Stormwater Utility Fee Estimation Method for Individual Land Use Areas

In South Korea, a reasonable rate system that can be domestically applied to calculate the sewage and stormwater separately from the domestic sewage fee system is needed. This study proposes a phased pricing scenario to separate sewage and stormwater in Bupyeong-gu, Incheon, and the rate changes are compared based on a simulated calculation of the stormwater utility fee. In this investigation, stormwater runoff cases from other countries and the current domestic system are analyzed. A stormwater utility fee introduction scenario is presented that considers the impervious surface area. Water and sewage usage statistics and hydrant spatial data were collected from the Incheon Metropolitan City Waterworks Authority, and the total amount of water and sewage fees from the land use area were calculated. The stormwater utility fee was calculated, and the rates of each step were compared. The total sewage fee of Bupyeong-gu during 2014 was 21,685,446,578 won and the phased stormwater utility fee was calculated, assuming that 40% represents the stormwater cost. The sewage fee for the residential area in phase 3 decreased by 0.77% compared to phase 1. For the commercial areas, the sewage fee decreased by 36.87%. Because the impervious surface ratio was high, the overall area contributing to the impervious surface area was small. In the industrial area, the sewage fee increased by 8.35%. In the green area, the sewage fee increased by 37.46%. The sewage fee for the apartment complexes decreased by 10.6%. Finally, the possibility of estimating the actual stormwater utility fee is conrmed.


Introduction
Rapid urbanization is presently causing a variety of environmental problems in South Korea. In particular, having an increase in impervious surfaces has resulted in increasing nonpoint pollutants and the river environment is becoming polluted. This is because the impervious surfaces can increase the stormwater runoff, thereby carrying a variety of pollutants on the surface into the rivers or lakes (Kim et al. 1993; Kang et al. 2014).
Consequently, the development of low impact development (LID) and green stormwater infrastructure (GSI) technologies and the installation of the related facilities are increasing worldwide to reduce the non-point pollutants and to control the impervious surfaces (Dietz 2007;Che et al. 2014). Developed countries such as the US and Germany are enforcing the stormwater utility fee to raise funds for technology development and other related projects. The stormwater utility fee charges a fee to individual parcel owners by estimating the stormwater runoff in proportion to the impervious surface area of each parcel that is based on the "polluters pay principle," which is an environmental policy principle in most countries (Parikh et (Kim et al. 2015). However, South Korea still has many obstacles in terms of introducing and implementing stormwater utility fees and many technical and legal problems must be resolved. The stormwater utility fee is calculated based on the impervious surface area that is used to cover the cost by collecting more charges as the number of nonpoint source emissions increases, which takes into account the land cover condition. Hence, it is essential to secure spatial data with the statistics of the impervious surface area, which constitute a scienti c basis.
Recently, the Ministry of Environment has suggested methods that use land cover maps, digital topographic maps, and aerial orthophotographs in order to create an impervious surface map to support the stormwater utility fee system (MOE, 2012). Lee (2018a, b) examined the methods of producing an impervious surface map as suggested by the MOE and veri ed the advantages and disadvantages of each method through a pilot production. As a result, a map production method that reclassi es the land cover map properties has the highest accuracy. Due to the production cycle of the land cover map, it is impossible to accurately and precisely classify the impervious surfaces that only uses single space data. Thus, to improve the spatial resolution and accuracy, Lee (2018a, b) suggested optimal methods to produce large-scale impervious surface maps by using aerial photographs, digital topographic maps, and road views on Internet portal sites. Then, a geographic information system (GIS)-based impervious surface ratio estimation method that uses a large-scale impervious surface map was de ned, and the impervious surface ratio statistics were calculated for the land use and parcel units.
The impervious surface area is a measure that can indirectly represent the stormwater runoff amount and nonpoint pollutant generation, and it is a rational criterion for the stormwater utility fee. For this reason, most developed countries use the impervious surface areas as a criterion in order to charge a fee. The current domestic sewage fee system is unreasonable because the water users must pay the cost of treating the stormwater runoff. To solve this problem, a reasonable sewage fee system is required in South Korea that can calculate the stormwater treatment cost separately from the sewage fee.
