Reducing the Susceptibility of Flood Supplier Areas in Gondang Micro-Catchment, Bojonegoro, East Java Province: a Flood Disaster Mitigation Effort

Flood disaster mitigation begins with reducing the susceptibility of flood supplier areas. This task can be accomplished by mapping the flood-supplier susceptibility and then deciding on long-term flood mitigation actions following the susceptibility level of the land and the legal status of its area. Pacal reservoir has Gondang as one of its catchment areas. During the rainy season, the spillway releases water immediately because the inflow exceeds capacity. To maximize water storage, reduce Pacal reservoir input, and prevent downstream flooding, the land cover must be improved. This paper will investigate the Gondang micro-flood-prone catchment’s locations and propose mitigation strategies. The legal status of the land is used, as well as Climate Hazards Group Infrared Precipitation with Station data (CHIRPS), Digital Elevation Model (DEM), and Google Imagery from 2014 and 2020. Paimin’s Equation was used to assess the flood risk. The findings indicate that the Gondang micro-catchment is susceptible to flood suppliers on a low to a high level. Changes in land cover during 2014-2020 resulted in alterations to the susceptibility level of the area. The legal status of the area has had an impact on land cover management. The simulation results of reforestation and maintenance of dense forests in forest areas, as well as regreening activities in agricultural areas, can improve land with high susceptibility to moderate levels, which accounts for 20.9% of micro-catchment areas. All of these proposed activities are expected to control the Pacal Reservoir’s inflow during the rainy season by increasing soil infiltration and reducing runoff.


Introduction
From upstream to downstream, watershed management must be comprehensive to prevent environmental deterioration and guarantee water supply [1].The hydrological cycle is what connects a watershed's upstream and downstream regions [2].Characteristics of watershed are strongly influenced by the hydrological cycle.Because each watershed is unique, it will also react differently to changes in the climate and land cover [3].
Flooding is a hydrometeorological catastrophe that shows a lack of effective watershed management.Extreme rainfall can also result in flooding, so forest cover is no longer able to reduce surface runoff [4].There are various locations where biophysical and geographic factors may result in floods downstream.Understanding flood-prone and inundated areas are essential for flood mitigation.As a 1313 (2024) 012022 IOP Publishing doi:10.1088/1755-1315/1313/1/012022 2 result, the examination of flooding is split into two parts: the flooding area and the flooding supplier [2].Flooding is more common in a flat or urban area, where rainfall turns into surface runoff, while flood sources are those areas upstream [2,5,6].
The Gondang micro-catchment is situated in the upstream area of the Pacal reservoir within the Bojonegoro regency.This regency relies on the reservoir as a source of water for irrigation purposes.. Bojonegoro is a national food warehouse and a rice supplier for the East Java province [7].Additionally, the Pacal reservoir contributes to 35% of this region's agricultural productivity [8].Due to sedimentation, this reservoir's ability to contain water has reduced, and during the rainy season, water is discharged immediately via the spillway [9,10].Continued sedimentation will reduce the reservoir's capacity to store water, increasing the risk of floods downstream.Reducing the amount of water entering the reservoir during the rainy season requires thorough management of the upstream area.
The upstream region acts as a rainwater catchment, which helps to replenish the aquifer through possible infiltration and percolation mechanisms [11].If this function is disrupted, the upstream area has the potential to become a flood supplier.Storage function may be ideal if sufficient forest land is available.It has been observed that the forest acts like a massive sponge, storing water during the wet season and gradually releasing it during the dry [12].Land cover is also one of the bases for determining the priority scale for watershed management [13].Although land use change is a dynamic process, it can be exacerbated by human intervention.Therefore, activities that contribute to the acceleratation of watershed degradation are frequent occurences.
Reducing the characteristics of disasters is an alternative to adaptation and mitigation activities [14].The first step is mapping the susceptibility of flood supplier areas.This paper will analyze areas that have the potential to be flood suppliers in the Gondang micro-catchment, as well as propose several activities to reduce its susceptibility level.The susceptibility of flood supplier areas is determined based on rainfall datasets, morphometry, and land cover [2].The proposed rehabilitation activities are prioritized to improve manageable factors, namely land cover.The degree of vulnerability as a cause of flooding and the area's legal status are both taken into consideration while carrying out rehabilitation following the priority scale.

Study area
This research was conducted in the Gondang micro-catchment.This micro-catchment is in the Gondang sub-district, Bojonegoro Regency, East Java province, Indonesia, precisely at 7º23'38"-7º23'39" South Latitude and 111º47'35"-111º52'15" East Longitude.Gondang is a catchment area for the Pacal reservoir [15] as seen in Figure 1.The Gondang River serves as the primary surface water resource within the Gondang micro-catchment, facilitating the flow of water towards the Pacal Reservoir.The altitude is 127 -880 m asl, while the slope is between 0 to 50%.

