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Potential of Groundwater to Supply Domestic Water Necessity in Evacuation Shelters of Merapi Volcano Eruption

Since the occurrence of eruption in 2010, volcanic activity of Merapi volcano increased several times. Cangkringan district was the most affected area with highest level of damage. Learned from the 2010 eruption, mitigation action was developed by preparing more and adequate evacuation shelters. During period of staying in evacuation shelter, primary necessity of clean water for the refugees needed to be fulfilled. Groundwater was considerated to be the alternative source to fulfill the needs of clean water for the refugeess in evacuation shelters. However, the amount of groundwater sources on the area of evacuation shelter had to be able to meet the domestic water necessity of all refugeess. This research aimed to measure the potential’s amount of groundwater on the surrounding area of evacuation shelter, particularly in Cangkringan district, and to identify the capability of groundwater to fulfill the refugees’s domestic water necessity in the evacuation shelter in Cangkringan district. Dynamic groundwater discharge method by Darcy model was used to measure the prediction of potential groundwater. The result showed potential of groundwater in Cangkringan were as much 88,417.2 m3/day or 88,417.200 liter/day. Total water necessity of refugees in Cangkringan was 335,700 liters/day. Compared to the prediction value of groundwater potential, the total water necessity of refugees in Cangkringan was able to be fulfilled.


Introduction
One of the most active volcanoes in the world and located in Indonesia is Merapi volcano, precisely in Java Island. More than 70 eruptions occurred since 1548 [1] and killed over 7000 people [2]. In 2010, Merapi volcano erupted explosively then caused area within radius 20 km from the summit had to be emptied. It means 1,335,885 inhabitants that lived within the radius 20 km from the summit [3] must be evacuated to safe places. Evacuation is challenging process because "moving" many people during disaster occuring might be very chaotic. Evacuation is considered to be one of the most difficult action of response operations, it could be worse if preparedness level was inadequate [4]. At that time, when the zone of hazard was extended to 20 km, from initially 15 km, there was no refugee shelter prepared within the radius to accommodate the refugees [3].
Learned from the 2010 eruption, mitigation action was developed by preparing more and adequate evacuation shelters. Based on data from BNPB (Badan Nasional Penanggulangan Bencana/ National Agency for Disaster Management), there were many evacuation shelters on area around Merapi volcano. The refugees stayed in evacuation shelters during volcanic activity increased and instruction from the government to empty the disaster affected area was applied. A place is capable for evacuation shelter if it meets the requirements of safety from disaster risk, capability to accommodate numbers of refugees, and availability of resources to fulfill human's necessity, among others is clean water. Human needs clean water for personal needs, or called domestic water necessity, such as for drinking, showering, washing, and cooking. Clean water is primary necessity for human, hence it must be fulfilled.
The certain condition during evacuation span, which is addition numbers of newcomers (refugees) at a certain time, surely affects on demand of the clean water in the area. In 2010 eruption, clean water for refugees in Sleman regency was supplied from groundwater through wells. In quantity and quality, groundwater is suitable to supply clean water for domestic needs [5]. To fulfill the domestic water necessity of all refugees in the evacuation shelter, it needs sufficient potential of groundwater. Consequently, the potential of groundwater in the eruption affected area, particularly in Cangkringan district, and the capability of the groundwater to supply refugees's domestic water necessity have to be identified.
Based on Merapi volcano's disaster hazard map and eruption affected area in 2010, Cangkringan district was the most affected area with highest level of damage. It was caused by material of the eruption flowing towards southern slope of Merapi volcano [5] [6]. Cangkringan district lied on eruption hazard area level III, II and I (Fig  1.) that passed through by Gendol river [6]. Lahar from Merapi eruption flowed downward through Gendol river. Once the material overflowed, Cangkringan was the most affected area.
Places that used for evacuation shelter in Cangkringan were mostly big building with wide open spaces, such as school or village hall. Based on effusive scenario of evacuation shelter, five spots in Cangkringan district were used for evacuation shelter to accommodate refugees from Cangkringan district [7], as seen on Table 1.

Potential of Groundwater
Potential of groundwater in an area could be measured by dynamic groundwater discharge method [5][8][9] [10]. The method uses basic mathematic equation that invented by Henri Darcy or widely known as Darcy Law [11] [12]. The equation of Darcy Law is : Based on the equation, groundwater dynamic discharge (Q, unit of measure m 3 /day) is the result of multiplication from three components, are aquifer hydraulic conductivity (K, unit of measure m/day), hydraulic gradient (I, constant), and area of cross-section aquifer (A, unit of measure m 2 ). Each component is calculated by different data. Specification of data to calculate value of Q is explained detail in Table 2. Drilling log profile data was collected from two sources. One was provided by Ministry of Public Works through Water Resources Office division of Raw Water Supply. Other data source was from research publication on potential of unconfined aquifer in partly Klaten Regency [13]. Spatial distribution of drilling log profile in Cangkringan is presented on Figure 2, and the composition materials of each location is presented on Table 3.  Due to lack of drilling log profile data in the research area, then it used neighbour approach. Therefore from the map can be seen that location of spots to take drilling log profile data were at the outside of research area. Secondary data of drilling log profile taken from surrounding research area that were used to represent existing condition in research area. Since identical landform characteristic in Cangkringan and its surrounding, then the drilling log profile in the area were considered similiar.
Groundwater flownet map have two main components, namely equipotential line or countur line of hydraulic head and groundwater flowline. Equipotential line is interpolation line of hydraulic head data. The hydraulic head data that structured as point is resulted from watertable depth data and elevation data (altitude). Substraction the value of elevation data with watertable depth equals value of hydraulic head. Direct measuring on some wells as samples was done to get watertable depth and elevation data. Location of wells were determined with systematic random sampling method and grid method.
Data distribution and result of data processing in form of groundwater flownet map was presented spatially with software of geospatial data processes, specifically ArcGIS 10.3 and Surfer 11. The maps of data distribution in research area are presented in Figure 3. The flownet map was used to determine components of hydraulic gradient (I) and area of cross-section aquifer (A). Furthermore, after all components were collected, then the value of groundwater dynamic discharge (Q) can be measured.