Therefore, this study calculated the stormwater utility fee by simulating a GIS-based large-scale impervious surface map that was produced from a previous study. In addition, it is applied as a pilot to the land use areas on a large scale and to apartment complexes on a small scale. First, stormwater utility fee systems from the US and Germany are analyzed and the implications are derived, and the current domestic system is examined. The fee estimation scenario is established in three phases to separately calculate the sewage fee and stormwater runoff processing fee. Then, the monthly water and sewage usage in the study area and the hydrant spatial data in Shape le format were collected. The properties of the water and sewage usage and the rates were input into the hydrant data, which overlapped with the serial cadastral map. The sewage fee was separately calculated from the water and sewage usage by using Microsoft Excel in accordance with the rate table and it was used as a basis for the stormwater utility fee estimation scenario. Finally, the changes in the stormwater utility fee were checked by applying it to each land use area and apartment complex while considering the number of households and the unit area size, and the results are discussed.

Study Area
Bupyeong-gu, Incheon, South Korea, which has an area of 31.98 km 2 , was selected as the target area for this study (Fig. 1). Bupyeong-gu is a typical downtown area, which includes a variety of sectors such as residential areas, commercial areas, and industrial areas. Since 2009, impervious surfaces have increased in this area along with the continuous promotion of several projects that include improving residential and urban environments ).
Furthermore, river ooding due to the localized heavy rains has caused the pollutants of urban sewage to ow into the Gulpo stream in Bupyeong-gu, which can lead to serious problems in terms of the generation and out ow of non-point pollutants. Thus, impervious surface and non-point pollution management are urgently needed in this area (Yang 2006). Lee (2018b) created an optimal large-scale impervious surface map of Bupyeong-gu, Incheon, and determined that the area consisted of a pervious surface area of 11.43 km 2 , an impervious surface area of 20.5 km 2 , and a water system area of 0.15km 2 . An impervious surface map of the land use area was drawn by overlapping the impervious surface map with the land use area map. The study area corresponds to an "urban area" in the classi cation of the properties of the land use area map. As shown in Fig. 2, the impervious surface ratio was 82.1% in the residential area, 91.7% in the commercial area, 94.1% in the industrial area, and 30% and in the green area. In particular, the impervious surface ratios of the residential, commercial, and industrial areas were higher than 80%. This indicates that there is an urgent need to manage and improve the impervious surfaces.
2.2. Analysis of the Domestic and Overseas Application Cases of the Stormwater Utility Fee 2.2.1 Application case of the stormwater utility fee in the US In the US, the federal government assigned the responsibility of managing the stormwater runoff that contains pollutants and sediments to the regional governments. As a result, this increased the stormwater runoff management cost of the regional governments, and they needed to establish a nancing plan. The stormwater utility fee system was introduced in the 1960s, and in the beginning, the rates were charged based on the water usage. In the 1990s, the Water Environment Federation issued the "User-Fee Funded Stormwater Utilities." As the emergence of GIS technology enabled the detailed measurement and management of the impervious surfaces, many municipalities changed the rating system to charging based on the impervious surface area. The collected fees are being used by the municipalities for the sewage pipe management cost as well as the facility investment, controlling the stormwater runoff, and treatment.
The city of Charlotte, which is an urban area, has a two-level rating system. Charlotte charges $5.18 USD per month if the impervious surface area is 2,000 ft 2 or less if it is a single family residence, and $6.72 USD per month if it is larger than 2,000 ft 2 . Montgomery County's charging system has seven levels depending on the impervious surface area, and it is based on the equivalent residence unit (ERU). For apartment houses and nonresidential buildings, they charge $88.40 USD per year per 1 ERU (2,406 ft 2 ) for the actual calculated impervious surface area ( Table 1). The city of Portland developed charges that are based on the average impervious surface area of 2,400 ft 2 for single-and two-family houses, and an average impervious surface area of 1,000 ft 2 for threeand four-family houses. For multi-family houses, commercial areas, and industrial areas that exceed them, the rates are charged based on the impervious surface area (Table 2). Washington D.C. uses a progressive rating system with six levels of impervious surface areas. The charging criterion for the residential area is $2.57 USD per month per 1 ERU, and for nonresidential areas, the rates are charged in proportion to the actual impervious surface area (Fig. 3). On-site $3.69 per month per 1,000 ft 2 for the impervious surface area *The stormwater runoff fee is separately charged between on-site (private property) and off-site (right to use public facility).
