GIS-Based flood susceptibility mapping in Wawar Watershed, Purworejo Regency

Purworejo is one of the regency in Central Java which vulnerable to being affected by flood in rainy season. Wawar watershed is one of the watersheds in Purworejo Regency, the land use in Wawar watershed is dominated by agricultural land so the potential for flooding is quite high in the downstream area. This study aims to determine the distribution of flood-prone locations and classify the level of flood susceptibility that occurs in Wawar Watershed. The method used in this study is data processing on GIS (Geographic Information Systems) using scoring method and weighting of each parameter. These parameters include land use, altitude, rainfall, slope, soil type, and river flow buffers. The resulting hazard map has 3 categories, low, moderate, and high vulnerable level. The results also show that highly vulnerable areas in the southern part of Wawar Watershed that dominated by lowlands area, flat slope, settlement and ricefield land use.


Introduction
Natural disasters caused by hydrometeorology, such as floods, droughts, landslides, forest fires, and extreme climates, are the most frequent disasters in Indonesia [1].Floods are becoming more severe and frequent, affecting more people than any other natural disaster [2].This is largely due to climate changes, land use, infrastructure and population.
There were several flood incidents on Purworejo regency.The Regional Disaster Management Agency stated that in 2020-2021, 307 householder spread across 9 subdistrict and 804 hectare paddy field were affected by the flood disaster that occurred.
Several factors, such as rainfall, watershed characteristics, landuse landcover changes, and an increase in degraded land, have influenced the occurrence of floods on Purworejo regency.There is watershed that located in Purworejo regency with approximately area 761 km2.The land use in Wawar watershed is dominated by agricultural land.This imply that farming activities causing high potential flooding.
Based on the background described above, this study aims to determine the distribution of flood prone location and classify the level of flood suceptibility that occurs in Wawar watershed, Purworejo regency.Identifying susceptible flood-prone area is importance to prevent future occurrences and considered as a critical method for assessing vulnerability.

Literature Review
Several methods, such as statistical and hydrological modelling can be used to map the flood susceptibility level [3].Geographic Information System (GIS) is used to classify the flood-prone area.This method require the support of some data series.The most influencing factors of flood were rainfall, land use, Flooding is frequently brought on by a lot of rain in a short amount of time.With more rain, there is a greater chance of a flood [1].Land use and land cover is another important factor that influences the incidence of flooding in a watershed.Landuse and land cover changes are one of the triggers for an environmental change, which can increase the erosion process [4] The hydrological composition of soil significantly influences the flood risk potential of such an area.The permeability of the soil in an area to a large extent determines the infiltration rate.Different soil types manifest different magnitudes of permeability [5].Furthermore, slope affects the amount of surface runoff and infiltration since water flows from higher to lower altitudes.Flat low-elevation areas may flood faster than higher-elevation areas with steeper slopes [1].

