besides, we used the principle of groundwater flow lines that intersect perpendicularly with the groundwater contour line (Todd, 2005).The formed flow nets were identified, and the flow net segment with a close to rectangular shape was used to determine the groundwater hydraulic slope or gradient (I) and aquifer cross-sectional area (A).The research procedure is illustrated in Figure 2.

Results and Discussion
This study was carried out in the Ngemplak sub-district, Yogyakarta, Indonesia.Ngemplak sub-district was chosen as the research area because the area is categorized in the groundwater transition zone.Estimating the availability of groundwater in different seasons needs to be done to identify symptoms of groundwater problems in the area.Ngemplak subdistrict is located in the 49 S zone, with UTM coordinates of 434000-443000 East and 914200-9152000 North, as shown in Figure 3.The geographic coordinate of our research location was at 7.41'54" south latitude and 110.26'42" east longitude.

Figure 3. Map of Research Location
The area of our research site was 35,71 km 2 .Meanwhile, the administrative border for our research location was the Ngaglik, Depok, Kalasan, Cangkringan, and Pakem sub-districts, along with the Klaten districts, as illustrated in Figure 3.Our research site was also part of the foot of Merapi Volcano.According to the results of Schmidt Fergusson climate classification, our research location had the average C climate index with the highest rainfall of 144 mm (Central Bureau of Statistics of Sleman Regency, 2018; Sejati & Adji, 2013).The data on groundwater level are illustrated in Figures 4 and 5.Meanwhile, the detailed data obtained from the dug well are presented in Table 1 and 2  The availability of groundwater in our research location is influenced by the lithology of unconfined aquifer components.Our analysis of the lithology data showed that the unconfined aquifers in the Ngemplak sub-districts consist of unconsolidated materials, such as fine, medium, and coarse sand.These materials are correlated with the activities of Merapi Volcano in the past.The aquifer system in our research location is classified as Merapi Aquifer System, which is dominated by loose material (Hendrayana & Vicente, 2013).Groundwater flows appropriately through the empty spaces or pores formed in those aquifer layers.The results of lithostratigraphic data analysis showed that the average unconfined aquifer hydraulic conductivity in our research site was 17.7 meters/day, with the average aquifer thickness reaching 45.7 meters.The configuration of the unconfined aquifers materials is listed in Table 3.In addition, the constructed flow nets suggested that the groundwater flows from the north to the south of our research location.This groundwater flow is significantly correlated with the area altitude.Accordingly, the north area of our research location is higher than the south area.The groundwater flow pattern in our research site is similar to the pattern of groundwater flow in the Manglayang Mountain area, Sumedang, West Java, Indonesia.The groundwater flow pattern in this study is classified as the radial flow pattern which is commonly found in volcano or mountain slope areas (Trisdiyansyah et al., 2022).Besides, the comparison of flow nets in the dry and rainy seasons suggested that, generally, the seasons possess no effects on the groundwater flow pattern.The variable of the season only influences the groundwater level.In the rainy seasons, we observed higher groundwater levels than in the dry season.In the rainy season, the groundwater level is closer to the ground level or shallower, while the groundwater gets deeper during the dry season.The higher equipotential of groundwater was observed in the rainy season than in the dry season, as shown in Figures 6 and 7. that the groundwater dynamic is higher during the rainy season than in the dry season.In the rainy season, the groundwater dynamic was 4,333,906.1 Liter/day, while in the dry season, it was 3,898,850.4Liter/day.The detailed results of dynamic groundwater calculation are shown in Tables 4.  ,412,496.3 3,898,850.4In general, individuals have easy access to the groundwater at our research site.Meanwhile, the presence of sands with distinctive categories forms thick unconfined aquifers capable of properly flowing the groundwater through the inter-pore spaces.Additionally, the average groundwater depth during the rainy and dry seasons was 3.6 and 7.9 meters, as presented in Tables 1 and 2. Linearly the groundwater quantity during the rainy season was also more significant than in the dry season.The change of season, from the rainy to dry season, induced a decrease in groundwater quantity by 435,055.8Liter/day.The dynamic of groundwater quantity causes no groundwater source scarcity.During the dry season, the groundwater quantity barely decreases by 2% from the rainy season.This decrease occurs due to the lower rainfall or precipitation during the dry season.Further, the reduced precipitation affects the infiltration and percolation process, decreasing the groundwater quantity (Sejati, 2021).
Although there has been a decrease in quantity, the availability of groundwater in the study area is still good.There were no symptoms of groundwater scarcity during the transition from the rainy season to the dry season (2019)(2020).This decrease of groundwater quantity is normally affected by the natural physical situation, such as rainfall and lithology (Irawan et al., 2022;Riasasi & Sejati, 2019).Our research location is categorized as the C climate category or comparatively wet climate based on the Schmidt Fergusson climate classification (Sejati & Adji, 2013).Besides, according to the climate classification using the dry and wet months calculation, rain occurs throughout the year in our research site, with different intensities.Besides, the lithology in our research location was dominated by loose materials that were capable of absorbing the rainwater through the infiltration and percolation mechanism, resulting in reserved groundwater.The reserved groundwater is in the form of accessible groundwater in the unconfined aquifers.
Groundwater also has dynamics affected by natural and non-natural factors.Therefore, maintaining the groundwater quantity for future continuous usage becomes a new challenge.Our research site possesses high rainfall as it is located at the foot of Merapi Volcano, enabling precipitation during the rainy and dry seasons due to the orographic rainfall.Rainwater harvesting can be investigated as an alternative media to minimize the dependency on groundwater.This study can be a reference for the use of geohydrology science in the geography field, especially for studies related to groundwater availability.The popular groundwater dynamic theory, Darcy's Law (Todd, 2005), can also be used in examining spatial groundwater availability.

Conclusion
There was a decrease in the quantity of groundwater in Ngemplak sub district during the rainy season in 2019 to the dry season in 2020.The quantity of groundwater decreased by 435,055.8liters/day.Based on the calculation of the dynamic discharge (Q), the decrease in the quantity of groundwater is affected by the hydraulic gradient of the groundwater (I) and the variable cross-sectional area of the aquifer (A).The numbers of these two variables are smaller during the dry season than the rainy season.The decrease in the quantity of groundwater during the dry season is of course closely related to reduced rainfall which is a source of groundwater filling.Reduced rainfall causes the groundwater level to decrease, then technically reduces the hydraulic gradient (I) and aquifer cross-sectional area variable (A).The difference in the quantity of groundwater during the rainy season and the dry season is normal.Even though its strength has decreased, the availability of groundwater in the study area is still good during the dry season.There was no indication of groundwater scarcity in the study area.Rainwater harvesting and intensification of water absorption can be carried out as a recommendation to anticipate problems related to the availability of groundwater in the study area.

Figure
Figure 2. Research Procedures .

Figure 4 .
Figure 4. Map of Groundwater Level During the Rainy Season Collected in March 2019

Figure 5 .
Figure 5. Groundwater Level During Dry Season Obtained in August 2020

Figure 6 .
Figure 6.Flow Nets During the Rainy Season Obtained in March 2019