As a result, most municipalities charge a stormwater utility fee based on the impervious surface area of each parcel, and the imposition criterion was changed from the parcel area or the water usage to the impervious surface.
When comparing the impervious surface area, the representative value (e.g., ERU) of the impervious surface area was set for each municipality and the fees were calculated accordingly. For the residential areas, some municipalities applied the same rate of 1 ERU to every parcel while considering the size and management cost of the city. Meanwhile, other municipalities applied different rates by setting different levels of the impervious surface area.

Application case of the stormwater utility fee in Germany
In 1985, the Federal Administrative Court and Local High Courts of Germany declared that charging sewage fees based on the water usage was problematic in terms of fairness (Kwon and Hur 2010). The Federal Administrative Court of Germany ruled that in order to improve the legal fairness of the fees that are based on the polluters pay principle, the sewage fees must be collected separately for the stormwater runoff processing fees and general sewage fees. Consequently, the municipalities changed their fee collection system and established a legal basis for urging the utilization and in ltration of stormwater. In addition, they re ected the speci c matters for the application of the separate calculation method for the sewage ordinance in accordance with the circumstances of the city.
In the case of Berlin, the water bill is divided into a basic fee and usage-based fee, and the sewage fee is divided into wastewater, stormwater, manure, and manure sludge fees. The wastewater fee is charged in proportion to the water discharge (water usage) in addition to the basic fee, whereas the stormwater fee is charged based on the impervious surface area (Table 3). Berlin changed to a sewage separate fee system in 2000 and they charged 1.897 Euros per 1 m 2 for the impervious surface in addition to the sewage fee. In general, the stormwater utility fee is charged based on the at rate, impervious surface area, development density, and the generation of stormwater runoff. The at rate charges the same fee for every household; however, it is di cult to apply the same rate to areas that have different discharge amounts. The stormwater runoff generation creates high administrative costs for the monitoring and measurements. The development density method charges the fee according to the ratio of the impervious surface area for the total land. However, it is di cult to implement this in comparison to the ERU method because the pervious and impervious surfaces must be considered. In contrast, the method of charging the fee that is based on the impervious surface area of each parcel is mainly used in developed countries because it is an indirect indicator of the stormwater runoff and generation of non-point pollutants. Furthermore, the speci c implementation methods of the system, such as the charging and collecting methods, did not differ signi cantly between the two countries (Table 4).
Therefore, the optimal large-scale impervious surface map by Lee (2018a) can be used as the basic data for calculating the stormwater utility fee. If the impervious surface area is applied based on the stormwater utility fee, the fee decreases slightly for users with a small impervious surface area such as detached houses; however, it increases signi cantly for users that have a large impervious surface area, such as commercial buildings, which is fair. The stormwater utility fee system is advantageous in terms of being able to cover the impervious surface management cost of private properties that can cause stormwater runoff that ows into public sewage treatment facilities. Like the US and Germany, which have similar problems, South Korea should also prepare related management funds by charging fees that are based on the impervious surface of each parcel. Table 4. Comparison of the funding characteristics for the stormwater utility fee between the US and Germany.
Division US (stormwater utility fee) Germany (regenwassergebühr) Background -Deterioration of the water quality by the stormwater runoff -Reinforced stormwater runoff regulation of the environmental protection agency (EPA) and stable stormwater management funding -The rate system was reformed according to the ruling of the Federal Administrative Court that the existing stormwater and wastewater combined rate system is not fair.