Figure 1. The geographical position of the Gondang micro-catchment
The Gondang micro-catchment consists of three soil types: Calsiustolls accounting for 53% of the area, Dystropepts covering 37% of the region, and Ustropepts making up the remaining 10% of the soil composition.In terms of geographical distribution, it can be observed that Dystropepts are predominantly found in the upstream region, whereas Calsiustolls and Ustropepts are more prevalent in the middle and downstream regions, respectively.As per the Minister of Energy and Mineral Resources No. 17 of 2012, it has been established that the Kalibeng Formation is linked to approximately 14% of the Gondang region, which has been officially designated as a karst landscape area.Despite being covered in forest vegetation, the karst terrain exhibits an inability to retain water on its surface [16].
Based on CHIRPS, daily rainfall in the Gondang micro-catchment is divided into 5 different grids, with monthly rainfall as described in Figure 2. The mean rainfall in the last 4 decades was 2,275 mm/year.The dry season occurs from May to October with rainfall varying between 17 and 156 mm/month whereas the rainy season lasts between November and April with rainfall of 185 -398 mm/month.The rainfall intensity during the rainy season reaches 81 -83% of the total annual rainfall.

Data collection
This research uses maximum daily rainfall, watershed morphometry, and land use.This paper utilised three datasets: (1) the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS), which offers a grid resolution of 0.05º or approximately 5.6 km, (2) DEMNAS, which has a spatial resolution of 0.27-arcsecond, corresponding to an accuracy of 8.25 m per grid, and (3) imagery from Google Earth captured at an eye altitude of 1 km, which can be considered as having a scale of 1: 22,700.Considering the reliability of the three data sources, the spatial analysis conducted in this study was performed at a semi detail scale.
Maximum daily rainfall was obtained from the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) between 1981 and 2021 [17].The validation of CHIRPS has been conducted using measurement data from three rainfall stations located in Malang Regency, East Java Province and Kendal Regency, Central Java Province.Previous research have demonstrated that CHIRPS exhibits a good performance in estimating ground measurement rainfall values [18,19].The utilization of CHIRPS, a tool that offers daily precipitation data, facilitates spatial analysis in numerous regions of Indonesia, including the Gondang micro-catchment, where climate monitoring stations are not available.The morphometric characteristics which include watershed shape, stream gradient, drainage density, and slopes are derived from the national digital elevation model [20] through automated processing using the morphometric analysis toolbox [21] within the ArcGIS software.In this paper, the land cover map is the result of the on-screen digitization of Google Earth imagery from the last decade, especially the years 2014 and 2020, which was then validated through ground checks.

Data analysis
This study compares the susceptibility level of the Gondang micro-catchment as flood supplier areas based on land cover maps in 2014 and 2020.This analysis was carried out spatially using the scoring method, as in Equation 1 [2], where SFS (susceptibility for flood supplier) is the susceptibility level for the flood supplier area, MR is the maximum daily rainfall, SW is the watershed shape, SG is the stream gradient, DD is the drainage density, SL is a slope, and LC is land cover.We classify each parameter into 5 categories/scores, namely (1) very low, (2) low, (3) moderate, (4) high, and (5) very high as detailed in Table 1.The overlay process will produce a total vulnerability score for the flood supplier for each land unit.The flood supplier classification is detailed in Table 2  A simulation was carried out using three scenarios to determine the impact of rehabilitation through increasing land cover on the susceptibilityof flood suppliers.The 3 scenarios applied are (1) restoration of dense forests in forest areas, (2) planting perennial trees in the agricultural area, and (3) the combination of scenarios 1 and 2. The dense forest that is restored and maintained in scenario 1 is 30% of the Gondang area.This value is the minimum forest adequacy in a watershed, according to the Minister of forestry decree No. 70/Kpts II/2001 concerning the stipulation and changes in the status and functions of forest areas.

Rainfall variability
The spatial analysis of CHIRPS daily data resulted in the same maximum daily rainfall value for all Gondang micro-catchment, namely 76-150 mm/day.According to Table 2, the entire research area has a high potential as a flood supplier area.