Data used and Methodology
The location of this research is in the Wawar watershed in Purworejo Regency.This research is a quantitative descriptive research to describe the level of flood disaster susceptibility in the Wawar watershed using GIS.The data used in this study are maps obtained from both government agencies and data analysis.They can be seen in Table 1.From the datas in Table 1, they became parameters for analysis in this research.Each parameter has a score that describes the actual condition of the environment which will affect flood susceptibility in the Wawar watershed.Then each parameter will be given weight based on its contribution to the occurrence of flooding.The value of the criteria for each parameter is presented in Table 2. Land use greatly contributes to the occurrence of flood disasters.Various land uses will affect the infiltration process.The bare areas will be more vulnerable to flooding compared to areas with a lot of vegetation thrive on it.Because the infiltration rate of areas with vegetation is faster than bare area land [7].Therefore, based on Table 2, areas with few plants have a greater score than areas with many plants.Table 3 explain the contribution of slope in affect flood susceptibility.Slope means the condition of land topography affected by different of high.Slope will affect how the water flow in watershed areas.Based on Table 3, the flat area has the highest score while the very step area has the lowest score.The possibility of flooding decreases with increasing slope [3], so flat areas will be more vulnerable to flood disasters than steep slope.Beside slope, another topography factor which contribute to flooding effect is altitude.The classification of altitude shown in Table 4.This classification made from 0 meters to > 156 masl.Based on Table 4, it can be seen that the types of altitude will be affect the occurrence of flood disasters too.Elevation has an important role in determining flood susceptibility area, areas that have low and flat elevation are very vulnerable to flooding [5].Areas with high elevation have a low score, meaning that high areas are less vulnerable to flooding than flat area, while low areas have the highest score so they are vulnerable to flooding.Soil is materials covers the surface of the land.Soil type will determine how the water above infiltrate subsoil.That's why soil classifation on flood susceptibility analysis is also important to involve.Table 5 explain the soil type classification.In Table 5 explains that different soil types also have an impact on the level of flood susceptibility.Soil type affects the drainage process because each soil type has different infiltration [6].The greater the soil infiltration, the smaller the level of flood vulnerability [7].Flood inundation water source basically comes from the river.River plays important role on give water supply which could make flood.Distance from river became important factor to determine flood susceptibility, and it can be obtained from buffer analysis.Areas closer to the buffer distance are more vulnerable to flooding and therefore have a higher score, meanwhile areas that are further away from the buffer distance are not vulnerable to flooding and therefore have a lower score.Area on the riverbank closer to the river is more vulnerable than far from the riverbank.The buffer is determine by the distance of water inundation when flood happened in that area.In determining the flood hazard areas, rainfall is very important factor because high intensity rainfall will make higher potential for flooding than not [8].Table 7 show the rainfall classification.Based on Table 7, it is known that the greater the rainfall, the greater the score.It means in areas with little rainfall, the potential for flooding is smaller than the area with higher rainfall.Next analysis using each parameters which given weight for thematic maps based on consideration of how much their influence in the process of flooding [4].The weight of each parameters will be proportional to its influence of the impact of flooding.The classification of parameter weight is presented in Tabel 8.The value of flood susceptibility is obtained from a mathematical calculation with a combination of index and weight of each parameter (Equation 1).The result of the calculation will describe the level of flood vulnerability of an area.
Where; x = flood susceptibility level, Wi = weight for I parameters, Xi = score for I parameters, n = amount of data.
Based on the calculation with formula 1, a cumulative score is obtained which describes the level of flood susceptibility.Potential flood susceptibility areas are divided into three classes as shown in Table 9.  1) with area 13146.8ha (Table 10).Land use is one of the main aspects that cause flooding.Changes in land use from non-built land to built-up land such as moor or yard into settlements, offices or industry will reduce the ability of land to absorb rainwater [9].In built-up land areas such as settlements, the ability of soil infiltration will be reduced and runoff will be greater instead.This effect can lead to floodplain settlements becoming vulnerable to low-probability but potentially high-consequence flood events [10].
The area along the riverbank is usually used by residents for agriculture.This is because this area is a sedimentation area so that many nutrients carried by the river flow are deposited in this area [21].This also means that there are many land changes and developments along the river which can cause flooding. .Areas with flat slopes are prone to flooding, while areas with high slopes are safe from flooding.The slope of the flat slope to the slope will affect the puddle in the event of extreme rain and the flat to the sloping area is also commonly found in daily activities of residents in an area [12].The slope in the Wawar watershed is divided into several categories, the area with flat slope is the dominating area (Figure 2) with an area of 19032.75ha (Table 11).This condition indicates that more than 50% of the area in the research location has the potential for flooding because it is located in a flat slope.The elevation in the Wawar watershed is very diverse, the northern area of the Wawar watershed is the upstream area so it is higher than the southern area (downstream area).The nature of water always flows from higher to lower areas, so that areas that have lower elevations have a higher potential for flooding compared to areas that have higher elevations [13].The majority of areas in the Wawar watershed are at an altitude of > 156 masl which is a mountainous area, the classification of altitude in the Wawar watershed can be seen in Figure 3 and Table 12.Although the Wawar watershed area is dominated by mountainous areas, flat areas are areas that often experience flooding events.That's because the accumulation of the water will flow to the lower areas of this watershed.So, the lower area will be more susceptible than the higher area in this condition.4, the dominating soil type in the Wawar watershed is latosol.The infiltration rate of latosol soil is rather slow so that if inundation occurs, the potential for flooding is greater.Latosol soil types are generally located in humid and cold climates so that they can cause waterlogging [15].This impermeability factor could also affect period of flood because thick Latosol soil means thick of the layer water should be penetrate.4.1.5.River buffer classification.River buffers are created to identify areas that border or are adjacent to rivers, the closer an area is to a river, the greater the potential for the area to flood.River buffer affects the occurrence of flooding [16].River floods occurred in the river channel that included the fluvial landforms in the form of alluvial plains, floodplains, natural embankments, back swamps, river terraces, and river bars [17].About 60.34% of the river buffer area in the Wawar watershed is included in the small buffer distance that is vulnerable (0-200 m).A small river buffer is very vulnerable to the possibility of flooding.In the upstream Wawar watershed area there are many tributaries so that the area has the potential for flooding.Rainfall is one of the main factors that cause flooding.The amount of rainfall that is more than average is usually a trigger for flooding [18].High rainfall that is not followed by good drainage channels will trigger flooding.Rainfall has a relationship between surface flow and erosion.The greater the rainfall, the greater the rate of surface flow and erosion, an increase in the rate of surface flow will increase the potential for flooding.Rainfall in the Wawar watershed is divided into 3 categories, namely, dry, medium and wet.A total of 62.57% of the area in the Wawar watershed is included in the medium category (Table 15).The medium and wet rainfall classes are having the same probability of flood, and it totally depends on the land characteristic itself and also drainage management of some area to control and suppress water runoff during rain [1].4.1.7Flood Susceptibility.The analysis of flood susceptibility in the Wawar watershed is obtained from the results of scoring and weighting calculations with the overlay method using GIS.The results of the analysis resulted in a map of flood susceptibility in the Wawar watershed which is shown in Figure 7.