.2 Domestic Water Necessity
Information of the value of groundwater was needed to measure groundwater availability in the research area to supply clean water for refugees during staying in evacuation shelter. The clean water was important for the refugees to fulfill their personal needs, or called domestic necessity, such as drinking, cooking, showering, and washing. Capability of the existing groundwater to supply the domestic needs can be assessed by comparing the value of potential groundwater (Q) with total needs of domestic water of all refugees staying in the area.
Numbers of refugees data were referred from Contigency Document of Merapi 2012 issued by Local Agency for Disaster Management (BPBD) of Sleman

Regency. Based on the document, information of the number of refugees and evacuation shelters in Cangkringan District was provided on Effusive Scenario of Evacuation Shelters data.
National Standardization Board [14] devided domestic water necessity into two characteristics, for village inhabitant and town inhabitant. Domestic water needs for town inhabitant was determined 120 liter/person/day, while for the village inhabitant 60 liter/person/day.
The value of domestic water necessity of the refugees was calculated by multiplicating total numbers of refugees that capable to be accomodated in all evacuation shelters in Cangkringan district with standard value of domestic water needs for village inhabitant, as much 60 liter/person/day. Potential of groundwater in the research area was considered as capable to supply the refugees's domestic water necessity if the value of groundwater dynamic discharge was higher than total value of water that needed for domestic necessity of the refugees. Otherwise, if the prediction value of potential groundwater was lesser than total domestic water necessity of the refugees, then it determined as not capable. Unit of measure for groundwater dynamic discharge must be converted from m 3 /day into liter/day.

Prediction Value of Potential Groundwater
Potential of groundwater in the research area was calculated with groundwater dynamic discharge prediction method. In the research, the context of groundwater was unconfined groundwater. Unconfined groundwater was known as saturated water located beneath land surface that flowing on unconfined aquifer layer. Definition of unconfined aquifer was rock layer beneath land surface that capable to store and pass through water, moreover the layer was only separated by one impermeable layer. The existance of unconfined groundwater can be observed in wells that dug up by man [5] [15][16 [17].
Based on spatial distribution of wells sample map, shown that wells did not exist in whole area of the research area. The existance of the wells were getting rare toward area on above 600 masl (meters above sea level). The collected watertable depth and elevation data from well measurement, and also drilling log profile data were formulated to measure the prediction value of groundwater dynamic discharge.
The first component to be calculated was hydraulic conductivity (K), that defined as the capability of aquifer material to passing through water. In the research calculated five drill data. The result showed value of K was 48.12 m/day and value of mean thickness of the unconfined aquifer layer was 93.4 m. The material of unconfined aquifer in the research area was dominated by material of sand with various classifications. Sand was included in type of uncosolidated material or tended loose, mean it have space inter grains that capable to pass the groundwater to any directions, vertically or horizontally. The sand capability to loosen water made it as one of the best material to form unconfined aquifer layer.
The hydraulic gradient (I) and area of cross-section aquifer (A) components were discussed concurrently because both used groundwater flownet map and aquifer thickness data. Two segments were determined in flownet map to measure the value of the both components. Segments were determined randomly that the segments' border was two adjacent flownet, as presented on Figure 4.  Table 4. Condition in evacuation was a certain condition, in terms of unusual daily basic needs. Addition numbers of people in certain time surely affected on amounts of water necessity compared to usual daily needs in the evacuation area. In that some days of evacuation span, there would be increasing of water demand to fulfill domestic water of all refugees in the area.
Natural condition post disaster also affected on surface water condition. Surface water, such as river or lake, probably was used for consumption by the community to fulfill their daily necessity. Consequently, the only clean water sources to supply the community was from groundwater.
The calculation of total numbers of refugees was used data from local government of Sleman Regency. Data of evacuation shelters in Cangringan was stated in Contigency Document of Merapi Eruption 2012. There were five evacuation shelters in Cangkringan with various capacity to accommodate the refugees, the data shown in Table 5. In average, each person needed 60 liters/day to fulfill domestic necessity. Total domestic water necessity = 5,595 persons x 60 liters/person/day = 335,700 liters /day The number showed total water necessity of refugees in Cangkringan was 335,700 liters /day that needed to be supplied. Compared to the prediction value of groundwater potential, 88,417,200 liter/day, it showed that total value of refugees's water necessity in Cangkringan was able to be fulfilled.
For the note, numbers of the evacuation shelters listed in Contigency Document of Merapi 2012 was effusive scenario, which mean directly affected by lahar flow. Apart from the scenario, at the real condition probably there were more evacuation shelters in Cangkringan.
Based on the value of Q in Cangkringan, the groundwater was still capable to supply more refugees. The calculated result on refugees's domestic water necessity could be basic information to predict the numbers of refugees that can be accommodate in Cangkringan district.

Conclusion
1. Cangkringan District stores abundant of groundwater potential. Based on calculation used groundwater dynamic dischage (Q) approach, there is potential of groundwater as much 88,417.2 m 3 /day or 88,417.200 liter/day. 2. Total water necessity of refugees in Cangkringan was 335,700 liters /day that needed to be supplied. Compared to the prediction value of groundwater potential, total value of refugees's water needs in Cangkringan was able to be fulfilled. 3. The Q value could be basic information to predict the numbers of refugees that can be accommodated in Cangkringan district.