Morphometric characterization
Gondang has an area of 23.14 km 2 and is classified as a micro-catchment, based on the Regulation of the Director General of Watershed Management and Social Forestry No. P.3/V-SET/2013 governing requirements for defining watershed features.We can use the morphometric characterization method to assess a watershed's size and hydrological characteristics [22].Morphometry, one of the static characteristics, has a largely consistent impact in determining how the watershed reacts to rainfall events.
The Gondang micro-catchment has an oval shape and a dense coarse drainage pattern.Drainage density is an excellent predictor of the soil infiltration capacity [23].SM 1, SM 2, SM 3, and SM 4 all fall into the low category for drainage density (Table 3).Typically, the phenomenon manifests in hydrological catchments characterised by the presence of a permeable subsurface [24].The elongation ratio compares the circle diameter and the watershed's maximum basin length [25].The four SMs' elongation ratios fall to a very low level.It follows that the Gondang micro-catchment is less effective at regulating surface runoff.If a watershed's elongation ratio is close to 1 or its form is similar to a circle, it will be more effective in regulating surface runoff [26].

Land use and land cover
Land cover is a dynamic characteristic, as opposed to morphometry, mostly because of anthropogenic activity.Human endeavors to satisfy diverse demands, including housing, industry, agriculture, mining, and other infrastructure amenities, are a vital concern for regional sustainability and economic progress [27,28].Land cover influences riverbank health, particularly during the rainy season, flood discharge, and water availability during the dry season [29,30].The ecosystem's balance will be impacted by changes in land cover [31,32].The physical functioning of the soil can also be significantly altered by changes in land cover, which can then affect the rate of soil infiltration [33][34][35].
Various types of land cover found in the Gondang micro-catchment, namely dense forest, sparse forest, juvenile forest, settlement, paddy field, and dry field agriculture (Figure 3).The legal status of the land is split into protection forest, production forest, and agricultural areas [36].Table 4 shows the disparity between the area's legal status and its actual land cover in 2014 and 2020.Numerous upper watersheds in Indonesia are in catastrophic condition as a result of this difference [37].The majority of the Gondang micro-catchment area is a forest area, both protection (7.0%) and production (71.3%) forest, according to the area's legal status.This composition is greater than the minimum requirement of forested land in a watershed.If forests are in healthy state and experiencing degradation, they exhibit a low susceptibility to floods in the spatial study [2].However, other land uses that ought to have been covered in forest growth were found in this area.In the designated forest area, there are dry-field agriculture, habitation, and paddy fields (Table 4).Protection forest shouldn't be damaged to maintain the life support system.
Except for settlement, all types of land cover changed between 2014 and 2020.Paddy field and dry field agriculture also remained mostly unchanged.The majority of the Gondang area is classified as a productive forest.The cycle of logging results in modifications to the land's cover.The area is more likely to experience flooding due to the logging cycle's clearance of vegetation [38].Extremely extensive logging and planting operations result in dense and sparse forests in 2014 and juvenile forests in 2020 (Table 4).The young forest in 2014 also changed into a sparse forest in 2020.The establishment of juvenile forest vegetation in the agricultural area in 2020 is an intriguing finding in SM 4.Although it still retains the designation of an agricultural region, this property has been transformed into a young forest that will be used for seasonal agriculture operations.This may be a sign of how well regreening initiatives are working.

The susceptibility of flood supplier areas
The Gondang micro-catchment, which is upstream, is one of its downstream's flood suppliers.Several concurrent elements affect the area's level of susceptibility as a flood source.The outcomes of the spatial study revealed that Gondang has varying degrees of susceptibility, ranging from low to high (Figure 4).The level of susceptibility changed from low to moderate as a result of the dense forest in 2014 evolving into the juvenile forest (10.11 ha), and the sparse forest (61.54 ha) in 2020.On the other side, a 0.49 Ha dense forest that became a juvenile in 2014 was present.The slope where there was a change in land cover has an impact on how these changes differ.The dense forest in 2014 changed to a sparse or juvenile forest in 2020, which increased surface runoff and decreased litter volume, causing the rainwater to enter river channels more quickly [39].Transitions from dense forest to juvenile or sparse forest can also affect infiltration rate.According to several studies [40][41][42][43], older forests typically have higher infiltration rates.Furthermore, the 46.06 Ha area in SM 4 is improving as well, although having a lesser extent.While some changes in juvenile forests in 2014 into sparse forests in 2020 caused the susceptibility level to improve from moderate to low, other changes in land covers such as dry field agriculture, paddy field, and juvenile forests in 2014 into sparse forests in 2020 caused the susceptibility level to improve from high to low.  Figure 4 shows a shift from some regions in SM 4 with a high level of land susceptibility in 2014 to a level with a moderate susceptibility in 2020.It demonstrates the significant impact of including perennial tree planting in agricultural regions as a means to mitigate susceptibility and serve as a preventive measure against flooding.As was already indicated, regreening efforts in non-forest regions have been successful in lowering their susceptibility to flooding [44].The capacity of soil infiltration is influenced by the land cover [45].Therefore, in addition to heavy rainfall, incorrectly channeled rivers, river silting, and coastal flooding, land cover change is one of the primary causes of flooding in Indonesia [46][47][48].The transition from perennial trees to annual crops has a significant impact on the rise in surface runoff.Surface runoff increases with increasing open area size, increasing the risk of flooding and vice versa [49].

