Land Use and Surface Runoff Change in Babon Watershed Semarang Greater Area

Babon is a watershed in Central Java that passes through the Semarang Regency, Semarang City, and Demak Regency. Most of the areas in Babon Watershed are potential residential locations for the community because they are situated on the outskirts of Semarang City. Over the past 20 years, Babon Watershed has experienced a change in land use from agriculture and plantations to residential areas. There have also been flooding incidents in several housing areas along the Babon River. The Semarang City Government currently focuses on developing watersheds in the central and western parts. However, no government intervention has yet to develop the eastern watershed, especially the Babon watershed. Babon watershed management is tricky because three regions pass it, so good governance collaboration from the government and related stakeholders is essential. Therefore, the research aims to assess land use and surface runoff change in the Babon watershed. The research method is quantitative. Data collection methods are secondary data collection and field observations. Data analysis methods are spatial analysis using Geographic Information Systems to determine land use changes and the infiltration areas in the Babon watershed in 2013 and 2023; also analysis of surface runoff water discharge using the rational method. Based on the research results, the Babon watershed has experienced significant changes in land use, with non-developed areas being transformed into developed lands. The increasing transformation from non-developed to developed lands impacts water infiltration areas.


Introduction
The population in Indonesia has been increasing from year to year, with a population growth rate of 1.25% per year from 2010 to 2020 [1].One of the main variables causing population problems is rapid population growth and limited availability of land to meet needs [2,3].As the population increases, the usage of land for cultivation also increases.The greater the population density, the greater the intensity of land use change [4].The demand for land rises alongside the growing population.Improper land use depletes the natural carrying capacity in an area [5].Therefore, land use is crucial to consider in watershed management.
To acquire land for habitation, people tend to settle on the outskirts of cities.There is a tendency for the area of the city center to expand to the outskirts of the city; this is because land prices in the suburbs are lower than in the city center [6].The Babon Watershed is one of the watersheds in Central Java that has a vital role in the sustainability of the system in the Eastern Semarang region, which passes through Semarang, Semarang and Demak Regencies [7].Babon is a watershed in Central Java that passes through the Semarang Regency, Semarang City, and Demak Regency.Most of the areas in Babon Watershed are potential residential locations for the community because they are situated on the outskirts of Semarang City.
Over the past 20 years, Babon Watershed has experienced a change in land use from agriculture and plantations to residential areas.There has also been an exploitation of the riverbanks of the Babon River for industry and residential purposes [8].This has resulted in flooding incidents in several housing areas along the Babon River.The Babon River is a river that crosses the city of Semarang, which is located in the middle of residential areas and has the potential to overflow and flood settlements during the rainy season [9].Rapid urbanization causes an increase in settlements in flood plains, changes in land use to settlements cause an increase in the number of flood events and an increase in people directly affected by floods [10,11].The Semarang City Government currently focuses on developing watersheds in the central and western parts.However, no government intervention has yet to develop the eastern watershed, especially the Babon watershed.Babon watershed management is tricky because three regions pass it, so good governance collaboration from the government and related stakeholders is essential.
Climate change alter has driven to surge calamities expanding in terms of recurrence and damage caused, which truly undermines urban and country security [12].Future climate change projections will increase the number of people affected by flooding [10].Flooding refers to floods that occur due to local rainfall in specific areas, resulting in the formation of puddles [4].Such flooding generally happens in low-lying areas with poor drainage capacity and dense built-up land [13,14].Changes in land use in urban areas have caused some locations originally meant for water absorption and drainage to become densely built-up areas relatively saturated with water.As a result, they can no longer function as water absorbers or outlets (rivers) [15] in low-lying areas.These puddle floods have spatial distribution in the city center, trade areas, government centers, and some residential areas [16].Floods can cause both physical and non-physical losses.
Previous research stated that the Babon River has a low, medium, and high risk of flooding [17].There have been violations of building construction on the Babon River border, which can cause a decrease in river water quality, increase river bank erosion, and reduce the amount of water discharge [8].There is also research on identifying the planned flood discharge received by the Babon River using topographic data, hydrological data, and existing condition measurement data, which states that most of the Babon River is experiencing flood or overflow conditions [9].Apart from that, there is an influence of changes in land use on the water quality status of the Babon River, namely that the pollution index value increases every year [7].Based on previous research, there is the potential for flooding in the Babon River and its impact on the surrounding area.Still, these studies have not considered the influence of population and land use changes on increasing runoff in the Babon watershed.Hence, the research aims to assess land use and surface runoff change in the Babon watershed.

