Decision support system for determining the location of rubber weir construction as an effort to meet raw water needs in Rembang, Indonesia

Rembang is one of the regency that was significantly affected by the drought since 2019. The areas affected by drought have gradually expanded so that the demand for raw water in Rembang is increasing. The flow of the Karanggeneng River which empties to the north of Rembang can be used to make rubber dams as a solution to overcome the shortage of raw water availability. The aim of this study is to determine alternative locations for rubber weirs to be built on the Karanggeneng River. The standards or criterias by which the place was chosen of the rubber weir, namely distance from the estuary, topographical considerations, distance from the Water Treatment Plant (WTP), operational ease of access and the interests of fishing boats. There are 3 alternative locations, namely Karang Village, Waru Tengah Village and Waru Kawisto Village. Data were obtained by conducting interviews with respondents consisting of experts and local residents. After that, the Analytical Hierarchy Process (AHP) is used to process the data. The research results show that the order of priority criteria for planning rubber weirs is the importance of fishing boats and Waru Tengah Village is the most prioritized location of a rubber weir’s construction.


Introduction
Water availability is vital in human life, affecting various sectors such as households, agriculture, industry and the economy [1].Although important, water-related challenges are increasingly complicated along with population growth and the decline in clean water sources both in terms of quality and quantity [2].Rembang Regency, a region in Central Java Province, is not spared from the problem of drought, which has become increasingly acute, especially in 2019.The increasingly disturbing drought has become a serious challenge for Rembang Regency, where increasing population growth has resulted in increased water consumption.Meanwhile, clean water sources, both in terms of quality and quantity, are decreasing.This problem becomes more complicated especially during the dry season, when rainfall decreases drastically and the amount of groundwater decreases [3].The impact of drought not only has an impact on meeting domestic water needs, but also has a major impact on the agricultural and industrial sectors, threatening food security and the region's economy.Therefore, the need to build rubber weirs is very relevant in efforts to overcome the drought which continues to worsen in this region.In addition to overcoming drought, the construction of rubber weirs also has the potential to prevent sea intrusion in the coastal areas of Rembang Regency.This area is vulnerable to marine intrusion, which IOP Publishing doi:10.1088/1755-1315/1321/1/012012 2 can threaten groundwater quality and disrupt the sustainability of raw water supplies.Marine intrusion can result in groundwater being polluted by seawater, making it unfit for consumption and agricultural purposes [8].This can also threaten the river ecosystem and the surrounding environment.This research aims to determine the optimal location of a rubber weir's construction on the Karanggeneng River in Rembang.

Location of Study
Rembang Regency is located on the east coast of Central Java, directly opposite East Java Province, and is the eastern entrance to the province.The southern half of Rembang Regency is a hilly area which is part of the North Limestone Mountains, with Mount Butak as the highest point (679 m).In the north there are hills with Mount Lasem as its peak (806 m).Mount Celering Reserve currently protects the area [6].

Figure 1. Location of Study
Rembang Regency is located at the northeastern tip of Central Java Province and is traversed by the North Java Coast road, located on the coordinate line 111°00'-111°30' East Longitude and 6°30'-7°6' South Latitude.The Java Sea is located to the north.

Survey of Alternative Rubber Weir Location
There are 3 (three) alternative locations for placing rubber weirs on the Karanggeneng River, Rembang District.The alternative data for rubber weir locations are as follows:  The first alternative location is located in Karang Village, to be precise on the Karanggeneng river.This location is closest to the estuary, with a distance of about 1 km from point 1 to the mouth of the Karanggeneng river.This location is also the place where fishing boats anchor, for access to the location via the road beside the houses of Semberjo hamlet residents.

Locations 2
The second alternative location is located in Waru Tengah Village, to be precise on the Karanggeneng river.This location is about 1.3 km from the mouth of the Karanggeneng river and is adjacent to the Waru Village TPU (Public Cemetery), there are no fishing boats anchored on the river bank, access to the location is via the Waru Village road.

Locations 3
The third alternative location is located in Waru Kawisto Village, to be precise on the Karanggeneng river.This location is about 1.7 km from the mouth of the Karanggeneng river and is the most distant location from the estuary compared to alternative locations 1 and 2, access to this location is via a village road which borders the Waru Village.

Selection Criteria of rubber weir location
There are 5 (five) criteria that will be used to consider the best location for placing a rubber weir on the Karanggeneng River, which will be arranged in a hierarchical framework.The criteria for the location of the rubber weir are as follows: Topographic Considerations [7] The ideal location for placing the construction of rubber weir is a location on the Karanggeneng river which is straight or does not turn, because the location affects the volume of the weir body to a minimum.2. Distance to the estuary [8] Because the construction of this rubber weir has a function to retain seawater, a location close to the mouth of the Karanggeneng River was chosen.3. Ease of Operational Access [9] Location points that have easy access routes for the construction of rubber weirs and the operation of rubber weirs.Selected a location that is easily accessible and does not flood when it rains or high tides.4. Distance from WTP [10] Distance to a water treatment plant can be an important criterion in determining the location of a rubber weir, especially when ensuring that the weir will not interfere with or be affected by the operation of water treatment plants in the vicinity.5. Fishing Boat Interest [11] The interests of fishing boats are one of the factors that need to be considered in determining the location of a rubber weir, especially if the area is an active area for fishing activities by fishermen.