Charging criteria -Stormwater fee is charged based on the polluters pay principle.
-Charged in proportion to the past water usage.
-Charged in proportion to the impervious surface area of each parcel -Municipalities use different methods for the fee calculation However, the cost for the combined sewage pipes is classi ed as the wastewater treatment cost even though the wastewater and stormwater ow together and they are not classi ed by the appropriate ratio of the wastewater and stormwater. Thus, it appears on the surface that the domestic sewage fee system does not include stormwater; however, it can be internally interpreted that it includes stormwater. It has an unreasonable structure in which the water applications (including groundwater) generally pay for the stormwater treatment costs. To improve this unreasonable structure, a reasonable sewage fee system that separates wastewater and stormwater fees and replaces the stormwater fee with a stormwater utility fee is required.
In the "2050 Sewerage Vision Public Hearing" that was held in 2012, a "stormwater utility fee" design method for stormwater management funding was presented. The targets of the stormwater utility fee were land owners that have their land hydraulically connected to public stormwater (wastewater) pipes. It was suggested that the standard cost covers the cost that is required for stormwater management such as conduit and stormwater treatment, and the method of imposition is different according to the land cover, which is a factor that in uences the stormwater discharge. To introduce the stormwater utility fee, a method of amending the current law should be considered because enacting a new law can result in large resistance. In addition, the stormwater utility fee should be applied in a phased manner, and it must be preceded by the people's empathy and understanding. In 2012, at the "policy discussion meeting for introducing a rainwater tax in Seoul," there was a case of serious opposition such as a resistance to tax increases and transferring the responsibility to create an impervious surface to the citizens. To enhance the people's acceptance of the stormwater utility fee system, phased applications that are based on South Korea's current rate system must be considered. The separate calculation and settlement of the stormwater utility fee must be implemented step by step. This can be achieved by separating the sewage fee into wastewater and stormwater in the beginning and later switching to the fee calculation while considering the impervious surface area of each parcel.
When the fee is linked with the stormwater management, the fee must be calculated while considering the impervious surface area that is based on the current land cover information. Therefore, a concrete scenario is required, which consists of the following three phases. In phase 1, the current sewage fee is simply separated into wastewater and stormwater. In phase 2, the impervious surface ratio based on the land use area is applied. In phase 3, the fee is speci cally calculated for the individual parcels.

Simulated calculation of the stormwater utility fee for the study area
According to Lee (2018b), the total area of all the apartment complexes in Bupyeong-gu was approximately 3.65 km 2 , and the total impervious surface ratio was 75.8%. When considering the 189 complexes, 43 had no previous surface with a 100% impervious surface ratio. Although some complexes have ower gardens, they were classi ed as impervious surfaces if they did not play the role of a pervious surface. When considering the apartment complexes in Bupyeong-gu, the lowest impervious surface ratio was 57.1%, and the highest impervious surface ratio was 92.9%, which excludes 43 complexes with a 100% impervious surface ratio. According to the building ordinance of Incheon, parcels with an area of 2,000 m 2 or larger must include a 15% landscape area; however, some apartment complexes do not comply with this rule. Therefore, impervious surfaces need to be actively managed in apartment complexes with an impervious surface ratio that is 85% for higher.
Consequently, for the simulated calculation of the stormwater utility fee in the study area (Bupyeong-gu, Incheon), the sewage fee was separated into wastewater and stormwater fees, and the fee calculation that considered the impervious surface area was presented in phases. These calculated phased fees were applied to a large apartment complex while considering the number of households and the unit area type in order to check the changes in the stormwater utility fee. Bupyeong-gu, Incheon is currently charging wastewater and stormwater management costs together as a certain ratio of the water fee in proportion to the water usage. Therefore, the monthly water usage statistics of Bupyeong-gu, Incheon were collected, and the fees were determined by calculating the actual sewage fee.