Data
Babon Watershed is one of the river basins located in Central Java, situated on the northern slope of Mount Ungaran.The Babon River originates from several tributaries that flow from Mount Butak in the Ungaran Regency.Babon Watershed consists of three sub-watersheds: the upper part comprises the Gung Sub-Watershed (covering an area of 4,207 hectares) and the Pengkol Sub-Watershed (covering an area of 3,438 hectares).In contrast, the lower part consists of the Babon Hilir Sub-Watershed (covering an area of 6,712 hectares), with its western boundary marked by the East Flood Canal.Babon Watershed shares its boundaries with the Garang River Basin (in Semarang City) to the west and the Tikung River Basin (in Demak Regency) to the east.The Java Sea borders it to the north, and the Hulu River Basin (located in Semarang Regency) to the south.The elevation of the Babon Watershed varies, ranging from approximately 2 meters above sea level in the north (lower region) to around 200 meters in the upstream area.The regions traversed by Babon Watershed are as follows [18].In general, the river system of the Babon Watershed can be categorized into 3 (three) subwatersheds as depicted on the map of the Babon Watershed system: • Sub-DAS Gung: This sub-watershed is located in the upper part of Babon Watershed and comprises the main river, Gung River, which flows perennially throughout the year.It is accompanied by several tributaries, namely Lutung River, Jaten River, Porang River, Klangit River, and Sinanas River.• Sub-DAS Pengkol: Positioned in the middle part of Babon Watershed, this sub-watershed is dominated by the perennial Sungai Pengkol as its primary river, along with its tributaries, which include Wideng River, Watukodok River, and Seketok River.DAS Babon located in Semarang Regency, Semarang City, and Demak Regency (Figure 1).

Method
The research method used is a quantitative descriptive method.The data collection methods include collecting secondary data and field observation.The analysis in the study included the population growth of Babon Watershed, changes in land use in the Babon Watershed, and analysis of surface runoff water.The analysis of the population of Baboon Watershed is to determine population growth in three regions.In population analysis, there is a comparison between the population in 2013 and 2022.The data used for population growth analysis is population data from the Central Bureau of Statistics (Badan Pusat Statistik) for Semarang Regency, Demak Regency, and Semarang City with secondary data collection.There is also an analysis of land use change in the Babon Watershed used to determine changes in land use in 2013 and 2023.Analysis of land use change using satellite image data (Landsat 8) of the Babon watershed for 2013 and 2023 obtained from the United States Geological Survey, which undergoes supervised classification analysis to determine land use changes with Geographic Information System.Furthermore, the surface runoff water analysis determines changes in runoff water in 2013 and 2023.The data used in the runoff analysis are rainfall intensity from the Department of Public Works, Water Resources and Spatial Planning, Central Java Province, as well as the total area from the results of the Babon watershed land use analysis-calculation of surface runoff analysis using the rational method.In general, the process of the analysis steps is explained through the following flowchart (Figure 2).