Mitigation for flood supplier areas
Management of the land cover is essential for the Gondang micro-catchment.Three alternative scenarios were used to model flood prevention in the Gondang region.Land use changes in the upstream result in the movement of sediment and dissolved material, variations in discharge and water quality, which affect the downstream area [50].Therefore, using this tree-based scenario would be appropriate for managing the Gondang micro-catchment.Figure 5 and Table 5 detail the degree of land susceptibility as a flood supplier using the three scenarios.Forests play a significant part in regulating the release of floodwaters.The forest's capacity to regulate floodwater discharge increases with vegetation density.The production forest, which makes up 71% of the Gondang area, is subject to the logging cycle, which results in dynamic fluctuations in vegetation density.On the other hand, most of the protected forests, both found in SM 1, 2, and 3, have a non-forest land cover.Scenario 1 illustrate the importance of maintaining dense forest in ahealthy condition and not experiencing degradation in the protected forest area.About 30% of the Gondang micro-catchment is restored and kept as a dense forest cover type in scenario 1.On governmentdesignated forest area with a slope of over 15%, dense forest is distributed spatially.Scenario 1 was successful in decreasing the susceptibility of the area by 1.4% and 0.2%, respectively, for high and moderate levels.
The SMs in the Gondang micro-catchment are all subjected to scenario 2, as opposed to scenario 1's focused on SMs 1, 2, and 3. Planting perennial trees can lessen gully erosion, as seen in the Naruan micro-catchment of the Wonogiri reservoir.This is because dry field agriculture contributes the most silt to the upper watershed [50].Scenario 2 has a bigger impact on enhancing high levels of land susceptibility than scenario 1 does.About 98 Ha or 4.2% of Gondang is used for agriculture.Perennial tree planting in agricultural area were able to improve the land susceptibility from high to moderate.
A technique for conserving soil and water in watershed management is vegetative rehabilitation.Various activity, such as reforestation and agroforestry, can be used to implement this tree-based strategy [51].The simulation of managing land cover in scenario 3 combines elements of scenario 1 and 2. Compared to the first two scenarios, the outcomes are the best.Improvements can be made to the 20.9% of the Gondang area that has high susceptibility through reforestation, preservation of dense forests in forest areas, and regreening initiatives in agricultural areas.
Several previous studies have provided evidence that the presence of virgin forests or similarly dense forests can effectively mitigate the frequency and intensity of floods.This is achieved through the forest's ability to retain water during periods of heavy rainfall and subsequently release it into rivers in a controlled and gradual manner [52].Compared to other types of land cover, the virgin forest has the highest rates of infiltration, making it more likely to reduce the danger of flooding [53].Several studies also demonstrate that a decrease in forest area can contribute to some floods occurences [54][55][56].The maintenance of dense forests has been found to have a positive impact on mitigating flooding and reducing run-off and soil erosion [57], eliminating some of the rainfall, storms, and allows the withdrawal of soil moisture deficit [58].

Conclusions
Understanding the characteristics of a flood disaster, including the potential of the flood supplier area, is important as part of an effort for flood disaster mitigation.On a watershed scale, the area that supplies flood water is upstream, and it is crucial to safeguard it from harm and potential issues in the downstream area, especially since there are significant structures like irrigation regulator reservoirs.The identification results showed that the Gondang micro-catchment has a low to a high level of susceptibility as a flood supplier to the Pacal reservoir.Changes in land use and land cover have the greatest impact on the level of flood susceptibility and are also the most manageable to manipulate so that the level of susceptibility can be improved.Simulated reforestation and regreening activities, conducted in compliance with the legal status of the land, can decrease the highly susceptible level, which makes up 20.9% of the micro-catchment area.Recommendations to increase forest cover, particularly on sloping ground, will be able to lessen the likelihood that floodwater will reach downstream areas by enhancing infiltration and lowering runoff.

Figure 2 .
Figure 2. Temporal distribution of monthly rainfall according to CHIRPS

Figure 3 .
Figure 3. Spatial distribution of land cover in 2014 and 2020

Figure 4 .
Figure 4.The spatial distribution of flood supplier areas in 2014 and 2020

9 Figure 5 .
Figure 5.The spatial distribution of flood supplier area according to the scenarios

Table 1 .
. Categories and scores for each parameter

Table 2 .
The classification for flood supplier area

Table 3 .
The morphometric character of the Gondang micro-catchment

Table 4 .
The cross matrix between land cover changed and the legal status of the area

Table 5 .
The distribution of flood supplier areas according to the scenarios