To change the fee system to a fee linked with stormwater management, the fee must be calculated while considering the impervious surface area that is based on the land cover status. Hence, a scenario is required, which consists of the following three phases. In phase 1, the current sewage fee is simply separated into wastewater and stormwater. In phase 2, the impervious surface ratio that is based on the land use area is applied. In phase 3, the fee is speci cally calculated for the individual parcels. The phased hypothetical scenario was established and the fees were calculated by performing a simulation to check the change in the the stormwater utility fee based on land use area (Fig. 4).

Collecting water usage statistics and data processing
When considering the data to calculate the stormwater utility fee, the statistics for the monthly water and sewage usage and the fees in 2014, along with the hydrant spatial data for the Shape le format, were gathered from the Incheon Metropolitan City Waterworks Authority. The water and sewage fees were calculated by converting the water and sewage rate table into a function using Microsoft Excel. These were used as the basis of the stormwater utility fee estimation scenario. It was determined that there were missing data for the monthly water and sewage usage along with the fees due to the closure and movement of the hydrants. In this case, the business category was estimated and the aperture size was calculated in reverse based on the fee, and the related information was added. The improvement results were converted to space data by combining them with the hydrant spatial data. In addition, the properties of the water and sewage usage and fees were inputted into the serial cadastral map, which overlapped with the hydrant spatial data. In some cases, one hydrant was used by multiple parcels, in which the aerial orthophotograph and digital topographic map were checked. They were joined and converted to a single parcel on the serial cadastral map by using the "merge" function of ArcGIS 10.2. The impervious surface ratio of the joined parcels was calculated again, and the water and sewage usages and fees were inputted into the properties of the serial cadastral map. Finally, the water and sewage fees resulted in a calculation of 68,457 results in Bupyeong-gu.

Simulated calculation scenario for the stormwater utility fee
In the phase 1 scenario, when considering the people's acceptance, the current sewage fee was divided into the wastewater and stormwater fees by using a simple ratio. When the sewage fee is collected, the stormwater cost is charged as a stormwater utility fee. In this study, a general distribution ratio of 6:4 was applied. Table 5 shows the differences in the charging method between the current sewage fee and the phase 1 scenario. Meanwhile, in the phase 1 scenario, the fee is calculated irrespective of the impervious surface ratio for each land use area and parcel. However, the total amount of the stormwater utility fee in Phase 1 was used as the reference amount for the fee distribution in phases 2 and 3. -The existing total amount is separated as the sewage fee and stormwater utility fee.
(e.g., sewage (wastewater) fee of 4,800 won (60%), stormwater utility fee of 3,200 won (40%)) As for phase 2, Lee (2018b) reported that South Korean cities have a high impervious surface ratio for each land use area, and there is no signi cant difference between the parcels in the land use area. Furthermore, unlike other countries, South Korea has many apartment complexes, and most of the detached house districts have a high impervious surface ratio. When considering these facts, the fee by the land use area was applied in phase 2 while considering the administrative convenience. The stormwater utility fee in the total sewage fee is calculated by distributing the fee in consideration of the average impervious surface ratio for each land use area. The concrete fee calculation method is presented as follows.
For the "total stormwater utility fee" in Eq. (1), the total stormwater utility fee that is calculated in the phase 1 scenario (corresponding to 40% of the total sewage fee) is used. For the impervious surface area that is based on the land use area, the calculation result from the impervious surface map was produced from a previous study (Lee 2018a).
The phase 3 scenario is similar in comparison to the developed countries and it can be implemented after the fee for the stormwater runoff management is stabilized. The impervious surface ratio is calculated for the individual parcels, and the fee is determined in proportion to the calculated impervious surface ratio. For the total sewage fee, the stormwater utility fee is distributed and the contribution rate of each parcel is considered. The concrete fee calculation method is expressed as follows.
The phase 3 scenario can be used to calculate the impervious surface ratio of the individual parcels irrespective of the land use area. It is fair and reasonable because the contribution to the stormwater runoff management can be applied quantitatively.