Population of Babon Watershed
The comparison of populations is a thorough process of analyzing and comparing the human population in different regions, countries, or periods.Data on the population gathered from various sources are essential pillars for demographers, sociologists, economists, and other researchers to understand the characteristics and dynamics of a human population.Information about aspects relevant to population growth and distribution dynamics can also be obtained by analyzing population figures and subsequent comparisons.
Table 1 explains the dynamics of population growth processes in three regions.The selected time range is sufficient to illustrate how a region undergoes population growth.This is important because knowing a region's population growth can help identify and understand the population growth patterns [19].Another aspect that can be studied is understanding population mobility and migration patterns in a region, which aids in identifying urban and rural comparisons.This information has implications for formulating or terminating public policies related to family, health, education, and the environment to be designed more effectively, especially in addressing demographic changes [20].
The three study areas exhibit a similar population growth trend, as all three experienced an increase in population during the period.Semarang City experienced a minor increase in population compared to the other two regions, primarily due to its status as the core city for the surrounding areas.Consequently, most current growth tends to occur in the surrounding regions.Similarly, Semarang Regency also experienced an increase in population, followed by Demak Regency, which is part of a developing peri-urban area, resulting in a relatively high population growth rate.This is greatly supported by the fact that many people now occupy areas close to Semarang City due to economic activities being a significant driver.
Based on population density data, all sub-districts in the Babon watershed experienced an increase in population density from 2013 to 2022.The population density in all sub-districts has the same characteristics; in 2013 and 2022, Pedurungan District, Semarang City, had the highest population density compared to other sub-districts.Meanwhile, the lowest population density is in East Ungaran and Sayung Districts.Figure 3 shows the population density Babon watershed map for 2013 and 2022.As a result, the increasing population growth raises concerns about the potential increase in demand for land for housing, infrastructure, industry, and agriculture.With rapid population growth, areas previously used for agriculture or forests may convert into residential or industrial areas.Consequently, agricultural and forest lands that were once green and productive may decrease, leading to deforestation and environmental degradation.Changes in land use in the Babon watershed can also affect water availability and river water quality.For instance, if more land is converted into residential or industrial areas, the water infiltration area decreases, and surface runoff increases.This can cause flooding and soil erosion, damaging the land and river ecosystems.

Physical Characteristics of Babon Watershed
The physical characteristics of the Babon Watershed explain the altitude, slope, rainfall, and flood potential of the Babon Watershed.Based on the altitude map (Figure 4), the Babon watershed has a height of 0 -65 meters above sea level in the downstream and middle parts, 65 -142 meters above sea level in the middle and upstream regions, and 142 -213 meters above sea level, 213 -290 meters above sea level, 290 -481 meters above sea level in the upstream parts.Areas with a low elevation level will easily experience flooding [21]; this is because the nature of water will flow from the highlands to the lowlands, so areas with low altitudes will be more vulnerable to flooding [22,23].
Meanwhile, based on the slope map (Figure 5), the Babon watershed has 4 (four) slope classes, namely 0 -8% (flat), 8 -15% (sloping), 16 -25% (rather steep), and 26 -40% (steep).Flat slopes are in the upstream, middle, and downstream parts; gentle slopes are in the upstream and middle; steep slopes are in the upstream and middle, while the steep slope is upstream of the Babon watershed.The slope level of this watershed area influences the level of potential flooding that occurs [21].The flatter the area, the greater the opportunity for flooding to happen because the flow of rainwater toward the river runs slowly, so it can trigger flooding [24].
Based on the rainfall map (Figure 6), there are 2 (two) rainfall categories, namely 1750 -2250 mm per year in the middle and downstream parts and 2250 -2750 mm per year in the middle and upstream regions.The higher the intensity of rainfall, the greater the potential for flooding will increase [25,26].Also, based on the water catchment area map (Figure 7), the upstream and middle areas are water catchment areas.The land use in this area is a non-built area.Apart from that, based on the flood potential map (Figure 8), locations with high flood potential are in the middle and downstream parts.Meanwhile, places with low potential for flooding are in the upstream and middle areas.

Land Use Changes in Babon Watershed
Land use change has been a hot issue in the global world for several decades, including suburbanization and urban sprawl [27,28].Land use changes in a Watershed refer to transforming or shifting land functions from one use to another within the DAS area.These changes can be natural, such as geological or climate-driven processes, but often, they are influenced by human activities, mainly through the exploitation of natural resources and infrastructure development.Therefore, analyzing land use changes in a Watershed becomes essential to understanding the impacts or implications of land transformations on the environment and society.It said by [29] if contends that higher infiltration of the budgetary division to the provincial districts, including different social bunches, may altogether moderate the impacts of normal calamities.
The first step in analyzing land use changes in the Babon watershed is to investigate the dynamics of existing land changes.This is done by comparing the land use patterns over the selected period, also used to analyze population growth dynamics.Through this comparison, information about the dynamics of land utilization within that period can be obtained.The following presents information about the land use changes in the Babon watershed from 2013 to 2023 (Table 2).

