Application of Remote Sensing and GIS to Estimate Above Ground Carbon Stock Changes to Support Sustainable Development: A Case Study of Mijen Sub-District, Semarang, Indonesia

Changes of vegetated land to built-up land will affect the above ground carbon (AGC) stocks in an area. These changes have an impact on climate change. This study aimed to estimate AGC stocks changes in Mijen Sub-District during 2017-2022. Mijen Sub-District is one of the suburban areas of Semarang City which has the main function as the lungs of the city. To determine the AGC stock, remote sensing data and GIS analysis are used. The results showed that during 2017-2022 AGC stock in the area decreased by 3,234.24-ton C. Estimation of changes of AGC stocks in Mijen Sub-District needed as a reference for local governments for policy making in regional and urban planning that integrates sustainable development.


Introduction
People tend to live in cities because cities are major centers of development, innovation, culture, and economic activity.Increasing economic activity in urban areas will trigger urbanization and urban growth [1,2].Urbanization will result in an increasing demand for land, so more cultivated land, forests and other vegetated land are converted into built-up land [3,4].Continued urbanization may even lead to uncontrolled expansion of the city into the surrounding sub-urban [3,5].
Mijen Sub-District is one of the suburban areas which affected by the development of Semarang City which has experienced massive physical development [6].According to the Spatial Planning (RTRW) of Semarang City for 2011-2031 Mijen Sub-District is part of the City Region (BWK) IX with the main function as an area for the development of the city's lungs [7].On the other hand, development of road network, public facilities, housing estates, and the development of the New City of BSB (Bukit Semarang Baru) have increased the attractiveness of Mijen Sub-District.This will have an impact on changes in land cover, especially vegetated land cover to built-up land to accommodate the increasing population activities.
Vegetated land such as forest and green space has the important function to support urban ecosystem services by contributing to store carbon and reduce carbon dioksida (CO2) concentration in the atmosphere [8].The changes of vegetated land to built-up land can affect AGC stocks and have an impact on greenhouse gas emissions in an area [9]  Land cover in an area can be identified using satellite imagery data and analyzed with GIS [13,14].Based on land cover data, it is possible to estimate AGC stocks.This study was conducted to obtain an overview of changes in AGC stocks in Mijen Sub-District during 2017-2022.It is hoped that the results of study can be be used as a consideration for policy makers in analyzing the impact of a land use on AGC stocks.

Study Area
Mijen Sub-District in Semarang City was selected as the study area (Figure 1).The total area is 5,653.04Ha and consists of 14 urban villages.In 2022, the population was 85.818 people [15].As a sub-urban area, this area is developing quite fast which is marked by the rapid change of vegetated land cover to built-up land.

Land Cover Identification
Image processing uses QGIS software version 3.16 Image processing includes geometric and radiometric corrections, image cropping according to the study area, and image interpretation.Visual image interpretation uses on-screen digitization based on the interpretation key (tone or color, texture, shape, size, pattern, site and association) to produce a land cover map [17].In this study, land cover is classified into seven categories: forest plantations, dry land agriculture, mixed dry land agriculture, paddy fields, built-up land, open land, and water body.The interpretation of land cover then tested for accuracy by comparing the interpretation results and the actual situation in the field.Land cover change is calculated by overlaying the land cover map of 2017 and 2022.Land cover change will be displayed in a matrix that describes the type and area of land cover change.

Estimation of Changes Above Ground Carbon Stock
The amount of carbon stock above ground for each land cover is calculated based on land cover (Table 1).The next step is to divert the existing land cover area by the unit value of the carbon stock in the land cover [18].Carbon stock changes are calculated by comparing AGC stocks in 2017 and 2022, as mentioned in the matrix.Water Body 0 For calculate carbon stocks in land conversion, it is calculated using the equation carbon stock changes (Eq.1) [18].
where ΔC is the annual carbon stock changes in each pool (tC/year).Ct2 is the carbon stock of each pool at the beginning (tC), and Ct1 is the carbon stock of each pool at the end (tC).Calculation of carbon stocks carried out by calculating the area of each type of land cover multiplied by the carbon stocks number, then add up the total carbon reserves per year.The methods used in this research include data collection, data processing and presentation analysis results.More details at flow diagram in Figure 2.

Result and Analysis
Image interpretation for seven classes of land cover in Mijen Sub-District has an accuracy of 94%.The results of Sentinel imagery interpretation in 2017 and 2022 indicate various changes in land cover (Figure 3).
Based on Table 2 and Figure 4, it is known that mixed dryland agriculture cover the biggest area of land in 2017 and 2022.Plantation forest, dry land agriculture, mixed dryland agriculture, and paddy fields decrease in 2022.The most converted land area is dryland agriculture (153.64Ha).The area of built-up land increased significantly to 329.04 Ha.Open land also increased by 55.83 Ha.Overall, it may be stated that land cover dynamics between 2017 and 2022, the majority of the vegetated land (plantation forests, dryland agriculture, mixed dryland agriculture, and paddy fields) has been converted to built-up land (Figure 5).Based on matrix of land cover changes (Table 3), emissions and carbon absorption can be calculated.Carbon absorption is calculated by multiplying the area of land cover classes that have changed from low to high carbon content per hectare.Meanwhile, emissions are calculated based on changes in land cover area from high to low carbon stocks.Carbon emissions during 2017-2022 is 11,841.12ton, exceeding carbon absortion of 2,292.15ton (Figure 6).The highest carbon emissions are caused by land cover convertion from mixed dry land agricultural, dry land agriculture, and plantations forest to built-up land especially settlements which has a less carbon stocks.On the other side, high carbon absorption is caused by changes in dry agricultural land to mixed dry agricultural land.It is important to pay attention to increasing of carbon emissions and decreasing of carbon stocks, because it can reduce the function of the Mijen Sub-District which has a function as the "lungs" of the city.The influence of Semarang City in Mijen District has an impact on reducing vegetated land.This can explain the environmental impacts of urban expansion into suburban areas [5,20].On the other hand, it should be noted that the land available for expanding activities in the city center is decreasing.
The rapid growth of built-up land in Mijen District needs attention.Because it can reduce environmental quality and reduce its function as the city's lungs.The presence of vegetation in cities makes a major contribution to reducing the negative impacts of urban activities, and improving environmental quality including improving air quality, energy conservation, reducing air temperature and ultraviolet radiation [21,22].This is due to the fact that changes in land cover will have an impact on carbon stocks and global climate change.Concern is needed for the quality of the urban environment and increased knowledge about the impacts of climate change.In order to maintain environmental quality and support sustainable development, it is necessary to monitor land use and tighten the existing permits.