Figure 2. Slope classification map
From the results of the analysis, the level of flood susceptibility in the Wawar watershed is divided into 3 classes of susceptibility, low, medium and high.Based on Table 16, most areas in the Wawar watershed is located in the high level.There are also several location which set on low and medium classes.There are 3531.98ha of area in the Wawar watershed is located in the low classes.The low susceptibility class is spread in the upstream Wawar watershed area, where the area is an area with hills to mountains with an altitude of more than 156 masl with very step slope to rather step slope.Slope became the main factor which influence flood susceptibility in this area.Steep slope rarely cause flood, but there will be another potentially hazard which is landslides.It happened because water will tent to flow on the steep slope which are the headwater of drainage system [22] and the water will accelerate the soil movement than becoming flood [19].
The low flood susceptibility class is dominated by forest.Area with heavily planted vegetation will have slow rate of runoff which decrease the surface erosion too [23].It will make water slowly reach the river and give time to infiltrate the soil so the possibility of flooding become smaller [20].This will also influence by the material which compossed the land.That's why soil type became important to the rate of water infiltration.
Latosol is a soil type located in the area where this soil type has a low infiltration rate, due to natural conditions.This kind of soil located on steep slopes and mountains.This condition will make rain does not cause puddles but becomes surface flow that flows to downstream area.If flood happened in this area the flood will related to landslide or we often call it by flash flood [24].As much as 42.55% of the area in the Wawar watershed is in the moderate classes, these areas are located in the upstream and downstream of the Wawar watershed.The upstream has a moderate flood susceptibility class because in the upstream area are made of many tributaries [22] so these areas are located close to the river.
Besides that land use in the upstream area is also used for agricultural activities, such as plantations and fields.The moderate susceptibility class is also found in the downstream area, more precisely in the coastal area which is a flat area.The soil type in the coastal area is regosol, where the regosol soil type has a fast infiltration rate so that in the event of rain it is easier to absorb water so the class is classified as moderate.A total of 18028.79ha are located in areas with a high level of susceptibility.The high flood susceptibility class in the Wawar watershed is located in the middle and downstream areas.In the middle and downstream areas, the land uses domination are settlements and rice fields that are prone to flooding [25].The middle and downstream areas are flat areas with an altitude of less than 20 masl so that the potential for flooding is greater.The dominating soil types are gley and alluvial, gley soil is a soil with a low infiltration rate that is prone to flooding [24].

Figure 7
Figure 7 Flood Susceptibility Classification

Table 1 .
Data used

Table 2 .
Landuse index classification

Table 3 .
Slope index classification

Table 4 .
Altitude index classification

Table 5 .
Soil type index classification

Table 6 .
Table 6 show classification for distance from river by buffer analysis.1314 (2024) 012051 River buffer index classification

Table 7 .
Rainfall index classification

Table 8 .
Flood parameter weighting classification

Table 9 .
Flood susceptibility classification Land use in the Wawar watershed is divided into several types (forests, settlements, dry land agriculture, mixed dry land agriculture, rice fields and open land).Land use in the Wawar watershed is dominated by dryland agriculture (Figure

Table 10 .
Landuse classification Figure 1.Landuse classification map 4.1.2.Slope Classification.The slope is one of the most important factors in determining the level of flood susceptibility [11]

Table 12 .
Altitude classification Several soil characteristics must have their role in the soil types classification, such as physic, chemistry and biology specifically[2].Concerning the discussion of flood susceptibility level, soil's physical characteristic has a common impact (texture, permeability, porosity, etc.)[14].Soils with very fine textures have a higher chance of flooding.Because of the finer the soil texture, the more difficult it is for surface runoff water from rain or river overflow to seep into the ground, resulting in inundation.Based on table 13 and figure

Table 13 .
Soil types classification