Table 2. Percentage of Land Use Change in Babon Watershed
Source: analysis by the author, 2023 The noticeable decrease in land use availability is evident in the utilization of vacant land, rice fields, estates, and forests.The reduction of vacant land by 8%, rice fields by 6%, estates by 5%, and forests by 2% (Figure 9) is strongly related to the increasing population.The diminishing land allocation for these purposes is partly due to the demand for non-developed land to be converted into developed land.The rapid increase in residential land use by 14% contrasts with the diminishing nondeveloped lands.This factor further strengthens the understanding that the growing population will significantly impact the utilization and availability of non-developed lands due to the escalating demand for developed land.The increasing population and demand for residential land also led to a notable rise in industrial activities, evident from the 7% increase in industrial land use.This aligns with the urban development process characterized by the growth of urban activities, including residential and industrial zones.
Land use changes within a watershed often connect with reducing open spaces and urban development.A Watershed is an ecological system encompassing rivers, forests, and lands irrigated by those rivers.Changes in land use in a Watershed can convert forests, agricultural lands, or other open areas into residential or other infrastructures.This has several implications that need to be considered.The information regarding changes in land use in the Babon watershed over ten years also indicates a reduction in non-developed land.Conversely, an increase in land use area is observed in developed land.This situation needs attention because if the massive reduction of non-developed land continues, it may have other impacts on the suitability of the land to support new activities conducted on that land.One concern is the potential decrease in water availability, which results from the reduced ability of the Watershed to store water in the ground and the accumulation of water in rivers, periodically water availability for local communities and ecosystems that rely on river flow [30].If contend that higher infiltration of the budgetary division to the provincial districts, including different social bunches may altogether moderate the impacts of normal calamities [29].

Land
The land use changes in the Babon watershed over ten years (Figure 10), especially the reduction of non-developed land and its conversion into developed land, have occurred significantly.As shown in the image below, settlements in the Demak Regency are substantially grown, particularly in the areas around water bodies.The change in rice fields is noticeable, with many transforming into residential areas represented by yellow.The availability of vacant land starts to diminish around the water bodies in 2023.There is also a significant change in the expansion of industrial land use, spreading more extensively in 2023, replacing the vacant land available in the surrounding areas in 2013.Significant changes are also apparent in the Semarang Regency, where previously scattered vacant lands have transformed into residential areas in 2023.Consequently, by 2023, almost all vacant lands in the Babon watershed have been converted into developed lands, including residential and industrial areas.Regarding Semarang City, there is little change in land use, mainly due to its early development and transformation into an urban area.On the other hand, the two areas, Demak Regency and Semarang Regency, are still developing due to the continuous increase in population.The urbanization process in these two regions significantly impacts the land condition, as it is driven by the need for land to accommodate urban activities.
The massive growth of developed land is closely related to the potential for flooding.Several factors that arise from this process have a significant impact on flooding.One such factor is the reduction in water infiltration due to the hard surfaces of the urbanized land, which can no longer absorb rainwater and may increase the volume of water flowing into rivers.There are also changes in natural drainage patterns, soil erosion, increased surface runoff, and reduction of natural river spaces.
These factors contribute to the increased risk of flooding, especially in areas experiencing rapid urbanization like Demak Regency and Semarang Regency.The ongoing urbanization process in these regions requires careful planning and management to mitigate the potential impacts on water drainage and flood risks [31].Sustainable urban development practices, such as green infrastructure, stormwater management systems, and preserving natural drainage pathways, can help alleviate the flood risk and improve the overall resilience of the Babon watershed.