Water and sewage usage and the fee calculation based on the land use area
The water and sewage usage and the fees by the land use area were calculated based on the serial cadastral map. The total water usage, total sewage usage, and total water and sewage fees for Bupyeong-gu from January to December 2014 were 56,493,980 m 3 , 55,882,048 m 3 , and 75,790,569,320 won, respectively.
The water and sewage usage and fees were the highest in the residential areas and the lowest in the green areas (Table 6). When considering the total water and sewage fees, the sewage fee was calculated as 21,685,446,578 won, and it was used as the reference amount for the stormwater utility fee scenario.

The fee application result based on the land use area scenario
The calculation results for the sewage fee and stormwater utility fee based on the land use area for phases 1 to 3 are summarized in Table 7. In this table, the "total" means the total sewage fee, which includes the wastewater and stormwater fees, "wastewater" represents the wastewater treatment fee, and "stormwater" denotes the stormwater utility fee. The annual total sewage fee remained the same at 21,685,446,578 won ($19.7 million USD) in every scenario, and the sewage fee was determined by changing the ratio of the stormwater utility fee in each scenario. The results were then compared (Table 8). As it moved from phase 1 to phase 2, the stormwater utility fee of Bupyeong-gu, Incheon decreased in the commercial area; however, it increased in the residential, industrial, and green areas. Then, as it moved from phase 2 to phase 3, the stormwater utility fee of Bupyeong-gu, Incheon decreased in the residential and commercial areas, but it increased in other areas.
The sewage fee of the residential area decreased by 0.77% in phase 3 in comparison to phase 1. Since there are many apartment complexes in South Korea, the fee is expected to decrease when considering the number of households and the unit area type. When considering commercial areas, the impervious surface ratio is high, but as a whole, the area contributing to the impervious surface is small. Thus, the stormwater runoff fee decreased by 36.87%. Since the occupied impervious surface area is not large in comparison to the water usage, the stormwater fee decreased when it is proportional to the impervious surface area. In the industrial area, the water usage is similar to that of the commercial area; however, the total impervious surface area is 1.9 times larger than that of the commercial area. As a result, the sewage fee increased by 8.35%. In the green area, the water usage is small, but the total impervious surface area is large; thus, the sewage fee increased by 37.46%. Moreover, there are many public facilities in addition to military facilities in the green area in Bupyeong-gu; hence, the stormwater runoff management burden increased.  563  570  553  526  600  548  568  578  577  575  553  558   Stormwater  4,839  405  418  396  374  413  391  405  418  413  413  393  400   Commercial  area   Total  3,902  318  335  310  297  311  317  334  351  336  341  323  329   Wastewater 3,115  252  267  246  236  244  253  268  283  269  274  259  264   Stormwater  787  66  68  64  61  67  64  66  68  67  67  64  65   Industrial  area   Total  3,409  295  311  284  264  281  271  281  295  287  290  272  277   Wastewater 1,933  171  183  163  150  155  152  157  168  161  164  153  155   Stormwater  1,476  124  128  121  114  126  119  124  127  126  126  119  122   Green area  Total  2,771  238  242  228  214  248  225  229  231  235  234  218  227 Wastewater  563  570  553  526  600  548  568  578  577  575  553  558   Stormwater  4,426  371  383  361  341  378  358  371  382  377  378  360  366   Commercial  area   Total  3,793  309  326  301  288  302  308  325  341  327  332  314  320   Wastewater 3,115  252  267  246  236  244  253  268  283  269  274  259  264  Green area  Total  3,192  273  278  263  247  284  259  264  267  270  270  253  262   Wastewater 1,198  106  106  100  93  113  98  97  95  101  99  91  97   Stormwater  1,994  167  172  163  154  171  161  167  172  169  171 162 165 ** Area to be charged = Total area of Bupyeong-gu -Area occupied by the municipality and central government (e.g., road, mountain, river) 3.3 Results when applying the fee estimation scenario for an apartment complex while considering the number of households and the unit area type The sewage fee charged to the "Bugae P" apartment complex (1,102 households in total), the largest among the apartment complexes in Bupyeong-gu, was calculated for each phase. This apartment complex belongs to the type two general residential area when considering the land use areas. The total area was 56,823.8 m 2 , the impervious surface area was 35,079.9 m 2 , and the impervious surface ratio was approximately 61.7%. The monthly average sewage fees that were charged to the total apartment complex were 8,741,013 won in phase 1, 7,926,900 won in phase 2, and 7,814,013 won in Phase 3. As a result, the fee decreased by 10.6%. The monthly average stormwater utility fee was 3,496,405 won in phase 1, 2,682,292 won in phase 2, and 2,569,405 won in Phase 3. As it went from phase 1 to phase 3, it decreased by approximately 26.51% (Table 9). In addition, phased fees were applied and compared while considering the unit area type of the households in the apartment complex. A higher fee was applied to the households with a larger unit area, even in the same apartment complex. The fees were distributed while considering the number of households in each unit area type. The total impervious surface area of the apartment complex was divided by the total number of unit areas (1 pyeong ≈ 3.3058 m 2 ) to calculate the impervious surface area per pyeong.