Conclusion
Land cover in Mijen Sub-District changed between 2017 and 2022.Increasing built-up land and reduced vegetated land can reduce carbon stocks.Change from land cover with high carbon stocks to land cover types with low carbon stocks causes large amounts of carbon loss.The highest carbon emissions come from mixed dry land agriculture to the other land cover with lower carbon stocks.Land cover changes will continue in the future and become a challenge for policy makers to make Mijen District the lungs of Semarang City.Limitations in this study are related to the resolution of satellite imagery.Future research can use a larger image spatial resolution.In addition, research can also be carried out related to predictions of changes in carbon stocks in the coming years .

Figure 1 .
Figure 1.Mijen Sub-District as A Study Area

Figure 2 .
Figure 2. Flowchart of Research Method

Figure 4 .Figure 5 .
Figure 4. Area and Percentage of Land Cover in Mijen Sub-District in 2017 and 2022

Figure 6 .
Figure 6.Graph of Carbon Emissions and Absorption from Changes of Land Cover in Mijen Sub-District During 2017-2022 . Greenhouse gas emissions can cause global warming 1264 (2023) 012023 and climate change [10].Changes in AGC stocks in an area need serious attention.There is a close interrelationship between urbanization, climate change, and cities because changes in one can affect the other [11,12].Currently, climate change has become a critical issue in world cities [9], including the Semarang City which encourages every policy making must be integrated into sustainable development.

Table 1 .
Amount of Carbon Stock Based on Land Cover

Table 2 .
Area and Change of Land Cover in Mijen Sub-District During 2017-2022

Table 3 .
[19]ix of Land Cover Changes in Mijen Sub-District During 2017-2022From Table3.Matrix of land cover shows that increasing of built-up area during 2017-2022 came from mixed dryland agriculture (187.95ha),followed by dryland agriculture (54.23 Ha), open land (66.03 Ha), paddy field (19.79 Ha) and plantation forest (1.04 Ha).The plantation forest area decreased by 3.63 Ha and the majority changed to mixed dryland agriculture (2.94 Ha) and built-up land (1.04 Ha).Land cover changes demonstrate that the change from vegetated land to built-up land happens quickly as a result of increased activity.Above ground carbon stocks will be impacted by these developments.Table4.shows that in the Mijen Sub-District, the total carbon stock was 171,305.91 ton C in 2017 and 168,071.67 ton C in 2022.Overall, carbon stock decrease by 3,234.24tonsC, it means annual change in carbon stocks decrease by 646,85 ton C.This shows that changes in land cover in an area can indicate the dynamics of carbon stocks in that area[19]

Table 4 .
Above Ground Carbon Stocks based on Land Cover in Mijen Sub-District During 2017-2022 This research was financially supported by The Faculty of Engineering, Diponegoro University, Indonesia through Strategic Research Grant 2023.The authors would like to thank to The Government of Mijen Sub-District for the supporting data and to Citra Kusuma for the survey and collecting data.Pratama L D Y and Danoedoro P 2020 Above-ground carbon stock estimates of rubber (hevea brasiliensis) using Sentinel 2A imagery: A case study in rubber plantation of PTPN IX Kebun Getas and Kebun Ngobo, Semarang Regency IOP Conf Ser Earth Environ Sci 500 [11] Chan N W 2017 Urbanization, Climate Change and Cities: Challenges and Opportunities for Sustainable Development Asia-Pacific Chemical, Biological & Environmental Engineering Society (APCBEES) International Conference 1-12 [12] Buchori I, Pramitasari A, Pangi P, Sugiri A, Maryono M, Basuki Y and Sejati A W 2021 Factors distinguishing the decision to migrate from the flooded and inundated community of Sayung, Demak: A suburban area of Semarang City, Indonesia International Journal of Disaster Risk Reduction 52 101946 [13] Dahy B, Issa S, Ksiksi T and Saleous N 2020 Geospatial Technology Methods for Carbon Stock Assessment: A Comprehensive Review IOP Conf Ser Earth Environ Sci 540 [14] Sejati A W, Buchori I, Rudiarto I, Silver C and Sulistyo K 2020 Open-source web GIS framework in monitoring urban land use planning: Participatory solutions for developing countries Journal of Urban and Regional Analysis 12 [15] BPS Kota Semarang 2023 KOTA SEMARANG dalam Tahun 2023 [16] Munthe C R and Sulistiyono N 2023 Above Ground Carbon Estimation Using Sentinel-1B RADAR Satellite Imagery J Phys Conf Ser 2421 [17] Bakker and Jensen L L F 2001 Principles of Remote Sensing ed L L F and H G C Jenssen (Hengelosestraat: The International Institute for Aerospace Survey and Earth Sciences (ITC),) [18] Tosiani A 2015 Buku Kegiatan Serapan dan Emisi Karbon Serapan Dan Emisi Karbon 1-41