Analytical Hierarchy Process
This study's data were collected using a questionnaire to identify alternative locations for rubber weirs to be built on the Karanggeneng River, Rembang District.Ten responders with extensive knowledge of water were given the questionnaire.As part of efforts to protect against tides, the questionnaire responses were then analyzed using the Analytical Hierarchy Process (AHP) approach with the application of Expert Choice v.11 to obtain priority criteria and alternative dams.Overall, the actions conducted for this study were as follows: a. Creating a research questionnaire and sending it to water industry experts b.Analyzing the questionnaire responses and determining the geometric mean value for every criterion under comparison.c.Using expert choice software to conduct pairwise comparisons on each criterion at a single level.d.Data retrieval will be carried out if the inconsistency ratio is more than 10%.e. Comparing the pros and cons of each option f.Sorting the alternatives and criteria for each element in order of greatest weight to least weight.Hierarchy is used in determining the location where a rubber weir will be built, because the proposed site of the weir takes into account the activities of local residents and good topographical conditions [12].So opinions from village officials and experts are needed so that the weir that will be built is in accordance with the needs of the residents and its function [13].Below is a hierarchy of 6 criteria and 3 alternatives for selecting rubber weir locations :

Weighting Criteria for Selecting Rubber Weirs Locations
By comparing each criteria, the weights of the criteria were determined.In order to complete the questionnaire, respondents ranked the significance of each comparison between the two criteria variables on a scale from 1 to 9. The geometric mean of the survey results was then determined.Next, the Expert Choice application version 11 was used to process each component's relevance level score.The outcome of the pairwise comparison of the criteria is shown in Figure 4 below.The subsequent priority ranking criterion can be ascertained by examining the outcomes of pairwise comparisons.These findings can be utilized to determine the most crucial factors to take into account while choosing rubber weir locations.Figure 5 illustrates the priority ranking criteria that need to be taken into consideration in the following order: Topographical Consideration (0.123), Interest of Fishing Boats:(0.353),Ease of Operational Access ( 0.

Comparison of rubber weir locations based on distance to the estuary
Alternative rubber weir locations in consideration of the distance to the estuary have differences in each location, such as in Karang Village, it has the closest distance to the estuary than the other locations or is 1 km away, while in Waru Tengah Village it has a distance from estuary 1.3 km, and Waru Kawisto Village it is 1.7 km from the estuary.

Selected Rubber Weir Location
The overall results of the comparison of alternative rubber weir locations can be seen in Figure 11 as follows: Based on the results of the alternative weighting of all criteria, the most prioritized alternative is Waru Tengah Village with a weight of 0.517 (51.7%).In second place is Waru Kawisto Village with a weight of 0.267 (26.7%).Meanwhile Karang Village is in third place with a weight of 0.216 (21.6%).After calculating the results of the questionnaire, a recapitulation chart (Relative Priority) will appear on the results of calculating the criteria and alternative rubber weir locations (Figure 12).

Conclusions
The results analysis using the Analytical Hierarchy Process (AHP) show that the best alternative location for building a rubber weir to increase the availability of raw water in Rembang is Waru Tengah Village.This was chosen because this location has a river topography with a straight channel, a river depth elevation of about 1.2 meters with a sand bottom, easy operational access via village roads, a fairly close distance to the WTP of around 4.5 km, and does not interfere with the interests of fishing boat.

Figure 2 .
Figure 2. Alternative Locations of Rubber Weir 1. Locations 1The first alternative location is located in Karang Village, to be precise on the Karanggeneng river.This location is closest to the estuary, with a distance of about 1 km from point 1 to the mouth of the Karanggeneng river.This location is also the place where fishing boats anchor, for access to the location via the road beside the houses of Semberjo hamlet residents.2.Locations 2The second alternative location is located in Waru Tengah Village, to be precise on the Karanggeneng river.This location is about 1.3 km from the mouth of the Karanggeneng river and is adjacent to the Waru Village TPU (Public Cemetery), there are no fishing boats anchored on the river bank, access to the location is via the Waru Village road.3.Locations 3The third alternative location is located in Waru Kawisto Village, to be precise on the Karanggeneng river.This location is about 1.7 km from the mouth of the Karanggeneng river and is the most distant location from the estuary compared to alternative locations 1 and 2, access to this location is via a village road which borders the Waru Village.

Figure 3 .
Figure 3. Hierarchy Structure of Rubber Weir Location Selection

Figure 4 .
Figure 4. Pairwise Comparison between Criteria for Selecting Rubber Weir Locations

6
300), Distance to Water Treatment Plant (0.143) and Distance to Estuary (0.081).The criteria for the importance of fishing boats are ranked first with a weight value of 0.353 (35.3%).

Figure 5 .
Figure 5. Priority Ranking Criteria for the Selection of Rubber Weir Locations

Figure 6 .
Figure 6.Priority Ranking Rubber Weir Alternative Location Based on Topographic Consideration The results of calculating the comparison of alternative on topographic consideration criteria obtained a Consistency Ratio Value (CR) = 0.006.The priority of alternative rubber weir locations on the topographical consideration criteria is obtained as follows 1. Karang Village : 0.185 2. Waru Tengah Village: 0.558 3. Waru Kawisto Village: 0.257 Based on the topography consideration, the preferred alternative is Waru Tengah Village which has a weight of 0.558 (55.8%).

Figure 11 .
Figure 11.Priority Ranking of Alternative Weir Locations on All Criteria

Table 1 .
Coordinates of Rubber Weir Alternative Locations