The responsible impervious surface area per pyeong was determined to be 0.856 m 2 . When this value is multiplied by the number of pyeong in each household, the stormwater utility fee that is proportional to the impervious surface area for each household can be determined.

Conclusions
This study proposes a phased separate estimation scenario for wastewater and stormwater fees in order to create a stormwater utility fee system that is suitable for South Korea. This is achieved while considering the impervious surface area and the changes in the fees, which were compared by applying this scenario. To that end, a variety of overseas cases and domestic current systems were analyzed and discussed. In addition, the statistics for the impervious surface area of the optimal large-scale impervious surface map that was produced in a previous study were used. The impervious surface area based on the land use area and parcel was calculated with an overlap with the serial cadastral map, and the fees were calculated based on the water and sewage usage between January to December 2014 in Bupyeong-gu, Incheon. Finally, they were applied as a pilot to each land use area and an apartment complex, and the changes in the fees in each phase were compared.
The total sewage fee between January to December 2014 in Bupyeong-gu was determined to be 21,685,446,578 won($19.7 million USD). Under the assumption that the share of the stormwater utility fee was 40% of the total sewage fee and the total fee does not change, the stormwater utility fees in each phase were calculated. When the stormwater utility fees for each land use area were calculated, the fees of the residential area and commercial area decreased as the phase progressed, whereas the fees for the industrial area and green area increased. In particular, the stormwater utility fee for the residential area decreased by 1.9% in comparison to the existing fee. Since South Korea has many apartment complexes, the fees are expected to decrease even further when the number of households and the area unit type of the apartment complexes are considered. To verify these assumptions, a large-scale apartment complex was randomly selected and the total sewage fee charged to the apartment complex was calculated. Then, the fees were calculated based on the impervious surface area in the parcel while considering the number of households and the unit area type. It was determined that for a large-scale apartment complex with a large pervious landscape area and a large number of households, the stormwater utility fee decreased by approximately 26.5% in comparison to the existing fee. This study veri ed the possibility of estimating the stormwater utility fee by using an impervious surface map that was produced in a previous study and the impervious surface ratio calculation method.
In the future, for the proper management of non-point sources and stormwater runoffs, a stormwater utility fee system that considers the impervious surface area and is appropriate for South Korea needs to be introduced and operated. However, in order to apply the stormwater utility fee, thorough preparations are required, such as discussing nancing and the responsibilities of the public and private sectors for stormwater management. The reason for this is that when a stormwater utility fee is collected without the citizens' understanding, the citizens may provide resistance. As a result, it is necessary to improve the water environment by using the necessary nancial resources and actively investing in water environment management as well as the implementation of a stormwater utility fee system. Furthermore, this system should be promoted based on the basic fee estimation method after the cost is allocated and calculated according to reasonable standards and grounds for each municipality. In addition, it needs to take into account the future impervious surface area management cost, the capacity to bear the cost, and the property value.

Declarations
Funding: (information that explains whether and by whom the research was supported) -Not applicable Availability of data and material: (data transparency) -Not applicable