Surface Runoff Analysis
The reduction of open spaces, mainly due to changes in land use converting open lands into residential areas, infrastructure, or other developed lands, can lead to a decrease in infiltration areas.Infiltration areas are lands capable of absorbing and retaining rainwater, slowing down water flow, and reducing the potential for flooding.Another related concern is the reduction of infiltration areas due to soil erosion or degradation resulting from changes in land use and human activities that degrade the land's ability to hold water.When lands lose their ability to retain water, the overall water infiltration capacity of the Watershed decreases, increasing the risk of and impacting water availability in the region [30].
Therefore, in formulating Regional Spatial Planning, changes in land use from green open spaces to developed areas in urban regions have significant impacts on the environment, especially on the water management in the city and its surrounding areas.Learning from urban experiences, considering its geographic conditions (location and physical characteristics), it is essential to consider the influence of land use changes on water flow and infiltration.Changes in land use in the upstream and downstream areas of the Watershed significantly impact surface runoff.One land type that has a substantial effect is residential land.When the water discharge exceeds its capacity and carrying capacity, residential areas can lead to waterlogging or even flooding [19].
In this context, the increase in residential or developed areas in the upstream area of the Babon watershed can reduce the capacity for water infiltration, leading to faster surface water flow and increased runoff volume.This raises the risk of waterlogging or flooding when the water flow reaches the downstream area of the Babon watershed, especially if the drainage system or river channels cannot accommodate high water discharge.The influence of land use changes in the upstream area of the Babon watershed on surface runoff can directly affect the water management conditions in the downstream area of the Babon watershed.During heavy rain or flood periods in the downstream area, the high water flow from the upstream area can cause increased river discharge and waterlogging or flooding.
Land use has a significant impact on rainwater falling on the ground.In forested and grassland areas, rainwater tends to infiltrate the soil, be used by plants, or become groundwater.In contrast, rainwater has little chance of infiltration on paved surfaces and roofs, leading to high surface runoff that directly flows into the urban drainage system.Population growth not accompanied by water conservation efforts can cause serious problems related to water resources.Many ponds and marsh areas have been lost due to conversion into residential areas, offices, and buildings.Consequently, the land's ability to absorb, retain, and store water in the soil diminishes due to the reduction of Green Open Space (RTH) which should function as water infiltration areas.This issue leads to a decrease in the natural capacity to manage water availability and can cause water-related problems in the future.
Calculating surface runoff is a critical aspect of hydrology to understand the surface water flow generated by rainfall or melting snow.Surface runoff can impact flooding, soil erosion, and water availability.The following is a more detailed explanation of surface runoff calculations: The surface runoff is then calculated to determine the surface water flow generated from rainfall in the areas around the Babon watershed, using a comparison of ten years from 2013 to 2023.The following are the results of the calculations obtained (Table 3).Source: analysis by the author, 2023 For each land type, there is a development trend in the area based on the rainfall intensity during the concentration (I), where the trend for developed lands tends to increase, while the trend for nondeveloped lands tends to decrease.The results of the surface runoff calculations, comparing the results of the two years, also show a similar trend to the intensity of rainfall during concentration.There is a significant increase in surface runoff in industrial and residential lands.However, the overall calculations indicate an increase in surface runoff of 7.86 m 3 /second.This further strengthens the previous analysis, indicating that changes in land use have a direct impact on surface runoff.Lands that were initially functioning as water infiltration areas have been transformed into hard surfaces that cannot absorb water effectively, leading to fast surface water flow and increased surface runoff volume.

Discussion
The Babon Watershed is one of the watersheds in Central Java that has a significant role in the sustainability of the system in the Eastern Semarang region, which passes through Semarang, Semarang and Demak Regencies [7].Most of the areas in Babon Watershed are potential residential locations for the community because they are situated on the outskirts of Semarang City.Urbanization is essential in the urbanization process around Semarang City, namely Semarang Regency and Demak Regency.Through the increase in population, land is being converted into settlements.There has also been an exploitation of the riverbanks of the Babon River for industrial and residential purposes [16].This condition causes a reduction in the capacity of the Babon River to accommodate river water discharge.This is in line with previous research, which states that changes in watershed land use influence an increase in surface runoff rates [32].
Based on the research results, there was an increase in land use for residential use by 14%.This condition affects increasing water runoff from 161.73 m 3 /second in 2013 to 169.58 m 3 /second in 2023.There has been an increase in runoff water from the Babon Watershed by 5%, namely 7.86 m3/second in 10 years.In early 2023, flash floods hit the middle and downstream parts of the Babon watershed, namely Tembalang and Mranggen subdistricts [33,34].The flooding occurred due to the overflow of the Babon River due to high-intensity rain and damage to the Babon River embankment.Apart from that, the change in land use from non-built to built-up in the sub-district affects the impact of flood events.More than 500 families were affected by the flood.This condition aligns with research by [10], which states that the increase in settlements in flood plains due to urbanization causes changes in land use and additions in the number of flood events and people directly affected.
It is necessary to normalize the river so that its capacity can accommodate the discharge with the river flow [9].Spatial planning must consider the upstream and downstream areas of the watershed.Regarding land use, land use planning must cover the entire watershed [35].Flood management by applying conservation techniques in upstream watersheds, structuring upstream areas and flood plains, constructing flood control buildings, and increasing population capacity [17].This is because changes in land use in upstream areas influence the amount of runoff and discharge that occurs in downstream areas [36].Initiatives and policies are needed to encourage agricultural intensification so that agricultural land does not expand spatially and does not change the use of forest land as a catchment area [4].Based on the research results, management watershed needs collaboration between Semarang Regency, Semarang City, and Demak Regency Government in land use policy.The collaboration is an effective governance strategy to minimize problems and high costs [37], especially in handling floods in the Babon watershed.

Acknowledgement
We want to thank Diponegoro University for its support and research permits, as well as thank the Central Bureau of Statistics; United States Geological Survey; Department of Public Works, Water Resources and Spatial Planning, Central Java Province, which has supported and assisted in providing data and information for research purposes.Also, we thank Faruq Al Muqsit for his research assistance.

Conclusion
This paper has demonstrated population growth, land use changes, and surface runoff of Babon Watershed.The Babon watershed is one of the river basins in Central Java, located on the northern slope of Mount Ungaran.It passes through Semarang Regency, Semarang City, and Demak Regency.The majority of the area in the Babon watershed is a potential choice for residential locations for the community, as it is situated on the outskirts of Semarang City and falls within the peri-urban area.With this condition, there has been a significant increase in the population in the Babon watershed, leading to a higher demand for land for residential, industrial, and infrastructural purposes.Over the past ten years, the Babon watershed has experienced significant changes in land use, with nondeveloped areas being transformed into developed lands.Many previously vacant lands, plantations, or paddy fields have become developed lands.
The increasing transformation from non-developed to developed lands impacts water infiltration areas.Areas initially intended to absorb water can no longer do so due to urbanization and development.As a result, the increase in residential and industrial areas or developed lands in the upstream area of the Babon watershed can lead to faster surface water flow and increased surface runoff volume.This increases the risk of waterlogging or flooding when the water flow reaches the downstream area of the Babon watershed.Futhermore, Babon Watershed management is tricky because three regions pass it, so good governance collaboration from the government and related stakeholders is essential.The management of the Babon Watershed requires the role of various stakeholders in controlling the use of space according to the land's carrying capacity.It is necessary to regularly carry out more detailed monitoring and supervision from upstream to downstream by the Central Java Provincial Government.

Figure 1 .
Figure 1.Map of River System and Babon Watershed Provinsi Jawa Tengah (analysis by the author, 2023)

Figure 3 .
Figure 3. Map of Babon Watershed Population Density (analysis by the author, 2023)

Figure 9 .
Figure 9. Land Use Area in Babon Watershed 2013 and 2023 (analysis by the author, 2023)

Figure 10 .
Figure 10.Land Use Change in Babon Watershed 2013 and 2023 (analysis by the author, 2023) s) 0,00278 : Constant, used when the area unit is in hectares (Ha) C : Runoff coefficient I : Rainfall intensity during the concentration-time (mm/hour) A : Watershed area (Ha) • Sub-Babon Hilir: Located in the lower part of Babon Watershed, this sub-watershed is characterized by the main rivers Sungai Babon and Banjir Kanal Timur.The East Flood Canal (Banjir Kanal Timur) is a diversion from Sungai Babon in the Pucung area.Some tributaries that feed into Banjir Kanal Timur are Sungai Candi, Mongkong, and Sungai Dungadem.Some rivers directly flow into the Java Sea, between Banjir Kanal Timur and Babon Rivers, including Tenggang River, Siringin River, Leles River, Doro River, Kaidin River, and Prih River.

Table 1 .
The Population of Babon Watershed Source: Central Bureau of Statistics, 2023